The Ward Leonard Method of speed control is based on the fact that the speed of DC motors can be changed by varying the applied voltage to the armature. The schematic diagram of the ward-leonard method is shown in the figure.
In Ward-Leonard system, M is the DC motor whose speed is to be controlled and G is a separately excited DC generator which is driven by either a 3-three phase induction motor or synchronous motor. The combination of AC driving motor and the DC generator is known as the motor-generator set or M-G set.
By varying the field current of the generator, the generator voltage is changed. When this changing voltage is applied across the DC motor M, it changes speed of the motor. Here, the field current of the DC motor Ifm is kept constant to its rated value during the speed control and the field current of the DC generator Ifg is changed in such a way that the generator voltage Vt changes from zero to its rated value. Therefore, the speed of the motor changes from zero to its rated value.
As the speed control is carried out with rated armature current of the motor Ia and with constant motor field flux ϕm, hence the motor produces a constant torque (τ∝ϕmIa) upto its rated speed.Since the power developed by the motor is proportional to speed, thus it increases with the speed. Therefore, with the armature voltage control method constant torque and variable power drive is obtained for the speed below the rated speed of the DC motor.
In order to control the speed above the rated speed, the field flux control method is used. In this method, the armature current (Ia) of the motor is fixed at its rated value and the generator voltage Vt is also made constant. The field current of the motor (Ifm) is now decreased and hence, the motor field flux (ϕm) is decreased. Therefore, the speed of the DC motor is increased.
As the power (VtIa) remains constant, the electromagnetic torque reduces as the field flux is decreased. Hence, the torque reduces with the increase in speed of the motor. Thus, with field flux control method, constant power and variable torque drive is obtained for the speed above the rated speed of the motor.
When the speed control over a wide range is required, the combination of armature voltage control and field flux control being used. With this combination, the ratio of maximum to minimum available speeds are to be 20 to 40.
If instead of induction motor, an over-excited synchronous motor is used for driving the generator, the power factor of the system can be improved. As the over-excited synchronous motor operates at leading power factor, then the leading reactive power generated by the synchronous motor can be used to compensate the lagging reactive power taken by the other inductive loads in the system. Therefore, the power factor of the system is improved.
When the load is heavy and intermittent, a slip ring induction motor is used to drive the DC generator. A flywheel is mounted on its shaft to prevent the heavy fluctuations in supply current. This scheme is known as Ward Leonard Ilgener scheme.
There is one another form of Ward Leonard system, in which non-electrical prime movers (such as diesel engine or gas turbine) are used to drive the DC generator
Following are the main advantages of the Ward Leonard system −
The Ward Leonard method of speed control suffers from the following drawbacks −
The Ward Leonard drives are used where smooth speed control of DC motor over a wide range in both directions is required like in −