Ward Leonard Speed Control Method of DC Motor

In this method, the speed control of the dc motor is achieved by controlling the voltage applied across the armature terminals. It is introduced by the scientist sir Henry Ward Leonard in 1891 and named as a 'Ward Leonard Speed Control'. The variable voltage to the armature is supplied by using a motor-generator set. A desired wide range of sensitive speed control can be obtained by this method.

Principle of Ward Leonard Method :

In this method, the field winding of the motor M2 for which the speed is to be controlled is supplied by a separate permanently connected dc supply. The armature terminals of the motor are connected to a variable voltage supplied by a motor-generator set. The shafts of both motor and generator in the motor-generator set are mechanically coupled. The connection diagram for Ward Leonard's speed control is shown below.

Ward Leonard Speed Control Method of DC Motor

Here the motor M1 drives the generator G running at an approximately constant speed. The motor M1 used can be either a.c. or d.c. type. The output voltage of the generator G is given to the armature terminals of the dc motor M2 (whose speed is to be controlled). The field of the generator is separately excited from the same dc supply supplied to the field circuit of a speed-controlled dc motor.

The connection of the generator field circuit is made through a reversing switch (not shown in the figure) and field regulator R. So that the connection makes it possible to easily reverse the generator excitation. Thus variable generator output voltage from zero to a maximum value in both directions can be obtained.

Thereby reversing the direction of the generator field current with reversing switch the polarity of generated voltage will also reverse and thus speed control of dc motor M2 in both directions will be achieved.

Nowadays, SCR circuits are used instead of motor-generator set to obtain variable dc voltage by rectifying the input ac power supplied through a transformer. Though it costs more when compared to the motor-generator set it reduces the maintenance. Also, this method provides the possibility to adopt automatic speed control through a feedback system.

Speed-Torque and Power-Speed Characteristics :

By adjusting the field current to its maximum value and excitation with rated voltage. Constant speed-torque characteristics with reduced speed below the base value can be obtained by reducing armature voltage.

Ward Leonard Speed Control Method of DC Motor

For obtaining the speed above the base value, the field current is decreased by maintaining constant voltage across armature terminals. The decreased field decreases the torque on the motor as the speed increases.

Thus, a combined armature and field control is used over a wide range of speed control. Therefore nature of the power-speed characteristics is also revealed through the kind of control over torque-speed characteristics.

Advantages of Ward Leonard Method :

Some of the attractive features of Ward Leonard speed control are,
  • Smooth speed control over a wide range in either direction can be achieved.
  • It is well suited for applications where there is a need for frequent starting, stopping, and reversals.
  • Speed control of Large motor can be carried.
  • It eliminates the starting gear requirement of the motor because the induced voltage in the generator is gradually increased.
  • A modified Ward Leonard method known as the Ward Leonard-Ilgner method which uses a flywheel reduces fluctuations due to sudden change in load demand.
  • When the motor back emf becomes greater than the generator emf, electrical power flows back from the motor to the generator which enhances the overall efficiency of the system.

Disadvantages of Ward Leonard Method :

The chief disadvantages of this system are,
  • High initial investment i.e., for motor-generator set.
  • Low overall efficiency at light loads.
  • Maintainance is more as of three machines
  • Lare floor area is required to implement the speed control method
  • The use of three machines increases the losses and driving noise.

Applications of Ward Leonard Method :

Due to its unlimited speed control in both directions, it is used in various applications like,
  • For elevators
  • As the main drive in a steel mill
  • Electric excavators
  • Blooming and paper industry
  • Colliery winders etc.

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