Chapter 26, Problem 1SQ

What is the purpose of inserting resistance in the stator circuit during starting?

To determine

Explain the reason of inserting a resistor in a motor stator circuit.

Explanation of Solution

A resistor is inserted in a motor stator circuit in series. The voltage drop caused by inserting resistor reduces the voltage across the motor terminals. Reduced voltage results in reduced starting torque and reduced inrush current. Therefore, the resistor is inserted in stator to reduce the starting voltage of a motor and to avoid the winding damages.

Example:

Refer to Figure 26-1 in the textbook where the supply voltage of 480 V is connected across the resistor and the motor which is connected in series. The resistor voltage drop causes the low voltage $\left(<480\text{ V}\right)$ across the motor terminals. Therefore, the motor starts at low starting torque and low starting currents which prevent the motor from the damages. If the resistance in series is not inserted and the motor connected directly to the full supply voltage causes high starting torque and draws the more inrush current, it may damage the machinery during starting. Therefore, the resistance inserted in series with the stator is to reduce voltage during starting.

Conclusion:

Thus, the reason for inserting a resistor in a motor stator circuit is explained.