AC Theory

  1. Explain what is meant by the syncronous speed of an electric motor?
    It is the speed of the magnetic field produced in the stator windings
  2. What is meant by the slip speed of an induction motor?
    It is the difference between synchronous speed and the actual speed of the rotor
  3. What determines the speed of the magnetic field in the stator windings?
    At lower voltages, the slip speed is greater and the overall speed of the magnetic field is therefore lower. Likewise, if the voltage is higher, the slip will be lower and the speed of the magnetic field will be higher
  4. A 4pole, 50Hz motor runs at a 1440 rev/min. What is the percentage slip?
  5. Consider a wound rotor induction motor. When all the resistance is in the rotor circuit. Will the starting torque be high or low?
    Starting torque will be high
  6. One of the disadvantages of a double cage rotor induction motor is that its pull out torque is reduced. However, this can be an advantage in certain applications. State two of these applications
    -In the textile industry, a normal squirrel caged motor is used for starting a spinning frame. The peak torque obtained during acceleration can break the thread because of snatch. A double cage motor usually overcomes its difficulty

    -It is also an advantage as a drive for conveyor belts that may be stopped, fully loaded. The motor supplies the necessary high starting torque to get the belt underway again
  7. Name the 4 types of losses that occur in motors
    • -Heat losses caused by resistance of the windings
    • -Magnetising the poles peices and eddy current losses in steel laminations
    • -Friction due to air movement (Windage)
    • -Bearing friction
  8. At which loading is the induction motor more efficiant, high loads and low slip or during low speeds with high slip?
    At high speeds and low slip
  9. At which loading is power factor high, during light loading or normal loading?
    Normal loadingĀ 
  10. Is there any effect on the heat buildup in an induction motor if it runs beyond full load?
    The current increases and therefore increases heat
  11. What effect will a drop in the supply voltage have on an induction motor?
    The torque produced by a motor is determined by the motor load. A small drop in supply causes a large drop in motor torque which increases motor current. This increase in current can lead to motor burnoutĀ 
  12. What effect will the loss of one phase supply have on a 3-phase induction motor?
    The motor will run as a single phase motor and will gradually burn out
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AC Theory