309A Motor Controls 2 — Questions and Answers
Question 1: What must be done to reverse the direction of a three-phase motor?
- Reverse all three supply leads
- Reverse any two of the three supply leads (Correct answer)
- Change the motor's internal capacitor
- Reduce the supply voltage by half
Correct 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.
Question 2: What is an interlock in a motor reversing starter circuit?
- A mechanical lock on the enclosure door
- A device that prevents both forward and reverse contactors from being energized simultaneously (Correct answer)
- A thermal switch that monitors motor temperature
- A timing relay for sequential starting
Correct 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.
Question 3: What is the function of a variable frequency drive (VFD) in a motor control system?
- To provide overcurrent protection only
- To control motor speed by varying the frequency and voltage of the power supply (Correct answer)
- To convert single-phase power to three-phase power only
- To provide power factor correction only
Correct 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.
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?
- No. 14 AWG
- No. 12 AWG
- No. 10 AWG (Correct answer)
- No. 8 AWG
Correct 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.
Question 5: What does a motor's nameplate 'code letter' indicate?
- The motor's efficiency rating
- The locked-rotor kVA per horsepower (Correct answer)
- The motor's service factor
- The insulation class
Correct 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.
Question 6: What type of motor starter provides reduced voltage starting through a series of resistors?
- Across-the-line starter
- Primary resistor starter (Correct answer)
- Wye-delta starter
- Soft starter
Correct 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.
309A Motor Controls 2
What must be done to reverse the direction of a three-phase motor?