In an ac system it is possible to connect two or more number of individual circuits to a common polyphase source. Though it is possible to have any number of sources in a polyphase system, the increase in the available power is not significant beyond the three-phase system. The power generated by the same machine increases 41.4 per cent from single phase to two-phase, and the increase in the power is 50 per cent from single phase to three-phase. Beyond three-phase, the maximum possible increase is only seven per cent, but the complications are many. So, an increase beyond three-phase does not justify the extra complications.
In view of this, it is only in exceptional-cases where more than three phases are used. Circuits supplied by six, twelve and more phases are used in high power radio transmitter stations. Two-phase systems are used to supply two-phase servo motors in feedback control systems.
In general, a three-phase system of voltages (currents) is merely a combination of three single phase systems of voltages (currents) of which the three voltages (currents) differ in phase by 120 electrical degrees from each other in a particular sequence. One such three-phase system of sinusoidal voltages is shown in Fig. 9.1.
Advantages of Three Phase System:
It is observed that the Polyphase System, especially three-phase, system has many_ advantages over the single phase system, both from the utility point of view as well as from the consumer point of view. Some of the advantages are as under.
- The power in a single phase circuit is pulsating. When the power factor of the circuit is unity, the power becomes zero 100 times in a second in a 50 Hz supply. Therefore, single phase motors have a pulsating torque. Although the power supplied by each phase is pulsating, the total three-phase power supplied to a balanced three-phase circuit is constant at every instant of time. Because of this, three-phase motors have an absolutely uniform torque.
- To transmit a given amount of power over a given length, a three-phase transmission circuit requires less conductor material than a single-phase
- In a given frame size, a three-phase motor or a three-phase generator produces more output than its single phase counterpart.
- Three-phase motors are more easily started than single phase motors. Single phase motors are not self starting, whereas three-phase motors are.
In general, we can conclude that the operating characteristics of a three-phase apparatus are superior than those of a similar single phase apparatus. All three-phase machines are superior in performance. Their control equipment’s are smaller, cheaper, lighter in weight and more efficient. Therefore, the study of three phase circuits is of great importance.