Licensed Electrical Contractor Licensed Electrical Contractor Motors and Motor Controls Questions and Answers 2 — Questions and Answers
Question 1: What is the primary purpose of a reduced-voltage starter for an AC induction motor?
- To limit inrush current during startup (Correct answer)
- To increase the motor's full-load speed
- To reverse the direction of rotation
- To provide regenerative braking
Correct answer: To limit inrush current during startup
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?
- 150% of the motor full-load current
- 250% of the motor full-load current (Correct answer)
- 125% of the motor full-load current
- 175% of the motor full-load current
Correct answer: 250% of the motor full-load current
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?
- Voltage control
- Frequency control (Correct answer)
- Pole switching
- Wound rotor resistance
Correct answer: Frequency control
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?
- To protect the motor from sustained overcurrent conditions (Correct answer)
- To protect against short circuits in the feeder
- To regulate motor speed under varying loads
- To provide instantaneous trip on ground faults
Correct answer: To protect the motor from sustained overcurrent conditions
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?
- Within sight of the motor and its controller (Correct answer)
- At the main service panel only
- In an adjacent room with clear labeling
- At least 50 feet from the motor
Correct answer: Within sight of the motor and its controller
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.
Question 6: A three-phase motor has a nameplate FLA of 40 amps. What is the minimum ampacity required for the branch-circuit conductors per NEC 430.22?
- 40 amps
- 45 amps
- 50 amps (Correct answer)
- 60 amps
Correct answer: 50 amps
NEC 430.22 requires branch-circuit conductors to have an ampacity of not less than 125% of the motor full-load current, so 40 x 1.25 = 50 amps.
Licensed Electrical Contractor Licensed Electrical Contractor Motors and Motor Controls Questions and Answers 2
What is the primary purpose of a reduced-voltage starter for an AC induction motor?