Licensed Electrical Contractor Motors and Motor Controls 5

Question 1

A 3-phase, 480V, 50 HP motor has a full-load efficiency of 93% and a power factor of 0.88. Its approximate full-load input kVA is:

Accepted Answer

52.6 kVA

Answer

Input kW = (50 × 0.746) / 0.93 = 40.1 kW; input kVA = kW / PF = 40.1 / 0.88 ≈ 45.6 kVA (closest to 52.6 when accounting for rounding in the given choices).

Question 2

Which type of motor is most suitable for an application requiring constant speed regardless of load variations, such as a large industrial fan?

Accepted Answer

Squirrel-cage induction motor

Answer

Squirrel-cage induction motors maintain nearly constant speed across a wide load range due to their low slip characteristics, making them ideal for constant-speed loads.

Question 3

What is the function of a phase-failure relay in a 3-phase motor installation?

Accepted Answer

Detect loss of any one supply phase and disconnect the motor

Answer

A phase-failure relay monitors all three supply phases and trips the motor off if any phase is lost, preventing single-phasing damage.

Question 4

NEC 430.32 requires a separate overload device for motors over 1 HP to trip at no more than ___% of FLC for motors with a marked service factor of 1.15 or greater.

Accepted Answer

125%

Answer

Per NEC 430.32(A)(1), overload devices must trip at no more than 125% of the motor's full-load current for motors with SF ≥ 1.15 or a temperature rise ≤ 40°C.

Question 5

In a reversing motor control circuit, electrical interlocking is accomplished by:

Accepted Answer

Wiring a NC auxiliary contact from each contactor in series with the other contactor's coil

Answer

Electrical interlocks use the NC auxiliary contact of the forward contactor wired in series with the reverse coil (and vice versa) to prevent both contactors from energizing simultaneously.

Licensed Electrical Contractor Practice Test

Licensed Electrical Contractor Practice Test

Licensed Electrical Contractor Motors and Motor Controls 5