01-06-2012, 04:19 PM
Three-Phase Appliance Protector
3 phase applaiance protecter.pdf (Size: 250.64 KB / Downloads: 154)
Many of our costly appliances
require three-phase AC supply
for operation. Failure
of any of the phases makes the appliance
prone to erratic functioning and
may even lead to failure. Hence it is
of paramount importance to monitor
the availability of the three-phase
supply and switch off the appliance
in the event of failure of one or two
phases. The power to the appliance
should resume with the availability of
all phases of the supply with certain
time delay in order to avoid surges and
momentary fluctuations.
The complete circuit of a threephase
appliance protector is described
here. It requires three-phase supply,
three 12V relays and a timer IC NE555
along with 230V coil contactor having
four poles.
Relays RL1 and RL2 act as a
sensing devices for phases Y and
B, respectively. These relays are
connected such that each acts as an
enabling device for the subsequent
relay. Therefore the combination of
the relays forms a logical AND gate
connected serially.
The availability of phase R energises
relay RL1 and its normallyopened
(N/O) contacts close to
connect phase Y to the input of
transformer X2. The availability of
phase Y energises relay RL2 and its
N/O contacts close to connect phase
B to the input of transformer X3, thus
applying a triggering input to timer
IC NE555 (IC1).
Therefore the delay timer built
around NE555 triggers only when
all the phases (R, Y and B) are available.
It provides a delay of approximately
four seconds, which energises
relay RL3 and its N/O contact closes
to connect the line to the energising
coil of four-pole contactor relay RL4.
Contactor RL4 closes to ensure the
availability of the three-phase supply
to the appliance.