A smooth variation of speed from zero to above normal with inherent stability of speed at all loads is achieved through an adjustable voltage system of speed control called Ward-Leonard Method of speed control.
Fig. 1: Ward Leonard Method of Speed Control.
In all the Speed Control methods, it is clear that the speed cannot be varied from zero to above normal by any one method and atleast two methods are required to be combined to do so. Further, the efficiency of the above mentioned controls is much less, due to power loss and instability due to load variation.
Working of Ward Leonard Method of Speed Control
In this method, a DC generator is mechanically coupled to a constant speed DC motor or an AC 3-phase induction motor as shown in Fig 1. The generated supply from the DC generator is fed directly to the armature of the controlled DC motor. The fields of both the DC generator and the controlled DC motor are separately excited from a suitable DC supply. The field of the DC generator is controlled through a field rheostat and a change-over switch to vary the generated voltage and to change the polarity respectively. This enables the supply to the controlled DC motor to vary at a wide range and also makes it possible to reverse the supply voltage polarity. This, in turn, changes the speed of the controlled DC motor to vary from zero to above normal speed as well as change the direction of rotation, if necessary. The controlled DC motor speed can be brought down to zero by reducing the supply voltage of the generator to a suitable level.
Advantages of Ward Leonard Method of Speed Control
- By this system, a speed as low as zero and as high as two times the normal speed could be achieved.
- The direction of rotation of the controlled DC motor can be changed simply by reversing the controller in the field circuit of the generator.
- As there is not much power loss in the field rheostat, the speed variations are achieved at higher efficiency.
- The speed of the controlled DC motor is independent of the load.
Disadvantages of Ward Leonard Method of Speed Control
This method requires high initial cost and low overall efficiency due to three machines in operation.
Applications of Ward Leonard Method of Speed Control
This system is used in steel rolling mills and paper mill drives, hoists, elevators etc, where precise control of speed over a wide range is required.