Overcurrent at partial load
Overcurrent at partial load
(OP)
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,
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,





RE: Overcurrent at partial load
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
RE: Overcurrent at partial load
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?
RE: Overcurrent at partial load
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"
RE: Overcurrent at partial load
"Venditori de oleum-vipera non vigere excordis populi"
RE: Overcurrent at partial load
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?
RE: Overcurrent at partial load
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RE: Overcurrent at partial load
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.
RE: Overcurrent at partial load
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.
RE: Overcurrent at partial load
RE: Overcurrent at partial load
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.
RE: Overcurrent at partial load
BK
RE: Overcurrent at partial load
Do you have any previous data, using this analyzer, to compare it too? Or is this the first time you used it?
RE: Overcurrent at partial load
175kvar for a 125Hp motor? Doesn’t seem possible. It’s hard to imagine that the motor would need a 150kvar capacitor.
RE: Overcurrent at partial load
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.
RE: Overcurrent at partial load
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.
RE: Overcurrent at partial load
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..
RE: Overcurrent at partial load
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.