Advantages of Three Phase Systems over Single Phase System | Introduction to Polyphase AC Circuits | Three Phase Supply Waveforms

Advantages of Three Phase 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.
3. 3-phase motors are self starting: As the three phase ac supply is capable of producing a rotating magnetic field wheni 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 samei cross-sectional area.
5. Smaller cross-sectional area of conductors : If samei amount of power is to be transmitted, then the cross-sectionali 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 much greater than that for single-phase motors or transformers with a similar frame size.
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. This produces superior operatingi 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 RVA rating. This helps offset the cost of supplying the third conductor required by three-phasei systems.
9. Better power factor : The power factor of three phasei machines is better than that of the single phase machines.

Introduction to Polyphase AC Circuits:

We have already discussed about the single phase ac circuits. The domestic ac supply is single phase ac supply with 230 V / 50 Hz.
But this ac supply is not suitable for certain applications. i Some loads need a polyphase ac supply.
The word poly means many. Thus a polyphase ac supply is
the one which produces many phases simultaneously.
How to generate a polyphase ac supply ?
The single phase ac voltage is generated by using a single phase alternator. Single phase alternator consists of only one armature winding.
But in order to generate a polyphase voltage, we have to use many armature windings. The number of windings is equal to the number of phases. i

Why to use a Polyphase System ?

A polyphase system has certain advantages over the single phase system, hence they are preferred practically. Some of these advantages are discussed in section 3.2.

How Many Phases ?

We cannot practically increase the number of phases to a very large number. The advantages related with the number of phases do not increase proportionally with number of phases, i but the complications of interconnections increase. A three phase system has proved to be the most economical as compared to the other systems. Hence in practice the threei phase system is most preferred.
In rare cases a six phase system is also used. 3.1.3

Three Phase Supply Waveforms :

The three armature windings used for generation of a three phase supply are placed at 120• away from each other. The voltages induced in these windings are of same amplitude and frequency, but they are displaced by 1200 with respect to each other as shown in Fig. 3.1.1