Motor Controls Flashcards

What is the purpose of a pilot device in a motor control circuit?

Answer: To signal or control the motor starter through the control circuit

Pilot devices (pushbuttons, selector switches, limit switches, float switches) operate in the control circuit to signal the motor starter to start, stop, or change operating conditions.

What is the typical control circuit voltage for a motor starter in a 600 V industrial installation in Canada?

Answer: 120 V

In Canadian industrial installations, 120 V is the standard control circuit voltage, typically derived from a control power transformer (CPT) stepping down from the 600 V supply.

What does NEMA Size 1 motor starter refer to?

Answer: A starter rated for motors up to 10 HP at 600 V

NEMA Size 1 motor starters are rated for motors up to 10 HP at 600 V (three-phase). NEMA sizes define the maximum horsepower and current ratings for standardized motor starters.

What safety feature does a maintained-contact stop button lack compared to a momentary-contact stop button in a motor circuit?

Answer: It does not provide a fail-safe stop in case of control circuit failure

A momentary-contact (normally closed) stop button provides fail-safe operation because if the control circuit loses power, the stop circuit opens and the motor stops. A maintained contact does not offer this protection.

What is the purpose of a timing relay in a motor control circuit?

Answer: To delay motor starting or stopping by a preset time interval

Timing relays introduce controlled time delays in motor control circuits, used for sequential starting of multiple motors, delayed stops, or coordinating motor operations with other processes.

What happens to the speed of an AC induction motor if the frequency of the supply is increased from 60 Hz to 90 Hz?

Answer: Speed increases by 50%

AC motor speed is directly proportional to frequency: N = 120f/P. Increasing frequency from 60 to 90 Hz (a 50% increase) results in a 50% increase in synchronous speed.