electric motors

  1. List two reasons why a shaded pole motor is used to power the fan in a portable heater?
    • any two of:
    • it can be stalled
    • its quiet operation
    • low torque
    • cost effective
  2. Why is a three phase induction motor more efficient than the single phase equvilant size induction motor?
    • Three phase induction motors have a balanced rotating magnetic field
    • single phase have a pulsating one
  3. Give an application where a three phase motor would require phase reversal protection?
    • Where it may injure people or cause damage to property and equipment
    • eg, lathe or conveyor
  4. State two reaons for using reduced voltage starting for a three phase induction motor?
    • Limit mechanical shock
    • Reduce high starting currents
  5. What is the purpose of the maintaining contacts in a direct online reversing starter?
    • To maintain supply to the contactor coil allowing user to release start button
    • To prevent auto restarting when supply is interupted
  6. What is the cheapest type of reduced voltage starter for three phase motors, and what special requirement, if any, does this method of starting impose on the internal connections of the motor winding?
    • Star-Delta
    • Needs a six terminal connection
  7. State One important reason for inserting external resistance into the rotor circuit of a slip ring motor at start up
    • Any one of:
    • to reduce starting current
    • To produce a higher starting torque than that availiable from a squirrel cage motor started DOL
    • To provide a more gentle startup for high inertia loads
    • To provide a ramped acceleration for high inertia loads
    • To provide a higher start torque at low speeds starting through improved power factor
  8. What is the main advantage of a single phase synchronous motor over other single phase motors?
    It rotates at a fixed speed independant of load
  9. Sketch a Torque Speed graph for a Three phase sqirrel cage induction motor
    on graph show:
    Starting Torque
    Pull out torque
    Full load torque
    Synchronous speed
    Full load speed
    Image Upload 1
  10. A primary resistance starter reduces the motor terminal voltage during starting by inserting resistance into each supply line.
    The motor terminal voltage ramps smoothly up during accelaration as a function of the decreasing line current and voltage drop across the line resistances.
    Explain, in principle, how an electronic soft starter achieves the same thing?
Card Set
electric motors
electric motor theory