Motor Controls Flashcards

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

Answer: Reverse any two of the three supply leads

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.

What is an interlock in a motor reversing starter circuit?

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

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.

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

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

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.

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

Answer: No. 10 AWG

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.

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

Answer: The locked-rotor kVA per horsepower

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.

What type of motor starter provides reduced voltage starting through a series of resistors?

Answer: Primary resistor starter

A primary resistor starter inserts resistors in series with the motor supply during starting to reduce voltage, then shorts them out once the motor reaches speed. The resistance limits inrush current.