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Motors and Motor Controls Flashcards

7 cards from real IEC practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

Read the first 7 Motors and Motor Controls flashcards as text

  1. What is the synchronous speed of a 4-pole, 60 Hz induction motor?

    Answer: 1800 RPM

    Synchronous speed = 120 × frequency ÷ poles = 120 × 60 ÷ 4 = 1800 RPM.

  2. When a motor is rewound with a higher number of turns per coil using thinner wire, the motor will have:

    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.

  3. A motor's power factor is 0.75 lagging. To correct the power factor to near unity, you would install:

    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.

  4. Which type of DC motor provides constant speed regardless of load and is NOT suitable for starting under heavy loads?

    Answer: Shunt DC motor

    Shunt DC motors have field windings connected in parallel with the armature, providing relatively constant speed but low starting torque.

  5. What is the purpose of a motor's 'reduced voltage solid-state starter' (soft starter)?

    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.

  6. Under NEC 430.109, which of the following is an acceptable motor disconnecting means?

    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.

  7. Which condition describes 'single-phasing' in a three-phase motor?

    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.