Motor Controls Flashcards
7 cards from real ITMC practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Motor Controls flashcards as text
What is the purpose of an overload relay in a motor control circuit?
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.
In a two-wire control circuit, what happens when the control switch is opened?
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.
Which component in a NEMA full-voltage starter provides low-voltage protection?
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.
A motor control technician measures 0 V across the coil of an energized contactor. What is the most likely cause?
Answer: The coil is open-circuited
Zero voltage across an energized coil indicates the coil is open-circuited, preventing current flow and coil energization.
What does a normally closed (NC) contact do when the relay coil is energized?
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.
Which type of motor starter is most appropriate for a high-inertia load where reduced starting current is required?
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.
What is the function of a time-delay relay (TDR) in a motor control circuit?
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.