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I'm not a motor expert but i've had some first hand experience with lack of advance and the problems incurred. The reason can get hugely technical but here's my simple explanation. As a dc motor is used above its rated voltage and speed then the magnetic field generated by the field coils gets "pulled" out of position by the armature. The distorted field then causes the brushes to change bars on the com (called commutation) at a less than ideal point. Ideally , the motor should commutate when the armature bar that is in circuit is still under the influence of the field pole its just leaving. This reduces the inductive kick back as the current changes direction. Called armature reaction if i remember correctly.

So advancing the brushes give the motor a better chance to commutate successfully at higher voltages and speeds. I've a few videos on the subject that may help.

In this video the brushes are neutrally timed:
http://www.youtube.com/watch?v=AobcKoow8Z0

And here advanced about 12 degrees:
http://www.youtube.com/watch?v=V8fuaMJ8N1Q

hope this helps.
 
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