Rotor jumpering 1

[kjiles]

I am working on an older westinghouse synchronous motor. I found a rotor pole that is shorted and want to know if jumpering it out is a good idea. It a 14 pole, so even with 1 down
there would be a difference is rpm and torque.

 

[waross]

One out of 14? About 7%.
Torque? Depends on applied load.
RPM? Still the same number of poles, but one pole will be a little weaker. The magnetic flux generated by the other 13 poles will return through the unexcited iron so there will be no change in the number of poles.
Circulating currents? Once the load current exceeds about 7% or so, there will be no more circulating currents.
There may be some interesting current harmonics.
If this is a salient pole machine, better to rewind it.
If this is a cylindrical rotor, better to schedule a rewind soon.

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

 

[wcaseyharman]

I know in the hydro generation world with large synchronous generators unbalanced magnetic forces are a significant concern with unequal air gaps (rotor shift) and excessive vibration due to unbalanced magnetic forces on the rotor.
I’m not an expert but those might be something to consider in your application.

Is the pole shorted to ground or just has shorted turns?

 

[electricpete]

With regard to magnetic centering force, I believe it could be resolved by jumpering 2 poles 180 mechanical degrees apart. But that means losing two poles of the same magnetic polarity, potentially creating problems with homopolar flux. And not just losing them but as waross mentioned turning them into magnetic short circuits. Potentially carrying as much flux as is required to saturate that portion of iron.

There are also a lot of space and time harmonic effects to consider. One is time harmonics owing to the saturated unexcited poles as mentioned. Another is the spatial perturbation from siniusoidal of the rotating flux wave. I think these can have unpredictable effects on vibration and performance. In a similar way torquing and cogging and crawling are attibuted to spatial harmonics associated with slot combinations in induction motors (before OEM's learned how to avoid the problematic slot combinations to manage the harmonic effects)

Is this any kind of standard repair method? I'm aware failed stator coils can be electrically cut out of induction motor windings with careful review using stanrdardized evaluation criteria, but that does not allow cutting out an entire pole as far as I recall. If you are not following some standard repair method, then it sounds like a research project (which is codeword for risky in most industrial repair environments).

TLDR - unless you have advice from someone who has evaluated this and says it is acceptable, I would strongly advise against it.

 

[edison123]

Huh?

Muthu

http://www.edison.co.in

 

[electricpete]

> Huh?

I assume that was directed to the op, rather than to the preceding poster (me) ?

 

[Yuma]

Hi Electricpete,

When you say "jumpering 2 poles 180 mechanical degrees apart. But that means losing two poles of the same magnetic polarity", it seems to me that it depends whether the number of pairs of poles is odd or even.

Do you agree? Or am I missing something?

 

[electricpete]

Yuma - Thanks - you are correct, that was my mistake. 14 pole would have opposite polarity 180 apart so the homopolar flux discussion goes away.

I edited my post to correct with strikethrough. There's still a lot to be concerned about. Still strongly advised against as far as I'm concerned.