Electrical neutral on a 250hp dc motor
Electrical neutral on a 250hp dc motor
(OP)
Can anyone explain how to check the electrical neutral on a 250hp GE dc motor? I have one connected to a Controls Techniques QuantumIII digital dc drive and the commutator is sparking. In reading the troubleshooting section of the service manual, it mentions to check the neutral. Also, what are your opinions of commutator stoning procedures?
Thanks in advance for any help.
Thanks in advance for any help.





RE: Electrical neutral on a 250hp dc motor
There is usually a mark (red or white line) on the frame and on the bridge. They normally coincide when the bridge is in neutral position. Look for them. And trust them.
RE: Electrical neutral on a 250hp dc motor
However, if the motor does not have Interpoles or the Interpoles are defective or with wrong polarity, the armature reaction will shift the no load Neutral Plane position when the motor load is increased.
If the interpoles (or pole phase windings when included) are defective the commutation will not be succesfull until the deffect is corrected.
RE: Electrical neutral on a 250hp dc motor
RE: Electrical neutral on a 250hp dc motor
High armature current changes due to load or drive sintonization also could be the problem, try to measure armature current at the dc bus not at printed circuit board. I have found one drive with CT ratio not correct, with the same sparking at 50% load.
RE: Electrical neutral on a 250hp dc motor
You should check the following things ( most already mentioned ) if there is no adjustment possible.
1 Are the brush boxes all uniform height above the commutator? measurement is 1.8 to 2 mm ( 75 to 85 mill )
2 Are the brush boxes uniformly spaced around the commutator? Measure around the comm surface.
3 Are all the interpols electrically ok?. Measure with a megger to ground and with a "ductoR"
4 Are all the Series elements connected correctly? Uaually a connection diagram is included in the machine documentation if it still exists. Otherwise call GE motor product service with the model number +18148753469
5 Is the motor overloaded ?
6 are all the brushes the same grade ? (do not laugh)
7 Are all the Shunt fields Ok ?. Easiest way is to do an "AC Drop Test" disconnect the field supply and connect 110 V Ac (or 240) direct from an AC outlet. The current will only be low because of the high level of inductance on the fields. Measure the voltage across each of the fields. Usually they are connected , 2 in series, (in parallel with two more) so you would expect half the supply voltage across each field. Shorted turns will change the inductance significantly and upset the voltage balance. This will not work if all fields are in parallel.
8 check the neutral while you have the AC on the fields. Measure the voltage ( milli Volts) between two consequetive brush arms, it should be somewhere between 0 and about 60 or 70 milli Volts. The voltage should be the same between any consequetive two arms. If not then your brush spacing may be out. the position of the commutator under the brushes can affect this. If it is possible to turn all the brushes together, on a ring, then do so until the voltge is at it's minimum. remember, it will change when you tighten the bolts, so it is best to turn a little, tighten, and then measure. (time consuming but still the fastest way)
with regards to stoning, Mostly hand stoning will only work if you wish to remove a film form the comm. If the com is even slightly out of round, the insecure anchoring of the stone will actually make the out or roundness worse. (many years of experience behind this one ) if it is badly pitted or burned, proper machining is the only answer
A question for you. Is there burning, and if yes, then is it uniform or is it in definite bars or patterns around the comm? If a pattern exists then It could be a pointer to your problem.
happy hunting
Tom
RE: Electrical neutral on a 250hp dc motor
1. DC Motor - Testing per NETA MTS-01
Visit
http://www.opmug.net/last_meeting.htm
for DC motor tests