Ward-Leonard Method of Speed Control

Ward-Leonard Method

The ward-Leonard method of speed control is used to control the speed of a DC shunt motor. In this method, the speed control of DC shunt motor is achieved by controlling the armature voltage.

So, in the ward-Leonard method, the variable voltage is given to the armature of a motor.

In other words, this method works with constant excitation and variable armature voltage. The variable voltage supply is obtained from a motor-generator set or converter unit.

The schematic diagram of the ward-Leonard method is shown in the figure below.

Ward-Leonard Method of Speed Control
Ward-Leonard Method of Speed Control

In the above diagram, motor M1 is the main motor whose speed we need to control. And the generator G is a separately excited DC generator driven by a constant three-phase AC motor or DC motor (M2).

The field winding of main motor M1 is permanently connected with the DC supply. Therefore, the excitation current for the main motor and generator is given to the circuit, and the rheostat of the generator is gradually brought out of the circuit as the generator picks up the speed.

The variable voltage of the generator terminal is obtained by varying the excitation current of the generator with the help of shunt regulator R.

The direction of the main motor armature reverses by reversing the direction of the excitation current of a generator.

Braking of motor (speed reduction) can be carried out by reducing the generator excitation. The EMF is less than the counter EMF of motor M1 in braking conditions. Therefore, the motor M1 operates as a generator, and generator G acts as a motor. Also, the AC driving machine M2 serves as a generator.

During this situation, the kinetic energy of motor M1 and its load returns to the main supply and take place the braking action of main motor M1.

The application of the Ward-Leonard method is where almost complete speed control in the direction of rotation is required like steel rolling machines, paper machines, elevators, cranes, mine hoists, etc.

Advantages of Ward-Leonard Method

The advantages of the ward-Leonard method are listed below.

  • This method can achieve excellent speed control over zero to normal speed in both directions.
  • The speed regulation is good.
  • Uniform acceleration can be obtained.

Disadvantages of Ward-Leonard Method

The disadvantages of the ward-Leonard method are listed below.

  • In this method, two extra machines are required, increasing the cost of an entire setup.
  • The overall efficiency of this arrangement is very low, especially in light load conditions.

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