In 3 phase 4 wire system, neutral wire carry return current in case of unbalance only.
Transmission systems are usually balanced system. If large amount of power to be transmitted and the demand is constant on all phases then three phase three wire system is preferred which is economical also.(only 3 wire instead of 4 wire will save conductor cost). Hence generally for transmission of High Power, High Voltage AC(HVAC) is used. It also suppress 3rd harmonics as current circulates in close delta circuit.
In distribution also where load is balanced on all three phases (industrial load i.e. motor & furnaces), 3 phase 3 wire is used to supply the power.
3 phase 4 wire is supply for distribution to household as their load are not balanced.
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(3Ph) 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.
The Single pahse AC was for lighting purpose only. Laters on motors were to be used for irrigation and industrial purposes. This has led to development of polyphase or multiphase motors.. To drive these motors, three phase voltage is generated.
Advantages of three phase systems:
Balanced Supply, the magnitude of all the three phase voltages are equal and displaced by an angle of 120 degrees with respect to one another and the frequency of all the voltages is same.
Unbalanced supply, the magnitude of all the three phase voltages are equal and displaced by an angle of 120 degrees with respect to one another and the frequency of all the voltages is different.
For generation of three voltages , three coils are used . If the voltages developed by these coils are tapped separately, then six conuctors would be required to transmit the power over greater distance. This makes system more complex and expensive. Hence three phases are generally interconnected that results in saving of conductor material and reduced complexity.
Star-Delta connection in three phase circuits is used to vary voltage and current. If you use star connection then voltage is reduced to 60% of supply voltage and current remains same while in Delta connection current is reduced to 60% of the value of current in star connection. Its major application is in starting induction motor where it need starting at low voltage to keep starting current low, it is started in star ie its three windings are connected in star but induction motor needs to be operated at high voltage at steady state or normal state to deliver rated power, after starting in star its three windings are switched to delta.
The configuration can be changed internally by switches to change the values of parameters.
Basically these connections are used to get a lower value if voltage initially to prevent components from burning . after a few seconds when the parameters stabilize, the configuration is altered to get the higher value of parameters.