Electric Traction: Buck and Boost Method of Speed Control of Traction Motors

The circuit diagram of the buck and boost method of speed control of traction motors is shown in the figure.

There are two traction motors viz. traction motor-I and traction motor-II. In this method of speed control, the armature of both the traction motors I and II and the motor-generator set are connected in series. This whole series combination is connected across the main supply.

When the terminal voltage of the generator is equal to the supply voltage in magnitude but of opposite polarity and the main contactor (MC) is closed, the voltage across the traction motors will be zero and hence their speed is also zero.

On the decrease in the terminal voltage of the generator, the voltage across the traction motors will start increasing. Consequently, the speed of the traction motors also increases. When the terminal voltage of the generator becomes equal to zero, then full supply voltage will appear across both the motors, i.e., one-half of the supply voltage across each motor.

In case, when the polarities of both the supply source and generator are same and the generator voltage is equal to the supply voltage, then the voltage across each traction motor will be equal to the supply voltage. Therefore, the voltage across the traction motors can be adjusted, i.e., bucked or boosted up, by adjusting the excitation of the generator.

Advantages of Buck and Boost Method of Speed Control

The buck and boost method of speed control of traction motors have the following advantages −

• By employing the buck and boost method, the traction motors can be operated on any speed.

• In case of temporary interruption in the supply, the kinetic energy of the flywheel can be utilized in producing energy from the motor-generator set and supplied to the traction motors.

• When buck and boost method is used for speed control, there is no energy loss in the starting resistance of the traction motors.

Disadvantages of Buck and Boost Method of Speed Control

The main disadvantages of the buck and boost method of speed control of traction motor includes the following:

• This method requires a motor-generator set.

• There is a loss of energy in the starting resistance of the motor-generator set.