Licensed Electrical Contractor Licensed Electrical Contractor Motors and Motor Controls Questions and Answers 2

Question 1

What is the primary purpose of a reduced-voltage starter for an AC induction motor?

Accepted Answer

To limit inrush current during startup

Answer

Reduced-voltage starters limit the high inrush current that occurs when a motor is started across the line, protecting both the motor and the electrical system.

Question 2

Per the NEC, what is the maximum allowable setting for a motor branch-circuit short-circuit and ground-fault protective device using an inverse-time circuit breaker for a standard AC motor?

Accepted Answer

250% of the motor full-load current

Answer

NEC 430.52 permits inverse-time circuit breakers to be sized up to 250% of the motor full-load current for standard AC motors.

Question 3

Which type of motor control method uses a Variable Frequency Drive (VFD) to adjust motor speed?

Accepted Answer

Frequency control

Answer

A VFD adjusts the frequency of the AC power supplied to the motor, which directly controls the synchronous speed of the motor.

Question 4

What is the function of an overload relay in a motor control circuit?

Accepted Answer

To protect the motor from sustained overcurrent conditions

Answer

Overload relays are designed to protect motors from damage caused by sustained overcurrent conditions that exceed the motor's rated capacity.

Question 5

When installing a motor disconnecting means, the NEC requires it to be located where?

Accepted Answer

Within sight of the motor and its controller

Answer

NEC 430.102(B) requires the disconnecting means to be in sight from the motor location and the driven machinery location for safety during maintenance.

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Licensed Electrical Contractor Licensed Electrical Contractor Motors and Motor Controls Questions and Answers 2