Electrician Motors and Controls 1 — Questions and Answers
Question 1: What NEC article covers the installation of motors, motor circuits, and controllers?
- Article 410
- Article 430 (Correct answer)
- Article 440
- Article 450
Correct answer: Article 430
NEC Article 430 covers all aspects of motor circuits including conductors, overcurrent protection, controllers, and disconnecting means.
Question 2: What is the purpose of a motor starter?
- To convert AC power to DC for the motor
- To limit inrush current and provide overload protection when starting a motor (Correct answer)
- To increase motor speed above nameplate rating
- To measure motor power consumption
Correct answer: To limit inrush current and provide overload protection when starting a motor
A motor starter limits the high inrush current during startup and provides thermal overload protection during operation.
Question 3: According to the NEC, motor branch circuit conductors must be sized at what minimum percentage of the motor's full-load current (FLC)?
- 100%
- 115%
- 125% (Correct answer)
- 150%
Correct answer: 125%
NEC 430.22 requires motor branch circuit conductors to be sized at no less than 125% of the motor's full-load current.
Question 4: What is the function of a thermal overload relay in a motor control circuit?
- To start the motor remotely
- To protect the motor windings from excessive heat due to overload (Correct answer)
- To reverse motor direction
- To vary motor speed
Correct answer: To protect the motor windings from excessive heat due to overload
A thermal overload relay monitors motor current and trips to disconnect the motor if it draws excessive current that could overheat and damage the windings.
Question 5: What type of motor uses both a start winding and a run winding?
- Three-phase induction motor
- DC shunt motor
- Single-phase split-phase motor (Correct answer)
- Synchronous motor
Correct answer: Single-phase split-phase motor
A single-phase split-phase motor uses a start winding (with higher resistance) to create the phase shift needed for starting torque.
Question 6: What is the maximum rating of an inverse time circuit breaker used to protect a motor branch circuit per the NEC?
- 115% of motor FLC
- 125% of motor FLC
- 150% of motor FLC
- 250% of motor FLC (Correct answer)
Correct answer: 250% of motor FLC
NEC Table 430.52 allows inverse time (standard) circuit breakers to be sized up to 250% of the motor's full-load current for short-circuit protection.
Electrician Motors and Controls 1
What NEC article covers the installation of motors, motor circuits, and controllers?