Motors and Motor Controls Flashcards
7 cards from real Licensed Electrical Contractor practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Motors and Motor Controls flashcards as text
What is the purpose of a motor's service factor (SF) rating?
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.
A 480V, 3-phase motor draws 28A at full load. Per NEC, the minimum branch circuit conductor ampacity before applying any derating is:
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).
Which motor protection device responds to both overload current AND phase loss conditions?
Answer: Electronic (solid-state) overload relay
Electronic overload relays monitor all three phases individually and can detect phase loss, imbalance, and overcurrent conditions.
In a wye-delta reduced-voltage starter, motor torque during the wye (starting) step is approximately:
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.
What does NEMA frame designation 56 indicate about a motor?
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).
A motor nameplate shows 'Code G.' What does this indicate?
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.
Which NEC article primarily governs the installation of motors, motor circuits, and motor controllers?
Answer: Article 430
NEC Article 430 covers motors, motor branch circuits, motor feeders, and motor controllers including overload protection requirements.