Effect of Brush Shift

Author: E.E. Kimberly

In a commutating-pole generator the brushes are usually adjusted very accurately to commutate coils whose sides are exactly under the interpoles. If the brushes were shifted slightly in the direction of rotation, some of the interpole flux would enter the armature back of the conductors under commutation. Since the polarity of an inter-pole in a generator is opposite to that of the main pole which precedes it in the direction of rotation, the net flux cut by active conductors between brushes is the difference between the main-pole flux and the interpole flux. When the generator is loaded, the interpole flux produces an effect similar to a differentially connected series field and causes the terminal voltage to drop abnormally.

A slight cumulative compounding may be achieved by shifting the brushes slightly opposite to the direction of rotation; and such shifting is good accepted practice, provided the commutation is not impaired.