Page 1 :
3.1, , , , Introduction to Polyphase Circuit :, , A polyphase supply as the name says has multiple, phases. This is mainly required when the power, requirement is beyond the power that can be supplied, , by the single phase supply., , The most widely used poly phase supply is the three, phase ($) supply. The 3 supply has a standard colour, , combination for the three phases and they are Red,, Yellow and Blue., , Scanned with CamScanner
Page 2 :
3.1.1 Three Phase Voltage and Current, Generation :, , Principle :, , — _ The single phase supply is generated using a single turn, , alternator., , ~ Thus if armature consists of only one winding, then only, , one alternating voltage is produced., , - But if the armature winding is divided into three groups, which are displaced by 120° from each other, then it is, possible to generate three alternating voltages., , Construction :, , - As shown in Fig. 3.1(a) the armature winding is divided, into three groups. The three coils are R — R’, Y - Y' and, B-B’., , RR‘, Y Y' and B B' are, the armature windings, , displaced by 120°, , , , (A-724) Fig. 3.1(a) : Generation of a 3-phase voltage, , All these coils are mounted on the same shaft and are, , physically placed at 120° from each other., , — When these coils rotate in the flux produced by the, permanent magnet, emf is induced into these coils., These emfs as shown in Fig. 3.1 are sinusoidal, of equal, amplitudes and equal frequency but they are displaced, , from each other by 120°., , , , Scanned with CamScanner
Page 3 :
Eq, Ey and E, are the three phase voltages, If, considered as the reference then Ey lags Ep by 129, Eg lags Ey by 120°, in other words Ey lags E, by 24qo, , , , , , , , , , , , , , , , , , , , , , , , , The three phase system in which the three p, voltages are of identical magnitudes and frequen,, are displaced by 120° from each other is called, symmetrical system., Balanced load : A balanced load is that in y, magnitudes of all impedances connected in the load, equal (Z each) and the phase angles of them agg, equal and of same type (inductive, resistive, , capacitive)., , , , , , Mathematical equations :, , — The three phase voltages are given as below:, , E, = &,sinot Ey = Eysin(@t-120), , mi, i, , a = Fe sin (@ t- 240), , Il) Waveform :, , ~The waveforms of the three phases is as shown in the, Fig. 3.1(b) with a phase lag of 120° between them., , , , , , , , , , , , , , , , , , Fig. 3.1(b), Il) Phasor :, , - Incase of 34, the phasor diagrams use the vertical, as the reference, instead of the horizontal line. H, the red phase is shown along the reference, while, , yellow phase lagging by 120° and blue phase lagging, , 240°. This phasor diagram is shown in the Fig- 32., , , , , Scanned with CamScanner
Page 4 :
Fig. 3.2, IV) Use of polyphase circuits :, , Whenever the power requirements of a machine are, more than that provided by a single phase supply, then, we need to use polyphase supply like three phase. The, , Power given by a three phase supply is thrice as given, by a single phase supply., , Scanned with CamScanner
Page 5 :
3.2. Balanced 36 Stareonieees :, , , , , , , , , , , , (I) Connection :, , — In the 3 star connection as shown in the Fig. 3.3, the, source as well as the load are connected in star., , — The common point of the star source and load is also, , connected., , Scanned with CamScanner
