Overcurrent at partial load

[Tillman]

I have a 125 HP, 600 RPM, 480/60/3 225 FLA induction motor driving a piston compressor. It keeps tripping on O/L (set at 234A). I put a power analyzer to it, and this is what I got:
81 kW, 175 kVAR, 193 kVA, 0.42 PF, 3 phase A 231/231/231, 3 phase U 482/482/482.
Does this sound like a defective motor or there is something I can do to help with the PF?
Thanks,

 

[DRWeig]

Hi Tillman,

If it's getting to FLA at only about half load, I would first look at the wiring and see if it were wired Y instead of delta? Putting 480V across a winding built for 277 would yield weird results I think. I wouldn't think the motor would last very long at all if wired like this but it's the only way I can think of to explain what you're reading...

Look for other's suggestions here, and let us know what you find out!

Good luck,

Old Dave

 

[aolalde]

Hi Tillman:

The electric parameters show that the load is around 100 HP, but the Power factor is very low (0.42). Your motor must have something wrong. First check your terminals connection, since the line voltage is quite close to the nominal and balanced the supply seems OK. Measure the shaft RPM to check the slip.

If this motor was recently re-wound, I would suspect of a wrong winding. By the way a reciprocating compressor has intermittent torque load; that could make swing the current unless a good inertia is added to the motor rotor, is the current steady?

 

[jraef]

Where do you say that it is half load, and how do you know that? I see a motor with perfect voltage balance, perfect current balance, running at 103% of nameplate FLA, and tripping within 1.3% of the OL relay setting. Looks normal to me, especially since most bimetal OL relays are only accurate to withing +-5%.

I assumed you have a bimetal OL relay since you stated a "setting" (not available on melting-alloy) and you did not say it was solid state (and I assumed you would if it was). A NEMA OL relay trip curve should trip in about 2 hours at 100% of the setting point, and it is considered acceptable from a QC and UL testing standpoint if the relay trips early (most people don't know that) since it's primary function is to prevent a fire, not avoid a nuisance shutdown. If it has tripped several times, a bimetal OL relay will build up a thermal memory as well, taking less time to trip. If your motor is rated for a 1.15 SF you could look at using a higher setting, but no more than 258A, and even then you can expect reduced moto life. Even so, I would replace that OL (or the heater elements if they are removeable), and I personally would go solid state if it were my money.

If you are in the US (again, an assumption based on 460V/60Hz stated) it is unlikely that your motor was wound for Y operation, it just isn't done that often here. A similar situation however would be that it is a 230/460V motor, and you accidentally used the 230V connection pattern. If you are indeed at a fraction of the known rated load capacity of the motor, that is a likely problem.

"Venditori de oleum-vipera non vigere excordis populi"

 

[jraef]

Oops, missed the .42 power factor until I saw aolalde's post (we were typing at the same time again!). That could indicate partial loading, so I think too that there is some sort of connection or winding problem.

"Venditori de oleum-vipera non vigere excordis populi"

 

[buzzp]

I tend to agree with jraef. I am not surprised this is tripping on OL. It only takes one phase to make this happen. Giving the small margin between the trip level (234) and runing amps (231) I would bet it is doing its job.
I am not familiar with piston compressors but sounds like these are constantly varying loads. Hence, perhaps your data was taken when the load is at a minimum (not compression stroke). This would explain the pf. Also, I would bet the current is bouncing around a bit too and is going above the trip level or your OL and causing the trip. (Maybe I am way off base with the load varying over one cycle of the compressor).
Are those power analyzer readings averages or are they peak numbers?

 

[electricpete]

following up jraef's commnet - it could be a motor running near full load... except for the very large reactive component and low power factor (even for a low-speed 600rpm motor). Therefore it may be also worthwhile to doublecheck the measurement setup.

=====================================
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[Tillman]

Thank you all for your suggestions. It'll be a few more days before I can get my hands on the motor for further testing. Here are some more details that have been asked for through the thread:
The motor was purchased new 01/01 and has not been reworked.
Actual RPM is 588-591. Nameplate RPM is 594.
Yes, the current is fluctuating quite a bit.
My reading was average, not peak.
Local mechanical folks estimated power drawn by the compressor was about 100 HP.

Thanks again and I'll keep further development posted.

 

[digitrex]

I agree with jraef. There is nothing wrong with the motor. A low pf(0.42) at full load for this 12-pole motor is also normal.

The areas to be investigated for improvements should be:
1. The compressor (piston type draws non-sinusoidal current), to reduce load etc
2. Consider 10% margin on overload setting or alternative type of overload relay.

 

[edison123]

About 50 KVAR at the motor head will see a much reduced line current and avoid nuisance trips.

 

[buzzp]

Do not set the OL above the service factor amps of the motor. As I said before, it is doing its job protecting the motor. The load fluctuations are obviouosly causing the current to cycle up and down, in sync with the stroke of the piston. If you look at your highest current in this cycle, you will see it go above the OL setting.

Verify that the setting can NOT be increased my looking at the service factor amps or service factor (SF applies to the HP but you can get to SF amps from here). That or call the mfg and ask them what the OL setting should be.

Sounds like something has changed recently to give you these problems. This could be the voltage changing (high or low) or a mechanical change such as loss of lubrication. Or even the motor is just wearing out. Good luck.

 

[bklauba]

Also, make sure that the relief valve on the compressor is open during startup. Once the motor is up to speed, this should then close.

BK

 

[buzzp]

Is the PF averaged too? If so, what time fram was used or number of cycles used? Look at the specs of your analyzer a little closer and see what it is really telling you. If it is an average pf, then maybe it is correct. Seems the wide swing in load could potentially cause an avg pf such as you got. But I am not familiar with such applications.
Do you have any previous data, using this analyzer, to compare it too? Or is this the first time you used it?

 

[busbar]

 
175kvar for a 125Hp motor? Doesn’t seem possible. It’s hard to imagine that the motor would need a 150kvar capacitor.

 

[BJC]

Tillman
What kind of stater does the motor have? Could it be a delta-wye or part winding starter that is connectd wrong. I'm not sure what kinds of current you would get with the wrong combinations. I would check the starter and make sure it's connected and operating correctly. Unused or floating windings in the stator could be reflected in the low power factor.

I have seen lots of piston compressors with reduced voltage starters utilizing 3 or 4 contactors. It can be easy to hook them up wrong.

 

[aolalde]

Tillman.

If your analyzer works with two inputs; One line current and the voltage between two other lines, the polarities or phases selected could be wrong and you are getting false PF and Power readings. Check the instrument instructions.

 

[rbulsara]

I have some basic questions..

Is this a new problem that recently started? If so how long the motor ran OK?

Was this compressor tried with another motor, after the problem with this motor?

How does this motor behaves when ran uncoupled from the load?

Was there any major repair/overhaul or change done to the compressor or the equipment setup?

I have a feeling that the compressor is jamming up intermittently..your 'average' readings are meaningless..

 

[OnLineSystems]

I've seen 2 similar cases:

a) harmonic distortion (pulp and paper).
Back of the envelope calculation on the nameplate suggest that the pf at full load should be about 0.5.
Overloading should cause the pf of the motor to rise above nameplate, and not drop below it. Lower pf can be caused by harmonics. Have additional VFDs been installed in this bus recently? (this could reduce the power quality of the bus, and result in a lower pf and higher currents). Are your voltage THD values below 5%?

b) long leads (copper mine).
I suspect that you measured the currents and voltages at the MCC. Long leads to the motor can cause the motor terminal voltage to be lower than the bus voltage (resulting in an undetected undervoltage for the motor). This would explain the higher currents and the above-nameplate slip. However, it would not explain the pf being more this low.