01-03-2012, 04:35 PM
AUTOMATIC PHASE CHANGER
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In three-phase applications, if low
voltage is available in any one or
two phases, and you want your
equipment to work on normal voltage,
this circuit will solve your problem.
However, a proper-rating fuse needs
to be used in the input lines (R, Y and
B) of each phase. The circuit provides
correct voltage in the same power supply
lines through relays from the other
phase where correct voltage is available.
Using it you can operate all your
equipment even when correct voltage
is available on a single phase
The mains power supply phase R
is stepped down by transformer X1 to
deliver 12V, 300 mA, which is rectified
by diode D1 and filtered by capacitor
C1 to produce the operating
voltage for the operational amplifier
(IC1). The voltage at inverting pin 2 of
oprational amplifier IC1 is taken from
the voltage divider circuit of resistor
R1 and preset resistor VR1. VR1 is used
to set the reference voltage according
to the requirement. The reference voltage
at non-inverting pin 3 is fixed to
5.1V through zener diode ZD1.
As soon as phase-R voltage goes
below 200V, the voltage at inverting
pin 2 of IC1 goes below reference voltage
of 5.1V, and its output goes low.
As a result, transistor T1 conducts and
relay RL1 energises and load L1 is
disconnected from phase ‘R’ and
connected to phase ‘Y’ through relay
RL2.
