309A Motor Controls 3 — Questions and Answers
Question 1: What is the purpose of a pilot device in a motor control circuit?
- To directly carry motor current
- To signal or control the motor starter through the control circuit (Correct answer)
- To provide overcurrent protection
- To regulate motor voltage
Correct answer: To signal or control the motor starter through the control circuit
Pilot devices (pushbuttons, selector switches, limit switches, float switches) operate in the control circuit to signal the motor starter to start, stop, or change operating conditions.
Question 2: What is the typical control circuit voltage for a motor starter in a 600 V industrial installation in Canada?
- 24 V
- 120 V (Correct answer)
- 240 V
- 600 V
Correct answer: 120 V
In Canadian industrial installations, 120 V is the standard control circuit voltage, typically derived from a control power transformer (CPT) stepping down from the 600 V supply.
Question 3: What does NEMA Size 1 motor starter refer to?
- A starter rated for motors up to 5 HP at 600 V
- A starter rated for motors up to 10 HP at 600 V (Correct answer)
- A starter rated for motors up to 25 HP at 600 V
- A starter rated for motors up to 50 HP at 600 V
Correct answer: A starter rated for motors up to 10 HP at 600 V
NEMA Size 1 motor starters are rated for motors up to 10 HP at 600 V (three-phase). NEMA sizes define the maximum horsepower and current ratings for standardized motor starters.
Question 4: What safety feature does a maintained-contact stop button lack compared to a momentary-contact stop button in a motor circuit?
- It cannot interrupt the circuit
- It does not provide a fail-safe stop in case of control circuit failure (Correct answer)
- It costs more to install
- It requires a larger enclosure
Correct answer: It does not provide a fail-safe stop in case of control circuit failure
A momentary-contact (normally closed) stop button provides fail-safe operation because if the control circuit loses power, the stop circuit opens and the motor stops. A maintained contact does not offer this protection.
Question 5: What is the purpose of a timing relay in a motor control circuit?
- To delay motor starting or stopping by a preset time interval (Correct answer)
- To increase motor voltage during starting
- To monitor motor temperature
- To provide overcurrent protection
Correct answer: To delay motor starting or stopping by a preset time interval
Timing relays introduce controlled time delays in motor control circuits, used for sequential starting of multiple motors, delayed stops, or coordinating motor operations with other processes.
Question 6: What happens to the speed of an AC induction motor if the frequency of the supply is increased from 60 Hz to 90 Hz?
- Speed decreases by 50%
- Speed increases by 50% (Correct answer)
- Speed remains the same
- Speed doubles
Correct answer: Speed increases by 50%
AC motor speed is directly proportional to frequency: N = 120f/P. Increasing frequency from 60 to 90 Hz (a 50% increase) results in a 50% increase in synchronous speed.
309A Motor Controls 3
What is the purpose of a pilot device in a motor control circuit?