What are phases in electrical supply and how do they work?

Answer 1

'Phase' refers to a winding into which a voltage is induced in an alternator or in a transformer or motor. The voltage across a phase is known as a 'phase voltage', and the current through it is a 'phase current'.

'Three-phase', means that there are three such windings in a machine, physically displaced from each other by 120 degrees, with each phase voltage is displaced by 120 electrical degrees.

The term 'phase' is sometimes, incorrectly, used to describe the 'hot' wire in an electrical installation -the correct term, though, is 'line'.

Answer 2

Two phases in a three phase power system are sometimes used for small amounts of three phase loading in residential and other locations (such as a pool pump). Two phases are only needed, and two single phase Transformers are used. One is for normal home use (the lighting transformer), which provides single phase service. The other transformer is connected along with the first so there are two phases, and a floating neutral that acts as a third phase to provide power to the three phase loads. This transformer is typically much smaller. This is more economical than providing three phase, three transformer power for a small amount of three phase load.

Answer

The above answer describes an 'open delta' three-phase system, not a two-phase system.

A two-phase system, which is rarely used anywhere these days, is provided by a generator having two armature windings ('phases') which are located at right-angles to each other.

This provides two phase voltages that are at displaced from each other by 90o.

If the two windings are connected independently from each other, then the system is described as a 'two-phase four-wire' system. If, on the other hand, the one end of each winding are connected together, the system is described as being a 'two-phase three-wire system'.

For a two-phase three-wire system, the voltage between the outer two conductors (the line voltage) is 1.414 times the phase voltage.

Answer 3

Most residential premises are supplied with a single phase supply. However, in some countries, such as here is Cyprus, it's not unusual to see residences with a three-phase supply provided (the advantage of which is not altogether clear, as most homes don't normally have any three-phase loads such as motors!).

For a three-phase three-wire system, the supply conductors are correctly termed 'lines' (not 'phases'!). The individual windings of the supply transformer are connected between these lines and are the transformer's phases.

For a three-phase four-wire system, the supply conductors are called 'lines' and the fourth conductor is called the 'neutral'.

Voltages between lines are called 'line voltages', whereas any voltage measured between any line and the neutral is termed a 'phase voltage'.

In most European countries, the secondary of the three-phase distribution transformer is star or wye connected, and is a four-wire system. Line voltage is then 400 V while the phase voltage is 230 V. Single-phase supplies are provided using one line conductor and one neutral conductor. To balance the load, alternate residences are supplied from alternate lines.

In North America, the secondary of the three-phase distribution transformer is delta or mesh connected, and a single-phase supply is obtained by means of two line conductors, with a line voltage of 240 V, and a centre-tapped neutral conductor from the common phase winding. This provides two 120-V together with one 240 V service to residential premises.