309A Motor Controls 2

Question 1

What must be done to reverse the direction of a three-phase motor?

Accepted Answer

Reverse any two of the three supply leads

Answer

Reversing any two of the three supply leads to a three-phase motor reverses the phase sequence, which reverses the rotating magnetic field and thus the motor's direction of rotation.

Question 2

What is an interlock in a motor reversing starter circuit?

Accepted Answer

A device that prevents both forward and reverse contactors from being energized simultaneously

Answer

Interlocks (mechanical and/or electrical) prevent simultaneous energization of forward and reverse contactors, which would cause a phase-to-phase short circuit and catastrophic damage.

Question 3

What is the function of a variable frequency drive (VFD) in a motor control system?

Accepted Answer

To control motor speed by varying the frequency and voltage of the power supply

Answer

A VFD controls motor speed by converting AC power to DC and then back to AC at a variable frequency and voltage. Motor speed is directly proportional to the frequency of the supply.

Question 4

According to the CEC, what is the minimum conductor size for a motor branch circuit supplying a motor with an FLC of 28 A?

Accepted Answer

No. 10 AWG

Answer

Motor branch circuit conductors must be sized at 125% of motor FLC: 28 × 1.25 = 35 A. No. 10 AWG copper rated at 30 A would be insufficient, so No. 10 AWG with 90°C insulation (40 A) is the minimum.

Question 5

What does a motor's nameplate 'code letter' indicate?

Accepted Answer

The locked-rotor kVA per horsepower

Answer

The NEMA code letter on a motor nameplate indicates the locked-rotor kVA per horsepower, which determines the inrush current when the motor starts. Higher code letters indicate higher inrush.

Red Seal Electrician 309A Practice Test

Red Seal Electrician 309A Practice Test

309A Motor Controls 2