CIRCUIT

IDEAS

E L E C T R O N I C S F O R Y O U • J U L Y 2 0 0 7 • 9 3W W W . E F Y M A G . C O M

The mains power supply phase Ris stepped down by transformer X1 todeliver 12V, 300 mA, which is recti-fied by diode D1 and filtered by ca-pacitor C1 to produce the operatingvoltage for the operational amplifier(IC1). The voltage at inverting pin 2 ofoprational amplifier IC1 is taken fromthe voltage divider circuit of resistorR1 and preset resistor VR1. VR1 is usedto set the reference voltage accordingto the requirement. The reference volt-age at non-inverting pin 3 is fixed to5.1V through zener diode ZD1.

Till the supply voltage available inphase R is in the range of 200V-230V,

the voltage at inverting pin 2 of IC1remains high, i.e., more than referencevoltage of 5.1V, and its output pin 6also remains high. As a result, transis-tor T1 does not conduct, relay RL1 re-mains de-energised and phase ‘R’ sup-plies power to load L1 via normally-closed (N/C) contact of relay RL1.

As soon as phase-R voltage goesbelow 200V, the voltage at invertingpin 2 of IC1 goes below reference volt-age of 5.1V, and its output goes low.As a result, transistor T1 conducts andrelay RL1 energises and load L1 isdisconnected from phase ‘R’ andconnected to phase ‘Y’ through relayRL2.

Similarly, the auto phase-change ofthe remaining two phases, viz, phase‘Y’ and phase ‘B,’ can be explained.Switch S1 is mains power ‘on’/’off’switch.

Use relay contacts of proper ratingand fuses should be able to take-onthe load when transferred from otherphases. While wiring, assembly and in-stallation of the circuit, make sure thatyou:

1. Use good-quality, multi-strandinsulated copper wire suitable for your

� MUHAMMAD AJMAL P.

AUTOMATIC PHASE CHANGERS.C.

DWIVEDI

In three-phase applications, if lowvoltage is available in any one ortwo phases, and you want your

equipment to work on normal voltage,this circuit will solve your problem.However, a proper-rating fuse needsto be used in the input lines (R, Y andB) of each phase. The circuit providescorrect voltage in the same power sup-ply lines through relays from the otherphase where correct voltage is avail-able. Using it you can operate all yourequipment even when correct voltageis available on a single phase in the

building.The circuit is built around a trans-

former, comparator, transistor and re-lay. Three identical sets of this circuit,one each for three phases, are used.Let us now consider the working ofthe circuit connecting red cable (call it‘R’ phase).

CIRCUIT

IDEAS

9 4 • J U L Y 2 0 0 7 • E L E C T R O N I C S F O R Y O U W W W . E F Y M A G . C O M

current requirement.2. Use good-quality relays with

proper contact and current rating.3. Mount the transformer(s) and re-

lays on a suitable cabinet. Use a TagBlock (TB) for incoming/outgoing con-

nections from mains.EFY Note: 1. During testing in the

lab, we used a 12V, 200-ohm, single-phase changeover relay with 6A cur-rent rating. Similarly, ampere-ratedfuses were used.

2. If the input voltage is low in twophases, loads L1 and L2 may also beconnected to the third phase. In thatsituation, a high-rating fuse will be re-quired at the input of the third phasewhich is taking the total load. �