Three Phase Theory

Introduction Three Phase Electricity, which was considered to be a matter of pride for its possessor, merely fifty years ago, has become the most necessary and vital ingredient of twenty-first century human life .our dependency on electricity has increase to such an extent that we start our day by switching on an electrical appliance and end it by switching off the electric bulb. Innumerable electrical appliances such as a bulb, a tube light, fan, air-conditioner, mixer, washing machine, geyser, tele-vision and the list extends up to infinity. The household appliances don’t need much power (electricity) to operate, and hence run well on singlephase electrical supply. But in day-today life we come across many situations in which large amount of power (electricity) is needed to perform the specified task such as in a flour mill, borewell pumps, factory machines etc. Here comes into operation another mode of power supply known as three phase power supply. As you are aware, to transmit power with single-phase alternating current, we need two wires (live wire and neutral). However you would have seen that distribution lines usually have only 4 wires. This is because distribution is done using three phases and the 4th wire is the neutral. How does this help? Since the three phases are usually 120’ out of phase, their phasor addition will be zero if the supply is balanced.

Figure (a) Three phase waveforms

Figure (b) Phasor Diagram

It is seen from figure (a) that in the balanced system shown, the three phases, usually designated R, Y, B corresponding to Red, Yellow and Blue, are equal in magnitude and differ in phase angle by 1200. The corresponding phasor diagram is shown in figure (b). The voltage between any of the phases and the neutral is called the phase-to-neutral voltage or phase voltage Vp. It is usual to call the voltage between any two lines as the line-to-line voltage or line voltage VL. If the R-phase voltage is VR = Vp.0, then the remaining phase voltages would be VY =Vp.-2p/3 and VB = Vp.-4p/3.

About Diode

A diode is an electrical device allowing current to move through it in one direction with far greater ease than in the other. The most common type of diode in modern circuit design is the semiconductor diode, although other diode technologies exist. Semiconductor diodes are symbolized in schematic diagrams as such:

The half wave Rectifier A rectifier is used to convert A.C. to D.C. .The simplest of all is the Half-Wave Rectifier. A diode is conductive in forward–bias and it resembles open circuit in reverse–bias this property of diode is used to convert A.C. into D.C. As we can see in diagram

+ Single Phase Prevention For Three Phase appliances

230 V

Three Phase Power Supply

* Bulbs are shown as output i.e. on or cut-off of the appliance

Working In our circuit we have used three transformers each connected individual phase (denoted by R, Y, B,) and half-wave rectified. And these outputs (each 3 volts) are connected in series (as an array of dry cells makes a battery) (total

9 volts). This 9-volt is now connected to a relay (an electro-mechanical device). When all phase gives power, the relay shifts from normally-closed (N/C to normally-opened (N/O) mode and our three phase appliance is switched on. But if any phase fails suppose R-phase is fail then we get no output to T1 and hence series current, which drive relay, also brakes and relay shift to N/C mode this separate all other phases to appliance. Here we have shown red and green bulbs to show output of this circuit i.e. when green bulb glow it means all the phases are ok, and if red bulb glows means all or any phase is fail and appliance will be separated. In practice three relay connected in parallel will be required. Although such praetors are available in market but they are very costly (2000-3000/-) but our circuit is very cheep abut 300/- and very effective.