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Motor current imbalance
Three phase motor's current is imbalance

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.

1 out of 1 members thought this post was helpful...

Adil... What are the 3 phase-to-phase, and the 3 phase-to-neutral voltages?

Regards,
Phil Corso(Cepsicon[at]aol[dot]com)

3 phase-to-phase voltages are 201V 202V 201V, and phase to neutral voltages are 400V 400V 400V.

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

No, simple Y/delta connection.

By curt wuollet on 7 October, 2009 - 9:52 pm

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.

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.

By curt wuollet on 8 October, 2009 - 1:46 pm

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

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

Get the motor surge tested and also interchange the rotor with similar specification motor

1 out of 1 members thought this post was helpful...

Adil, what is motor's rated current?

Phil Corso

2 out of 2 members thought this post was helpful...

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

1 out of 1 members thought this post was helpful...

Curt, surely you didn't mean to imply in your 08-Oct-09 (13:46) post that the wye-point in the starter should be grounded... or did you?

Regards, Phil


By curt wuollet on 10 October, 2009 - 1:34 am

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

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.

1 out of 1 members thought this post was helpful...

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

By Roy Matsoin on 9 October, 2009 - 6:37 pm

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

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,

Adil,

If you just measure the winding resistance with an Ohmmeter because the windings are quite low in resistance it would be hard to detect a shorted turn. If you pass an AC current though the winding a shorted turn will show up as a high current. Because you don't want the motor to start or burn out use a lower voltage with Ammeter in series. As an example of what I mean take a normal low voltage transformer, measure the primary resistance with your Ohmmeter, now short out the secondary - no difference in primary Ohms!

But you know without even testing it that if you connect the transformer to the AC supply and short out the secondary you will get a huge difference in primary current. So what I am trying to explain is how to measure the AC resistance (reactance).

The amperage you get is not important just that each winding should be the same indicating that there are no shorted turns. I don't say this is infallible, it's possible that the winding could short under running and not at rest but if you get a high current on one winding it points to a problem for sure.

Is it possible that one winding is reversed, i.e. swapped end for end?

Roy,

I've checked that with low variable voltage transformer. With its primary being at 220V and secondary at 110V. The current that I found on primary side of the transformer while connecting the secondary with each of the three windings of the motor is as follows:

4.6A 5.0A 5.5A

>Roy,

> I've checked that with low variable voltage transformer. With its primary <

--- snip ---

Any idea what next to check ?

1 out of 1 members thought this post was helpful...

Adil... there are additional tests you can carry out. However, more data is required; please provide the motor's rated speed, power-factor, and efficiency if known!

Regards, Phil Corso

1 out of 1 members thought this post was helpful...

Adil... there are additional tests you can carry out. However, more data is required; please provide the motor's rated speed, power-factor, and efficiency if known!

Regards, Phil Corso


Here's the additional info

Rated RPM 2940
Rated KW 7.5kW
Measured RPM 2890

1 out of 1 members thought this post was helpful...

Adil... The amperage test, while providing additional information, did not provide any more knowledge than was provided by AUF calculation. So, hopefully you are interested in learning more about failure-mode. If so, then bear with me regarding the following questions:

o Is the motor still in service?

o Is there a duplicate installation in service?

o Did the unbalanced-current problem exist from its initial commissioning?

o The rated slip is 2 percent while the actual slip is 3.7%. Yet, the amperage does not reveal an "overload!" Is this a "special" motor?"

o How was "measured" RPM determined?

o What is the motor driving?

o What is its duty-factor? 24/7; intermittent operation; frequent stop/starts/reversals?

o Have you measured input power?

Phil Corso (cepsicon[at]aol[dot]com)

o Is the motor still in service?

Yea, it is still in service .

o Is there a duplicate installation in
service?

yea, there's another motor similar to that installed for the same purpose, and also working as a standby to it.

o Did the unbalanced-current problem
exist from its initial commissioning?

I can't confirm. But, this problem is being experienced for quite some time (i.e. 2-3 years) according to my investigation.

o The rated slip is 2 percent while the
actual slip is 3.7%. Yet, the amperage
does not reveal an "overload!" Is this a
"special" motor?"

No, there's nothing special that I can notice in this motor.

o How was "measured" RPM determined?

By contact tachometer

o What is the motor driving?

Water pump (centrifugal)

o What is its duty-factor? 24/7;
intermittent operation; frequent
stop/starts/reversals?

Almost 12 hours a day

o Have you measured input power?

No, I don't have any power quality meter. While, theoratically i don't know how to measure the power of unbalance current. I guess it can be done by the method of symmetrical components but i don't know how to use it.

1 out of 1 members thought this post was helpful...

Adil... unless the second drive has comparable measurements, then I, like Roy confirm the motor is at fault.

Phil Corso

1 out of 1 members thought this post was helpful...

Adiul, re: your comment regarding power measurement in an unbalanced system..

Contact me Off-List if you want info on how to do it with single-phase clamp-on meter!

Regards, Phil

Adil,

You gave the current readings with a single phase voltage applied to each winding in turn as 4.6A 5.0A 5.5A. That tells me the windings are of different impedance, the question is why?

Shorted turn somewhere perhaps.

Perhaps the winder didn't make them all the same.

Anyway there's probably not a lot you can do to fix it. If it's still running OK I would just keep it running, perhaps have a spare ready.

Regards,
Roy

By rahmad_yusni on 8 November, 2009 - 3:39 am

Adil,

check and measure stator winding. there is difference measurement (ohm) stator winding phase to phase and neutral to phase??

Rahmad,

You missed the whole point of my low voltage tests. Measuring Ohms with an Ohmmeter you will see little difference between windings. By testing for current at low voltage we get the impedance of 23.91, 22 & 20 Ohms.

It only takes one turn shorted to have a significant effect on impedance whereas you would not be able to detect it measuring Ohms.

Regards,
Roy

What exactly u mean by stator winding phase-to-phase and phase-to-neutral?

I've checked the resistance of the stator windings once again, and this time the difference is quite clear.

A1A2 -> 2.2 ohms
B1B2 -> 2.9 ohms
C1C2 -> 2.2 ohms

1 out of 1 members thought this post was helpful...

Adil... Roy reminded me of another point.

If this motor is located in a Classified Area, and it is not explosion-proof, then its operation for longer than 1 hour in 10,000 is illegal!

Phil Corso

By Charles Webster on 18 December, 2011 - 9:34 pm

Adil,

It could be a uneven air gap caused by mechanical misalignment such as soft-foot. Soft foot would show up in vibration analysis as (I think) a 2X n vibration - best to get a VA expert to check this possibility.

Regards
Charles

Adil,

Is very clear, the motor needs rewinding.
Thank you.

1 out of 1 members thought this post was helpful...

Adil... Posts by Charles and Zouhen indicate there is still interest in your problem. How was it resolved?

Regards, Phil

By Bobby Ebilane on 23 December, 2011 - 10:48 am

Hoping this motor has been running before right? And if it is, I think the windings are alright. Current imbalance on motors can cause by so many possibilities, but reading all the information you sent make me think one possibility. Have you heard about Rotor Bars? Rotor bar problem on motors is very seldom that's why it is being ignored most of the time. It can cause current imbalance if there is any crack or possibly high resistance joints. There are ways to find rotor bar problems, most of the new testing equipment that are being sold today can detect rotor bar issues.

By Ake Goransson on 23 January, 2012 - 3:16 pm

The main problem detected by imbalance/unsymmetry in current and phase shift indicate windings intercoil problem. This problem could have been caused by some broken rotor bars because the low frequency vibrations that it could generate. I have never seen imbalance caused by only rotor bar problems. By following the rotor bar signal, phase and current imbalance over time the motor could be run some years more if it not do to many direct starts. Search for "rotorstavsbrott och statorfel i motorn, axelbrott, kardanknutar"