Electrical Engineering ⇒ Topic : Constancy of Total Power in Balanced 3phase System

Daniel
 
Constancy of Total Power in Balanced 3phase System It was shown in chapter that the instantaneous power p supplied by a singlephase source to a load (*lagging p.f.) is given by Since phase and line values and power factor for a balanced 3phase system are fixed, total 3phase power shall remain constant at all times. Therefore, an important advantage of a 3phase system over the singlephase system is this constant and smooth power flow into a load such as a 3phase motor. For this reason, 3phase motors are used when the load is greater than 3 h.p.Moreover, 3phase system presents overall economic and operating advantages compared with singlephase system or other polyphase systems. Therefore, the 3phase system is the most widely used of all the polyphase systems
figure (a) Graphical Explanation. Fig. (a) shows the graphical explanation for the constancy of total power in a 3phase balanced system. Thus p_{R} represents the power supplied to phase R. The power graph for phase Y lags by 120° behind p_{R} and is represented by p_{Y},. Similarly, graph p_{B} represents the power supplied to phase B. If we sum up the three curves point by point, it will be seen that total power is constant as shown by the dotted horizontal line in Fig. (a). Thus, although the three component powers fluctuate, the total power is constant. This is due to the phase difference between the fluctuations of the components. Due to the constant power supplied by a balanced 3phase system, large loads are always driven by 3phase motors rather than single phase motors  
 
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