Question

what are the advantage of a 3 phase system over single system

Answer 1

The advantages of polyphase system over single phase systems are given below:

Power delivered is constant. In single phase circuit the power delivered is pulsating and objectionable for many applications.For a given frame size a polyphase machine gives a higher output than a single phase machine.Polyphase induction motors are self starting and are more efficient. Single phase motor has no starting torque and requires an auxiliary means for starting.Comparing with single phase motor, three phase induction motor has higher power factor and efficiency.
Three phase motors are very robust, relatively cheap, generally smaller, have self-starting properties, provide a steadier output and require little maintenance compared with single phase motors.For transmitting the same amount of power at the same voltage, a three phase transmission line requires less conductor material than a single phase line. The  three phase transmission system is so cheaper.
For a given amount of power transmitted through a system, the three phase system requires conductors with a smaller cross-sectional area. 
This means a saving of copper and thus the original installation costs are less.Polyphase motors have uniform torque whereas most of the single phase motors have pulsating torque.Parallel operation of three-phase generators is simpler then that of single phase generator.Polyphase system can set up rotating magnetic field in stationary windings.
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Answer 2

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