Ward Leonard Method of Speed Control

The arrangement consists of a seperately excited d.c motor M whose speed is to be controlled. G is a seperately excited dc generator driven by a prime mover, usually an induction motor. Motor generator set converts ac to dc.

Ward leonard method diagram

Ward Leonard Method Procedure

1. DC motor field is excited by a dc source. Now, the generator field flux is kept to a low value to produce a small output voltage, to limit armature current through motor.
2.  A change in the generated field current varies the applied voltage to the motor and therefore, the speed is changed. So, motor speed is controlled merely by changing generator field current.
3. Thus by varying the voltage applied to the motor from a small value to rated value, speeds below rated speed can be obtained.
4. Further, by varying motor field and keeping voltage constant speeds above base speed can be obtained.
5. If you reverse generator field terminals, opposite polarity voltage is generated by the generated and speed get reversed.

Advantages of Ward Leonard Method

1. No starting rheostat are used therefore less power wastage.
2.  Accurate, wide range and bidirectional speeds are obtained easily.  
3. Speed regulation is very good.
4.  Motor direction is changed merely by changing generator field current. Thus, It has inherent regenerative braking property.

Disdvantages of Ward Leonard System

1.  The method is very costly as two extra machines (motor-generator set) are required and requires more floor area.
2.    Overall efficiency of the system is not sufficient especially if it is lightly loaded due to large mechanical losses in three machines.

ward leonard system applications

Rolling mills, colliery winding motor and Lifts and elevators are some of the examples employing ward leonard method of speed control. 


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• Speed control methods of DC motor