ITMC Motor Controls 4

Question 1

What condition causes a bimetallic overload relay to trip?

Accepted Answer

Sustained overcurrent causing the bimetallic strip to bend and open the contact

Answer

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?

Accepted Answer

One phase of the supply voltage is missing (single-phasing)

Answer

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?

Accepted Answer

Allow the motor to cool, identify the overload cause, and correct it before restarting

Answer

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?

Accepted Answer

The auxiliary (holding) contact wired in parallel with the start button

Answer

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?

Accepted Answer

Loss of one or more supply phases

Answer

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.

(ITMC) Industrial Technology Maintenance Certification Practice Test

(ITMC) Industrial Technology Maintenance Certification Practice Test

ITMC Motor Controls 4