IEC Motors and Motor Controls 5 — Questions and Answers
Question 1: What is the synchronous speed of a 4-pole, 60 Hz induction motor?
- 900 RPM
- 1200 RPM
- 1800 RPM (Correct answer)
- 3600 RPM
Correct answer: 1800 RPM
Synchronous speed = 120 × frequency ÷ poles = 120 × 60 ÷ 4 = 1800 RPM.
Question 2: When a motor is rewound with a higher number of turns per coil using thinner wire, the motor will have:
- Lower voltage rating and higher current draw
- Higher voltage rating and lower current draw (Correct answer)
- Same electrical characteristics with higher efficiency
- Same voltage but higher starting torque
Correct answer: Higher voltage rating and lower current draw
More turns increase the winding impedance, raising the voltage required and reducing the current drawn for equivalent power output.
Question 3: A motor's power factor is 0.75 lagging. To correct the power factor to near unity, you would install:
- Capacitors in parallel with the motor (Correct answer)
- Inductors in series with the motor
- A resistor bank across the motor terminals
- A larger overload relay
Correct answer: Capacitors in parallel with the motor
Capacitors supply reactive power locally, offsetting the lagging reactive power demand of inductive motors and improving the overall power factor.
Question 4: Which type of DC motor provides constant speed regardless of load and is NOT suitable for starting under heavy loads?
- Series DC motor
- Shunt DC motor (Correct answer)
- Compound DC motor
- Permanent magnet DC motor
Correct answer: Shunt DC motor
Shunt DC motors have field windings connected in parallel with the armature, providing relatively constant speed but low starting torque.
Question 5: What is the purpose of a motor's 'reduced voltage solid-state starter' (soft starter)?
- To vary the motor speed continuously during operation
- To gradually ramp up the voltage to the motor during starting to reduce inrush current and mechanical stress (Correct answer)
- To convert three-phase power to single-phase for the motor
- To provide power factor correction during running
Correct answer: To gradually ramp up the voltage to the motor during starting to reduce inrush current and mechanical stress
Soft starters use SCRs to gradually increase the voltage applied to the motor, reducing starting current and mechanical shock to the driven equipment.
Question 6: Under NEC 430.109, which of the following is an acceptable motor disconnecting means?
- A standard lighting switch rated for the motor's FLA
- A motor circuit switch rated in horsepower (Correct answer)
- A 15-amp receptacle for motors under 1 HP only
- Any switch with an ampere rating equal to the motor nameplate current
Correct answer: A motor circuit switch rated in horsepower
NEC 430.109 requires the disconnecting means for motors to be a motor-circuit switch rated in horsepower, a circuit breaker, or other approved device.
Question 7: Which condition describes 'single-phasing' in a three-phase motor?
- The motor is accidentally connected to single-phase power
- One of the three supply phases is lost while the motor is running (Correct answer)
- The motor is running at reduced voltage on all three phases
- The motor is operating on two separate single-phase circuits
Correct answer: One of the three supply phases is lost while the motor is running
Single-phasing occurs when one supply phase is lost (due to a blown fuse or open conductor) while the motor continues to run on two phases, causing severe overheating.
IEC Motors and Motor Controls 5
What is the synchronous speed of a 4-pole, 60 Hz induction motor?