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 three phase 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 energizes relay
RL1 and its normally opened (N/O) contacts close to connect phase Y to the
input of transformer X2.
The availability of phase Y energizes 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 energizes relay RL3 and its N/O contact, closes to connect the line to
the energizing coil of four-pole contactor relay RL4. Contactor RL4 closes to
ensure the availability of the three-phase supply to the appliance.
The rating of contactor RL4 can be selected according
to the full-load current rating of the appliances. Here the contact current
rating of the four-pole contactor is up to 32A. The availability of phases R, Y
and B is monitored by appropriate LEDs connected across the secondary windings
of transformers X1, X2 and X3, respectively. Hence this circuit does not
require a separate indicator lamp for monitoring the availability of the three
phases. When phase R is available, LED1 glows.
When phase Y is available, LED2 glows. When phase B is
available, LED3 glows. The main advantage of this protector circuit is that it
protects three-phase appliances from failure of any of the mounted on the
backside of cabinet. Connect the appliance through external wires.
Caution. To avoid the risk of electric shock, ensure that AC
mains is disconnected during assembly of the circuit and double check
everything before connecting your circuit to the mains. phases by disconnecting
the power supply through the contactor and automatically restores the three-phase supply to the
appliance (with reasonable time delay) when all the phases are available. Assemble the circuit on a general purpose PCB
and enclose in a cabinet with the relays and contactor.
