Periodic oscilation in power consumption of a reciprocating compressor

[sinectica]

What could be the reason of a periodic consumption(we see an armonic wave of the intensity, amperes) of an electric motor coupled to a reciprocating compressor of 4 cylinders?
Could it be an error in the calculated inertial wheel?

thank you so much, any contribution will be appreciated.

 

[HVAC68]

It could be due to unloading of the compressors in response to lower load conditions.

HVAC68

 

[Barry1961]

Every time a piston strokes up you will have a surge in required power. The flywheel will supply some of the power to compress the air by putting someone of it's stored energy to work. The only way the flywheel can do this is with a speed change, slow down. When the flywheel slows down so does the motor.

A larger flywheel will help to smooth out the the surges but there will still be surges since there must be a speed change for a flywheel to work.

How constant do you want the current to be and why?

Barry1961

 

[ivymike]

how does the period of the oscillation compare to the period of crankshaft revolution?

 

[sinectica]

The period of the oscillation is the same as the shaft speed, that it's 7 Hz.
on the other hand, the oscillation is of about 30% of a constant intensity, that I don't know if it's acceptable or not.

Thank you so much for your help!
Marc

 

[Barry1961]

If you can use a standard over current device without over sizing I would not worry about it. Does the motor over heat while running?

Barry1961

 

[zdas04]

It could be actual load shifting. Have you looked at the compressor cylinder discharge temperatures? Often you see an unbalanced load when compressor valves are too stiff on one cylinder (either incorrect design for the conditions or the valve spring gets coated with scale) and the cylinder partially fills. Partial filling creates a situation where the actual compression ratio (and rod load) is much higher than external measurements would indicate. This results in higher cylinder-discharge temperature (explained by the heat of compression calculation) that is really easy to see. A load that varies 30% on a 7 Hz cycle might indicate a really bad suction valve on one cylinder (the spike is on the compression stroke).

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

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