Compensation winding

A compensation winding in a DC shunt motor is a winding in the field pole face plate that carries armature current to reduce stator field distortion.

Its purpose is to reduce brush arcing and erosion in DC motors that are operated with weak fields, variable heavy loads or reversing operation such as steel-mill motors.

By adding a compensating winding in the pole face plate that carries armature current in the opposite direction of current in the adjacent armature windings, the position of the flux at the pole face plate can be restored to the position it would have with zero armature current.

[1]: 393  [2]: 65–66 Figure A. shows a cross-sectional view of a two pole DC shunt motor.

The result is that the center of flux in the gap between the pole face plate and the armature has shifted.

A. Cross section of DC motor with compensation windings. A = armature windings, C = compensation windings, F = field windings, R = rotor (armature), S = stator (field).
B. Cross section of DC motor with compensation windings showing magnetic flux due to field windings.
C. Cross section of DC motor with compensation windings showing magnetic flux due to armature windings.
D. Cross section of DC motor with compensation windings showing magnetic flux due to field and armature under heavy load. The flux in the gap has shifted.
E. Cross section of DC motor with compensation windings showing magnetic flux due to field and armature under heavy load with compensation windings. The flux in the gap has been restored.