3-Phase AC

Three phase supplies

In single phase circuits we have seen that we have two conductors: a line (usually at 230V) and a neutral. In three phase systems however we have 3 different lines, each phase voltage 120 degrees out of phase with each other and sometimes a neutral.

In three phase circuits the three lines are coloured brown, black and grey with a black neutral. Under the previous standard they were labeled red, yellow and blue, with a black neutral.

Star and Delta

There are two ways of connecting 3-phase supplies and loads: STAR and DELTA. The diagrams below show the two different methods of connecting a load to a 3-phase supply.

The phase voltage is the voltage measured across each load, the line voltage is the voltage measured between the phases - eg between L1 and L2.

In a  balanced star connected load the phase voltages are different, in the use the phase voltage is 230V and the line voltage is 400V, you can prove this yourself using the formula below.

In a delta connected load and phase voltages are the same but the line and phase currents are different.

Balanced and unbalanced star loads

A balanced system is where each load draws the same amount of current and the neutral conductor carries no current. An unbalanced system is where the current drawn by the loads is different, this results in the residual current taken by the neutral conductor. It is possible to determine the neutral current of an unbalanced system by drawing an equilateral triangle to scale and then adding the phase currents to scale. Measuring the gap between the open ends of a triangle will tell you the current flowing through the neutral.

A balanced load

An unbalanced load

Advantages of 3-phase

Here are some advantages of 3-phase supplies:

3-Phase is used for the distribution of electricity across the National Grid. 3-phase distribution requires less copper or aluminium to transmit the same amount when compared to a single phase system.

3-phase powered motors are more efficient than single phase motors and start automatically.

3-phase motors are more efficient and produce less vibration.

When AC is converted into DC (rectified) the resultant DC is much smoother than with a single phase supply.

A 3-phase distribution transformer

A high power 3-phase motor