Motor Controls Flashcards

What is the purpose of a disconnect switch on a motor circuit?

Answer: To provide a visible means of isolation for maintenance safety

A motor disconnect switch provides a visible means of disconnecting the motor from the power supply for safe maintenance, as required by CEC Rule 28-600. It must be within sight of the motor.

What condition does a phase-loss relay protect against?

Answer: Loss of one phase in a three-phase supply

A phase-loss relay monitors all three phases and trips the motor off-line if one phase is lost (single-phasing). Running on two phases causes excessive current draw and rapid overheating.

In a two-wire motor control circuit, what provides the motor with low-voltage protection (LVP)?

Answer: Nothing — two-wire control provides low-voltage release (LVR) only

Two-wire control (using a maintained contact device like a float switch) provides only low-voltage release (LVR). The motor restarts automatically when power returns. Three-wire control provides true LVP.

What is the standard marking for the terminal connections of a dual-voltage, nine-lead three-phase motor?

Answer: T1 through T9

NEMA standards designate motor lead terminals as T1 through T9 for a nine-lead dual-voltage motor. The connection pattern (series or parallel) determines whether the motor runs at high or low voltage.

What type of motor protection device uses a bimetallic element that bends when heated?

Answer: Thermal overload relay

Thermal overload relays use a bimetallic strip that bends when heated by motor current. When the current exceeds the set point for a sufficient time, the bending trips the relay contacts.

According to the CEC, where must a motor disconnect be located relative to the motor it serves?

Answer: Within sight of the motor and within 9 m

CEC Rule 28-600 requires the motor disconnect to be within sight and not more than 9 m from the motor, ensuring the person working on the motor can verify the disconnect is open.