# What limits the speed of an air compressor?



## strantor (May 3, 2020)

Is it heat buildup in the compressor head? Availability of lubricant to the crank? Other?

Let me explain the reason for my question. Compressors typically run off a simple pressure switch; on or off, 60Hz, fixed speed, fixed displacement. The CFM at 40psi and the CFM at 90 psi are usually pretty close, at least for those which use an induction motor.

For example a 5hp compressor, when you first turn it on, starts spinning at a more-or-less fixed speed, delivering a more-or-less fixed CFM, but consuming much less than 5hp. The amount of power consumed begins to climb as pressure climbs, and probably don't utilize all 5HP until right before the pressure switch drops out. The rest of the time before that, the true potential of that big motor is never utilized.

BUT if I were to put that motor on a VFD and (assuming motor is 3ph vector duty, with bearings rated for it) set the max frequency output to say 240Hz instead of 60Hz, and put the drive in torque mode instead of speed mode, then:
1. as soon as you turn on the compressor it ramps up to 4x the normal speed.
2. As pressure starts to build and the motor starts to see some resistance, it starts to slow down. This point at which it starts to slow down is the point at which it hits its 5hp limit, which it will stay at for the remainder of the cycle.
3. As the pressure continues to build, the speed continues to drop, until (assuming I've got all the parameters set correctly) it should fall just down to 60Hz right before the pressure switch cuts it off.

So the result is:
1. a much better utilization of the motor
2. A much faster charge-up time
3. Higher CFM (in fact much higher than nameplate rated) at lower pressures (and nameplate rated CFM at nameplate rated max pressure) instead of a more-or-less constant CFM.

Instead of a linear HP graph starting at zero and ending at 5hp right before pressure switch cutoff, we see a fast climb to 5hp and a plateau at 5hp, continuing until pressure switch cutoff.

So that's what leads me to my question. Is it safe to 4x overspeed a compressor head when output pressure is below 1/4 of max pressure? Is it cooling that's the issue? Because if it is, then I suspect it's a non-issue. The heat from compression at lower pressures will be less than at max pressure (right?) And in addition the cooling fan/pulley will be spinning so much faster than normal, that if anything, I suspect the compressor might actually experience cooler operating temps than normal.

But if the concern is oil starvation then I probably need to be daydreaming of something else entirely. I don't know if there's anything that can be done about that. There's probably also more important questions that I don't know enough to ask. So I petition this panel of online experts. What say you?


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## FOMOGO (May 3, 2020)

I think you would see the same result as running your car for extended periods at 8k rpm, i.e. early demise. Compressors are designed to run at a fixed r.p.m., and the newer ones aren't built nearly as well as the older ones. Mike


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## strantor (May 3, 2020)

FOMOGO said:


> I think you would see the same result as running your car for extended periods at 8k rpm, i.e. early demise. Compressors are designed to run at a fixed r.p.m., and the newer ones aren't built nearly as well as the older ones. Mike


Any insight into exactly why? Is it bearings? Is it something that could possibly be upgraded to withstand the new higher speed?


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## silverhawk (May 3, 2020)

Air compressors work with a piston, and usually a single piston at that. There is no real way to balance that out so you can ramp up speed without occurring to much vibration. You will be replacing bearings often in the driving motors when you push the speed up, and likely even re- sleeving and changing pistons out with some regularity. If you stop to think about it, the technology is old enough that some one would be selling a high speed compressor for a lot of money by now if it could take the speed changes.

joe


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## FOMOGO (May 3, 2020)

A combination of stroke, generally long, oil supply, often splash, and if pressure fed, generally only 10-15psi at speed, balancing of rotating assembly, minimal, ring pack, and piston to bore clearances, etc.. Mike


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## Lo-Fi (May 3, 2020)

One key thing here: small compressors are usually splash lubricated, they do not have pressure lubricated big end bearings. The reciprocating forces climb steadily with rpm, which will overwhelm the oil film at some point. The connecting rod won't be designed for those forces either, and you may find that the reed valves won't like running that fast. A little way over rated rpm is going to be fine, but several times is going to end in disaster. 

Look at it another way, though: on any given supply, running an induction motor is limited by the startup current. With the soft start the VFD offers, you can run a much larger compressor than you would on a switch. That's worth looking at!


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## strantor (May 3, 2020)

silverhawk said:


> Air compressors work with a piston, and usually a single piston at that. There is no real way to balance that out so you can ramp up speed without occurring to much vibration. You will be replacing bearings often in the driving motors when you push the speed up, and likely even re- sleeving and changing pistons out with some regularity. If you stop to think about it, the technology is old enough that some one would be selling a high speed compressor for a lot of money by now if it could take the speed changes.
> 
> joe


Most of the compressors in the HP range that I'm looking at are 2 cyl. And even the single cylinder models I believe have a counter weighted flywheel. Still, I'm sure being designed for such low RPMs, they are not balanced to the specs that would be called out for something spinning 4x as fast. Maybe the speed range could be improved by pulling the crank+flywheel and doing a very exact balancing of it? 

You mention having to re-sleeve the compressor often. Why is that? Does higher speed cause more wear per stroke? Because this increased speed would not equate to more strokes in the lifetime of the compressor. Same number of strokes to reach full tank, we just get there faster.

The motor bearings can be upgraded. Whether or not the rotor will hold together in one piece is another story. Many vector duty motors these days are built to go well above 60hz and that's what I would be using. Pushing them to 240hz is probably a stretch but 120hz is quite common. I used 240hz (4x speed) as an example and I probably should have used a more realistic example.

Your point about old technology and someone would be doing it already, is a pretty safe point to make, but there are a lot things that go into it. The VFD and vector duty motor are expensive; things most consumers would probably not pay extra for. I just happen to have them laying around waiting for a purpose. And this would almost certainly have to be a consumer-level product. In the industrial market, there are screw compressors which would beat the pants off of any piston compressor in terms of reliability/longevity.


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## ddillman (May 3, 2020)

go with a screw compressor it would hold up to the higher rpm's


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## Ulma Doctor (May 3, 2020)

you would do better trying to figure how to get a larger volume pump, rather than trying to overspeed a small pump to gain performance.
if you overspeed a small pump, there is less time to draw in air to be compressed
the overheated air will transfer that heat into the compressor, to attempt to dissipate the best it can
the air charge will be less dense and require more volume to occupy the same space
the charge will most likely condensate and now you have a higher chance of water contamination and the stresses that causes

i can go on but, hopefully the picture has been painted


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## strantor (May 3, 2020)

Lo-Fi said:


> One key thing here: small compressors are usually splash lubricated, they do not have pressure lubricated big end bearings. The reciprocating forces climb steadily with rpm, which will overwhelm the oil film at some point. The connecting rod won't be designed for those forces either, and you may find that the reed valves won't like running that fast. A little way over rated rpm is going to be fine, but several times is going to end in disaster.
> 
> Look at it another way, though: on any given supply, running an induction motor is limited by the startup current. With the soft start the VFD offers, you can run a much larger compressor than you would on a switch. That's worth looking at!



Ok reed valves, that's a gotcha that I don't have an answer for. Also the inertia of the connecting rods, that's a stump. Thanks for the brain fodder.

Sooo.... let's see. I suppose I could oversize the compressor head in relation to the motor and gear it down. Since I already used 5hp and 240hz as my example I'll just continue with those numbers... instead of a 5hp motor and a 5hp compressor head, I use a 5hp motor and a 20hp compressor head. I use 4:1 double-reduction between motor and compressor. At start-up, the motor is spinning 4x its nameplate 60Hz RPM and the compressor is spinning exactly at its rated RPM. At start-up the 5hp compressor is delivering the volume of a 20hp compressor (like 60 CFM vs 12CFM). As pressure builds, the motor motor slows down as before, and as pressure approaches max, the compressor has slowed to where it's delivering about what what a 5HP compressor should be delivering right before the pressure switch cuts out.

How bout that?


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## strantor (May 3, 2020)

Ulma Doctor said:


> you would do better trying to figure how to get a larger volume pump, rather than trying to overspeed a small pump to gain performance.


I think we had the same idea at the same time...


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## JimDawson (May 3, 2020)

Doing a little research it seems that most recip compressors in that HP range limit out at 1050 to 1300 RPM. Even the ones with pressure lubrication.  Not sure why that is, maybe mechanical limits.

Your logic is sound, but it seems that an appropriate pump is not available.  The variable speed compressors are all rotary screw, and I know that some of those will run up to 6700 RPM.

What might make sense is to run two 5HP pumps with pulleys sized to turn the pumps at 1050 RPM at maybe 120 HZ on the motor then slow it down to 525 RPM as the pressure builds.  I think the net effect might be the same.


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## strantor (May 3, 2020)

ddillman said:


> go with a screw compressor it would hold up to the higher rpm's


I've looked into it. I cannot find even a small screw compressor which is affordable. Seems $3k-$4k is the entry point for a complete unit. I can find small replacement screw units for cheaper ($1500) but then I have to design an oil recirculation system around it. But in piston compressors for example I can get a 20hp Eaton piston compressor head brand new for $1500 (that's still outside my budget, I would be looking for used).


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## Downunder Bob (May 3, 2020)

ne thing you might want t consider is heat in the heads, which can carbonise the oil, I understand that most hobby compressors don't use much oil, but as you speed it up it will tend to use more, and running faster will increase the heat in the heads to the point of carbonising the oil, this will destroy the reed valves and clog up the intercooler and after cooler if you have one if not it will end up in your tank. 

I have seen this happen one ship I worked on the compressor motors were replaced with higher speed motors. The plan was to increase the rate of compressed air. it did slightly but it quadrupled the maintenance on the compressors. In the end we had to put the original motors back on and get an extra compressor. So my suggestion is don't do it.


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## BtoVin83 (May 3, 2020)

Reciprocating things have a finite FPM plus your flywheel may not take the RPM


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## Packard V8 (May 3, 2020)

strantor said:


> Ok reed valves, that's a gotcha that I don't have an answer for.


 Don't worry about the reed valves.  In kart and motorcycle engines, they function at higher RPM than a compressor head would ever see. 



> Also the inertia of the connecting rods


  Connecting rods don't fail from their own inertia, but that of the piston, rings and pin.  Inertial load goes up as a function of the square of the RPM.  A compressor rod will have been designed for an RPM range.  4X is most likely beyond that envelope.



> Sooo.... let's see. I suppose I could oversize the compressor head in relation to the motor and gear it down. Since I already used 5hp and 240hz as my example I'll just continue with those numbers... instead of a 5hp motor and a 5hp compressor head, I use a 5hp motor and a 20hp compressor head. I use 4:1 double-reduction between motor and compressor. At start-up, the motor is spinning 4x its nameplate 60Hz RPM and the compressor is spinning exactly at its rated RPM. At start-up the 5hp compressor is delivering the volume of a 20hp compressor (like 60 CFM vs 12CFM). As pressure builds, the motor motor slows down as before, and as pressure approaches max, the compressor has slowed to where it's delivering about what what a 5HP compressor should be delivering right before the pressure switch cuts out.  How bout that?


That could be made to work.  Just remember the difference between an empty startup at 0 PSI and an operating startup at 125 PSI.  In the real operating range, your gain is much less than your theoretical from empty startup.

jack vines


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## Mitch Alsup (May 3, 2020)

FOMOGO said:


> I think you would see the same result as running your car for extended periods at 8k rpm, i.e. early demise.



My Ferrari has an 8,500 RPM red line, and I have bounced it off the 8,750 fuel cutoff limit numerous times.

As far as running an air compressor at 4× rated flow--I wish you luck, and I hope it does not come apart in use.


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## matthewsx (May 3, 2020)

There are plenty of good answers here but if you're really coming up on the limits of air available from piston air compressors the real answer is to go with a screw type. Surplus units can be had fairly cheap at auction but require lots of power and take up lots of space.

To answer the actual question about what limits things it's physics. It might be possible to modify an existing compressor pump as you've suggested but the fact of asking about it probably means the engineering required isn't available in-house. 

I've spent lots of time modifying Briggs & Stratton utility engines to run up to 8500rpm for kart racing. It takes changing most of the internal parts and lots of time/money making sure everything is as close to perfect as possible. And this is for running only 15 or 20 minutes at a time with variable rpm, not a constant load for hours. It's unlikely any reciprocating compressor pump would last a reasonable amount of time with the type of mods required to double it's rpm. 

Either modify the process that takes excessive amounts of air, or bite the bullet and buy an appropriate compressor for your needs.

JMHO,


john


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## pontiac428 (May 3, 2020)

I suspect the reason you shouldn't run your compressor as fast as Joe Walsh's Maserati is heat.  Each stroke of the piston is going to generate a given amount of heat as the air is compressed.  If you increase the RPM, you increase the amount of heat that the head has to dissipate.  When you consider plastic pistons and splash oiling, things get complicated as RPM rises.  For high RPM applications, you see vane pumps and screw pumps with intercoolers or even roof-mounted cooling stacks.  I think your piston pump may have 20% overhead or less to overdrive it before it eats itself for dinner.  Who wouldn't want to sell compressors that out pump the competition, if all it meant was a motor swap or a pulley change?  There's probably a reason that nobody is doing it, and I would bet it is the speed limitations of the piston compressor head.


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## strantor (May 3, 2020)

Packard V8 said:


> That could be made to work.  Just remember the difference between an empty startup at 0 PSI and an operating startup at 125 PSI.  In the real operating range, your gain is much less than your theoretical from empty startup.
> 
> jack vines


I'm planning multiple modes of operation and other goals in mind that I didn't get into at first because I didn't want the thread to get too far off the original question before I had a good understanding. 

What has been discussed until now is what I would call start-up mode. Balls to the wall, as fast as the motor can go without self destruction, to attain max system pressure. 

One that's happened, we enter "maintain pressure" mode. That's a PID loop where pressure is monitored by an analog sensor rather than a pressure switch with large hysteresis/headband. In this mode, if I connect a low volume air tool which causes tank pressure to drop by only say 5 PSI, the motor will come gradually to life, creeping along at just a few tens of RPM until max pressure is reacquired. 

The reason for this, my other goal is noise reduction. My current oilless compressor is taking years off my life, always kicking on violently with no warning in a 90db banshee scream. With the new setup I know it's still going to be loud at startup, maybe even loaderbthan what I have now, but I can take the loud noise at startup because I'll be expecting it. I just turned the switch on, it's going to be loud; but once that's over, I don't have to worry that thing is going explode into action 90 minutes from now and make me spill coffee in my crotch.


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## brino (May 3, 2020)

strantor said:


> I just turned the switch on, it's going to be loud; but once that's over, I don't have to worry that thing is going explode into action 90 minutes from now and make me spill coffee in my crotch.



An admirable goal........well, at least for *my* crotch.....

This is a very interesting thread and I will be following along closely.

Thanks @strantor for the original question and everyone for their posts!

-brino


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## JimDawson (May 3, 2020)

One thing that has not been discussed here is scroll type air compressors.  They are typically much quieter than either recip or rotary screw.  Might also be adaptable to variable speed, not sure about that.  Eastwood is one manufacturer.  https://www.eastwood.com/eastwood-elite-qst-30-60-scroll-air-compressor.html  I have wondered if it possible to salvage the scroll compressor out of a heat pump unit and make an air compressor out of that.  I think most of them are in the 10HP range for an average size home unit.  Some of the newer ones are variable speed.

One thing that would help your situation immensely would be adding extra storage capacity.  I have about 260 gallons of air storage (160 gal before the air drier and 100 gal after) and after the initial start up, mine only kicks on occasionally even with the differential set at 5 PSI.  I have the max pressure set to 125 PSI even though the system is rated at 175 PSI.  Normally the compressor runs about 3 minutes or so to top up the system even with moderately heavy air usage.  I am running my 5 HP compressor with a VFD, and I use a 10 second ramp up to speed with coast to stop set.  I have thought about adding analog controls, just been too lazy to do it.


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## strantor (May 3, 2020)

JimDawson said:


> One thing that has not been discussed here is scroll type air compressors.  They are typically much quieter than either recip or rotary screw.  Might also be adaptable to variable speed, not sure about that.  Eastwood is one manufacturer.  https://www.eastwood.com/eastwood-elite-qst-30-60-scroll-air-compressor.html  I have wondered if it possible to salvage the scroll compressor out of a heat pump unit and make an air compressor out of that.  I think most of them are in the 10HP range for an average size home unit.  Some of the newer ones are variable speed.
> 
> One thing that would help your situation immensely would be adding extra storage capacity.  I have about 260 gallons of air storage (160 gal before the air drier and 100 gal after) and after the initial start up, mine only kicks on occasionally even with the differential set at 5 PSI.  I have the max pressure set to 125 PSI even though the system is rated at 175 PSI.  Normally the compressor runs about 3 minutes or so to top up the system even with moderately heavy air usage.  I am running my 5 HP compressor with a VFD, and I use a 10 second ramp up to speed with coast to stop set.  I have thought about adding analog controls, just been too lazy to do it.


I have thought about scroll compressors. I've long lusted after that beautiful beast Eastwood sells. I recently had my entire home HVAC system replaced and I told them to leave me the 2.5ton condenser. I had every intention to make that my new shop compressor. It's a quiet unit with a huge cooler to boot. But the more I researched the more I was turned off to the idea. Refrigeration systems circulate oil in addition to freon. The compressor needs oil going through it just like a screw compressor does. In order to do it properly I would need to design an oil separator and oil cooling/recirculation system. You can find example of guys just taking the scroll units out and slapping them directly onto an air tank, and it works, but I suspect not for long. I could find absolutely no data on how long they last when used in this manner.


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## Buffalo21 (May 3, 2020)

We sell and service industrial boiler room equipment, we are constantly bombarded by power utilities to install VFDs on every 3 phase electric motor possible, sometimes its practical, sometimes it is not. We install VFDs on the boiler blower motors, that maybe save $500/yr in electricity, but the approved control system to run the VFDs, can cost anywhere between $35,000 to $85,000, depending on complexity and other control issues.

They constantly ask about the air compressor modules, that are sourced from either Quincy, I-R and Atlas-Capco, all of them say the same thing. The operating speeds of the modules, is optimized to maximum performance, required air flow rates and long term reliability, they say its a balancing act to get the best of all 3 requirements at the same time. Altering the motor speed gains you nothing. We install them on the modules all the time, but they are locked in at 60 Hz, all they really gain, with the installation, is a soft start for the motor.


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## strantor (May 3, 2020)

Buffalo21 said:


> We sell and service industrial boiler room equipment, we are constantly bombarded by power utilities to install VFDs on every 3 phase electric motor possible, sometimes its practical, sometimes it is not. We install VFDs on the boiler blower motors, that maybe save $500/yr in electricity, but the approved control system to run the VFDs, can cost anywhere between $35,000 to $85,000, depending on complexity and other control issues.
> 
> They constantly ask about the air compressor modules, that are sourced from either Quincy, I-R and Atlas-Capco, all of them say the same thing. The operating speeds of the modules, is optimized to maximum performance, required air flow rates and long term reliability, they say its a balancing act to get the best of all 3 requirements at the same time. Altering the motor speed gains you nothing. We install them on the modules all the time, but they are locked in at 60 Hz, all they really gain, with the installation, is a soft start for the motor.


I'm not out to copy Quincy, Atlas, or anyone else. What I've described here is, as far as I know, not done by anyone. I'm not locking anything in at 60Hz. This not a soft start, and power factor correction is not among the design goals.


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## Superburban (May 3, 2020)

One should also be cautious with lowering the speed of the compressor. Splash lubricated compressors may not get enough lubrication. Also, the pulleys with a fan built in may not flow enough air at slower speeds.

Otherwise, I like the idea of a bigger head unit, then slowing it down as the pressure increases. The air flow could easily be fixed by a separate fan. If you are looking at heavy usage, you could run a separate external oil pump, with jets to cool the bottom of the pistons, and even an external cooling system for the oil. Both with temps monitored as part of the main motor control system.

Is it overkill for a air compressor system? is that not our way of doing things?


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## strantor (May 3, 2020)

Ok, allow me to steer this out of theory zone and into action. Disregard all previous talk of 240Hz and overspeeding the compressor head by 4x its rated RPM.

I've chosen a 3hp motor from among my cache and just picked up a 5hp compressor head from HF. As you can see on the motor nameplate, it's a 1760RPM/60Hz motor motor with a stated maximum safe speed of 3600RPM. So I should safely be able to run it up to 123Hz. But I'm getting a strong "I live life 10% over the edge" vibe from this motor so let's say 135Hz/3,960RPM is my max.

My compressor head is 1050RPM max, 650RPM min (for cooling I guess?), 17.3 CFM @ 40 PSI, 15.2 CFM @ 90 PSI, and has a 14.5" flywheel. Since the compressor overspeed condition only happens at the beginning of charging an empty tank, I feel comfortable overpseeding it by ... uh.. (arbitrarily) 25%. Unlike the motor which stands a real chance of centrifugal forces pulling itself apart, I'm not too worried about the compressor failing with an occasional 25% overspeed condition. So call it 1313RPM max.

So... (1313RPM × 14.5")/3960RPM = 4.8" motor pulley diameter. They didn't have a pulley that size at Tractor Supply so I got a 4.5" pulley. Looks like with a 4.5" motor pulley at 3960RPM I'll only be able to overspeed the compressor by 17% (1229RPM). Shame. Anyway, still looking good.

In the beginning before any pressure builds, I think the volume is a direct function of cylinder displacement. So with a 3.14" bore, 2.36" stroke, and two cylinders (36.52C.I.D.), at 1229 RPM, I should be moving 44,883 cubic inches/min (26CFM) from zero PSI. By the time 40PSI is reached, I estimate we should still be below the 3HP threshold and therefore still running 17% excess compressor RPM and therefore the rated 17.3CFM becomes 20.2CFM.

Somewhere between 40PSI and 90PSI, I estimate, is where we max out the 3HP motor's power and RPM starts to drop off. Wherever that point is (I haven't done the math on that yet, I think I'll just discover it empirically) this turns into a 3HP compressor. But it's still a 3HP compressor spinning way above 1760 RPM and moving a lot more air, a lot sooner in the process, than a normal 3HP compressor. Right at the point before cut-off, is when the speed finally is pulled down to the 1760 nameplate RPM, and when it finally becomes a 3HP compressor, moving the same amount of air as a standard 3HP compressor; it's just a 3HP compressor geared down to turn a 5HP head. And then it shuts off. So it only spends a few seconds being the good little boy that the nameplate advertised. All the rest of time it's being a very, very bad boy.

All that sound more reasonable than the wacky numbers I spouted in the OP? Anyone wanna make a prediction about operation? Be careful now; if you opt to say nay, realize that we are no longer in theory mode and you might be eating crow in short order.


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## projectnut (May 3, 2020)

Not all compressors are created equal so to speak.  Kellogg for instance at one time made 6 different models.   However they came in 70 different  cfm and pressure ratings.

As an example the popular model 321 dual stage series compressors came in 16 different sizes and used motors ranging in size from 1 hp to 2 hp.  They had pressure ranges from 100 psi to 250 psi, compressor speeds from 300 to 823 rpm, and piston displacements from 4.32 cu ins to 11.9 cu ins.  The basic rule of thumb is that the lower the pressure rating, the lower the piston displacement, and the higher the speed.  Conversely the higher the pressure rating the higher the piston displacement, and the lower the speed.


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## JimDawson (May 3, 2020)

Just a thought here, you may be able to use the current limit on the VFD to control the speed at least in the initial phase for pump up.  Typically as you exceed the motor base speed (1760 in this case) the current goes down as the RPM increases.  Below the base speed the current goes up as the motor slows down.  This may only happen when operating in sensorless vector mode, because the VFD is trying to produce constant torque below the base speed, and constant HP above the base speed.

Not sure is this would be useful and I'm not sure if current limiting would actually slow the motor down or just trip out the VFD on overcurrent.  It might depend on the available settings on the VFD.  The theory is, as the load on the motor increases as the pressure comes up, the VFD starts backing down the speed to keep the output current at the max set point.


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## strantor (May 3, 2020)

Superburban said:


> One should also be cautious with lowering the speed of the compressor. Splash lubricated compressors may not get enough lubrication. Also, the pulleys with a fan built in may not flow enough air at slower speeds.
> 
> Otherwise, I like the idea of a bigger head unit, then slowing it down as the pressure increases. The air flow could easily be fixed by a separate fan. If you are looking at heavy usage, you could run a separate external oil pump, with jets to cool the bottom of the pistons, and even an external cooling system for the oil. Both with temps monitored as part of the main motor control system.
> 
> Is it overkill for a air compressor system? is that not our way of doing things?


My thoughts exactly. If it's worth doing, it's worth overdoing. And in some cases it's worth overdoing even if it isn't worth doing at all. But I think this is worth doing. 

I will keep the lube system mods in mind but (keep in mind I have no idea what I'm talking about) at only 25% intermittent/rare overspeed I don't think they will be necessary. 

As for separate cooling fans and monitoring temps, absolutely. A yet unmentioned (maybe obvious) piece of this puzzle is a PLC. PLC will establish the operating modes and run PID. PLC will monitor temps, pressure, maybe flow, I don't know what all else yet. I don't want to get too deep into the weeds until I at least get it moving air. But with quiet in mind, I think I will house this in a sound dampening cabinet with muffled cooling fans drawing in outside cooling air which should allow me to run below the 650 minimum rated compressor RPM once in "maintain pressure" mode. ...but wait, you implied the *oil* might be the issue at low RPMs. Why is that? Can you expound? Does "splash lubricated" mean that the crank goes down and slaps the surface of the oil bath, *splashing* it back up onto the pistons/cylinders, and _that's_ why the low RPM might be a problem? If so, then I see where you are coming from. I will give this more thought, pending clarification. Thanks for the input.


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## matthewsx (May 3, 2020)

Okay, I'm not going to get into any of the engineering aspects of this and go straight for the practical side of things.

Why not just do what professional shops do and build a shed on the outside of your shop? Increase the holding tank capacity if you need more volume and call it a day.




Harbor Freight makes some good equipment but I doubt their compressor pumps are that great. If they will give you an extended warranty that would be a good purchase regardless of how fast you want to run it. 

I guess for me an air compressor is a single purpose tool that just has to do what I need day in and day out. The companies that build them have engineering staff that have probably tried doing what you propose and either found out it didn't make a big difference or it increased the price above what their customers would pay.

Another advantage of moving the compressor out of the shop besides keeping coffee off your pants is a tank failure is much less likely to kill you or bust up the shop.

Just some thoughts from a practical guy who would like to move his compressor outside and use the floor space for more machine tools


John


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## strantor (May 3, 2020)

JimDawson said:


> Just a thought here, you may be able to use the current limit on the VFD to control the speed at least in the initial phase for pump up.  Typically as you exceed the motor base speed (1760 in this case) the current goes down as the RPM increases.  Below the base speed the current goes up as the motor slows down.  This may only happen when operating in sensorless vector mode, because the VFD is trying to produce constant torque below the base speed, and constant HP above the base speed.
> 
> Not sure is this would be useful and I'm not sure if current limiting would actually slow the motor down or just trip out the VFD on overcurrent.  It might depend on the available settings on the VFD.  The theory is, as the load on the motor increases as the pressure comes up, the VFD starts backing down the speed to keep the output current at the max set point.


Right. My understanding is that it's the same exact concept as field weakening in a DC motor (and some VFD manufacturers even use that term although I think it's a misnomer). You can go far above nameplate rated Hz on a 3ph motor, you just can't do it at nameplate amps. You put in nameplate rated 60Hz values, then set the 135Hz frequency limit and 8amp current limit, put it in constant torque mode, and it will ramp up to 135Hz until it starts to see a load. The load will pull the speed down; amps will go up and Hz will go down, until they converge at nameplate rated Hz&amps


----------



## strantor (May 3, 2020)

matthewsx said:


> Okay, I'm not going to get into any of the engineering aspects of this and go straight for the practical side of things.
> 
> Why not just do what professional shops do and build a shed on the outside of your shop? Increase the holding tank capacity if you need more volume and call it a day.
> 
> ...



I've considered putting the compressor outside. I don't like it because this is an extremely humid climate. I have a (half-ass) air conditioned shop and the humidity in here is much lower. I don't want to take ambient moisture from outside the shop and turn it loose inside the shop where all my machines are. I realize I will still get some condensation in the tank even with the compressor indoors but I expect it to be much less.

I'm sure this HF compressor head is a turd next to a Quincy or even Campbell or an Ingersoll, but it was $164! If it lasts me a couple of years, I say money well spend considering this is an experiment and all. (And yeah, I got the 2yrs replacement program. Something told me I might need it).

Why not just get a bigger compressor? Money. Those things ain't cheap. I'm looking at over $1,000 to get what I want. On that note, I'm working with what I have. I have a 3Hp motor and i have VFDs. I don't have 80 gallon tanks and 5hp single phase motors.

And at foundation of the matter, simply, this is what I want to do. The inefficient operation of commercial/consumer compressors has irritated me since I first learned of it. The room for improvement is right there in front me, in front of everyone, and it never gets improved upon. People don't want to pay for the improvement. I do, and I don't even need to because I have the pieces of the puzzle, so I see no reason not to move forward.


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## matthewsx (May 3, 2020)

strantor said:


> I've considered putting the compressor outside. I don't like it because this is an extremely humid climate. I have a (half-ass) air conditioned shop and the humidity in here is much lower. I don't want to take ambient moisture from outside the shop and turn it loose inside the shop where all my machines are. I realize I will still get some condensation in the tank even with the compressor indoors but I expect it to be much less.
> 
> I'm sure this HF compressor head is a turd next to a Quincy or even Campbell or an Ingersoll, but it was $164! If it lasts me a couple of years, I say money well spend considering this is an experiment and all. (And yeah, I got the 2yrs replacement program. Something told me I might need it).
> 
> ...



If you already have the parts and knowledge you need then have at it. Compressors are like any other tool, manufacturers build what they think the market wants and will pay for whether it's a $200 junker or a $20k beauty....

Looking forward to pictures of your build


John


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## Buffalo21 (May 3, 2020)

A lot of the splash lubrication systems, there is a paddle/spoon, built into the bottom of the connecting rod, that dips into the oil at every rotation and flings the oil, around in the crankcase


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## strantor (May 3, 2020)

matthewsx said:


> If you already have the parts and knowledge you need then have at it. Compressors are like any other tool, manufacturers build what they think the market wants and will pay for whether it's a $200 junker or a $20k beauty....
> 
> Looking forward to pictures of your build
> 
> ...


Thanks. Pic #1 LOL. Just getting an idea for the spacing and dimensions of my base plate. I'll need to make a 90deg plate for the motor because it's a face mount.


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## Superburban (May 3, 2020)

strantor said:


> ...but wait, you implied the *oil* might be the issue at low RPMs. Why is that? Can you expound? Does "splash lubricated" mean that the crank goes down and slaps the surface of the oil bath, *splashing* it back up onto the pistons/cylinders, and _that's_ why the low RPM might be a problem? If so, then I see where you are coming from. I will give this more thought, pending clarification. Thanks for the input.


Yes, splash lubrication usua;;y has a rod extending from the crank shaft, which di[ps into the oil bath, and splashes the oil around the crank case.


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## strantor (May 3, 2020)

Buffalo21 said:


> A lot of the splash lubrication systems, there is a paddle/spoon, built into the bottom of the connecting rod, that dips into the oil at every rotation and flings the oil, around in the crankcase





Superburban said:


> Yes, splash lubrication usua;;y has a rod extending from the crank shaft, which di[ps into the oil bath, and splashes the oil around the crank case.


Thanks. So to be safe I think I must assume that the minimum speed (650rpm) has more to do with lubrication than with cooling. I _shall not_ go below 650RPM. 

A few things are converging here. I'm rethinking exceeding 3600rpm on the motor. It has a service factor of 1.15 so I think it would be safer to periodically over-amp it than over-speed it. So to drop it down to 3600RPM max, and bring my compressor max RPM up to 125% at beginning-of-charge where I wanted it, I would need a 5.25" pulley on the motor.

There is no motor torque requirement specified in the manual. It assumes you're connecting a 5hp motor (go figure) and it only talks about pulley diameter based on nameplate motor RPM. 

If my motor were 5Hp, then with a nameplate RPM of 1760, that would call for an 8.6" pulley per the formula in the manual. But since it isn't, if I multiply 8.6 by (3/5) that would be a 5.19" pulley for a 3hp motor instead of 5hp. Pretty close to 5.25"

Now if I _shall not_ run the compressor below 650RPM and I aim for a cut-out just when the motor speed drops to its nameplate value of 1760RPM, so in other words I need 650 compressor RPM for 1760 motor RPM, that would be a 5.35" pulley. Pretty close to 5.25"

So think the cosmos has spoken. 5.25" pulley is what the motor is destined to have. I was not liking these stamped steel pulleys and weld-on hub that I got from tractor supply. Not for 3600RPM. Rather have a machined one for better balance. Just so happens, surplus center has those.





						5.25 OD 1-1/8 Bore 2 Groove Pulley | Finished Bore Pulleys | Pulleys | Power Transmission | www.surpluscenter.com
					

5.25 OD 1-1/8 Bore 2 Groove Pulley, Finished Bore Pulleys, 5.25 O.D. 1-1/8 BORE 2 GROOVE PULLEY,  DOUBLE GROOVE PULLEY  Brand new.  Fractional HP,Concentric International,TOMP2BK55-G         ,



					www.surpluscenter.com


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## extropic (May 3, 2020)

I quit reading after reply #6.
I hope the OP is the result of acute cabin fever due to the stress of the current shelter-in-place restrictions.

In reply to your initial question. First of all, you have not specified what particular compressor pump you have in mind or the environmental factors  where it would operate. All "5 HP" compressor pumps are NOT created equal, therefore your question is unanswerable.
A compressor pump is a SYSTEM. All components are intended to operate within their specified limits, if it's well engineered, designed and manufactured. Exceeding a design limit by a factor of FOUR would probably induce high risk of premature pump failure. The exact mode of failure could be studied (by you?) but I decline to venture a WAG.

I can not agree with your #1 listed result: "a much better utilization of the motor".
I don't know exactly what you mean by the statement. If you want to consume more watts (at low pressure) so the motor always runs under 100% capacity (a foolish goal IMHO) there may be a more durable way. It would be relatively large, mechanically and electrically complex, as well as expensive (compared to a conventional compressor package). You might be able to create a system consisting of multiple compressor pumps which are individually clutched in and out depending on available HP (current draw). Knock yourself out.


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## Mini Cooper S (May 3, 2020)

Interesting question. Air is free at least until you compress it so I would have to ask "Why don't industrial piston type compressors do this?" At least I've never experienced any that do, but maybe some do. Of course, screw compressors may be a different story.


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## extropic (May 3, 2020)

matthewsx said:


> snip
> 
> To answer the actual question about what limits things it's physics. It might be possible to modify an existing compressor pump as you've suggested but *the fact of asking about it probably means the engineering required isn't available in-house.*
> 
> snip



Thank you John. You said it so well. I hope to remember that phrasing.
I learned something really useful today.


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## Downunder Bob (May 3, 2020)

I see you have clearly made u your mind to go ahead with this mod, Be my guest, but don't say you weren't warned. With that said. Good luck.


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## strantor (May 3, 2020)

extropic said:


> I quit reading after reply #6.
> I hope the OP is the result of acute cabin fever due to the stress of the current shelter-in-place restrictions.
> 
> In reply to your initial question. First of all, you have not specified what particular compressor pump you have in mind or the environmental factors  where it would operate. All "5 HP" compressor pumps are NOT created equal, therefore your question is unanswerable.
> ...



Your skipping ahead is understandable. This started in a rabbit hole and went even deeper. But I think you would have benefited from reading reply #27. If you go back and read that post I think you might see that I'm less stupid than I might have sounded. I was using hyperbolic numbers to illustrate the concept. And you might also see my point about better utilization of the motor.

I've always been this way. The cabin fever is admittedly bringing it to the surface though.


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## matthewsx (May 3, 2020)

extropic said:


> Thank you John. You said it so well. I hope to remember that phrasing.
> I learned something really useful today.



Not to diminish the abilities of the OP, it's just I've spent a lot of time modifying small engines and know how much effort goes into coming up with the right "recipe" for a reliable mod. If you had a half dozen of those HF pumps you could probably figure it out but you would break most of them. It sounds like he's on his way though 

John


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## strantor (May 3, 2020)

Mini Cooper S said:


> Interesting question. Air is free at least until you compress it so I would have to ask "Why don't industrial piston type compressors do this?" At least I've never experienced any that do, but maybe some do. Of course, screw compressors may be a different story.


I've touched on that a couple times but I don't think I've really explained myself fully.

I don't think there's a market for what I'm building, and I didn't set out on this voyage for the purpose of bringing something to market. The result of this will be a consumer grade compressor built with some industrial parts that make it expensive. Probably so expensive that it would have to be marketed to industrial customers who would rather spend their money on a screw compressor which will run 100s of thousands of hours. This won't do that, and I don't need it to. I need it to be ready to go quickly when I want to use it, and cheap. It's cheap for me to build because I already have the industrial stuff.

An analogy: if it's possible for a pickup truck to have 800hp, 50" tires, and 2ft of ground clearance, then why isn't Ford or anyone else making trucks like that? Since no commercial enterprises market such a truck, does that imply it can't be built? Heck no! Guys do it every day.

I just have strange priorities.


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## pontiac428 (May 3, 2020)

Well, I think you're on the right track for a rabbithole project.  It's clear that you intend to modify the shape of the tank pressure-time graph by adding power to the system using a VFD while the system is under low-pressure load, then returning to stockish power levels at the top.  You are exploiting an efficiency by changing how the motor is putting out it's power that isn't done in an analog scenario.  You've got a complete idea, and kudos for breaking out the parts and trying it out.  I won't judge if the juice isn't worth the squeeze, as long as you learn something.


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## Mini Cooper S (May 3, 2020)

strantor said:


> I've touched on that a couple times but I don't think I've really explained myself fully.
> 
> I don't think there's a market for what I'm building, and I didn't set out on this voyage for the purpose of bringing something to market. The result of this will be a consumer grade compressor built with some industrial parts that make it expensive. Probably so expensive that it would have to be marketed to industrial customers who would rather spend their money on a screw compressor which will run 100s of thousands of hours. This won't do that, and I don't need it to. I need it to be ready to go quickly when I want to use it, and cheap. It's cheap for me to build because I already have the industrial stuff.
> 
> ...




I wasn't trying to say that it couldn't or shouldn't be done, I was thinking that there must be a down side that would make it impractical. With that said, new innovations can come from projects like this, Go for it!
Richard


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## FOMOGO (May 3, 2020)

Mother of invention, and all that. Frank Zappa was always an advocate of new and different. Keep us posted. Mike


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## Superburban (May 3, 2020)

I think we are all guilty of over engineering projects. This one sounds like the better uses of a a 3 phase variable speed motor in a home shop.


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## matthewsx (May 4, 2020)

Just make sure it has a sound tank, and a good pressure relief valve....

Safety fast  


John


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## matthewsx (May 4, 2020)

FWIW, I'm about to hook up my little dual tank Hitachi compressor to a 30 gal tank that will live in the rafters of my little shop. Not worried about how quickly it charges or how loud it is at this point. 

Why? 

My dad's old craftsman 20 gal compressor still works fine and I don't really need anything more.

But I have the parts


John


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## strantor (May 4, 2020)

pontiac428 said:


> Well, I think you're on the right track for a rabbithole project.  It's clear that you intend to modify the shape of the tank pressure-time graph by adding power to the system using a VFD while the system is under low-pressure load, then returning to stockish power levels at the top.  You are exploiting an efficiency by changing how the motor is putting out it's power that isn't done in an analog scenario.  You've got a complete idea, and kudos for breaking out the parts and trying it out.  I won't judge if the juice isn't worth the squeeze, as long as you learn something.


Well you have that knack for brevity that I lack. That's a beautiful summary of something that would (did) take me several pages to explain. Thanks for the encouragement.


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## strantor (May 4, 2020)

Mini Cooper S said:


> I wasn't trying to say that it couldn't or shouldn't be done, I was thinking that there must be a down side that would make it impractical. With that said, new innovations can come from projects like this, Go for it!
> Richard


I'm sure there is a down side, I will find out what it is. Maybe there's a workaround for it.


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## strantor (May 4, 2020)

matthewsx said:


> FWIW, I'm about to hook up my little dual tank Hitachi compressor to a 30 gal tank that will live in the rafters of my little shop. Not worried about how quickly it charges or how loud it is at this point.
> 
> Why?
> 
> ...


I'm glad you said that. It runs parallel to a question that came up last night while I dreamed sweet compressor dreams. Why is the compressor always mounted on top of the tank? This makes it top heavy. In my case, putting a 5HP head and 3HP motor, either of which easily weighing double what my existing 2hp compressor+motor weighs, is going to make my 26gal craftsman compressor *very* top heavy. I would rather mount the motor & compressor at ground level and place the tank on top. But nobody else does that. I've never once seen a compressor configured that way. Why?

I assume it's the condensation that would surely happen along a long vertical air line from the compressor on bottom to the air entry on top, especially when the compressor switches off. That water would trickle down and maybe into the cylinders where (maybe?) It would create a hydro-lock situation and blow head gaskets?

Would it be ok if there were a cooler and a water trap between compressor and tank? Would it be ok in your situation, considering you aren't going from a compressor head directly to an overhead tank, but rather from a tank to another overhead tank?


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## matthewsx (May 4, 2020)

I'm not sure of the engineering but I suspect either something like you say, or it might just be easier. In my case I'll just be running a hose from the regulator into the extra storage tank I have. The small compressor will live underneath the table where my mill/drill is.

I don't have any experience with coolers or water traps but I'm sure copper tubing will provide some of what you need.

John


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## talvare (May 4, 2020)

strantor said:


> I'm glad you said that. It runs parallel to a question that came up last night while I dreamed sweet compressor dreams. Why is the compressor always mounted on top of the tank? This makes it top heavy. In my case, putting a 5HP head and 3HP motor, either of which easily weighing double what my existing 2hp compressor+motor weighs, is going to make my 26gal craftsman compressor *very* top heavy. I would rather mount the motor & compressor at ground level and place the tank on top. But nobody else does that. I've never once seen a compressor configured that way. Why?
> 
> I assume it's the condensation that would surely happen along a long vertical air line from the compressor on bottom to the air entry on top, especially when the compressor switches off. That water would trickle down and maybe into the cylinders where (maybe?) It would create a hydro-lock situation and blow head gaskets?
> 
> Would it be ok if there were a cooler and a water trap between compressor and tank? Would it be ok in your situation, considering you aren't going from a compressor head directly to an overhead tank, but rather from a tank to another overhead tank?



I have worked on compressed air systems in industrial environments where the air storage tanks were above the compressors, some times several stories above the compressors with inter-connected storage tanks. These systems work just fine but you do have to insure that the plumbing takes into account the accumulation/draining of condensate.

Ted


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## Reddinr (May 4, 2020)

From my experience compressor speed is limited to about 60 MPH.  A few years ago I was helping a friend move a large, tall compressor he bought at a yard sale.  He tied it in and we took off in his pickup.  About half way home there was a tire chunk in the road, then a curve.  The combination broke the compressor loose and it went flying over the side, down a bank and into a tree.  Must have been 60 MPH, limited by a tree. ..


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## pontiac428 (May 4, 2020)

I second @Reddinr 's assertion.  Back in the '90s I moved a lot.  I had my big vertical compressor strapped upright like a motorbike in the bed of my trusty ol' stepside.  Similarly, I hit a bump, took a corner, and saw my compressor drop through the weakened, rotted bed planks hidden under a sheet of plywood.  Definitely would have been worse had I been doing over the established compressor speed of 60 mph.


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## tq60 (May 4, 2020)

Supercharge...

An old vacuum cleaner pump can be used as a supercharger.

Air filter on suction side and pressure side to air intake.

Did that with an old gas station pump and greatly reduced fill time.

Place a meter on moter current and add cut out switch to turn off it it causes issue with higher pressure.

Was 2 stage made into 3 stage...

Sent from my SAMSUNG-SM-G930A using Tapatalk


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## epanzella (May 4, 2020)

The problem with driving a reciprocating motor over speed is that it, uh, reciprocates. Piston speed goes up but the piston still has to reverse direction twice for every stroke. The load and impact imparted to the main and rod bearings go up exponentially as the RPM goes up.  Would it throw a rod or sieze? I dunno but I would bet the farm that the life of the compressor head would be drastically reduced. It sounds like you need a bigger compressor. Failing that, increase the storage capacity of you existing machine. This will give you more intermittent output but it won't help with continuous operation.


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## macardoso (May 4, 2020)

strantor said:


> I'm planning multiple modes of operation and other goals in mind that I didn't get into at first because I didn't want the thread to get too far off the original question before I had a good understanding.
> 
> What has been discussed until now is what I would call start-up mode. Balls to the wall, as fast as the motor can go without self destruction, to attain max system pressure.
> 
> ...



If the noise is a main concern and your air needs are typically moderate, then an "Ultra Quiet" air compressor may be for you. I have two compressors chained together. A 5HP piston compressor has the big 30 gallon tank and turns on at 90 psi and off at 110. I added a HF 2HP ultra quiet compressor which turns on at 100 psi and off at 135. The output is pressure regulated to 110 psi. With them chained together, 98% of the time only the quiet one runs (60dBA) and uses the other compressor as a 30 gallon pony bottle. If I really use a ton of air the big one will kick on at 90dBA. I *love *the ultra quiet one - worth every penny. 

Just have to be careful to never turn the output of the little compressor beyond 110 psi. The pressure relief valve on the big tank *should* protect it against that but better to not push it,


----------



## nnam (May 4, 2020)

Nice idea.  Heat would be one issue, another would be lubrication.  Commercial unit often has filter and some already mentioned active lubrication method.  The last one would be depends on which compressor you'll be using.  Add a large cooling fan, oil filter and pump, and slap a large evaporator coil on it     

So if you can solve that, and have time, add programming to it, to monitor the current, and increase the RPM until certain RPM or current is reached then stop there.  You can also have heat sensor.  The Arduino boards are really cheap.  I am in a process of playing it.
 
In any rate, experiment takes time, effort and money.  Just stay safe, and go at it, document everything, and hopefully you'll post it up here .


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## Joe in Oz (May 6, 2020)

Sorry to pipe in so late in the rabbit hole.
I like your idea a lot in principle.
First thing I wouold do is try running the motor on your VFD without any load at all at various frequencies. You will find that most tree-phase motors will only run up to about twice design frequency, certainly not four times. When they hit their limit they slow down, making a higher and higher pitched sound while actually slowing down. Find the max frequency at which the motor runs reliably.
Second, run your motor connected to the compressor, with the outlet plumbing disconnected. That will give you an idea of the torque required to drive the compressor head. Now try that at the maximum frequency determined earlier. You will find it may no longer run at that frequency, since the torque curve drops off rapidly after about 150% of motor rated frequency.
Once you find the maximum frequency your motor will drive your compressor, reconnect the plumbing and see what happens against a load. My vest guess is that the VFD will hit the name plate Amp limit quite early in the filling of the tank, and reduce the motor rpm to around name plate rpm.
If your theory is right, then you have found the most efficient way to charge your tank with the compressor head you have and the motor you have.
All the theories and concerns offered up to here are all just theories - including yours.
I've used VFDs on LOTS of machines and motor new and old. My contribution to this rabbit hole is not theory but practice. I've put a VFD on a compressor and found the main advantage being the soft start and being able to control the motor current more accurately, so as to keep the motor safe.

As far as compressor head speed is concerned, I've run a compressor head at around 2000rpm without any noticable increase in vibration. I tried this without any pressure side plumbing to see if the vibrations stayed under control - and to ensure that the cast iron (in my case) flywheel/fan was safe. Then reconnected the pressure side and let it run. It required a MUCH larger motor to drive it and I reduced the speed again to match the output of the motor that came with it.....


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## Lo-Fi (May 6, 2020)

If you don't know the channel, don't let the language fool you: he's a very clever guy.


----------



## projectnut (May 6, 2020)

You might want to rethink your choice of compressor heads.  The reviews on the one you linked are horrible.  More than 1/3 of those offering a review said it lasted less than a year.  Some said it was defective out of the box, and others said it lasted less than 3 months.  How they ever arrived at a 3.8 rating out of 5.0 is beyond me.

It might be worth your time to consider a bit more substantial unit.  I have 2 compressors I use regularly.  The most used one is a Kellogg 311.  It dates back to 1965, and is still going strong.  Over the years I've had to replace a couple valves, change the oil, and air filters, as well as replace a couple belts.  In the time I've had it I doubt I've paid $300.00 for maintenance and replacement parts.  Keep in mind this unit is 55 years old. The second compressor is a 3 hp Craftsman oil less.  It's noisy but does the job.  In the 25+ years I've had it maintenance has cost less than $50.00.

If the reviews on the one you're looking at are anywhere near accurate it sounds like you'll definitely be replacing it every couple years or less.  Keep in mind it uses a splash oil system.  While they work reasonably well at low rpm's they are definitely not a good choice when attempting to run the compressor multiple times it's rated speed.  Heat is the enemy of a compressor.  Running it at speeds beyond it's rated capacity will certainly generate considerable heat.  Since the only cooling system is ambient air dissipating that heat will be a real problem.


----------



## Joe in Oz (May 6, 2020)

I had forgotten I saw that video a year or so ago.... but Ave never shows you any of his interesting projects finished.... Pity. So I promptly forgot about another interesting but inconclusive Ave project.
Anyway, looks like he was able to run his compressor at 100Hz. That's something. We hav 50Hz mains power here and my compressor ran at 120Hz. But went back to about 65Hz under final pressure load.


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## strantor (May 6, 2020)

Lo-Fi said:


> If you don't know the channel, don't let the language fool you: he's a very clever guy.


Well I'm a fan and I thought I had seen all of his videos; I guess not. Or maybe I did. I leave things playing while I work; music, audiobooks, youtube videos, etc. and just listen to them in a mental subroutine - perhaps something percolated up into the frontal lobe and I mistook it for an original idea. We are describing the same thing. I wonder if he had any long term success with it.



projectnut said:


> You might want to rethink your choice of compressor heads.  The reviews on the one you linked are horrible.  More than 1/3 of those offering a review said it lasted less than a year.  Some said it was defective out of the box, and others said it lasted less than 3 months.  How they ever arrived at a 3.8 rating out of 5.0 is beyond me.
> 
> It might be worth your time to consider a bit more substantial unit.  I have 2 compressors I use regularly.  The most used one is a Kellogg 311.  It dates back to 1965, and is still going strong.  Over the years I've had to replace a couple valves, change the oil, and air filters, as well as replace a couple belts.  In the time I've had it I doubt I've paid $300.00 for maintenance and replacement parts.  Keep in mind this unit is 55 years old. The second compressor is a 3 hp Craftsman oil less.  It's noisy but does the job.  In the 25+ years I've had it maintenance has cost less than $50.00.
> 
> If the reviews on the one you're looking at are anywhere near accurate it sounds like you'll definitely be replacing it every couple years or less.  Keep in mind it uses a splash oil system.  While they work reasonably well at low rpm's they are definitely not a good choice when attempting to run the compressor multiple times it's rated speed.  Heat is the enemy of a compressor.  Running it at speeds beyond it's rated capacity will certainly generate considerable heat.  Since the only cooling system is ambient air dissipating that heat will be a real problem.



If this concept works as well as I hope, then I might consider upgrading to a better compressor head in the future. There are two objectives currently; low cost using as many on-hand parts as I have, and squeezing out every ounce of performance as practice proves possible. The cheap China compressor was one of the only options. I know, pay for a quality product and only pay once, but this is quite experimental and the only thing more tragic than it not working, would be to prove it doesn't work by grenading a brand new $1k compressor.


----------



## strantor (May 6, 2020)

Joe in Oz said:


> Sorry to pipe in so late in the rabbit hole.
> I like your idea a lot in principle.
> First thing I wouold do is try running the motor on your VFD without any load at all at various frequencies. You will find that most tree-phase motors will only run up to about twice design frequency, certainly not four times. When they hit their limit they slow down, making a higher and higher pitched sound while actually slowing down. Find the max frequency at which the motor runs reliably.
> Second, run your motor connected to the compressor, with the outlet plumbing disconnected. That will give you an idea of the torque required to drive the compressor head. Now try that at the maximum frequency determined earlier. You will find it may no longer run at that frequency, since the torque curve drops off rapidly after about 150% of motor rated frequency.
> ...


Yes that sequence of events is pretty much the plan. I expect to be able to spin the motor up to double speed no problem, maybe spin it double speed with the compressor coupled but unplumbed. Hopefully stay at double speed until a few tens of PSI. That might be unrealistic. we will see...


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## strantor (May 9, 2020)

Made some progress today. Built a frame to house the motor and compressor head. I've decided on a "cart" configuration. The motor, compressor, and air dryer will stand upright one on top of each other, and the compressor tank will stand upright beside them, about the same height. I filled the compressor with oil and ran it from my 480v supply just because. I'm working on a turnbuckle to raise/lower the motor for changing belts. The motor+compressor assembly will be all rigidly mounted together on their frame which will isolated from the rest of the cart by rubber vibration dampeners. Once I'm happy with the unistrut frame I will weld it together and recover my hardware.


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## westerner (May 9, 2020)

strantor said:


> And at foundation of the matter, simply, this is what I want to do.


And so, you should. 

I like the concept. I see the pros presented as well as the cons. 
VFD technology has changed the entire electric power landscape. (At least for hacks like me)

Since no one here has done exactly what you propose, we all will await your presentation of actual data. 

I am intrigued.


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## stupoty (May 10, 2020)

Lo-Fi said:


> If you don't know the channel, don't let the language fool you: he's a very clever guy.



I was thinking of that video when I read OP's post


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## Superburban (May 10, 2020)

Lo-Fi said:


> If you don't know the channel, don't let the language fool you: he's a very clever guy.


I don't know, he runs the compressor backwards, talks about it as if it is a single piston compressor, Does not give me the feeling of clever.


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## strantor (May 10, 2020)

Superburban said:


> I don't know, he runs the compressor backwards, talks about it as if it is a single piston compressor, Does not give me the feeling of clever.


He has his own kind of clever. If you watch more of his videos you'll see what I mean. Those things you pointed out, this isn't a good example. He's a jack of all trades, more knowledgeable in a vast number of topics than some experts are in only one.


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## strantor (May 16, 2020)

Well I have some data to present. Nothing on the topic that started this thread, but I am making progress. I haven't connected it to a VFD yet. I'm running it across the line off my 30HP RPC. I ran it probably an hour today, taking temperature measurements. I want to have an aftercooler between my compressor and tank; I had that on my old compressor and I don't want to give it up. My old aftercooler was an A/C condenser out of a chevy truck.




It was VERY effective but with only (5/16?") ID I think it would be too restrictive and throw off my numbers. I got some 3/4" copper pipe to make a zig-zag cooler but I wasn't confident it would be effective so before chopping my pipe up into a bunch of pieces I decided to see how much cooling 10ft of copper could achieve, by running it in a straight line.




Turns out it drops the temperature by about 25deg in still air, and when I put a fan somewhere in the middle, it went to about 55deg.





I think in a zig-zag with fan cooling across the entire thing, it might be just as effective as the condenser was. And this is just a pre-stage to an actual compressed air dryer (Quincy QRHT-25) that I got for free. It had suffered an instant death and instant replacement at one of the places I do work, and they were glad to have me dispose of it; I revived it (I think) and hopefully the only thing going into my tank henceforth will be clean dry air.




I want to get all this in place before I start playing with RPMs, CFMs, VFDs, etc. Because the temperature of the air affects SCFM and I don't want to start the testing off with one set of control conditions and then switch it up, or have to re-do all my tests with & without the dryer setup. It's hard to hold back but I feel like if I'm going to do anything right here, I want it to be this.

Anyway, I don't have the dryer inline yet but I have some numbers, might as well post them

Here's what it Looks like at 60HZ across the line 







At zero pressure it was about 14CFM I believe, didn't take a picture


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## jwmelvin (May 16, 2020)

Thanks for continuing to report. I’m following along and enjoying learning.


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## Superburban (May 16, 2020)

Be careful, the copper pipe you get at home Depot, or Lowes is usually the type M, I do not believe it is rated for compressed air. You want is type K.

You can increase the cooling ability by wrapping non insulated wire around the tube, to effectively give you fins. If the setups they use for hot water heating systems is type K, then they would be the ideal setup, they have the fins already on, just get the length you need, and solder or braze away (Not sure if solder is good enough for compressed air).


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## Superburban (May 16, 2020)

Here, I found this link for you.



			https://www.copper.org/publications/pub_list/pdf/copper_tube_handbook.pdf


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## strantor (May 16, 2020)

Superburban said:


> Be careful, the copper pipe you get at home Depot, or Lowes is usually the type M, I do not believe it is rated for compressed air. You want is type K.
> 
> You can increase the cooling ability by wrapping non insulated wire around the tube, to effectively give you fins. If the setups they use for hot water heating systems is type K, then they would be the ideal setup, they have the fins already on, just get the length you need, and solder or braze away (Not sure if solder is good enough for compressed air).


I'm using type K, rated 1280PSI. But even the annealed copper tube I believe was rated somewhere around 300psi. I don't know if you're referring to the "hydronic baseboard element" products with fins; I did think hard about buying a few lengths of that, but I couldn't bring myself to purchase anything else until I had verified that what I already have isn't going to cut it, and I think it will cut it.

Good tip about the wire around the pipe. I had wondered how effective that would be. I will test it. I really wanted to buy some of the "slinky" stuff that the pipe between the cylinders has wrapped on it, but I couldn't find that for sale anywhere. If you find it at all, it's wrapped on a pipe already and it's not something you just order a couple of feet of on Amazon; it's more of the "contact us for a quote; just let us know how many truckloads per month you need."


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## Superburban (May 16, 2020)

Yea, I thought the M was closer to the 150 PSI, been a long time since I read the charts. Thats why I had to look after posting that. The link is a good reading for anyone using copper pipe.

I was surprised at how much heat extraction you get with just a wrap of wire. I used a length of 6 strand copper ground wire. Another thing you can do, is use a couple of T'ees, and run two or more sections of pipe. Running them in parallel, is better then a longer single section. It allows the same amount of air to go through the cooler slower then in a single long section, and has less internal friction loss. Also, the air flowing slower, allows more heat to be absorbed by the pipe walls.


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## strantor (May 17, 2020)

Superburban said:


> Yea, I thought the M was closer to the 150 PSI, been a long time since I read the charts. Thats why I had to look after posting that. The link is a good reading for anyone using copper pipe.
> 
> I was surprised at how much heat extraction you get with just a wrap of wire. I used a length of 6 strand copper ground wire. Another thing you can do, is use a couple of T'ees, and run two or more sections of pipe. Running them in parallel, is better then a longer single section. It allows the same amount of air to go through the cooler slower then in a single long section, and has less internal friction loss. Also, the air flowing slower, allows more heat to be absorbed by the pipe walls.



I'm glad you brought that up as I've been wondering about it. Almost all radiator/condenser/heat exchanger designs (and I've studied them a lot lately) have multiple parallel paths I can't figure why. I'm not arguing with you, or the designers of all these products; obviously you all know something I don't- just thinking out loud. To me, the only reason you cited that makes sense is less internal friction. In my mind, turning a zig-zag into a ladder would indeed slow down the air moving through, but now the air only passes through the cooling section once. Same amount of air passing through the same amount of pipe. Seems like nothing gained. If anything, it seems like the temperature outlet could be warmer, or least less of a guarantee of it being cooler, as turbulence/eddy effects might result in one of the parallel paths becoming a "preferred" path with most of the air passing through it and the others being less used. I have no idea if that's "a thing" but it seems plausible to me. With a zig-zag I wouldn't have to hurt my head thinking about it.

Maybe the gains in the cooling effect of multiple parallel paths is something not obvious? Some sort of exponential function or affinity law like the torque requirements of a fan rising with the square of shaft RPM?

If I seem like I'm pushing back at all, it's because I've bought a box of elbows, not tees, but mainly because I want to understand why I'm doing what I'm doing, whatever it is I'm doing.


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## Superburban (May 17, 2020)

Think of a river, the narrow spots the water moves through faster, then when you hit a wide area, it slows down. If it is the same river, it is the same amount of water flowing downstream in both conditions. Its just that the wide areas, act kinda like a small lake, and allow the water speed to slow down. And of course, the narrow areas the water needs to speed up to get the same flow.

Or think of a dam, the water above is calm, or close, and going through the dam, it is not a place you would want to be. Yet, both are flowing the same gallons per minute.

Do not think of flow (gallons per minute), the same as speed (feet per second)

Or some simple math. You are pushing 4 GPM. through the one pipe, you will still have 4 GPM. Now split it through 4 pipes, and you will have 1 GPM going through each pipe. And by doing that, the air speed through each pipe will be 1/4 of what it is going through one pipe. By going slower, you have more chance for the air particles to dissipate its heat to the pipe wall.

After all that, why do we still see single tube heat exchangers? I think it is the manufacturing cost. one long tube bent 50 times, or a ton of short tubes cut, and brazed to a ton of t'ees. 

One long path, especially with a lot of elbows, will create internal friction. 1 you are pushing all the air through one tube, and 2, each elbow creates some friction. IIRC, for liquids each 90 deg elbow is equivalent to 1 to 2 feet of tubing. I do not know what it is for a gas.

Not trying to put the brakes on your project, it is a great idea, and I want to see the end game.


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## extropic (May 17, 2020)

strantor said:


> I'm glad you brought that up as I've been wondering about it. Almost all radiator/condenser/heat exchanger designs (and I've studied them a lot lately) have multiple parallel paths I can't figure why. I'm not arguing with you, or the designers of all these products; obviously you all know something I don't- just thinking out loud. To me, the only reason you cited that makes sense is less internal friction. In my mind, turning a zig-zag into a ladder would indeed slow down the air moving through, but now the air only passes through the cooling section once. Same amount of air passing through the same amount of pipe. Seems like nothing gained. If anything, it seems like the temperature outlet could be warmer, or least less of a guarantee of it being cooler, as turbulence/eddy effects might result in one of the parallel paths becoming a "preferred" path with most of the air passing through it and the others being less used. I have no idea if that's "a thing" but it seems plausible to me. With a zig-zag I wouldn't have to hurt my head thinking about it.
> 
> Maybe the gains in the cooling effect of multiple parallel paths is something not obvious? Some sort of exponential function or affinity law like the torque requirements of a fan rising with the square of shaft RPM?
> 
> If I seem like I'm pushing back at all, it's because I've bought a box of elbows, not tees, but mainly because I want to understand why I'm doing what I'm doing, whatever it is I'm doing.



Regarding radiators (heat exchangers), for a given flow (SCFM) capacity, a greater number of smaller diameter tubes improves heat transfer because, for each tube, more fluid (air) is coming into contact with the tube wall. Take for example a .375" ID tube; the area is about .11 sq in. and the circumference is about 1.18 in. Also, a 1.00" ID tube has about 7.18 times (.79 sq in) the area (flow capacity) but only 2.67 times the circumference (3.1416 in). Using 7 x .375 ID tubes (7 x .11 = .77 sq in) is almost the same area (-.02 sq in) but (7 x 1.18 = 8.26 / 3.1416) = 2.63 times the surface area inside the tube(s).


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## jwmelvin (May 17, 2020)

Yes, decreasing tube improves heat transfer. Check out Mezzo heat exchangers, made from very small tubes. https://mezzotech.com/why-micro-tubes/

Seems like even the thin-wall (type M) is fine for the working pressures of a shop-air system. In areas the pipe may suffer impact, the thicker varieties would offer a benefit.


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## strantor (May 18, 2020)

I made my aftercooler today. Between all my chores I didn't get a chance to tease any new numbers out of it, but at least it's there now. I decided to go ahead and move forward with my zig-zag design. I did not outright disregard the recommendation of multiple parallel paths; I was strongly considering it, too strongly. I was entering the analysis paralysis region and I had to slap myself out of it. I had a plan and I had the parts to accomplish the plan and just enough time to build what I had planned, so I built it. Thanks for the suggestions; please don't think I'm not open to suggestion. I am still planning on the wire wrap, even if not needed, just to see how much difference it makes. All I really need to do is get the air down to 140f; from there the air dryer can handle it. I ran it for a few minutes with a bit of restriction, time was growing short, no time for anything scientific, but coming out of the compressor the pipe was too hot to touch, and going into the air dryer it felt room temperature, maybe a little warmer. I think the unistrut frame it's mounted to sinks a great deal of heat, may not even need it's own cooling fan.


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## strantor (May 18, 2020)

I wanted to move the CFM gauge to the input side of the compressor for a few reasons:
1. It has a maximum pressure rating of 100PSI so with it on the compressor outlet as I had it, I wouldn't be able to connect any tools downstream of it, as the instant I stop using the tool, the gauge will be overpressured.
2. Measuring CFM @ specific PSI on the output is finicky. The tank introduces a lag; it's like a big capacitor. I set my flow valve to achieve a certain CFM and then I must watch my tank pressure for quite a while, see if it's going up or going down; am I drawing from the tank more than the compressor is replenishing? It can take a surprisingly long time to ascertain that. Am I drawing _less_ than the compressor is replenishing? That takes time to determine as well. With the gauge now on the input I have an instant indicator of CFM.
3. I know that desired measure of CFM is the *delivered* air, that measured at the output as I had it initially, but nearly all consumer grade compressors are rated in SCFM at the input, as I have it now. My measurements now may not be as accurate, but they will be more comparable to published stats of competing compressors.

So I did move the gauge, and since i was fiddling with the intake I decided to rig up the silencer I had put in mind. I didn't even finish the design; I just got it to the point where I could draw air through it and through the CFM gauge. Despite this, the compressor is greatly quieted. So much so that I may even consider this good enough and leave it as is (at least for now).
















My plan was to have the bucket lined with sound deadening foam, and a few baffles through which the air must flow. But apparently just an empty bucket is pretty darned effective.


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## strantor (May 19, 2020)

*Finally! The Data We've All Been Waiting For...*

I screwed with the compressor all day in a fit of obsession, finally got what I wanted out of it. In what follows, there are some "screenshots" (cameraphone pictures of my laptop screen) of the scope graph spit out by the VFD software. The green Horsepower trace is very noisy because of the reciprocating nature of the compressor compounded by periodic sampling rate. It looks like crap but you can get the general idea. Also occasionally one or more values drop to zero; that's a bad connection of my USB>serial converter or a brain fart of the virtual machine I'm running, not sure which, but it doesn't mean the drive stopped; it ran flawlessly.


Here's what the HP looks like over a tank charge-up from 0 to 145 PSI *with the motor running at a fixed 60Hz*, as it would connected directly to mains power with no VFD. Green line is HP output, blue line is Hz (speed).







Here's what it looks like in constant horsepower mode. Again, Green line is HP output, blue line is Hz (speed).







Here is the actual data (no "approx") from both tests on one chart:






That is just what I had hoped the chart would look like; actually the results are better than I had hoped. I think my original math predicted 26CFM @ 0 PSI and something like 20CFM @ 40 PSI, but I knew real life would be much less. This is actually closer to the calculated values than it is to my gut feeling of what it would be.

That said, it turns out that the diminishing effect of higher CFM at lower pressures does not cumulatively amount to a whopping gain when you look at tank charge-up time. Here's the data on that:

*Tank charge up, 0-140PSI (26gal tank):
Constant speed (60Hz) - 3min 21sec
Constant HP (120-60Hz)- 2min 26sec

Constant HP is 55 seconds (38%) faster. *
I was hoping to fill the tank in half the time. Not sure why; that's obviously (now) stupid.

To put the above chart in some context, I've added some other compressors from the market:
1. Quincy Q13160VQ (3.5HP/60Gal/$850) 
	

		
			
		

		
	



2. DeWalt DXCMLA3706056 (3.7HP/60gal/$779) 
	

		
			
		

		
	



3. Campbell Hausfeld VT6271 (3.2HP/30gal/$599)
	

		
			
		

		
	










Here's a video of a charge/discharge cycle if you're bored enough to watch. Feel free to criticize the unsafe disheveled lair; I've been in full-on obsession mode with this compressor and haven't tidied up since I started. Also it was a minor disaster before I started.









One important thing to note if anyone wants to try and replicate this: there is not exactly a "constant HP" mode in a VFD; at least not in mine. Maybe there is a VFD out there which has such a mode, but I don't know which one. When I was speaking before about constant HP, overspeeding the motor at lower amps, etc., I had maybe lost a bit of my mind. Drives typically have a *constant speed* mode and a *constant torque* mode.

Constant speed mode will hold the motor at (you guessed it) - a constant speed. Amps/torque can go up or down (*within the envelope of the motor's nameplate parameters*) depending on changes in load, but speed will remain the same - so long as the load doesn't get so great that it causes a high amps fault.

Constant torque mode will apply a fixed amount of torque from the motor. Hz/speed can go up or down (*within the envelope of the motor's nameplate parameters*) depending on changes in load, but torque will remain the same. If load goes down, speed will increase. If load increases, speed will decrease.

I set out thinking all I needed to do was put the VFD in constant torque mode, set base speed as 60Hz and max speed as 130Hz, and it would automatically behave as I had previously described. It turns out that that ain't how it works. The Yaskawa V1000 VFD that I used does not actually have a bona fide constant torque mode, but it does have a torque limit parameter which theoretically (I thought) should have allowed me to me to achieve an equivalent mode of operation: set a fixed 130Hz reference, and as it bumps up against the torque limit, the speed will decrease. It didn't. What happened was, above 60Hz in the region where available torque is diminished, it was still trying to match the sub-60Hz torque limit. It was trying to achieve nameplate rated torque at double the nameplate rated RPM. No bueno. Here's what that looks like:






Maybe if I had a VFD with a bona fide constant torque mode, the operation above nameplate rated RPM would have been handled automatically as I expected. It's been a long time since I've set up a drive in this manner. I really thought that above nameplate rated RPM they transitioned into a constant power mode. Pretty sure I didn't make that up. But whatever, I knew what I needed: A constant *power* mode, wherein speed *and* torque are variable, *and* it all works even _outside_ the envelope of the motor's nameplate parameters. Since this drive didn't have such a mode, I made it myself. Yaskawa has two different softwares for their VFDs. "Drive Wizard Industrial" is what you saw in the screenshots above; that's the normal drive commissioning/parameter changing software available to anyone. But they have another trick up the sleeve: "DriveWorksEZ." DWEZ is a different, secret-but-not-quite-secret software that speaks to a different, secret-but-not-quite-secret processor on the control board. With this separate software, within this separate processor, you can do some custom programming; write some quasi-"PLC" type functions. Here's the function I wrote to achieve the "Constant Horsepower" mode:




Basically it does this:
1. If output power is greater than 125%, gradually lower speed (override commanded 120Hz speed reference)
2. If output power is between 100% & 125%, gradually raise speed (override commanded 120Hz speed reference)
3. If output power is less than 100%, return to commanded 120Hz speed reference

I bring all this up for 2 reasons; firstly to brag about my kludgey hack solution in the face of poorly made assumptions, and secondly to caution you to go into this better prepared than I did. Make sure that the drive you choose has some kind of constant HP mode which works above 60Hz, or that is has some higher-level programming function like DriveWorksEZ that isn't unobtainium. DriveWorksEZ is unobtainium. I would be remiss if I didn't point that out. They only want Yaskawa-certified techs wielding it. I used to work for a Yaskawa authorized distributor and I was a certified installer; that's why I have the software. You might not be able to get it. Don't just jot down a parts list from this thread and try to replicate it; it could be a costly disappointment.



OK... concept proven I think. Now that's out of the way, I can exit obsession mode and get on with my life, clean my shop, do the things I'm supposed to do. I am back to having a working compressor, even if a hideous abortion of one. I will come back and finish it in small bits as time permits. To Do:
- Weld up unistrut wonder, recover my hardware
- mount unistrut wonder (atop vibration arresting mounts), silencer, air dryer, tank, and an electrical enclosure, on a cart
- troubleshoot Quincy air dryer; may not be operating at 100% capacity
- Install analog pressure sensor, tied into drive or separate PLC. Wire up existing pressure switch as failsafe
- improve zig-zag heat exchanger, wire wrap, etc.
- finish silencer concept
- Install temperature sensors in key areas
- program other modes of operation, like re-charge
- Fabricate a metal shroud for it


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## brino (May 19, 2020)

Amazing amount of time, effort and knowledge invested here.
Thanks for sharing it with us!

-brino


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## jwmelvin (May 19, 2020)

I agree, fantastic effort and report, thank you. Please do post the final configuration for our enjoyment too.


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## fixit (May 19, 2020)

The only problem I have with this whole project is in use the compressor will run in a very narrow psi range. Does the cost of this conversion justify the apparent small increase in output? I was involved in compressed air systems for a major Glass container Mfg. we ran 3500 plus ( yep 3500) horsepower of compressors 24/7 to maintain the forming machines. I constantly looked at more efficient means of providing compressed air. Two methods stood out. One was to use a variable inlet controlled centrifugal machine as the system control. The other was my favorite but corporate didn't like the initial cost. It involved conditioning the intake air to the compressor by cooling & dehumidifying. This resulted in your choice of two results. One being a reduction of total horsepower or the other being is the increase of total air volume. Both gave you clean DRY air and reduced downstream treating of the air & maintenance to the forming machines caused by wet air. So to suggest an addition to your project, condition your intake air using a window air conditioner getting the air down to 30% humidity and 50/60 deg temp this set up should also pressurize the intake slightly also. I know this works I've seen it.


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## 7milesup (May 19, 2020)

Very cool thread.  Thank you for all the effort and data.  

I used two "A" coils from a house central air unit, soldered together as my condenser.  Yours looks really great too.   One of my favorite simple formulas...P*V=P*V


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## Downunder Bob (May 19, 2020)

A well thought out and executed experiment, However i can't help thinking it's a lot of work for a very small gain, particularly in the hobby set up, However you had the pleasure of testing your ideas, well done. The old story whatever floats your boat.


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## strantor (May 19, 2020)

brino said:


> Amazing amount of time, effort and knowledge invested here.
> Thanks for sharing it with us!
> 
> -brino


Time & effort sure, knowledge is questionable. 


jwmelvin said:


> I agree, fantastic effort and report, thank you. Please do post the final configuration for our enjoyment too.


Will do!


fixit said:


> The only problem I have with this whole project is in use the compressor will run in a very narrow psi range.


I don't understand your meaning. It runs in the same PSI range as any other commercially available single stage compressor.


fixit said:


> Does the cost of this conversion justify the apparent small increase in output?


For me, yes. I had most of the parts on hand. For most people, probably not. If you had to go out and buy everything, absolutely not.


fixit said:


> I was involved in compressed air systems for a major Glass container Mfg. we ran 3500 plus ( yep 3500) horsepower of compressors 24/7 to maintain the forming machines. I constantly looked at more efficient means of providing compressed air. Two methods stood out. One was to use a variable inlet controlled centrifugal machine as the system control. The other was my favorite but corporate didn't like the initial cost. It involved conditioning the intake air to the compressor by cooling & dehumidifying. This resulted in your choice of two results. One being a reduction of total horsepower or the other being is the increase of total air volume. Both gave you clean DRY air and reduced downstream treating of the air & maintenance to the forming machines caused by wet air. So to suggest an addition to your project, condition your intake air using a window air conditioner getting the air down to 30% humidity and 50/60 deg temp this set up should also pressurize the intake slightly also. I know this works I've seen it.


I'll keep it in mind, thanks!


7milesup said:


> Very cool thread.  Thank you for all the effort and data.
> 
> I used two "A" coils from a house central air unit, soldered together as my condenser.  Yours looks really great too.   One of my favorite simple formulas...P*V=P*V


Your HVAC coils are certainly better than this. Now that I have a wealth of data I might try subbing my old A/C condenser coil back into the mix and see if I suffer any loss of CFM. Hooray for gas laws! If you add temperature to the mix (_P1V1/T1_=_P2V2/T2_ ) you start to get answers about missing CFM at higher pressures.


Downunder Bob said:


> A well thought out and executed experiment, However i can't help thinking it's a lot of work for a very small gain, particularly in the hobby set up, However you had the pleasure of testing your ideas, well done. The old story whatever floats your boat.


I am likewise a bit underwhelmed with the gains. However if you regularly do something requiring lots of CFM at not much pressure (like HVLP painting) this could be very beneficial. With 18.5CFM@40PSI, this 3HP compressor fills a niche that otherwise would demand a 7.5HP compressor.


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## Downunder Bob (May 19, 2020)

7milesup said:


> Very cool thread.  Thank you for all the effort and data.
> 
> I used two "A" coils from a house central air unit, soldered together as my condenser.  Yours looks really great too.   One of my favorite simple formulas...P*V=P*V



I imagine your condenser is also going to cool the air so your favourite formula should read P*V/T = P*V/T .


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## nnam (May 19, 2020)

I think the gain is rather good.  We can't expect magic.  However, with better cooling of out going air, the tank actually store more air.

Better cooling of the compressor head and active filter oil, you may get higher rpm as long as you watch the temperature.

Furthermore, put a larger motor can push it higher.

On the graph, I don't get the up and down, should it be smooth.  I am not talking about noise, but the second graph where constant HP appears to show up and down CFM.  Can you explain that?


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## nnam (May 19, 2020)

on another note, a larger tank solves most of the problem, I think.


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## strantor (May 19, 2020)

nnam said:


> I think the gain is rather good.  We can't expect magic.  However, with better cooling of out going air, the tank actually store more air.
> 
> Better cooling of the compressor head and active filter oil, you may get higher rpm as long as you watch the temperature.
> 
> ...


I don't think this thing needs a bigger motor. In some of my testing I ran for 20+ minutes at the VFD's "redline" rather than the motor's, running the motor at 150%+ of its rated power, and it never even got warm.

Ideas of oil pumps, jets, etc. still plague my thoughts despite prior warnings about piston/rod inertia. This compressor is NOT balanced. In some frequency bands (ranges of RPM) it threatens to shake the unistrut wonder apart. Curious thing though, it seems smoother and happier the faster it goes. In fact, at its rated RPM it's pretty shaky, but at 125% of rated RPM it purrs like a kitten. Maybe it would like to go 150%? 200%? It's almost cheap enough to warrant exploring. 

On the graph I left little notes explaining the humps. I was opening a valve and purging all the air from the tank.


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## strantor (May 19, 2020)

nnam said:


> on another note, a larger tank solves most of the problem, I think.


I disagree; let's discuss. The way I see it, the only reason compressors have a tank is so that you can periodically, briefly, get more CFM than the fixed-speed machine can deliver. You exceed its flow capacity at a cost; a trade-off - you get it at lower pressure. Ever ran your ugga-dugga until it just won't dugga anymore and just hisses at you? You've bleed your tank down to like 20PSI and now you must take a smoke break while the compressor replenishes. This is the "wasted capacity" that plagues the typical compressor. While you're out there giving yourself cancer, the motor is putzing along at the only RPM it knows; you paid for 3HP or 5HP or whatever, but for the first half of your cigarette you're only getting 1HP; maybe 2? 

The large tank is (IMO) a poor compromise of a solution. And scaling the tank upwards seems to be popular but I don't understand why. If you're exceeding the CFM capacity of your pump, then you're exceeding the CFM capacity of your pump, plain and simple. It doesn't matter how big your tank is; you're going to drain it down, and when you do, you're going to have to wait for it to recharge. The bigger the tank the longer it takes to drain down AND the longer your smoke breaks have to be. Small tank, less ugga-duggas between smoke breaks, and shorter smoke breaks. Either way, same number of ugga-duggas to get the job done, and in the same amount of time.

Now introduce the constant HP concept and maybe, just maybe, your ugga never ceases to dugga. Maybe pressure drops and it slows, the but the motor rises to meet the CFM demand with no smoke breaks atall. I don't know; it will be tested...

So with this setup I think I favor my 26gal tank. If given a choice I may have even gone smaller to save space. AND if it were only say 10gal, it would be ready to go at full system pressure less than a minute after plugging it in. It would be ready before I even found my air tool and that one QD that I have, that gets passed between all the tools because none of them ever has a friggin QD on it despite the fact I buy at least a dozen every time I go to HF... but I digress. 

Agree? No? Let's talk it over. I suspect there's something I'm missing, as I quite often do, when I look around and see that everyone making compressors (and making money by making compressors) is making them with bigger tanks and people seem to want bigger tanks. What am I not seeing?


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## Larry$ (May 19, 2020)

If your primary goal has been to quiet the thing down, putting it someplace to do that is the cheapest, easy solution. Just make sure it has plenty of cooling. I've got a 25hp screw in a small sound conditioned space, with lots of air flow generated with the air to air cooler. It has now run that way for over 12,000 hours, must work! I've got quite a few motors on equipment that run on 400 cycle power 14,000+-rpm but designed for that.  Our motors on VSDs are all designed for that.  I would guess that the limiting factor in running at other than design speed is over heating at slow and balance/bearings/lube at high. 
Splash lube "systems" have a small "scoop" on the crank that splatters the oil around. At design speed it will throw some oil high enough to lube the rings. As speed is slowed will enough oil get to the rings? Heat damages oil, best to use a full synthetic. 
Some oil & water (condensate) always exit a compressor. They cause damage to many things that air is used for. Treat your air to limit the damage done. Cool it and separate the water, filter out the oil and dirt, regulate the pressure to the lowest level that works. Compressed air is expensive.


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## nnam (May 19, 2020)

strantor said:


> On the graph I left little notes explaining the humps. I was opening a valve and purging all the air from the tank.


Thanks.  I was reading via a phone, I couldn't see the text, and forget to check it again.  If you worry about wear, start with very clean inside, even gasoline or whatever safer to clean it up.  Then try some experiment, then check the oil against a very clean piece of paper and flash light to check for metal.  I think there are better ways, and even more costly, which I say, not recommend due cost, oil analysis.

If the balance is an issue, I think try solving it may help.


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## nnam (May 19, 2020)

strantor said:


> Agree? No? Let's talk it over. I suspect there's something I'm missing, as I quite often do, when I look around and see that everyone making compressors (and making money by making compressors) is making them with bigger tanks and people seem to want bigger tanks. What am I not seeing?



The efficiency reduced if the motor starts and stops often.  A larger tank would help if you increase the kick off pressure.  So say you use 90 psi on your tool, and you want to kick it off at 90 psi, that means, if the pump can handle that CFM of your tool, it would kick off, and you can keep working.

A larger tank keeps it longer before it kicks off again, improve efficiency.
It also helps the motor with longer life.  The vfd with constant torque helps alot in that regard.

It's only a problem if you empty the tank.  In that case, why not keep it full, and have an auto water bleeding valve, not sure if that solves the rust issue.


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## westerner (May 19, 2020)

strantor said:


> Agree? No? Let's talk it over. I suspect there's something I'm missing, as I quite often do, when I look around and see that everyone making compressors (and making money by making compressors) is making them with bigger tanks and people seem to want bigger tanks. What am I not seeing?


I was taught that the benefit of a larger tank on ANY system was fewer start cycles which saves wear and tear on the motor, starter and belts. 

Great set of data, and an interesting study. Thank You for your effort in posting this well designed experiment. I think we ALL learned something.


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## Downunder Bob (May 20, 2020)

The waY I see it is that a larger tank will help if your air demand is high at intermittent intervals, if the demand is always high then a larger compressor is the way to go.


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