Chapter 27, Problem 1SQ

Why is it desirable to remove the autotransformers from the line when the motor reaches its rated speed?

To determine

Explain the reason why it is desirable to remove the autotransformer starters from the line when the connected motor reaches its rated speed.

Explanation of Solution

Autotransformer starter is placed between the motor and supply power lines to limit the supply voltage that is provided across the terminals of the motor during accelerating of the connected motor. With the lower accelerating voltage, motor draws less current and produces less torque in motor. Taps are provided on the autotransformer starters to permit the user to initiate the motor at an increasing speed from 50 percent, 60 percent, 75 percent or 85 percent of the line voltage.

The autotransformer starters are removed from the line connection when the motor attains the rated speed to obtain the required electrical energy by removing the magnetizing current, iron, and copper losses. Generally, the insulating life of the autotransformers may extend due to reduced heat and voltage applications.

Conclusion:

Thus, the reason why it is desirable to remove the autotransformer starters from the line when the connected motor reaches its rated speed is explained.