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Single Phasing in AC Motors

Single Phasing in AC Motors

The loss of current through one phase of a 3 phase supply is described as single phasing.
During single phasing no current passes through one line while other two lines carry excess current depending on the load.
As shown in figure other two lines carry 2.4 times normal line current for the load.
Also phase current distribution also will be altered.
Coil 'B' and 'C' carries 1.3 times normal phase current while coil 'A' carries 2.9  times normal phase current. Reason is clear from the diagram itself ( Since current from one phase finds coil 'A' as low resistance path )


Causes of Single Phasing

  1. Line fuse blown off
  2. Terminal contacts broken due to vibration or ageing
  3. Terminal contacts become loose
  4. Line wire broken
  5. In contactor relay, one of the contactor is not making proper contact
  6. Relay contact spring may be damage or weak
  7. Contact joints covered with oxidation product, hence not conducting

Effects of Single Phasing
  1. Uneven distribution of current in the motor windings which may damage them
  2. Motor will keep on running with noise and vibration but once stopped, it will not start
  3. Due to uneven distribution of phase current, there will be fluctuation in torque produced
  4. Motor will vibrate vigorously
  5. Fluctuation in motor rpm
  6. Cause humming noise
  7. Motor winding may burn out
  8. Possibility of electrical fire
  9. Winding insulation may melt, causing short circuiting or earthing, results in electric shock and casualty
  10. Generator may overload and associated problems.

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