A poly phase system is that distributing alternating current or electrical power. In an ac system it is possible to connect two or more number of individual circuits to a common poly phase source. Poly phase systems have three or more energized electrical conductors carrying alternating currents with a defined phase angle between the voltage waves in each conductor; for three-phase voltage, the phase angle is 120° or ~2.09 Radians. Though it is possible to have any number of sources in a poly phase system. Poly phase systems are particularly useful for transmitting power to electric motors. The power generated by the same machine increases 41% from single phase to two phases. The maximum possible increase beyond three phase is only 7% and also the complications .The most common example is the three-phase power system used for industrial applications and for power transmission. In general a three phase system of voltages is merely a combination of three single phase systems of voltages of which the three voltages are differ in particular sequence.
A major advantage of three phase power transmission is that the remaining conductors act as the return path for any single conductor, the power transmitted by a balanced three phase system is three times that of a single phase transmission but only one extra conductor is used. Thus, a 50% increase in the transmission costs achieves a 200% increase in the power transmitted.
The power in a single phase circuit is pulsating when the power factor of the circuit is unity the power becomes zero the power becomes zero 100 times in a second in a 50 HZ SUPPLY therefore single phase motors have 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 motor 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 circuit.In a given frame size a three phase motor or a three phase generator products more output than its single phase counterpart.Three phase motors are 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 to those of the similar single phase apparatus.