Pole Voltage drop test on 6 pole synchronous motors

[Nitin Sharma]

Dear All,

I require your kind recommendations regarding the test results of pole voltage drop test carried out on one of our 6 pole synchronous motor as below.

In Feb-20, we have carried out the Pole Voltage drop test for rotor of motor and results found unsymmetrical to the tune of around 25% in one of the pole, which indicates shorting inside the rotor. The motor was sent for dry ice cleaning and then testing done again, no change in results found and then motor taken in line without any issues.

Subsequently Pole Voltage drop test done during the June-20,Jan-21 , on both occasions results found symmetrical, that is no short circuit. But recently during the Sep-21, we have again tested and again found pole voltage drop as unsymmetrical with around 12% variation in the same pole as found during Feb-20. Motor taken in line.

Motor is operating fine without any abnormal vibrations and all the operating parameters within limits.

I require your opinion on this and any further actions suggested.

 

[Gr8blu]

First question - AC ort DC drop test?
Second question - did you verify correct operation/calibration of the test equipment on a known "good" unit? (Either before or after the second "suspect" results?)
Third question - did the same tech do all the testing? Whether they did or not - was the test repeatable with another tech doing the work on the same unit? Was it repeatable with the same tech doing similar testing on another unit?
Fourth question - where (in the rotor circuit) are you connecting to perform the test? For example - did you make/break any of the "normal" connections?

More food for thought:
a) why did you perform the test in the first place?
b) Was it part of a preventive maintenance program carried out at site (or at a repair facility)?
c) Was the machine exhibiting torque, vibration, or thermal issues beforehand?
d) If you thought the pole was as unbalanced as first perceived (25%) - or second (12%) - why did you put it back in service without rewinding the pole either time?

Converting energy to motion for more than half a century

 

[electricpete]

Good question ac or dc. I think pole drop tests are typically done with dc, but if it was a low voltage ac test than certainly the magnetics could enter into it... possibly the residual magnetism characteristics could change depending upon conditions of last deenergization.

Shorts on AC windings of large machines usually progress to failure quickly due to voltage induced in the shorted turn by the main flux (also called an autotransformer effect). That doesn't apply to shorts of dc windings, they can sometimes operate for long periods of time with turn to turn short (the more turns are shorted the more likely it will show up in other ways).

If it's not an ac test, it could be a turn to turn short that is coming and going with thermal changes or vibration. I know generator guys have lots of interest / options for on-line monitoring shorted turns of sync generators although I'm not that familiar with those.

=====================================
(2B)+(2B)' ?

 

[waross]

If it's not an ac test, it could be a turn to turn short that is coming and going with thermal changes or vibration.

I had that issue with an exciter armature. It ran well cold, but developed excessive sparking when it came up to temperature.
Cold it tested good on a growler.
Heated in an oven it tested good when hot.
We ran the generator until the excessive sparking occured.
Then we used a brush position test and rotated the armature while checking the brush voltage.
When the brushes are in the proper position the voltage under this test should be null.
We found a varying voltage as the armature was rotated.
This verified that the armature was in need of a rewind.
The point is that sometimes a fault will only develop when a machine is heated under actual operating conditions.
The other lesson learned:
When doing a brush position test, rotate the machine if possible. That should demonstrate that there are no winding faults present in the armature.
When a machine is heated under load, the heat is generated in the windings and conducted to the iron.
There will be differential heat expansion.
As Pete pointed out; shorted turns will be more apparent under an AC test due to the auto-transformer effect.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Gr8blu]

Pete / Bill: The DC drop test is the most common - but it only works for gross differences in resistance. When looking at a field pole winding with several hundred turns of relatively small-gauge wire, a few shorted turns will likely get lost in the possible noise of measurement error - particularly if looking at multiple poles in circuit together. Of course the AC test looks at total (R + jXL) impedance - and here a few turns can be readily seen. In my experience, most users and repair shops would start with the DC test and then do the AC test on an as-necessary basis. Most OEMs would skip right to the AC test.

Good point on the thermal / vibration factors: I wanted to get some more detail out of the original poster, but that was the general direction I was heading toward.

And yrs - it is pretty difficult to get reliable "live" data from a synchronous rotor field winding under operating conditions. About the only way I know of to do that is to literally instrument the windings in question (at the winding stage) and run the sensor feeds out to a termination point on the rotor shaft somewhere between the main rotor body and the bearing. Then stop the rotor, and plug into the termination to get your results. Still not a true "live" feed, but can be a better picture than a "tear the whole thing apart to get at it" approach. Note: most of the WIFI methods aren't really looking at direct temperature measurement - and can see "data distortion" from the presence of the strong magnetic fields within the operating machine - particularly for larger power/torque ratings.

Converting energy to motion for more than half a century

 

[oldfieldguy]

We always did the polke drop test using a AC, the rationale being that AC might be better to detect issues with the magnetic components as well as shorted turns.

old field guy

 

[electricpete]

I agree with Gr8bluac is more sensitive to a small number of shorted turns since inductance is proportional to turns squared and resistance proportional to turns to the first power

I can't quite see what is interesting there would be to see in the magnetics. If ac voltage magnitude is not high enough, it might be fooled by residual magnetism but it shouldn't be hard to get voltage high enough to overcome that.

It looks like op hasn't been back since he posted.

=====================================
(2B)+(2B)' ?

 

[waross]

A shorted turn becomes a transformer with one less turn on the primary and a one turn shorted secondary.
The primary current to support the secondary short may be a greater indicator than the inductance.
A DC coil will take the same voltage AC as DC but with much less current on AC.
Bit beware of moving iron devices.
The same voltage AC will not develop enough force to pull the moving iron in, but will draw excess current trying to magnetize the air gap and burn out the coil in a minute or so.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!