309A Motor Controls

Question 1

What is the maximum overcurrent protection permitted for a motor branch circuit with a full-load current of 40 A using an inverse-time circuit breaker?

Accepted Answer

100 A

Answer

CEC Rule 28-200 permits inverse-time circuit breakers to be sized up to 250% of the motor FLC for standard motors. 40 A × 2.50 = 100 A maximum.

Question 2

What is the purpose of a motor overload relay?

Accepted Answer

To protect the motor against sustained overcurrent conditions

Answer

Overload relays protect motors from sustained overcurrent conditions (overloads) that would cause overheating and insulation damage. They are sized based on the motor's nameplate FLA.

Question 3

In a three-phase motor starter, what is the function of the holding coil (contactor coil)?

Accepted Answer

To keep the contactor contacts closed after the start button is released

Answer

The holding coil (contactor coil) creates a magnetic field that holds the main contacts closed after the start pushbutton is released, maintaining the circuit through a seal-in auxiliary contact.

Question 4

What is the standard overload relay trip setting for a motor with a nameplate FLA of 28 A and a 1.15 service factor?

Accepted Answer

32.2 A

Answer

For motors with a service factor of 1.15 or greater, the overload relay is set at 125% of nameplate FLA: 28 × 1.25 = 35 A. However, the standard setting is typically 115%, giving 28 × 1.15 = 32.2 A per CEC requirements.

Question 5

What type of motor starter reduces the starting current to approximately one-third of the full-voltage starting current?

Accepted Answer

Wye-delta starter

Answer

A wye-delta starter connects the motor windings in wye during starting, reducing the voltage across each winding by 1/√3, which reduces starting current to approximately 33% (one-third) of DOL current.

Red Seal Electrician 309A Practice Test

Red Seal Electrician 309A Practice Test

309A Motor Controls