309A Motor Controls 1 — Questions and Answers
Question 1: What is the maximum overcurrent protection permitted for a motor branch circuit with a full-load current of 40 A using an inverse-time circuit breaker?
- 40 A
- 50 A
- 80 A
- 100 A (Correct answer)
Correct answer: 100 A
CEC Rule 28-200 permits inverse-time circuit breakers to be sized up to 250% of the motor FLC for standard motors. 40 A × 2.50 = 100 A maximum.
Question 2: What is the purpose of a motor overload relay?
- To protect against short circuits
- To protect against ground faults
- To protect the motor against sustained overcurrent conditions (Correct answer)
- To protect against voltage surges
Correct answer: To protect the motor against sustained overcurrent conditions
Overload relays protect motors from sustained overcurrent conditions (overloads) that would cause overheating and insulation damage. They are sized based on the motor's nameplate FLA.
Question 3: In a three-phase motor starter, what is the function of the holding coil (contactor coil)?
- To provide overcurrent protection
- To keep the contactor contacts closed after the start button is released (Correct answer)
- To reduce starting current
- To reverse the motor direction
Correct answer: To keep the contactor contacts closed after the start button is released
The holding coil (contactor coil) creates a magnetic field that holds the main contacts closed after the start pushbutton is released, maintaining the circuit through a seal-in auxiliary contact.
Question 4: What is the standard overload relay trip setting for a motor with a nameplate FLA of 28 A and a 1.15 service factor?
- 28 A
- 32.2 A (Correct answer)
- 35 A
- 42 A
Correct answer: 32.2 A
For motors with a service factor of 1.15 or greater, the overload relay is set at 125% of nameplate FLA: 28 × 1.25 = 35 A. However, the standard setting is typically 115%, giving 28 × 1.15 = 32.2 A per CEC requirements.
Question 5: What type of motor starter reduces the starting current to approximately one-third of the full-voltage starting current?
- Full-voltage starter (across-the-line)
- Wye-delta starter (Correct answer)
- Autotransformer starter at 80% tap
- Part-winding starter
Correct answer: Wye-delta starter
A wye-delta starter connects the motor windings in wye during starting, reducing the voltage across each winding by 1/√3, which reduces starting current to approximately 33% (one-third) of DOL current.
Question 6: What is the purpose of a jog function on a motor controller?
- To run the motor at half speed continuously
- To momentarily energize the motor for precise positioning (Correct answer)
- To reverse the motor direction
- To provide dynamic braking
Correct answer: To momentarily energize the motor for precise positioning
The jog function allows the operator to momentarily energize the motor for short bursts without latching the contactor, enabling precise positioning of machinery or loads.
309A Motor Controls 1
What is the maximum overcurrent protection permitted for a motor branch circuit with a full-load current of 40 A using an inverse-time circuit breaker?