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Motors and Controls Flashcards

7 cards from real Ramsay Test 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 Controls flashcards as text

  1. What does a Variable Frequency Drive (VFD) do to control an AC motor's speed?

    Answer: It varies the frequency and voltage of the AC power supplied to the motor

    A VFD rectifies AC to DC, then inverts it back to variable-frequency AC; since synchronous speed is directly proportional to frequency, varying frequency varies motor speed.

  2. Which type of AC motor operates at exactly synchronous speed under any load?

    Answer: Synchronous motor

    Synchronous motors use DC-excited rotor poles that lock in step with the rotating stator field, running at precisely synchronous speed regardless of load (up to the pull-out torque limit).

  3. In a control circuit, what happens to a normally closed (NC) auxiliary contact when its associated contactor coil is energized?

    Answer: It opens, breaking the circuit path through that contact

    A normally closed contact is closed when the coil is de-energized; energizing the coil causes the armature to pull in, which mechanically opens the NC contact.

  4. What is 'plugging' as a method of stopping or reversing an electric motor?

    Answer: Applying reverse-phase voltage to a running motor to rapidly decelerate it to a stop

    Plugging reconnects the motor with reversed phase sequence while still rotating, creating a braking torque that rapidly stops the motor; a zero-speed switch must disconnect power before reversal occurs.

  5. Why does a single-phase induction motor require a starting capacitor or auxiliary winding?

    Answer: To create a phase shift that produces a rotating magnetic field necessary for self-starting

    A single-phase supply produces a pulsating (not rotating) magnetic field; the auxiliary winding and capacitor create a second phase, shifted approximately 90°, producing the rotating field needed to develop starting torque.

  6. What is dynamic braking of a DC motor?

    Answer: Disconnecting the armature from supply and connecting a resistor across it so the motor acts as a generator dissipating energy

    In dynamic braking, the armature is disconnected from the power supply and connected across a braking resistor; the still-spinning motor generates voltage and the resistor dissipates the kinetic energy as heat.

  7. A motor nameplate lists a NEMA enclosure designation of 'TEFC.' What does TEFC mean?

    Answer: Totally Enclosed Fan Cooled — the motor is sealed and cooled by an external fan on the shaft

    TEFC (Totally Enclosed Fan Cooled) motors have a sealed housing preventing air exchange with the environment; an external fan mounted on the shaft blows air over the motor frame for cooling.