PC Motors and Motor Controls 2 — Questions and Answers
Question 1: What is the minimum horsepower rating required for a motor disconnecting means per NEC 430.110?
- Equal to the motor's HP rating (Correct answer)
- 115% of the motor's HP rating
- 125% of the motor's HP rating
- 150% of the motor's HP rating
Correct answer: Equal to the motor's HP rating
NEC 430.110(A) requires the motor disconnect to have an HP rating at least equal to the motor's HP rating at the applied voltage.
Question 2: Per NEC 430.102, where must the motor disconnecting means be located?
- Within 10 feet of the motor controller
- Within sight of the motor and within 50 feet (Correct answer)
- In the same electrical room as the motor
- On the same wall panel as the branch circuit breaker
Correct answer: Within sight of the motor and within 50 feet
NEC 430.102(B) requires the motor disconnect to be within sight of the motor, meaning visible and not more than 50 feet away.
Question 3: What type of motor starting method is used to reduce inrush current during startup?
- Full voltage across-the-line starting
- Reduced voltage starting (Correct answer)
- Single-phase starting
- Direct-on-line starting
Correct answer: Reduced voltage starting
Reduced voltage starters (autotransformer, wye-delta, or electronic soft starters) limit inrush current by applying less than full voltage during starting.
Question 4: What is the standard secondary voltage for a motor control circuit transformer per NEC 430.72?
- 120V (Correct answer)
- 240V
- 480V
- 600V
Correct answer: 120V
NEC 430.72 permits control circuit conductors supplied by a grounded transformer secondary at up to 120V without additional overcurrent protection in many cases.
Question 5: A squirrel cage induction motor is classified as what type of motor?
- DC series motor
- Universal motor
- AC induction motor (Correct answer)
- Synchronous motor
Correct answer: AC induction motor
A squirrel cage induction motor is the most common AC induction motor, named for its rotor conductor arrangement that resembles a squirrel cage.
Question 6: What does NEMA MG1 classify regarding motor construction?
- Motor winding insulation classes only
- Motor enclosure types such as ODP and TEFC (Correct answer)
- Motor voltage and frequency ratings only
- Motor starting torque categories only
Correct answer: Motor enclosure types such as ODP and TEFC
NEMA MG1 is the standard that classifies motor enclosure types including Open Drip Proof (ODP) and Totally Enclosed Fan Cooled (TEFC) among others.
PC Motors and Motor Controls 2
What is the minimum horsepower rating required for a motor disconnecting means per NEC 430.110?