Advantages of Polyphase Systems over Single Phase System :
The three-phase system has certain advantages over the single-phase system. They are as follows:
1. More output: For the same size, the output of the polyphase machines is always higher than that of the single-phase machines. Typically the three-phase machines produce about 45% higher output.
2. Smaller size: For producing the same output, the size of three-phase machines is always smaller than that of the single-phase machines. This factor is important in the urban area where space is an important parameter.
3. 3-phase motors are self-starting: As the three-phase ac supply is capable of producing a rotating magnetic field when applied to stationary windings, the three-phase AC motors are self-starting. The single-phase motors need to use an additional starter winding.
4. More power is transmitted: In the transmission system it is possible to transmit more power using a three-phase system, than the single phase system, by using the conductors of the same cross-sectional area.
5. Smaller cross-sectional area of conductors: If the same amount of power is to be transmitted, then the cross-sectional area of the conductors used for the three-phase system is small as compared to that for the single-phase system.
6. The horsepower rating of three-phase motors and the KVA (kilo-volt-amp) rating of three-phase transformers is about 150% greater than for single-phase motors or transformers with similar frame sizes.
7. The power delivered by a single-phase system fluctuates. This power falls to zero three times during each cycle. The power delivered by a three-phase circuit also fluctuates, but it never falls to zero. In a three-phase system, the power delivered to the load is the same at any instant This produces superior operating characteristics for three-phase motors.
8. In a balanced three-phase system, the conductors need to be only about 75% the size of conductors for a single-phase two-wire system of the same KVA rating. This helps offset the cost of supplying the third conductor required by three-phase systems.
9. Better power factor: The power factor of three-phase machines is better than that of single-phase machines.