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
- 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 windings 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.