Licensed Electrical Contractor Motors and Motor Controls 3 — Questions and Answers
Question 1: A two-speed, single-winding (consequent pole) motor achieves different speeds by:
- Changing supply frequency
- Reconnecting stator windings to change the number of poles (Correct answer)
- Adjusting rotor resistance
- Varying the supply voltage
Correct answer: Reconnecting stator windings to change the number of poles
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?
- Limit inrush current during starting
- Dissipate regenerative energy when the motor decelerates (Correct answer)
- Improve power factor during running
- Protect against voltage spikes from the supply
Correct answer: Dissipate regenerative energy when the motor decelerates
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:
- 125% of FLC
- 150% of FLC
- 250% of FLC (Correct answer)
- 400% of FLC
Correct answer: 250% of FLC
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:
- Interlock contact
- Seal-in (memory) contact (Correct answer)
- Normally closed auxiliary contact
- Anti-plugging contact
Correct answer: Seal-in (memory) contact
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:
- 80°C
- 105°C
- 115°C (Correct answer)
- 140°C
Correct answer: 115°C
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.
Question 6: What is the purpose of a jogging (inching) circuit in a motor control system?
- Allow repeated brief starts without engaging the seal-in contact (Correct answer)
- Reduce starting current by inserting resistance momentarily
- Reverse motor direction automatically
- Monitor motor temperature and trip on overtemperature
Correct answer: Allow repeated brief starts without engaging the seal-in contact
A jog circuit energizes the contactor only while the jog button is held, bypassing the seal-in contact so the motor runs only briefly for positioning.
Question 7: Which enclosure type is required for a motor installed in a Class I, Division 1 hazardous location?
- NEMA 4 (watertight)
- NEMA 7 (explosion-proof) (Correct answer)
- NEMA 3R (raintight)
- NEMA 12 (industrial)
Correct answer: NEMA 7 (explosion-proof)
NEMA 7 (explosion-proof) enclosures are designed to contain any internal explosion and prevent ignition of surrounding flammable gases in Class I, Division 1 locations.
Licensed Electrical Contractor Motors and Motor Controls 3
A two-speed, single-winding (consequent pole) motor achieves different speeds by: