Compensating Windings

Author: E.E. Kimberly

Stabilizing windings do not prevent crowding of the main-field flux to one side of the pole. Commutating poles maintain the commutating zone at a point midway between the main poles, but they do not correct distortion of the main flux at the main poles. Under extremely severe operating conditions of reversal of rotation and wide range of speed variation, it is necessary to add a compensating winding. If distortion of the main-field flux is not controlled by a compensating winding, the violent shifting of the flux across the pole faces and the armature conductors may induce high voltages in the armature coils and may cause flashover between commutator bars when the motor is reversed quickly. Compensating windings are embedded in longitudinal slots in the pole faces and are connected in series with the armature. Fig. 11-6 (a) shows the arrangement of compensating conductors and the flux ϕc that would be produced by them if they acted alone. By making the compensating ampere-turns just equal to the armature ampere-turns, the main-field distortion is practically eliminated, as indicated in Fig. 11-5 (d).

Compensating windings require a costly construction and are justified only under extreme conditions, such as those encountered in steel-mill motors.