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State advantages of three phase over single phase.
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Advantages of three phase system

Three phase system give study output and power delivered is constant. In single phase system power delivered is pulsating.

The output of three phase machine is greater than single phase machine of same size.

Three phase induction motors are self starting. Single phase motor are not self starting.

We can obtain single phase supply from three phase but three phase supply cannot be obtained from single phase.

For transmission of three phase system conducting material required is less as compared to single phase system for given volt ampere and voltage rating.

Efficiency of 3 phase motor is more as compared to single phase motor.

Parallel operation of three phase alternator is simpler as compared to single phase alternator.

Power to weight ratio of three phase alternator is high as compared to single phase alternator which reduces the cost for generating same amount of power.

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Advantages of Polyphase circuit over Single phase circuit: i) Three-phase transmission is more economical than single-phase transmission. It requires less copper material. ii) Parallel operation of 3-phase alternators is easier than that of single-phase alternators. iii) Single-phase loads can be connected along with 3-ph loads in a 3-ph system. iv) Instead of pulsating power of single-phase supply, constant power is obtained in 3-phase system. v) Three-phase induction motors are self-starting. They have high efficiency, better power factor and uniform torque. vi) The power rating of 3-phase machine is higher than that of 1-phase machine of the same size. vii) The size of 3-phase machine is smaller than that of 1-phase machine of the same power rating. viii) Three-phase supply produces a rotating magnetic field in 3-phase rotating machines which gives uniform torque and less noise.

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Advantages of Three phase circuits over Single phase circuits: i. Three phase transmission line requires less conductor material for same power transfer at same voltage. ii. For same frame size, three phase machine gives more output. iii. For same rating, three phase machines have small size. iv. Three phase motors produce uniform torque. v. Three phase induction motors are self-starting. vi. For same rating, three phase motors have better power factor. vii. Three phase transformers are more economical. Power capacity to weight ratio is more. viii. Three phase machines have higher efficiencies. ix. Three phase system is more economical with regards to generation, transmission and distribution of power. x. Three phase system requires less maintenance and it increases the life of the system. In three phase system rotating magnetic field is produced rather than the pulsating field produced by single phase system.
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Advantages of 3-phase system over 1-phase system: - 

1. More output: For the same size, output of poly-phase machines is always higher than single phase machines. 

2. Smaller size: For producing same output, the size of three phase machines is always smaller than that of single phase machines.

3. More power is transmitted: It is possible to transmit more power using a three phase system than single phase system. 

4. 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 three phase system is small as compared to single phase system. 

5. Better power factor: Power factor of three phase machines is better than that of single phase machines. 

6. Three phase motors are self- starting: The three phase ac motors are self starting, while single phase induction motor are not.
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