309A Motor Controls 4

Question 1

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

Accepted Answer

To provide a visible means of isolation for maintenance safety

Answer

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.

Question 2

What condition does a phase-loss relay protect against?

Accepted Answer

Loss of one phase in a three-phase supply

Answer

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.

Question 3

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

Accepted Answer

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

Answer

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.

Question 4

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

Accepted Answer

T1 through T9

Answer

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.

Question 5

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

Accepted Answer

Thermal overload relay

Answer

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.

Red Seal Electrician 309A Practice Test

Red Seal Electrician 309A Practice Test

309A Motor Controls 4