Licensed Electrical Contractor Motors and Motor Controls 2 — Questions and Answers
Question 1: What is the purpose of a motor's service factor (SF) rating?
- Indicates the motor's insulation class
- Indicates allowable overload capacity without damage (Correct answer)
- Specifies the minimum supply voltage
- Defines the locked-rotor current multiplier
Correct answer: Indicates allowable overload capacity without damage
Service factor indicates the permissible overload multiplier a motor can handle continuously without damage, e.g., SF=1.15 means 15% overload is acceptable.
Question 2: A 480V, 3-phase motor draws 28A at full load. Per NEC, the minimum branch circuit conductor ampacity before applying any derating is:
- 28A
- 35A (Correct answer)
- 42A
- 49A
Correct answer: 35A
NEC 430.22 requires branch circuit conductors to be sized at 125% of the motor's full-load current (28A × 1.25 = 35A).
Question 3: Which motor protection device responds to both overload current AND phase loss conditions?
- Thermal magnetic circuit breaker
- Eutectic-alloy overload relay
- Electronic (solid-state) overload relay (Correct answer)
- Dual-element time-delay fuse
Correct answer: Electronic (solid-state) overload relay
Electronic overload relays monitor all three phases individually and can detect phase loss, imbalance, and overcurrent conditions.
Question 4: In a wye-delta reduced-voltage starter, motor torque during the wye (starting) step is approximately:
- 100% of full-voltage starting torque
- 67% of full-voltage starting torque
- 33% of full-voltage starting torque (Correct answer)
- 50% of full-voltage starting torque
Correct answer: 33% of full-voltage starting torque
Wye starting reduces voltage to each winding by 1/√3, reducing torque to 1/3 (33%) of full-voltage starting torque.
Question 5: What does NEMA frame designation 56 indicate about a motor?
- The motor has a 56 hp rating
- The shaft height is 56/16 inches (3.5 inches) from the base (Correct answer)
- The stator bore diameter is 5.6 inches
- The motor weighs 56 pounds
Correct answer: The shaft height is 56/16 inches (3.5 inches) from the base
NEMA frame numbers encode the shaft centerline height: divide the first two digits by 4 (56 ÷ 4 = 14/4 = 3.5 inches).
Question 6: A motor nameplate shows 'Code G.' What does this indicate?
- Motor efficiency class
- Locked-rotor kVA per horsepower range (5.6–6.29) (Correct answer)
- Insulation temperature class
- Motor design letter (torque characteristic)
Correct answer: Locked-rotor kVA per horsepower range (5.6–6.29)
NEMA Code letters indicate locked-rotor kVA per horsepower; Code G corresponds to 5.6–6.29 kVA/hp, used to size fuses and breakers.
Question 7: Which NEC article primarily governs the installation of motors, motor circuits, and motor controllers?
- Article 240
- Article 310
- Article 430 (Correct answer)
- Article 440
Correct answer: Article 430
NEC Article 430 covers motors, motor branch circuits, motor feeders, and motor controllers including overload protection requirements.
Licensed Electrical Contractor Motors and Motor Controls 2
What is the purpose of a motor's service factor (SF) rating?