Licensed Electrical Contractor Motors and Motor Controls 2

Question 1

What is the purpose of a motor's service factor (SF) rating?

Accepted Answer

Indicates allowable overload capacity without damage

Answer

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:

Accepted Answer

35A

Answer

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?

Accepted Answer

Electronic (solid-state) overload relay

Answer

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:

Accepted Answer

33% of full-voltage starting torque

Answer

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?

Accepted Answer

The shaft height is 56/16 inches (3.5 inches) from the base

Answer

NEMA frame numbers encode the shaft centerline height: divide the first two digits by 4 (56 ÷ 4 = 14/4 = 3.5 inches).

Licensed Electrical Contractor Practice Test

Licensed Electrical Contractor Practice Test

Licensed Electrical Contractor Motors and Motor Controls 2