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

Changing supply frequency

Answer

Adjusting rotor resistance

Answer

Varying the supply voltage

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

Limit inrush current during starting

Answer

Improve power factor during running

Answer

Protect against voltage spikes from the supply

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

125% of FLC

Answer

150% of FLC

Answer

400% of FLC

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

Interlock contact

Answer

Normally closed auxiliary contact

Answer

Anti-plugging contact

Question 5

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

Accepted Answer

115°C

Answer

80°C

Answer

105°C

Answer

140°C

Question 6

What is the purpose of a jogging (inching) circuit in a motor control system?

Accepted Answer

Allow repeated brief starts without engaging the seal-in contact

Answer

Reduce starting current by inserting resistance momentarily

Answer

Reverse motor direction automatically

Answer

Monitor motor temperature and trip on overtemperature

Question 7

Which enclosure type is required for a motor installed in a Class I, Division 1 hazardous location?

Accepted Answer

NEMA 7 (explosion-proof)

Answer

NEMA 4 (watertight)

Answer

NEMA 3R (raintight)

Answer

NEMA 12 (industrial)