309A Motor Controls 4 — Questions and Answers
Question 1: What is the purpose of a disconnect switch on a motor circuit?
- To provide overcurrent protection
- To provide a visible means of isolation for maintenance safety (Correct answer)
- To reduce starting current
- To reverse motor direction
Correct answer: To provide a visible means of isolation for maintenance safety
A motor disconnect switch provides a visible means of disconnecting the motor from the power supply for safe maintenance, as required by CEC Rule 28-600. It must be within sight of the motor.
Question 2: What condition does a phase-loss relay protect against?
- Overcurrent on all three phases
- Loss of one phase in a three-phase supply (Correct answer)
- Overvoltage conditions
- Harmonic distortion
Correct answer: Loss of one phase in a three-phase supply
A phase-loss relay monitors all three phases and trips the motor off-line if one phase is lost (single-phasing). Running on two phases causes excessive current draw and rapid overheating.
Question 3: In a two-wire motor control circuit, what provides the motor with low-voltage protection (LVP)?
- The overload relay
- Nothing — two-wire control provides low-voltage release (LVR) only (Correct answer)
- The circuit breaker
- The disconnect switch
Correct answer: Nothing — two-wire control provides low-voltage release (LVR) only
Two-wire control (using a maintained contact device like a float switch) provides only low-voltage release (LVR). The motor restarts automatically when power returns. Three-wire control provides true LVP.
Question 4: What is the standard marking for the terminal connections of a dual-voltage, nine-lead three-phase motor?
- L1, L2, L3 through L9
- T1 through T9 (Correct answer)
- M1 through M9
- A1 through A9
Correct answer: T1 through T9
NEMA standards designate motor lead terminals as T1 through T9 for a nine-lead dual-voltage motor. The connection pattern (series or parallel) determines whether the motor runs at high or low voltage.
Question 5: What type of motor protection device uses a bimetallic element that bends when heated?
- Magnetic overload relay
- Thermal overload relay (Correct answer)
- Electronic overload relay
- Fuse
Correct answer: Thermal overload relay
Thermal overload relays use a bimetallic strip that bends when heated by motor current. When the current exceeds the set point for a sufficient time, the bending trips the relay contacts.
Question 6: According to the CEC, where must a motor disconnect be located relative to the motor it serves?
- In the motor control centre only
- Within sight of the motor and within 9 m (Correct answer)
- In the electrical room only
- Anywhere in the same building
Correct answer: Within sight of the motor and within 9 m
CEC Rule 28-600 requires the motor disconnect to be within sight and not more than 9 m from the motor, ensuring the person working on the motor can verify the disconnect is open.
309A Motor Controls 4
What is the purpose of a disconnect switch on a motor circuit?