ET201: THREE PHASE SYSTEMS (PART 2)

1.1.1    Three phase system advantages

There are several reasons why three phase system is superior to single phase system for transmission and distribution of electricity;

a)   In a balanced three phase system, the conductors need be only about 75% the size of conductors for a single phase two-wire system of the same power (KVA) rating. This helps to reduce the cost because less copper (or aluminium) is needed.

b)    The power delivered by a single-phase system pulsates and falls to zero three times during each cycle. The power delivered by a three-phase circuit pulsates also, but it never falls to zero

Figure 1.1.1. Three phase power never falls to zero

a)     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 a similar frame size.

b)   The efficiency and power factor of three phase motors much better than single phase motors for the same power transferred.

c)   Three phase motors have the ability to “self-start”; caused by the phase difference between three phase coils, but not in single phase motors.

d)  Three phase transformers are lighter, more efficient and cheaper compared to single phase transformer for the same size.

e)     When connected in parallel, single phase generator present difficulties which do not occur with three phase generators.

h)  The instantaneous three phase power flow is constant.  This is advantageous for both the end user and the generator, which does not have to supply a pulsating torque.