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If single phasing occurs when the motor is running, it should not be loaded beyond _______ percent of the rated load.
1. 5
2. 25
3. 50
4. 75

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Correct Answer - Option 3 : 50

Single phasing:

  • A three-phase motor must be connected to the rated load voltage and load for proper working. If due to some reason, one phase of the motor gets disconnected, the motor will continue to run from the active 2-phase supply. This is called single phasing.
  • The motor will continue to run with vibration and reduced speed. However, depending upon the loading condition, the motor may/may not start on two phases.
  • It will work satisfactorily if the motor carrying the load not more than 0.5 times rated load
  • Single phasing is not desirable for the proper operation of the induction motors and appropriate measures should be taken to protect the machine.


Causes of single phasing:

  • One of the three back up fuses blow (or fuse melts)
  • One of the conductors of the motor is open-circuited.
  • Wrong setting of the protection device provided on the motor.
  • Relay contacts may be damaged or broken.


Effect of single phasing:

  • Motor runs with reduced speed.
  • It operates with uneven torque and produces a humming noise.
  • Due to loss of current from one phase the current flowing through the remaining two phases increases. The winding insulation, at times, may not be designed to withstand the increased current and heat thereby damaging the insulation and causing short-circuited between the winding and the motor burns out.
  • It may cause an overloading of the Generator.
  • If the motor is arranged for stand by and automatic starting than the motor will not start and if the over relay provided fails to function the motor may burn.

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