In Ward Leonard’s control of DC motor, the lower limit of speed is imposed by

A. Residual magnetism of the generator

B. Core losses of the motor

C. Mechanical losses of motor and generator together

D. Cannot be determined

Answer: A. Residual magnetism of the generator

Explanation:

In the ward Leonard method when the field excitation is controlled (i.e. capable of supplying current in one direction only), the minimum speed that can be obtained is 10% of the nominal speed. This is due to the presence of residual magnetism in the field circuit of the generator field. Thus, a lower limit of speed is imposed by the residual magnetism of the generator.

Due to the presence of residual magnetism, even with no field excitation enough voltage is generated to create crawling in the machine, especially at a light load. To prevent crawling the following methods are adopted:

(i) The armature circuit is opened.

(ii) A differential field winding in the generator is connected across the armature terminals.

This field will oppose the residual flux and thereby reducing crawling.

In Ward Leonard’s control of DC motor, the lower limit of speed is imposed by

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