Troubleshooting DC Machines

My book says that if your dc machine's commutator brush sparks, it is caused by defective commutation windings . I don't understand what they mean by defective commutation windings .

As Colin indicated there are multiple reasons for arcing at the commutator. I would read the statement of "commutator windings" as the armature windings. Shorted armature windings can cause arcing but unless the armature has been obviously overheated it's not likely. More likely is as worn commutator surface. Look carefully at it and see if there is carbon or copper shorting between the commutator segments. If you can see that condition then have motor reconditioned if possible. The full process is a little to complicated for me to want to explain here.

Some sparking is inevitable especially if the motor is a heavy-duty type (or one that draws a large amount of current - for example low-voltage/high power types). However inadequate brush pressure may also cause sparking. Also if the commutator has worn out you will get sparking - especially true in old motors where the carbon brushes have reached almost end of travel. New brushes will solve the problem but I'm not sure for how long - this since the new brushes will be significantly longer than the older ones and the spring pressure will therefore be a lot more uniform and steady.

There is one other reason not mention so far and that it is possible that the brush rigging has shifted. The position of the brushes is critcal for ideal operation. The brush postion is adjusted for minimun sparking. All DC motors will spark as the brushes short out a winding when it crosses 2 communator segments. This is normal though. Ideally the voltage is zerp so no sparking would occur but that is near impossible to achieve.
As chuck stated the most common reason for excessive sparking though is the communator needs re-conditioning.

Commutation windings means armature windings The abnormal spark between the carbon brush and commutator indication of partially shorted windings in the armature.

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