DC generator issues after rewind
DC generator issues after rewind
(OP)
Hello,
I have a DC compound wound self-excited generator which was recently rewound with a new armature and new commutator. The shunt and series fields were not changed.
The shunt field is wired in parallel to the series field through a rheostat. At normal operation, the rheostat used to operate in one particular position - I don't know the exact ohms, but there is a marking on the panel. But now in order to achieve the same output voltage, the rheostat has to be set much lower. So the shunt field needs more current than it used to in order that the gen can produce the same output voltage.
The brushes are well seated. The machine has been running for two weeks. There is no arcing that I have heard of. I don't have the opportunity to shut the machine down.
The only things I can think of is that the rotor was wound improperly, as in the windings are not all the correct polarity, or the brush angle is off because of the new commutator.
If the new commutator was smaller than the old commutator, resulting in the brushes touching one too many com bars, what would that do? Could that cause this problem? If so, are there any patterns on the com bars which would reveal/indicate this problem?
thanks,
EE
I have a DC compound wound self-excited generator which was recently rewound with a new armature and new commutator. The shunt and series fields were not changed.
The shunt field is wired in parallel to the series field through a rheostat. At normal operation, the rheostat used to operate in one particular position - I don't know the exact ohms, but there is a marking on the panel. But now in order to achieve the same output voltage, the rheostat has to be set much lower. So the shunt field needs more current than it used to in order that the gen can produce the same output voltage.
The brushes are well seated. The machine has been running for two weeks. There is no arcing that I have heard of. I don't have the opportunity to shut the machine down.
The only things I can think of is that the rotor was wound improperly, as in the windings are not all the correct polarity, or the brush angle is off because of the new commutator.
If the new commutator was smaller than the old commutator, resulting in the brushes touching one too many com bars, what would that do? Could that cause this problem? If so, are there any patterns on the com bars which would reveal/indicate this problem?
thanks,
EE





RE: DC generator issues after rewind
Also,the interpole polarity might be set wrong. But that would cause sparking.
If you can post all the nameplate data, I'll check and see if I've wound one before.
RE: DC generator issues after rewind
I realize this is not a lot of information, but that's all I have right now.
EE
RE: DC generator issues after rewind
A check can be made (unit shutdown) by applying an AC voltage across thr shunt field (say 110V) and shifting the brushholder assembly until zero millivolts AC is read across the brushes. External circuits disconnected. This will give the new neutral point, which can be marked on the brushholder assembly.
BTW you said the shunt field is parallel to the series field. That is wrong. Check the connections again.
It can be that the new winding has a different turn count from the old also.
rasevskii
RE: DC generator issues after rewind
If the armature had some shorts before rewinding, Having a new winding, it would increase the output for a given excitation.
The commutator diameter wouldn't have an effect on the output, or the brush angle. As long as the brushes are fully seated, and spanning all the bars it should.
You might check the neutral by exciting the shunts with AC current, and measuring across your brush holder for the lowest voltage.
RE: DC generator issues after rewind
The AC test sounds like a very good idea, despite the fact that I'd have to shut down the unit. Would a small variation, like 1 or 2 degrees, cause this much difference in operation?
The rheostat is in series with the shunt field. The smaller the resistance of the rheostat, the more current goes to the shunt field.
RE: DC generator issues after rewind
Replacing the commutator, and rewinding.
Comms are aligned to either a bar or a mica segment. If the comm isn't keyed it's easy to get it off. That would throw it off by a half a bar.
If the winder didn't line out his data right. The winding could be off by a bar. Skewed slot generators are easy make a mistake on.
RE: DC generator issues after rewind
RE: DC generator issues after rewind
RE: DC generator issues after rewind
RE: DC generator issues after rewind
RE: DC generator issues after rewind
RE: DC generator issues after rewind
rasevskii
RE: DC generator issues after rewind
I did some more investigating and found the smoking gun. The shunt field was wired so that it could be run in manual with a rheostat, or in auto using an automatic voltage regulator. When using the AVR, the rheostat was still in the circuit. So, the AVR was in parallel to the shunt field, and that parallel combination was in series with the rheostat.
There was a diode in the AVR which had failed, allowing the current to flow through the AVR AND the shunt field. The current measured at the rheostat (2.5A) represented the current in the shunt field and the parallel path through the AVR.