Polyphase System is a combination of two or more than two voltages having same magnitude and frequency but displaced from each other by an equal electrical angle. As poly means, many (more than one) and phase means windings or circuits. Each of them has a single alternating voltage of the same magnitude and frequency.
The angular displacement between the adjacent voltages is called a Phase Difference and depends upon the number of phases.
However, the above equation does not hold good for the two-phase system where the voltages are displaced by an angle of 90 degrees electrical. Thus, in other words, a Polyphase System can be defined as an AC system having a group of (two or more than two) equal voltages of same frequency arranged to have an equal phase difference between the adjacent EMFs.
Single phase systems are employed for the operation of almost all the domestic and commercial applications. For examples – Fans, Televisions, Refrigerators, Washing machines, Mixer-grinder, Computers, Exhaust Fans, Lamps, Electric Toasters, Electric Irons, etc. But the single phase system has its limitations in the field of generation, transmission, distribution and industrial applications. Thus, because of such limitations single phase system is replaced by Polyphase System.
Polyphase or we can say Three Phase System is universally adopted for a generation, transmission and distribution of electric power because of its practical utility and various advantages, which is discussed in the topic Advantages of 3 Phase Over Single Phase System.
ALSO SEE Advantages of 3 Phase Over Single Phase System.
The Polyphase System may be two-phase system, three phase system or six-phase system. But mainly three phase system is used for all practical purposes. So, whenever the term Polyphase is used, it means a Three Phase System unless it is mentioned otherwise.