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Winding resistance tolerances in three phase motors


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Hi guys,

Is there a standard tolerance percentage that should be followed when checking the winding resistance of each coil on a three phase electrical motor?

The general rule of thumb I followed when measuring winding resistance was that each phase winding had to be identical or close to identical, the resistance would 

vary on each motor due to the cross sectional area of the winding.

I've searched the web and there is varying opinions from +/- 5% and  +/- 10%.

I just want to know what your thoughts are? The IEC to my knowledge haven't posted anything regarding this.

Thanks,

Ewan 

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Hi Ewan

The length of the winding wire on each phase should be close to equal and the cross section should also be equal, so there should be no significant variation.
I have never considered the percentage difference and indeed, from my experience, the resistances are usually very close, or wildly different and wildly different means insulation failure.

I think that most people in the field treat winding resistance as a go / no-go test looking for big differences.

A more accurate indicator for things like shorted turns, is to measure the inductance of the winding.

Usually, it is difficult to get an accurate resistance reading because the resistance is so low and a standard meter is used giving variations due to the integrity of the connections of the meter leads.

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Hi Marke,

Thanks again for the reply. 

That's great, I was just curious if there was a rule of thumb to follow for winding resistance checks, like there is for insulation resistance testing. 

I suppose you could use the "REL" function on a fluke multimeter, where the resistance of the meter leads is subtracted from the measured resistance reading for a more precise resistance measurement

I have never measured winding inductance.

I would use an insulation resistance tester to measure the insulation integrity between phase windings to check for possible insulation deterioration between phases, if the motor was larger and the winding resistance was small if I suspected a possible phase to phase short. The MCB would trip immediately when the motor was is started which would a sure sign of a short circuit fault. 

Do you have any circuit diagrams of motor starter circuits that you could possibly send? 

Thanks,

Ewan.

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I guess I am old school and rely on experience to say that "it does not look right to me" rather than apply a rule of thumb.

In many situations, the tolerance is very dependent on the conditions and what is OK in one, is far from OK in another.

When you use a high voltage insulation test, it is often easy to test phase to earth, but sometimes impossible to test between windings because the motor is hard connected and the terminations are inaccessible. For example, a submersible bore hole pump where the pump may be 100Meters down a hole and only three wires come up.

With a submersible pump, it is common to see 500V resistance to earth less than "half a meg"  (500 Kohms) but this is OK, but on a standard motor in a clean environment, that would indicate a major problem brewing.

The hardest fault to find, is the shorted turn. It will not show up on a resistance check except under extreme cases, but this will show up in an inductance check.

Many is time time when questionable motors do not show static faults, but do show faults causing trips while running. I recall one such motor that had to be heated in a kiln to get the temperature up before it showed an insulation fault. It would typically run for 15 - 20 minutes before blowing fuses.

A poor mans test is to connect a current limiting impedance in series with each phase to the motor, then run it with no load and measure the voltage across the motor terminals. For example, connect a 1KW heating element in series with each phase, then run it open shaft. The voltage across the motor terminals should be balanced. If there is a severe imbalance, then there is usually a shorted turn.

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When you use a high voltage insulation test, it is often easy to test phase to earth, but sometimes impossible to test between windings because the motor is hard connected and the terminations are inaccessible. For example, a submersible bore hole pump where the pump may be 100Meters down a hole and only three wires come up.

What you could do is carry out a resistance test line to line, if the motor is connected in star, then the measured resistance is doubled because you're measuring two star winding coils in series by measuring line to line.

If the motor is delta then line to line, the measured resistance would be 66% of the one phase coil

Thanks, Ewan 

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True, but you are also measuring the resistance of the cables down to the motor and too often, these cables are undersized (water cooled) and the resistance of the cables is significant compared to the motor windings and can give false expectations.   Sound judgment is needed!!

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You would need to log the measured resistance of the coils before the motor is submerged 100 meters.

And like you said, the insulation resistance of that motor will vary greatly from being in a workshop compared to being 100 meters below and subjected to harsh conditions.

 

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