The supply system which contains more than one phase, is known as polyphase system, e.g. two phase, three phase supply systems.
Advantages
There are the following advantages of polyphase system over single phase supply system:
(i) The polyphase alternator has more output than a single phase alternator for the same frame size of machine. A two phase alternator has approximately 40% more output and a three phase alternator has 70% more output than a single phase alternator.
(ii) To transmit the required amount of power over a given distance a polyphase system i.e. polyphase transmission line requires less copper than a single phase transmission line.
(iii) In case of three phase system the lighting and power circuits can be drawn from the same line but in single phase separate lines are required.
(iv) Single phase motors are not self-starting where the three phase motors are self-starting.
(v) Three phase motors have smooth torque where single phase motors have the pulsating torque.
(vi) Polyphase motors can be build in large capacity while single phase motors cannot.
(vii) For the same size of machine the output of single phase motors is always less than the output of three phase motors.
(vii) The power factor and efficiency of three phase motor is always better than the single motor.
(ix) Fault location is always easy in thme phase system.
(x) Starting of three phase motor is always easy than single phase motors.
