Motor current imbalance

A

Thread Starter

Adil

I'm facing a problem in diagnosing the main reason behind the 7kW motor's current imbalance. It's line current is 14.2A 13A 10.6A. The voltages are balanced. I've rotated the phase, and checked that the currents move with the conductor change. This confirms that the problem is with the motor. The resistance of the windings of the motor is also balanced, i.e 2.6 ohms for all windings.

I just have clamp meter, multimeter, & contact tachometer as my analysis instruments.

Thanks
 
Is there a VFD controlling this motor?

You could possibly have a bad IGBT not gating. Might have to bring somebody in for offline testing if it is the motor and not something else.
 
C

curt wuollet

I don't follow your reasoning that if the currents move with a conductor change, it points to the motor. If anything, it points to line phase irregularities.

Take a look at the waveforms, I've see this with machines where there is a non-linear single phase load connected. It changed the waveshape and the motor responded to the change. Phase shifts can cause this also.

Regards
cww an itinerant automation worker.
 
R
Possibly a turn or two shorted in the winding with the high current. The resistance reading isn't picking that up because the resistance difference is so small however to AC it's like a shorted secondary on a transformer.

A test you can do is take a low voltage transformer and compare the amperage you get through each winding when you put the voltage across it.

Roy
 
> Is there a VFD controlling this motor? You could possibly have a bad IGBT not gating <

No, simple Y/delta connection.
 
I've rotate the phases of the motor at the output of the magnetic starter. So, the winding, which is consuming high current, also rotated.
 
C

curt wuollet

If the center of the wye is not grounded, look at the voltages when running wye. An abnormally low voltage could indicate a shorted turn, a scraped pole or even a manufacturing mistake.

An inductance test will show this also. You could also have a bent shaft which causes the values to change at a rate that fools your meter. I have also opened a motor where the lead wire connections to the
stator were just twisted and taped. And have seen those connections go bad when crimped.

Regards
cww
 
Adil, the data you provided thus far, indicates that the supply voltage VUF (Voltage-Unbalance-Factor) is nil, while the line current AUF (Amperage-Unbalance-Factor) is 15.9% or 16.6% per NEMA or IEC Standards, respectively.

The data rules out supply voltage unbalance as the cause. Instead, motor winding failure the most likely cause. The degree of failure is severe to critical and motor replacement is the prudent course-of-action. Temperature measurements around the surface of motor showing significant deviation in circumferential readings should confirm the internal failure.

However, before you replace the motor I question the accuracy of voltage measurements. The phase-to-neutral reading of 201V does not correlate with the phase-to-phase voltage reading of 400V. That is, if 400V is correct, then corresponding phase-to-neutral readings should be about 230V. Conversely, if the 201V reading is correct, then the corresponding phase-to-phase voltages should read about 348V. Are the instruments you are using "true RMS" reading?

Regards,
Phil Corso
 
I'm extremely sorry for posting wrong readings by mistake.

The correct readings are as follows:
Phase-to-Phase voltages are 402V 400V 402V
Phase-to-neutral voltages are 232.2V 234.1V 232.8V

And the rated current of the motor is 15.2 Amps.

Thanks
 
Which point did you measure the current at motor connection or terminal block at control panel. It could be of motor contact.

Please measure voltage at incoming, breaker, contact , terminal block.

 
Adil... voltage data provided confirm supply is not the cause of the problem.

VUF is about 0.3 percent. Also, analysis indicates input power is about 83-84% of the motor's rating.

If you wanted additional proof that the problem is internal, then determine the motor's Temp-Rise-Over-Ambient, and compare it to its nameplate rating.

For additional info I suggest Control List Thread # 1026153138 "Unbalanced Voltage Effects!"

Regards, Phil Corso
 
R
Adil,

Did you do the test I suggested, low voltage transformer with Ammeter?
This will give you the relative impedance of each winding. If you have a shorted turn or two it should show up as a higher current

Roy
 
> Did you do the test I suggested, low voltage transformer with Ammeter? <

Please explain that test in detail.

thanks
 
C

curt wuollet

No, only that it might be grounded. They do some strange things around the world. If it were, it would preclude seeing any imbalance, provided that their phases are also symmetrical about ground which they would have to be in that case. What I am saying is that with the motor running in the wye configuration, the bad "winding" should have less voltage from the phase to center. I am still suspicious of a metering or iron issue with a small difference. A shorted turn or two usually upsets things by more than this.

Regards

cww
 
M

mahesh mandwarya

Dear,

Whether any load of the 1 /2 phase nature like Xerox machine, Lighting etc is connected? if so, it can result in the supply voltage variations, leading to the Current imbalance.

Regards,
 
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