Single Phasing Preventer

Single Phasing Preventer:

✪The concept of single phasing is already explained in section 6.2.

✪When one of the phase is blown off due to open circuit or due to operation of fuse of that phase then three phase Induction motor still works with two phases.

✪The whole power is shared by remaining two phases, Thus, the current in these two

healthy phases increases b y √3 times.

Causes of Single Phasing

✪increase in heating in motor windings.

✪unbalanced rotor currents produces negative phase sequence components. This component produces magnetic flux which rotates in opposite direction to main flux. There by double frequency currents are

induced in rotor causes rotor heating.

✪This heating is not detected by replica type thermal relays used for protection of stater winding.

✪Also the phase over current relays act slowly. Hence single phasing causes major damage to rotor.

✪Single phasing preventers are generally used for small capacity motors.

✪Fig.shows the connection scheme for single phasing preventer.

✪Single phasing preventers are connected in secondaries of line CT.

✪These mainly contains a negative sequence filter.

✪The output of 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.