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Electrical neutral on a 250hp dc motor

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.

RE: Electrical neutral on a 250hp dc motor

The easiest way to do that on any DC machine is to connect ordinary 50 or 60 Hz to the main field and then check the voltage across the armature with an AC millivoltmeter (i.e. an ordinary DMM). It is wise to use a transformer and apply 24 V, even if the winding easily can take 100 - 240 V AC. Turn the bridge until you get a minimum reading. You should not have to move the bridge more than a few millimeters from where it is. DO NOT FORGET to fasten the bridge after the turning!

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

This procedure locates the no load Neutral Brush position and assumes that the brushes are equally spaced and the armature has not short circuits.  
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

Suggestion: If the brush braces are properly aligned against the check marks, then it is probably other problem that causes the commutator sparking.

RE: Electrical neutral on a 250hp dc motor

That sparking could be caused by problems with interpoles, if there is a problem initiating or if there is a wrong connection, in a motor the interpole polarity is the same as that of the main pole preceding in the direction of rotation.
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

If the motor is only 250 Hp then It may not have a brush ring that will allow easy turning of the brushes.

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

Suggestions: Industry standards might be used to test the DC motor, e.g.
1. DC Motor - Testing per NETA MTS-01
Visit
http://www.opmug.net/last_meeting.htm
for DC motor tests

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