What limits the speed of an air compressor?

[macardoso]

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.

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]

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 .

 

[Joe in Oz]

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.....

 

[Lo-Fi]

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

 

[projectnut]

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]

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.

 

[strantor]

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.

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]

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.....

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...

 

[strantor]

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.

 

[westerner]

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.