What limits the speed of an air compressor?

[matthewsx]

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

 

[strantor]

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]

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.

View attachment 323110

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

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.

 

[matthewsx]

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.

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

 

[Buffalo21]

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

 

[strantor]

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

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.

 

[Superburban]

...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.

 

[strantor]

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

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

 

[extropic]

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.

 

[Mini Cooper S]

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.