When
one of the 3 line supplying current to a 3 phase induction motor becomes open
circuited single phasing occurs. If motor is already running it will continue
to run. The excess
current
will passes through remaining 2 wires and over heating will occur. If motor is
in stopped position it will not start due to open circuit fault in one line.
In delta connected
motor if supply line L1 is open circuited then phase C
will carry more than 3 times normal current
under full load conditions. While phase A & B are in series carry more than
full load current.
In star connected motor if
supply line L1 is open circuited then current flowing at full load in lines L2
& L3 will be in the order of 250% of normal
full load current.
The protection given against single phasing is a
combined overload and single phasing relay.
This include 3 overload relay with trip contact so arranged that it will trip
if displacement of one overload element differs from other.
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