Motor current imbalance

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 ?
 
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

 
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


 
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?

<i>Yea, it is still in service .</i>

o Is there a duplicate installation in
service?

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

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

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

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?"

<i>No, there's nothing special that I can notice in this motor.</i>

o How was "measured" RPM determined?

<i> By contact tachometer </i>

o What is the motor driving?

<i> Water pump (centrifugal)</i>

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

<i> Almost 12 hours a day </i>

o Have you measured input power?

<i> 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. </i>
 
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
 
R

rahmad_yusni

Adil,

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

Charles Webster

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
 
B

Bobby Ebilane

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.
 
A

Ake Goransson

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"
 
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