Single phasing :
When one of the supply lines get disconnected of the three phase supply connection then this situation is known as single phasing.
Single phasing secures mainly due to rupturing of a fuse or open circuit in one of the supply lines. Under this conditions, motor continuous to operate on a single phase supply
If the motor is loaded to its rated full load, it will draw excessive current on single phasing.
Single phasing may cause extreme magnetic unbalance, reduction in torque and overheating due to negative phase sequence currents.
This condition may cause damage to the motor therefore, protection against single phasing is necessary.
Single Phasing Preventer :
When one of the phases is blown off due to an open circuit or due to the operation of the fuse of that phase then three phase Induction motor still works with two phases.
The whole power is shared by the remaining two phases, Thus, the current in these two healthy phases increases by √3 times.
This causes :
Increase in heating in motor windings.
Unbalanced rotor currents produce negative phase sequence components. This component produces magnetic flux which rotates in opposite direction to the main flux. Thereby double frequency currents are induced in rotor causes rotor heating,
This heating is not detected by replica-type thermal relays used for the protection of stator winding.
Also, the phase overcurrent relays act slowly. Hence single phasing causes major damage to the rotor.
Single phasing preventers are generally used for small-capacity motors.
Fig. shows the connection scheme for a single phasing preventer.
Single phasing preventers are connected in secondary's of line CT's.
These mainly contain a negative sequence filter.
The output of the negative sequence filter is fed to a level detector. Further which sends tripping command to the starter or circuit breaker,
Thus it protects the motor from damage.