The advantages of polyphase systems over single-phase systems are:
1. A polyphase transmission line requires less conductor material than a single-phase line for transmitting the same amount power at the same voltage.
2. For a given frame size a polyphase machine gives a higher output than a single-phase machine For example, output of a 3-phase motor is 1.5 times the output of single-phase motor of same size
3. Polyphase motors have a uniform torque where most of the single-phase motors have a pulsating torque.
4. Polyphase induction motors are self-starting and are more efficient. On the other hand single phase induction motors are not self-starting and are less efficient.
5. Per unit of output, the polyphase machine is very much cheaper.
6. Power factor of a single-phase motor is lower than that of polyphase motor of the same
7. Rotating field can be set up by passing polyphase current through stationary coils.
8. Parallel operation of polyphase alternators is simple as compared to that of single-phase alternators because of pulsating reaction in single-phase alternator.
It has been found that the above advantages are best realised in the case of three-phase systems. Consequently, the electric power is generated and transmitted in the form of three-phase system.