IEC Motors and Motor Controls 3 — Questions and Answers
Question 1: In a star-delta (wye-delta) motor starting method, the motor initially starts in wye and then switches to delta. What is the starting current relative to across-the-line starting?
- Reduced to 1/2 of full-voltage starting current
- Reduced to 1/3 of full-voltage starting current (Correct answer)
- Reduced to 2/3 of full-voltage starting current
- Equal to full-voltage starting current
Correct answer: Reduced to 1/3 of full-voltage starting current
Wye-delta starting reduces the starting current to 1/3 of full-voltage (across-the-line) starting current.
Question 2: A two-speed motor uses a consequent pole winding. Switching from low speed to high speed changes the:
- Supply voltage
- Number of poles (Correct answer)
- Number of phases
- Motor frame size
Correct answer: Number of poles
Consequent pole motors change speed by reconnecting the winding to create a different number of magnetic poles, since motor speed is inversely proportional to the number of poles.
Question 3: What is the function of a control transformer in a motor control circuit?
- To boost voltage to the motor during starting
- To step down line voltage to a safer control voltage (typically 120V) for the control circuit (Correct answer)
- To provide power factor correction
- To regulate motor speed
Correct answer: To step down line voltage to a safer control voltage (typically 120V) for the control circuit
Control transformers step down the higher line voltage to a safer, standardized control voltage (typically 120V) used by the pilot devices and coils.
Question 4: Which motor protection device opens the circuit when the motor windings reach an excessive temperature?
- Current relay
- Thermal protector (thermistor or thermostat embedded in windings) (Correct answer)
- Ground fault relay
- Voltage relay
Correct answer: Thermal protector (thermistor or thermostat embedded in windings)
Thermal protectors or PTC thermistors embedded in motor windings directly sense winding temperature and open the circuit when unsafe temperatures are reached.
Question 5: What does NEMA design letter 'B' indicate about a motor's torque and starting current characteristics?
- High starting torque, high starting current
- Normal starting torque, normal starting current, suitable for most applications (Correct answer)
- High starting torque, low starting current
- Low starting torque, very high slip
Correct answer: Normal starting torque, normal starting current, suitable for most applications
NEMA Design B motors have normal starting torque, normal starting current, and low slip, making them the most commonly used general-purpose motor design.
Question 6: A motor's locked rotor current (LRC) is typically how many times its full-load current (FLC)?
- 1 to 2 times FLC
- 2 to 3 times FLC
- 6 to 10 times FLC (Correct answer)
- 15 to 20 times FLC
Correct answer: 6 to 10 times FLC
Typical induction motors draw 6 to 10 times their full-load current during starting (locked rotor condition).
Question 7: A variable frequency drive (VFD) controls motor speed primarily by varying the:
- Supply voltage only
- Supply frequency and voltage supplied to the motor (Correct answer)
- Number of motor poles
- Rotor resistance
Correct answer: Supply frequency and voltage supplied to the motor
VFDs control motor speed by simultaneously varying the output frequency and voltage (maintaining a constant V/Hz ratio) supplied to the motor.
IEC Motors and Motor Controls 3
In a star-delta (wye-delta) motor starting method, the motor initially starts in wye and then switches to delta.
What is the starting current relative to across-the-line starting?