Electric motor for a compressor.

[Mill23]

I working on sizing an electric motor to run a reciprocating compressor. We are in the 10kW range at 2000RPM, 45-50Nm. We are looking at frameless motors as they are light weight and have a small profile.
Someone told me that an electric motor will be eaten up by the reciprocating compressor since the torque goes from 50Nm to (-), acting as a brake when some of the air pressure pushes the piston back down.
He says at most it will last a year.

I'm wondering if anyone can confirm or shoot down this train of thought?

 

[dpc]

There are a lot of motor-driven reciprocating air compressors in service. Most of the ones I see are belt-driven and have motors that appear to be older than I am, and that's pretty old.

I'd say if you correctly apply a new motor, it should last a long time.

There may be some application issues unique to recips that I'm not aware of, but someone should be along shortly to give you a better answer.

Cheers,

Dave

 

[DickDV]

If the motor is to be single phase, you should plan on using a cap-start, cap-run motor in order to have enough starting torque to get through the first compression stroke.

 

[Mill23]

we plan on using 3-phase. I'm wondering on the torque peak to trough at max compression to negative torque. Anyone have any experience with problems in this area? thanks,

 

[ccjersey]

Your analysis that shows the negative torque seems wrong. The volume of compressed air exhausted to the discharge would nearly eliminate any remaining "power" in the compressed air charge just because there will be such a small volume of air remaining in the compression chamber.

If there were no outlet, then sure, you would have a significant re-expansion of the compressed air. I know it is traditional to have a large, heavy flywheel on a reciprocating compressor which would tend to help with your concern.

A 10 kW machine would surely have multiple cylinders so another piston could more than absorb any "negative torque" produced as a piston goes over TDC.

 

[itsmoked]

Will this be one of those painfully loud direct drive comps or will you have some flywheel storage?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Mill23]

because of packaging restraints, there is no real room for a flywheel. We would be using a direct drive frameless dc motor.

 

[MikeHalloran]

That sounds like a hard life for a DC motor, and its power supply. How many cylinders does the compressor have?



Mike Halloran
Pembroke Pines, FL, USA

 

[waross]

Well, with DC torque should not be a problem. As was stated, as soon as the air pressure in the cylinder exceeds the tank pressure the air starts exiting. There is not much left in the cylinder at the end of the stroke. Have you considered an unloader to assist starting? Most air compressors in that size range start unloaded. a flywheel or multiple cylinders will reduce the torque surges.

Bill
--------------------
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