ITMC Motor Controls 4 — Questions and Answers
Question 1: What condition causes a bimetallic overload relay to trip?
- High ambient temperature only
- Sustained overcurrent causing the bimetallic strip to bend and open the contact (Correct answer)
- Voltage imbalance exceeding 10%
- Loss of one phase
Correct answer: Sustained overcurrent causing the bimetallic strip to bend and open the contact
Sustained overcurrent heats the bimetallic strip, causing it to deflect due to differential thermal expansion until it trips the overload contact.
Question 2: When troubleshooting a motor starter that hums loudly but the motor does not run, which is the MOST likely cause?
- Overload relay is tripped
- One phase of the supply voltage is missing (single-phasing) (Correct answer)
- The coil is open
- The control fuse is blown
Correct answer: One phase of the supply voltage is missing (single-phasing)
A loud hum from a contactor with the motor not running or running poorly typically indicates single-phasing, where one supply phase is lost causing the contactor to vibrate and the motor to be unable to develop full torque.
Question 3: What is the correct procedure after manually resetting a tripped thermal overload relay?
- Immediately restart the motor at full load
- Allow the motor to cool, identify the overload cause, and correct it before restarting (Correct answer)
- Replace the overload heater element
- Increase the overload relay trip setting
Correct answer: Allow the motor to cool, identify the overload cause, and correct it before restarting
After a thermal overload trip, the motor must cool, the root cause of the overload must be identified and corrected, then the relay reset before restarting to prevent repeat trips and damage.
Question 4: In a three-wire control circuit, which component maintains the contactor energized after the start button is released?
- The stop button NC contact
- The overload relay NC contact
- The auxiliary (holding) contact wired in parallel with the start button (Correct answer)
- The main power contacts
Correct answer: The auxiliary (holding) contact wired in parallel with the start button
The auxiliary (holding) contact, wired in parallel with the momentary start button, seals in the control circuit and keeps the contactor energized after the start button is released.
Question 5: A phase-failure relay in a motor control circuit is designed to protect against which condition?
- Motor overheating from continuous duty
- Loss of one or more supply phases (Correct answer)
- Reverse motor rotation
- Ground faults in the motor windings
Correct answer: Loss of one or more supply phases
A phase-failure relay monitors all three supply phases and trips the circuit if any phase is lost or severely unbalanced, protecting the motor from single-phase damage.
Question 6: What is the purpose of a control power transformer (CPT) in a motor control center (MCC)?
- To step up control voltage for long wire runs
- To step down line voltage to a safe control voltage (typically 120 V) for the control circuit (Correct answer)
- To provide DC power for the motor field
- To filter harmonics from the VFD output
Correct answer: To step down line voltage to a safe control voltage (typically 120 V) for the control circuit
A CPT steps down the high line voltage (e.g., 480 V) to a safer control circuit voltage (typically 120 V AC) to power contactors, relays, and pushbutton stations.
Question 7: When a solid-state overload relay is compared to a bimetallic type, which advantage does the solid-state type provide?
- Lower initial cost
- Immunity to high ambient temperature effects on trip accuracy (Correct answer)
- Simpler reset procedure
- Wider current range without changing heater elements
Correct answer: Immunity to high ambient temperature effects on trip accuracy
Solid-state overload relays use electronic sensing and are not affected by ambient temperature variations, unlike bimetallic types whose trip points shift with ambient temperature.
ITMC Motor Controls 4
What condition causes a bimetallic overload relay to trip?