Low PF on Compressor
- Thread starter reed35
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Are the motors otherwise identical?
Same rating, same voltage, same number of poles (check rpm)?
Is the pf you give measured pf or that given on the nameplate?
Are the compressors and their loading identical?
hardworkingstiff
Senior Member
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- Wilmington, NC
Capacitor going bad?
Yes, could be. I wondered whether each compressor had local pfc.
But if it has, wouldn't you want to correct to better than 0.8?
Rick Christopherson
Senior Member
I was going to ask the same thing. If it is a measured pf, then it could simply be that this motor is not operating near its rated capacity. The lower the load on a motor, the lower the powerfactor.
weressl
Esteemed Member
The power factor decreases with the thermal age of the motor, in other words a motor running at half a load will be 'younger' that the one at full load after say 10000 running hours. I squared T.
Same size? Same model? Compressors can use regular motors or integral, hermetically sealed windings in the compressor body. The later ones have much poorer natural power factor, but more efficient. Strangely enough.:smile:
Compressor motors load and unload continuously, so they start at a low load then run up - often above their nameplate rating - before start unloading again. Since the power factor is load dependent, it is important to measure the actual load when measuring the power factor. If identical motors at identical load exhibit dsignificantly different power factor, say +/- 0.05, I would consult a motor shop, because I would think that my motor is going South.....
It may, but I doubt that a motor would go from 0.8 to 0.6 without some serious, or even terminal, degredation.
weressl
Esteemed Member
Have you looked at some of the hermetically sealed compressor PF curves? Or even regular US motor curves? At no load? If the compressor loader/unloader failed it can be just sitting there, spinning.
Yes, obviously power factor varies with load.
But my comment in post #7 was directly in response to yours about thermal aging, not loading.
weressl
Esteemed Member
Except that I never claimed that power factor would go from .8 to .6 DUE to thermal aging. Those were general comments on variatiables that have different impact and a combination of several could be the result of such large difference. As you pointed out such drastic change due to thermal aging ONLY would indicate permanent damage to the machine.
I can't help thinking that might have been a more appropriate response in post #8 instead of commenting about loading. And, I didn't say that you claimed that pf would go from 0.8 to 0.6 due the thermal aging. I was just pointing out that I thought it unlikely.
Anyway, we are broadly in agreement which is good.
On power factor variation with load here's some measured data on a couple of motors motor for which we provided the drives.
Both centrifugal loads on reasonably large three-phase motors.
I suppose a salient point is that these were applications where the machines don't often run very lightly loaded. Over the normal operating range, typically 50% up, there isn't a huge change in PF.
On degradation of PF due to thermal stressing, I have seen no actual data that gives the extent of the problem.
Many of our customers log motor parameters either by manual observation or, more commonly, by a plant wide SCADA system with no reported reduction in PF.
I'm not saying that motor PF isn't affected thermal stress and time.
Just that I have no experience of it.
That was my first thought., and IMO the first place to start looking and go from there.
yes to all the questions and the PF is measured by the ct's measuring amperage.
The answer is yes to all questions. The PF is measured by the ct's capturing the amperage and voltage. They are also calculating kvar.
There are no correcting capacitors.
Thanks for all the information.
I suppose it could be a motor or compressor problem.
How practical would it be to swap the motor with one of the others to see if the problem moves with the motor or stays with the compressor?
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