9 Oct 11 11:56
Wondering about "Insulating the brushes from the commutator? We used to isolate the field and apply grid voltage to the field as Gunnar suggests. Read the voltage at the brushes and adjust for null voltage. As a second check the armature may be rotated with the voltmeter on the brushes. Any variation of the voltage indicates a shorted or open armature. Neglect possible small changes as the brushes transfer from one commutator bar to the next, damage will be indicated by the voltage rising and falling as the bad winding passes from one pole to the next.
This gives you a good initial setting. Even a compensated motor may need a little trimming under load. Move the brushes in a direction so as to tend to cover the sparks. Too much movement and sparking will start at the other side of the brushes.
You must use some judgement for reversing motors. We had some motors that only ran in reverse less than 5% of the time. The loads in reverse were often lighter. We set the brushes for forward rotation only and let them spark a little in reverse.
We had a motor which ran the same amount forwards as in reverse. The load was heavy both ways. We adjusted the brushes for the same in either direction.
The main point is to reduce, limit or avoid damage to the commutator and brushes.
Regardless of the type of motor the null voltage test is a good starting point for the initial brush setting.
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