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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. 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?

    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.

  2. A two-speed motor uses a consequent pole winding. Switching from low speed to high speed changes the:

    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.

  3. What is the function of a control transformer in a motor control circuit?

    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.

  4. Which motor protection device opens the circuit when the motor windings reach an excessive temperature?

    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.

  5. What does NEMA design letter 'B' indicate about a motor's torque and starting current characteristics?

    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.

  6. A motor's locked rotor current (LRC) is typically how many times its full-load current (FLC)?

    Answer: 6 to 10 times FLC

    Typical induction motors draw 6 to 10 times their full-load current during starting (locked rotor condition).

  7. A variable frequency drive (VFD) controls motor speed primarily by varying the:

    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.