The loss of current through one phase of a 3 phase supply is described as single phasing in three phase induction motors.
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
Line fuse blown off
Terminal contacts broken due to vibration or ageing
Terminal contacts become loose
Line wire broken
In contactor relay, one of the contactor is not making proper contact
Relay contact spring may be damage or weak
Contact joints covered with oxidation product, hence not conducting
Effects of Single Phasing
Uneven distribution of current in the motor winding which may damage them
Motor will keep on running with noise and vibration but once stopped, it will not start
Due to uneven distribution of phase current, there will be fluctuation in torque produced
Motor will vibrate vigorously
Fluctuation in motor rpm
Cause humming noise
Motor winding may burn out
Possibility of electrical fire
Winding insulation may melt, causing short circuiting or earthing, results in electric shock and casualty
Generator may overload and associated problems.
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