# difference between single double and three phase

If single phase is 230 V and 3 phase is 440 v then what about for 2 phase? And why is it so?

• The number of phases have nothing to do with the voltage. Commented Jun 18, 2013 at 7:21
• @IgnacioVazquez-Abrams that isn't strictly true - if the single phase is 230V and the system is balanced perfectly, the 3 phase line voltage will be 398.4V (irrespective of what the OP thinks it is. Commented Jun 18, 2013 at 7:28
• Where do you get the "3 phase is 440 v" assumption from, please? Commented Jun 18, 2013 at 10:41
• The voltage from the utilities is rather dependent on your location and the quality of the supply (I regularly take measurements that are surposed to be 415VAC but are closer to 400VAC). However mains matching transformers do allow you to have adjustment to your "mains" supply voltage to compensate for lower than ideal utility voltage. Thank fully I don't have to mess with this any more... Using the right transformer you can have whatever you like . Commented Jun 18, 2013 at 11:54

Here is what wikipedia says about two-phase power.

Two-phase electrical power was an early 20th century polyphase alternating current electric power distribution system. Two circuits were used, with voltage phases differing by 90 degrees. Usually circuits used four wires, two for each phase. Less frequently, three wires were used, with a common wire with a larger-diameter conductor. Some early two-phase generators had two complete rotor and field assemblies, with windings physically offset by 90 electrical degrees to provide two-phase power. The generators at Niagara Falls installed in 1895 were the largest generators in the world at the time and were two-phase machines.

The advantage of two-phase electrical power was that it allowed for simple, self-starting electric motors. In the early days of electrical engineering, it was easier to analyze and design two-phase systems where the phases were completely separated. It was not until the invention of the method of symmetrical components in 1918 that polyphase power systems had a convenient mathematical tool for describing unbalanced load cases. The revolving magnetic field produced with a two-phase system allowed electric motors to provide torque from zero motor speed, which was not possible with a single-phase induction motor (without extra starting means). Induction motors designed for two-phase operation use the same winding configuration as capacitor start single-phase motors.

Single phase and 3 phase are unrelated to 2 phase described above. Basically, 3 phase is what the power stations produce now and ultimately this gets distributed as 3 single phases to our homes: -

There are three line voltages shown in red that are 120º apart in phase orientation. This can be seen as three single phase voltages (blue) and if you did the trigonometry you'd see that the length of red is sqrt(3) times bigger than the length of blue hence, if you have 230V phase voltage, the line voltage would be 398V. Here's how the voltages look in time: -

Going back a few years when the UK had 240V, the line voltage was 415V and sometimes 440V line voltages were referred to ans they yielded a phase voltage of 250V.

The 230 V is phase-to-ground voltage. 400 V (not 440) is phase-to-phase voltage, because 230 * sqrt(3) = 400.

For two-phase system eg. 100 V the phase-to-phase would be sqrt(2) * 100 V, because phases differ in angle of 90 degrees and you can calculate this from Pythagorean theorem.

Voltage is measured relatively from a point. You cannot say how much voltage is passing through a point, instead you may say how much more voltage a point has than any other point.

Here in single phase, where you are supplied with a neutral wire and a phase wire, if you measure the voltage between the phase and the neutral, it would be 230 V.
But in the case of three phases, where you are supplied with three phase wires and a single neutral wire, if you measure the voltage between phase to neutral it would still be 230 V.
And if you measure the voltage between any two of the phase wires you would get 440 398 V.

Thanks @Transistor for the correction.

• $230~V * \sqrt 3 = 398~V$. Commented Jun 27, 2016 at 16:49