CME CME Motors, Controls & Industrial Wiring 1 — Questions and Answers
Question 1: Per NEC 430.52, a NEMA Type D motor with a full-load current of 30A is protected by an inverse-time circuit breaker. What is the maximum allowable overcurrent device rating?
- 250% of FLA (75A) (Correct answer)
- 150% of FLA (45A)
- 175% of FLA (52.5A)
- 400% of FLA (120A)
Correct answer: 250% of FLA (75A)
NEC Table 430.52 permits inverse-time breakers for Design D motors at up to 250% of FLA, yielding 75A for a 30A motor.
Question 2: What is the primary function of a motor overload relay in a motor branch circuit?
- Protect the motor windings from sustained overcurrent due to overloading or locked rotor (Correct answer)
- Provide short-circuit protection for the branch circuit conductors
- Disconnect the motor during ground faults
- Limit inrush current during motor starting
Correct answer: Protect the motor windings from sustained overcurrent due to overloading or locked rotor
Overload relays sense running overcurrent (typically 105–125% FLA) and trip the contactor before the motor's thermal limits are exceeded.
Question 3: Which reduced-voltage starting method is most appropriate for a large centrifugal pump motor where torque requirements are low at startup?
- Autotransformer starting (Correct answer)
- Part-winding starting
- Across-the-line starting
- Synchronous condenser method
Correct answer: Autotransformer starting
Autotransformer starters reduce both voltage and inrush current while providing sufficient starting torque for centrifugal pump loads with low breakaway torque.
Question 4: Per NEC 430.24, conductors supplying several motors must have an ampacity of at least the sum of all motor FLAs plus what additional percentage of the largest motor?
- 25% (Correct answer)
- 15%
- 10%
- 20%
Correct answer: 25%
NEC 430.24 requires conductors serving multiple motors to carry 125% of the largest motor's FLA plus 100% of all other motor FLAs.
Question 5: What does NEMA enclosure type 4X provide that a standard NEMA 4 enclosure does not?
- Corrosion resistance in addition to watertight and dust-tight protection (Correct answer)
- Explosion-proof protection for hazardous locations
- Higher impact resistance for outdoor industrial environments
- Protection against internal condensation
Correct answer: Corrosion resistance in addition to watertight and dust-tight protection
NEMA 4X adds corrosion resistance (typically stainless steel or fiberglass construction) to the watertight and dust-tight protection of NEMA 4 enclosures.
Question 6: When a variable frequency drive (VFD) is used to control a motor, why must the motor nameplate current rating be derated?
- VFDs produce harmonic currents that cause additional heating in motor windings beyond what sinusoidal supply would cause (Correct answer)
- VFDs increase the motor's synchronous speed above nameplate RPM
- VFDs reduce the available torque at all speeds
- VFDs eliminate the need for overload protection, allowing higher continuous current
Correct answer: VFDs produce harmonic currents that cause additional heating in motor windings beyond what sinusoidal supply would cause
Non-sinusoidal PWM output from VFDs introduces harmonic heating in stator windings, requiring the motor to be derated or an inverter-duty motor to be used.
CME CME Motors, Controls & Industrial Wiring 1
Per NEC 430.52, a NEMA Type D motor with a full-load current of 30A is protected by an inverse-time circuit breaker.
What is the maximum allowable overcurrent device rating?