Three Phase Power

Three-phase electric power is a common method of alternating current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by grids worldwide to transfer power. It is also used to power large motors and other heavy loads. A three-phase system is generally more economical than others because it uses less conductor material to transmit electric power than equivalent single-phase or two-phase systems at the same voltage. The three-phase system was introduced and patented by Nikola Tesla in the years from 1887 to 1888.

In a three-phase system, three circuit conductors carry three alternating currents (of the same frequency) which reach their instantaneous peak values at different times. Taking one conductor as the reference, the other two currents are delayed in time by one-third and two-thirds of one cycle of the electric current. This delay between phases has the effect of giving constant power transfer over each cycle of the current and also makes it possible to produce a rotating magnetic field in an electric motor.

Three-phase systems may or may not have a neutral wire. A neutral wire allows the three-phase system to use a higher voltage while still supporting lower-voltage single-phase appliances. In high-voltage distribution situations, it is common not to have a neutral wire as the loads can simply be connected between phases (phase-phase connection).

Three-phase has properties that make it very desirable in electric power systems:

The phase currents tend to cancel out one another, summing to zero in the case of a linear balanced load. This makes it possible to eliminate or reduce the size of the neutral conductor; all the phase conductors carry the same current and so can be the same size, for a balanced load.

Power transfer into a linear balanced load is constant, which helps to reduce generator and motor vibrations.

Three-phase systems can produce a magnetic field that rotates in a specified direction, which simplifies the design of electric motors.

It is precisely for this reason that it is so often used in the Commercial and Industrial properties. Whether it's your local garage, shop,factory or office block you will most often find a 3 phase 415 volt supply. Extra care must be taken when working with this system as an electric shock from 415 volts can quite often be fatal, where as 230/240 volts, without RCD protection, in the majority of cases will just give you a hard belt.

The three winding end connected together at the centre are is called the neutral (denoted as 'N'). The other ends are called the line end (denoted as 'L1', 'L2' and 'L3').
The voltage between two lines (for example 'L1' and 'L2') is called the line to line (or phase to phase) voltage. The voltage across each winding (for example between 'L1' and 'N' is called the line to neutral (or phase voltage).
The line to line voltage is the vector sum of the phase to phase voltage across each winding. This is not the same as the arithmetic sum and is given by the following equation:
Line to Line Voltage (V_LL)

Example: Line to Line Voltage (VLL)   Line to Neutral Voltage (VLN)

3 Phase WYE (Y)
A 3 Phase-Wye connected system consists of three hot lines, or phases, commonly referred to as X, Y, Z, a neutral, and a ground wire for a total of five wires in a power distribution cable.
In North America the most common 3 Phase-Y voltages are either 120/208 VAC or 277/480 VAC, while internationally the most common 3 Phase voltage is 230/400 VAC. The lower voltage in each case is the country’s standard utilization voltage and is measured Line-to-Neutral, while the higher voltage is measured Line-to-Line. The Line-to-Line voltage is always 1.732 times higher than the Line-to-Neutral voltage in a Wye configured 3 Phase system.
The line current supplied to the load is also the same as the phase current. It is important to note that when all three “Hot” phases of the system are loaded equally, the net current draw in the neutral line is zero!

3 Phase Delta (Δ) A 3 Phase-Delta connected system consists of three hot lines, commonly referred to as X, Y, Z, and a ground wire for a total of four wires in a power distribution cable. In North America the most common 3 Phase-Δ voltages are either 208VAC or 240VAC, while internationally the most common 3 Phase voltage is 230 VAC. These phase voltages are measured Line-to-Line and are typically the country’s standard utilization voltage. Since there is no neutral line in a Delta-connected system, there is no Line-to-Neutral voltage! However, the line current in a Delta-connected system is 1.732 times the phase current supplied to the load(s). Proper care must be taken to correctly size cables in a Delta system because the line currents are much higher than the load (or phase) currents. Delta systems typically have lower line voltages but higher line currents than Wye-connected systems.