Licensed Electrical Contractor Motors and Motor Controls 3

Question 1

A two-speed, single-winding (consequent pole) motor achieves different speeds by:

Accepted Answer

Reconnecting stator windings to change the number of poles

Answer

Consequent pole motors achieve two speeds by reconnecting stator windings to create a different number of magnetic poles, which changes synchronous speed.

Question 2

What is the primary function of a dynamic braking resistor in a DC motor drive system?

Accepted Answer

Dissipate regenerative energy when the motor decelerates

Answer

During deceleration, the motor acts as a generator; dynamic braking resistors dissipate this regenerated energy as heat to stop the motor quickly.

Question 3

Per NEC 430.52, the maximum rating of an inverse-time circuit breaker used as motor branch-circuit short-circuit protection for a Design B motor is:

Accepted Answer

250% of FLC

Answer

NEC Table 430.52 allows inverse-time circuit breakers to be sized up to 250% of the motor's full-load current for Design B motors.

Question 4

In a 3-phase motor control circuit, a holding contact wired in parallel with the start pushbutton is called a:

Accepted Answer

Seal-in (memory) contact

Answer

A seal-in or memory contact is wired in parallel with the start button so the contactor remains energized after the start button is released.

Question 5

A motor's nameplate insulation class 'F' has a maximum allowable temperature rise (by resistance method) of:

Accepted Answer

115°C

Answer

NEMA Class F insulation is rated for a maximum temperature rise of 115°C above a 40°C ambient (155°C total), per NEMA MG-1.

Licensed Electrical Contractor Practice Test

Licensed Electrical Contractor Practice Test

Licensed Electrical Contractor Motors and Motor Controls 3