SOLUTION

Ward-Leonard system.

The Ward-Leonard system comprises a fixed speed 3-phase AC induction motor driving; 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.

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.

In the case of the Ward-Leonard system, the output voltage of the DC generator, which is adjusted by controlling the field voltage, is used to control the speed of the DC motor. This type of variable speed drive had good speed and torque characteristics and could achieve a speed range of 25:1.  It is no longer commonly used because of the high cost of the 3 separate rotating machines. In addition, the system requires considerable maintenance to keep the brushes and commutators of the two DC machines in good condition.

Advantages of Ward-Leonard system.

1. A wide range of sensitivity can be obtained by using this method.
2. The absence of an external resistance improves the efficiency at all speeds.
3. No special starting gear is required
4. As the generator induced voltage is gradually raised from zero, the motor starts up smoothly
5. Speed reversal is smoothly carried out.

Disadvantages of Ward-Leonard system.

1. The initial cost of the system is high as there is a motor-generator set installed, of the same rating as that of the main DC motor.
2. Larger size and weight.
3. Requires large floor area
4. Costly foundation
5. Maintenance of the system is frequent.
6. The overall efficiency of the system is not sufficient, especially it is lightly loaded.

Application of Ward-Leonard System

• Colliery Winders
• Cranes
• Electric Excavators
• Mine hoists
• Elevators
• Steel rolling mills
• Paper Machines