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Motor KW Readings

Motor KW Readings

Motor KW Readings

We are commissioning the plant and carrying out no load run 3 similar 6KV / 1000 KW / 8 poles / 118A / motors. For 2 motors we have seen the motor KW of 28KW but for one of the motor motor KW of 13 KW. The no load current of motor per data sheet is 42A at 6000KV. We have seen 46 A for all three motors at 6400 V.

Attached are the motor current and voltage waveform captured through relay and plotted in excel file. As seen for good motor, current and voltage phase sequence is RYB. But for bad motor, voltage phase sequence is RYB and current phase sequence is RBY.

What can be the issue here? - incorrect Current transformer phase sequence? or phase sequence is correct but there is polarity for C phase is reversed? Did not see any temperature rise in relay or through RTD measurement during the 2 hours run.

Please share your thoughts.

RE: Motor KW Readings

Polarity for phase C CT is reversed. No doubt.

Phase angle (assuming same reference for VT and CT) is very close to 90 degrees (I get around 87.5 degrees) which is OK for an idling machine. Could actually even be a bit better.

There is some voltage distortion, which reflects in current. Voltage distortion is from outside. Not caused by MUT. Distortion components not in-phase and not 100% in synch. Do you have any large machines with high slip nearby? Or cascade drives?

Gunnar Englund
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Motor KW Readings

From a very rough calculation your motor supply voltage is about 6.6kV for a 6kV machine. Depending on how generous the manufacturer has been with the stator iron the current distortion and resultant voltage distortion could be due to the beginning of magnetic saturation in the core. The peaky nature of the waveform indicates a low-order odd harmonic which is consistent with early saturation. I don't think it is a significant problem, but it would indicate a motor operating at the upper end of its supply voltage range. Once the motor goes on load the volt-drop in the feeder cable will tend to reduce the effect as the terminal voltage will be slightly lower.

RE: Motor KW Readings


My sincere thanks for sharing your views.

ScottyUK - You are correct. This being new plant and not in service, the incoming voltage is higher (@10%) than rated voltage. Once plant is in service and more load comes in, we expect it to come down <5%.

Skogsgurra - You observations are correct - motor PF is very low - this is low RPM motor and running on load. I also believe that CT polarity is reversed.

Thanks again for your help.

RE: Motor KW Readings

Besides the CT being backwards, don't ignore the fact that with no-load the kW reading is a very small percentage of full scale so the combined CT/PT/meter accuracy can play a significant role in causing power measurements to vary between motors.

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