In this chapter we shall introduce the concept of polyphase systems. Polyphase systems can be visualized as a group of individual sources of the same magnitude that are separated by a certain phase angle such that they are evenly divided across a single period. The polyphase load is similarly divided into individual sections or legs. By dividing the sources, the application of power can be much more smooth. Further, for the same total load power, the current delivered by each of the segments is reduced compared to a single phase system. For an analogy we could look at a bicycle. A single phase system is like pedaling with only one leg. That is, power is applied in a single burst per revolution of the pedal. Having two pedals is like a two-phase system; power is delivered twice per revolution, once for the right leg and once for the left. Because there are two pedals, it makes sense to separate them physically by 180 degrees or one half of a revolution, otherwise the power delivery will not be smooth. It should obvious to anyone who has pedaled a bike that you must pump a single pedal much harder using only one leg to achieve the same speed obtained when pumping with both legs.
Polyphase loads can be balanced or unbalanced. A balanced load means that all legs or sections of the load exhibit the same impedance. Consequently, the currents supplied by the sources will be the same except for the phase shifts between them. It is possible to create a polyphase system using any number of phases, however, the more phases we add, the more complex the construction of the polyphase source and load. Also, the number of required conductors between the source and load increases (one per phase). These all increase construction, installation and maintenance costs. Polyphase systems using three sections (hereafter simply referred to as three-phase systems) are popular because they deliver the benefits of polyphase while limiting the complexity and minimizing the costs. Therefore, we shall our limit our discussion to three-phase systems utilizing balanced loads.
Three-phase systems can be wired in either delta or Y configurations, or a combination. These are reminiscent of the delta and Y constructs seen in earlier chapters. We shall investigate all of the combinations to determine system parameters such as line voltage, line current and load power. We will also investigate power factor correction for balanced loads that have a non-negligible phase angle.