ITMC Motor Controls 2 — Questions and Answers
Question 1: What is the purpose of an overload relay in a motor control circuit?
- To protect against short circuits
- To protect the motor from excessive current due to mechanical overload (Correct answer)
- To limit starting voltage
- To regulate motor speed
Correct answer: To protect the motor from excessive current due to mechanical overload
Overload relays monitor running current and trip the circuit if the motor draws excessive current from a mechanical overload, preventing winding damage.
Question 2: In a two-wire control circuit, what happens when the control switch is opened?
- The motor continues running due to a holding contact
- The motor stops and will not restart automatically when the switch closes
- The motor stops and restarts automatically when the switch closes (Correct answer)
- The motor reverses direction
Correct answer: The motor stops and restarts automatically when the switch closes
Two-wire control uses a maintained contact device, so the motor restarts automatically when the control switch recloses without requiring a separate start button.
Question 3: Which component in a NEMA full-voltage starter provides low-voltage protection?
- Thermal overload relay
- Main contactor coil
- Holding contact (auxiliary contact) (Correct answer)
- Fuse block
Correct answer: Holding contact (auxiliary contact)
The holding contact (auxiliary contact) seals in the coil circuit after the start button is released; loss of voltage drops the coil and opens the holding contact, preventing automatic restart.
Question 4: A motor control technician measures 0 V across the coil of an energized contactor. What is the most likely cause?
- The coil is open-circuited (Correct answer)
- The coil is shorted
- The contactor power contacts are welded
- The overload heater is blown
Correct answer: The coil is open-circuited
Zero voltage across an energized coil indicates the coil is open-circuited, preventing current flow and coil energization.
Question 5: What does a normally closed (NC) contact do when the relay coil is energized?
- Closes to complete the circuit
- Opens to break the circuit (Correct answer)
- Remains in the same state
- Alternates between open and closed
Correct answer: Opens to break the circuit
A normally closed contact is closed in the de-energized (normal) state and opens when the coil is energized, breaking the circuit.
Question 6: Which type of motor starter is most appropriate for a high-inertia load where reduced starting current is required?
- Full-voltage (across-the-line) starter
- Auto-transformer starter (Correct answer)
- Manual starter
- Drum switch
Correct answer: Auto-transformer starter
An auto-transformer starter reduces inrush current by applying a reduced voltage during starting, making it suitable for high-inertia loads that need controlled acceleration.
Question 7: What is the function of a time-delay relay (TDR) in a motor control circuit?
- To limit motor speed
- To introduce a preset time interval before a contact action occurs (Correct answer)
- To protect against phase imbalance
- To convert AC to DC for the control circuit
Correct answer: To introduce a preset time interval before a contact action occurs
A time-delay relay introduces a specified time interval between coil energization (or de-energization) and contact action, enabling sequenced control operations.
ITMC Motor Controls 2
What is the purpose of an overload relay in a motor control circuit?