AC Theory cont...

  1. What is the difference between a multi-speed and variable speed single phase electric motor?
    A multispeed can operate at more than one predetermined set speed eg/half speed or full speed, whereas a varible speed motor is one that can provide almost infinite stepless possibilities up to full speed
  2. Name two typical multispeed single phase electric motors
    • -Tapped field universal motor
    • -Two speed split phase motor
  3. On what principal is speed change acheived in a two speed split phase motor?

    How is it acheived?
    Speed change is acheived by having two run windings

    A centrifugal switch is set up with double contact to switch between the full speed winding and the other speed winding
  4. What is the function of the fan that is mounted on the rotor shaft?
    It acts as a cooling device in a motor
  5. What is housed inside the cast iron yoke of a 3-phase induction motor?
    The laminsted iron core
  6. Explain why the rotor in a squirrel cage induction motor is called that?
    Because it looks like an excercise wheel used for squirrels
  7. What material is the squirrel cage rotor manufactured from?
    Copper or cast alluminium
  8. For what type of application is an induction motor with a double cage rotor used?
    Lower starting current but increased starting torque applications
  9. State the type of rotor in which the resistance can be varied to reduce starting current and increase starting torque
    Double cage rotor
  10. With regard to the rotating magnetic field in a three phase motor, what does the natural 120' phase shift between each phase of generated e.m.f give between poles?
    It gives the necessary current phase shift between poles to establish a rotation in the stator of the 3-phase induction motor
  11. On what does the direction of rotation of a three phase induction motor depend?
    It depends on the phase sequence of the supply
  12. Explain what is meant by the phase sequence of the supply?
    The order in which the line voltages attain their immediately succeeding maximum values
  13. How can the direction of a 3-phase motor be reversed?
    By simply interchanging any two of the line conductors supplying the motor
  14. Explain how torque is produced in the 3-phase induction motor?
    The induction motor is made up of several closed circuits causing currnets to run  in them, these currents set up a magnetic field that interacts with the rotating stator magnetic field to produce torque. The magnitude of this torque depends on the relative motion between the rotating field and the rotor
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AC Theory cont...