Motor Starters

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  1. What factors must a motor allow for?
    • -Ease of starting and stopping by operator
    • -Prevention of automatic restarting after power failure, if unexpected restarting might cause injury to the operator
    • -Automatic overload protection for motors exceeding 370W rating
    • -Phase failure or reversal of phase rotation protection, if this is liable to cause injury or damage
    • -Provision for the remote starting and stoppping of a motor, should this be required
  2. State two ways speed control can be obtained for motors that are single phase 
    • -Tapped field winding
    • -Continuously rated variable resistance
  3. State two ways speed control can be obtained for motors that are three phase
    • -Frequency
    • -Pairs of poles
  4. State the typical current levels when a motor is started DOL
    A DOL starter will draw around 600 per cent or 6 times its normal full load current
  5. State the relationship between voltage and torque
    Torque produced by an induction motor at a given speed is proportional to its supply voltage squared 
  6. In the protection of motors there are two types of overcurrent ralays used to protect a motor against overload

    State them:
    • -Magnetic relay
    • -Thermal relay
  7. 5 ways of reducing the high current starting a motor DOL
    • -Star Delta starter
    • -An autotransformer
    • -Primary resistance starter
    • -Secondary resistance starter
    • -Electronic starter
  8. With respect to other varying starters whta is the advantage of using an Autotransformer starter
    It has various tappings by which an appropriate starting voltage can be chosen to provide a starting torque and current to suit a specific application. They are also efficiant as no energy is wasted, as it is in other type starters
  9. Under what circumstances can two autotransformers be used for starting a 3 phase induction motor
    This method can be used when the required starting torque is greater than can be obtained by by star-delta starting
  10. Why does the primary resistance starter have a lower efficiancy than other starters?
    Because energy is dissapated as heat in the starting resistors
  11. When positioning a starter or controller as well as protection and accessibility state THREE other considerations that should be taken into account
    • -Protection
    • -Accessibility
    • -Protective devides
  12. Over-temperature protection for motors, in general, is deemed a requirement. However, there are situations where that type of protection is not required. Give an example where over-temperature protection can be omitted
    Unattended shaded pole motor 480VA
  13. Why is it important to consider thermal effects when positioning a starter or controler?
    Locating a controller outside iont he sun could cause the thermal overloads to become pre-heated and likely to trip whilst the motor is operating normally
Card Set:
Motor Starters
2012-12-16 02:32:14

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