Page 131 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

INDUCTION MOTORS

6.20 CHAPTER SIX

pressed, the coil of relay (or contactor) M is energized, causing normally open contacts M ,
1
M , and M to close. When these contacts close, power is applied to the motor. Also,
2 3
contact M closes, shorting out the starting switch, allowing the operator to release the
4
switch while relay M remains energized. When the stop button is pressed, relay M deener-
gizes, stopping the motor.
The built-in protective features in this type of magnetic motor starter circuit are
1. Short-circuit protection
2. Overload protection
3. Undervoltage protection
Fuses F , F , and F provide short-circuit protection for the motor. When a short circuit
1 2 3
occurs, the current increases instantly to many times the rated value. The fuses blow, dis-
connecting the motor from the power supply to prevent it from burning up.
However, these fuses are designed to tolerate currents many times greater than the
full-load current. They would not burn up during normal starting due to inrush current.
Therefore, if a short circuit occurs through a high resistance and/or excessive loads, it will
not be cleared by the fuses.

FIGURE 6.25 A three-step resistive starter for an induction motor.

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