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Motor Controls Flashcards

6 cards from real 309A practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

Read the first 6 Motor Controls flashcards as text

  1. What is motor slip and how is it expressed?

    Answer: The difference between synchronous speed and actual rotor speed, expressed as a percentage

    Slip is the difference between the synchronous speed and actual rotor speed, expressed as a percentage: Slip% = ((Ns - Nr)/Ns) × 100. Typical induction motor slip is 2-5%.

  2. What is the purpose of a reduced-voltage autotransformer (RVAT) motor starter?

    Answer: To reduce starting current by applying reduced voltage through an autotransformer

    An RVAT starter uses an autotransformer to apply reduced voltage (typically 65%, 80%, or 100% taps) during starting, reducing inrush current proportionally while maintaining higher torque than resistor starters.

  3. What does the term 'plugging' mean in motor control?

    Answer: Reversing the motor connections while running to provide rapid braking

    Plugging is a braking method where the motor supply connections are reversed while the motor is still running, creating a counter-torque that rapidly decelerates the motor. It generates significant heat.

  4. What is the function of a current transformer (CT) in a motor control centre?

    Answer: To provide a proportional scaled-down current signal for metering and relay operation

    Current transformers produce a scaled-down secondary current proportional to the primary (motor) current, allowing standard 5 A meters and relays to monitor high-current motor circuits safely.

  5. What is the purpose of dynamic braking on a DC motor?

    Answer: To disconnect and reconnect armature connections to a resistor, converting kinetic energy to heat

    Dynamic braking disconnects the motor armature from the supply and connects it across a braking resistor. The motor acts as a generator, converting kinetic energy to heat in the resistor.

  6. According to the CEC, what is the maximum rating of a motor that can be started with a manual (toggle) starter?

    Answer: 1 HP

    CEC and industry standards generally limit manual motor starters (toggle-type) to motors of 1 HP or less, as larger motors require magnetic starters for adequate switching capacity.