SOLUTION

Ward Leonard Speed Control

The Ward-Leonard system consists of a fixed speed 3-phase AC induction motor driving by a separately excited DC generator that, in turn, feeds a variable voltage to a shunt wound DC motor. So this is essentially a DC variable speed drive. Ward Leonard-controlled dc motors are preferred for automatic drives.

The basic principle of the DC variable speed drive is that the speed of a separately excited DC motor is directly proportional to the voltage applied to the armature of the DC motor. The main changes over the years have been concerned with the different methods of generating the variable DC voltage from the 3-phase AC supply.

One of the advantages of the Ward Leonard drive system is its ability for regenerative braking, thereby delivering power to the grid, corresponding to low motor speeds. For regenerative braking, the output voltage of generator “G” is reduced below the induced voltage of the motor “M” by decreasing the generator field excitation. This reverses the current flow in the armature of the machines “G” and “M”.

As the current is reversed, the machine “G” will act as a motor and the machine “M” will act as a generator. The mechanical energy supplied to the machine “M” is converted into electrical energy; which is now supplied to the machine “G’ [presently working as a motor]. The machine “G” will act as a prime mover for the ac machine which will now work as a generator feeding power to the grid. The nature of the speed-torque characteristic is similar to that shown in Figure. Drop-in motor speed due to change in load torque is caused by I_ar_a drop in both the machines. When the generator output voltage is high, the motor speed is automatically high. The I_ar_a drop in both machines will be negligible compared to their outputs. Therefore, the % speed regulation of the motor will be excellent. The Ward Leonard drive is used in automatic drives such as steel rolling mills, paper plants, elevators, machine tools.