Motor current unbalance

[rusape]

We have a large mill dc shunt motor with 2 armatures
connected in parallel electrically on the same shaft. Each have their own seperatly excited field. Current balancing is done between the armatures by decreasing the respective field current. The problem I have is this. We have just replaced the armature i.e both armatures due to the risers of the one armature being broken off as the dowels of the spider came out. However we now have a major current unbalance between the 2 armatures. The rated current per motor is 5750 Amps and the inital unbalance under load is in the region of 4000 Amps. I would like to believe that the field is causing this, but then with the previous armature the currents were balanced. The armature circuit is free of any hot connections. I would appreciate any suggestions in this matter.

Thanks.

 

[cbarn24050]

Hi, 2 arms on 1 motor thats a new 1 on me. Have you checked your brushgear position? If that is ok are there any equalising wingings? if so are you sure they are still phased correctly? I suspect that even a small field difference could make a large difference in the arm current so you might be able to adjust it out.

 

[electricpete]

I have't worked with dc motors... just thinking out loud.

I agree with cbarn the field current adjustment should be the first thing you try. Possibly minor variations between motors could cause a substantial differenc in required setting.

Assuming the variation is beyond the range of your field pot, it sounds like maybe your replacement armature is not an identical replacement. Perhaps a resistance test on both the armature and field circuits will reveal the difference. Different number of turns in either circuit would cause the behavior you are seeing.

 

[rusape]

Thanks for the repsonses. The fields of both armatures
have been checked out and are o.k. I am presently balancing the currents by reducing the one field but hence have a limitation on my top speed due to field weakening limit. The motor does have equalizing windings, but am not sure if they are phased correctly. Will check.The neutral was not checked due to time constraints but will be done during the next outage.

 

[jbartos]

Suggestion: Also, the windings shall be properly connected in terms of their orientations.