Three phase power

"Three-Phase Power" is industrial grade power supply! It consists of 4 wires. 3 'live' and one 'neutral'. Each live wire paired with the ground will typically provide normal AC (Alternating Current) of PD (Potential Difference) 110 volts (In the US anyway). The three seperate live wires are 120 degrees out of phase. So the PD between any 2 live wires is 208 volts.

Three-Phase is hardcore, and will provide the possibility of simple and cheaply constructed motors which are more powerful and in particular have more starting torque.

Obtaining a three-phase feed in a household situation (in a legal manner) will almost always be discouragingly costly, and mostly pointless... unfortunately!

It is unfortunate indeed that residential customers cannot easily get three-phase power--if you're a collector of vintage computer equipment, you will find that some devices require three phase to run the fans and motors. Our old Digital TU78 tape drive needed it, for instance, and I know lots of other mainframes and such needed it.

I have read about things you can use to convert two-phase to three-phase, but they're not for the amateur electrician.

Three phase power is the method by which electrical power is generated, transmitted, and in some cases, consumed.

As you may already know, alternating current reverses polarity 60 or 50 times a second, depending on where you are. If you plot the voltage as Y on a graph with the time being X, you will get a sine wave. Three phase power can be thought of as three different sine waves which are 120 degrees apart.

Three phase power is used because it is more efficient to generate and use. For instance, let's say you have a generator that is putting out single-phase power, that is, one waveform. For every revolution of the generator's rotor, you will only be converting mechanical energy into electrical energy part-time, IE, when the output voltage is non-zero. The waveform will only reach its peaks every 1/120th of a second; the rest of the time, output power will be sub-optimal, and the turbine won't be evenly loaded.

Enter three-phase power. If you look at a plot of all three phases, you will notice that at any given moment in time, one of the phases is nearing a peak. This means that a three phase generator will be converting mechanical energy into electrical output nearly 100% of the time, resulting in much more even loading of the turbine and better use of the generator's capability.

The same holds true for electric motors. A single-phase motor's power output varies wildly 120 times a second. This results in a lot of vibration. A three phase motor will run much smoother since almost full power is available from at least one phase most of the time.

Three phase power can be wired in one of two ways. There's WYE, in which the phases are connected in a "Y" shape with common neutral, and Delta, where the phases are connected in series, like a triangle. Delta arrangements are common in high voltage power line distribution, whereas WYE is more common in low voltage distribution within a building.

Residential and small business electrical installations are generally single-phase; one of the three phases on the utility poles is tapped by a transformer, providing a single low voltage phase. Three phase power is rarely needed outside industry because it's not cost-effective for small loads.

Three-phase power, or polyphase power is indeed distributed from the power company with three hot wires (with voltages 120 degrees out of phase) and a smaller, neutral wire, which is the ground.

Assuming a "balanced" load, the instantaneous power delivered to the load is constant in time, as opposed to pulsating with a single-phase power source. This is one of the attractions of three-phase power.

The typical three-phase power connections are the wye and the delta connections. These are the two "canonical" balanced connections that you often see in textbooks.

---

The wye connection is shown below. A, B and C represent the three hot lines and n the neutral. The phase voltage between each of A, B and C and the neutral is V_p, each 120 degrees out of phase from the others. The line voltage V_l, the voltage between two hot wires, is 3^(.5)*V_p.

                   _____
                  |     |
A   --------------|  Z  |---
                  |_____|  |
                           |
                   _____   |
                  |     |  |
B   --------------|  Z  |--|
                  |_____|  |
                           |
                   _____   |
                  |     |  |
C   --------------|  Z  |--|
                  |_____|  |
                           |
                           |
n   ------------------------

It turns out that if each of the loads Z are identical, no current flows on the neutral wire at all. This is called a balanced load. Therefore, the neutral wire may be totally omitted. Balanced loads are not usually the case in the real world of course, and a small amount of current flows along the neutral wire. This is why it is present (but smaller).

The delta connection is shown below. A, B and C represent the three hot lines. Note that the neutral is not included here, as it is unneccesarry.

A   ---------------------------|
                     |         |
                   __|__       |
                  |     |      |
                  |  Z  |      |
                  |_____|      |
                     |       __|__
                     |      |     |
B   -----------------|      |  Z  |
                     |      |_____|
                   __|__       |
                  |     |      |
                  |  Z  |      |
                  |_____|      |
                     |         |
                     |         |
C   ---------------------------|

There exist wye-delta and delta-wye transformations to represent one type of load in terms of the other. The voltage sources, which were not shown here, may themselves be connected to the load in either a wye or a delta configuration.

Most industrial three-phase voltage supplies are either 480 volts or 277 volts RMS, compared to 120 volts for residential usage. There are many three-phase applications where the ground wire is needed, and some, such as three-phsae motors, where it is not. You will find a mixture of these in the industry.