Previous Year Question

Correct answer: A — Fine speed control over whole range

How the Ward Leonard system controls speed

In the Ward Leonard system, a prime mover drives a DC generator, and the generator supplies the armature of the DC motor being controlled. Adjusting the generator field current changes its generated voltage. This provides a continuously adjustable armature voltage for the controlled motor.

Here V_a is motor armature voltage, I_aR_a is the armature resistance drop, and Φ is motor field flux. With approximately constant motor flux, increasing the generator output voltage increases motor speed. Reducing the voltage reduces speed smoothly, including operation at low speeds.

Below base speed, armature-voltage control normally gives a constant-torque operating region within current limits. Above base speed, weakening the motor field can extend the speed range, approximately at constant power. Reversing generator voltage reverses motor direction, and suitable arrangements also permit electrical braking.

The main benefit is therefore fine, smooth speed control over a wide range. The motor–generator set requires several machines, space and maintenance, and introduces additional conversion losses. Its historical appeal was controllability, particularly for duties such as hoists and rolling mills.