Three Phase application protector

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.

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 separateindicator 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

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