Author Topic: Three-phase vs. single-phase power  (Read 72557 times)

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Offline Steve

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Three-phase vs. single-phase power
« on: December 22, 2005, 10:31:31 AM »
It seems that some of us are confused about the differences between single-phase and three-phase power (please note that contrary to popular belief, there is no such thing as two-phase power.)

Most residential homes are fed with single-phase power (sometimes called split-phase.) The power company runs three wires into the home which comprises of two hot wires and a neutral wire. The neutral is actually a center-tapped feed off the transformer. Voltage measured across both hot wires is 240 VAC. Voltage measured from any hot to neutral is 120 VAC (split-phase). So, a regular 120 VAC circuit is fed with one hot and the neutral (split-phase). A dryer or oven is fed with both hots which provides 240 VAC (single or full-phase). Some people mistakenly believe that a 240 VAC circuit is "two-phase," but they are wrong; it's actually the full phase of a single-phase circuit whereas the 120 VAC feeds are half-phase (split-phase).

Most commercial buildings are wired with three-phase power. Three phase power has three hot wires with each being 120 degrees out of phase with each other. Three-phase power can be more efficient than single-phase power in certain applications. Three-phase motors, for example, are much more powerful (or efficient) than their single-phase counterparts. And three-phase motors are cheaper because they are less complex and do not require starting capacitors. Here is a good analogy which should help you visualize the differences between the two: Using a single-phase supply is like one strong man pushing a car uphill. At some point, the work is beyond what that one man can do; whereas a three-phase supply is like having three equally strong men each pushing the car in a relay system one after another. The end result is that each man is doing 1/3 the work of the one strong man so, together, the three of them push the car further. Or, you can think of it like this: In the U.S., our power is generated at 60 Hz. So, a single-phase circuit's sine wave will swing +/- at a rate of 60 times per second. Think of it as 60 "pulses" or "pushes" of work per second. A three-phase circuit combines three single-phase circuits (each 120 degrees out of sync with the other) so in the same one second period of time you've got three "pulses" or "pushes" ... or three times the work!

To determine whether you have three-phase or single-phase power, simply measure the voltage between two hots or a hot and ground. You're most likely to encounter a three-phase circuit that shows 120 VAC between any hot and ground, and 208 VAC between any two hots. A single-phase circuit will show 120 VAC between a hot and ground, and 240 VAC between any two hots.

Bringing in a three-phase feed to your house is usually ridiculously expensive, or impossible.

I hope this helps clear up any confusion.
« Last Edit: December 22, 2005, 10:38:07 AM by Steve »
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Offline Steve

  • Steve Zinski
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Re: Three-phase vs. single-phase power
« Reply #1 on: December 22, 2005, 10:39:43 AM »
Oh, and regarding ovens, three-phase power will result in a more "efficient" oven... it will probably heat up faster and might be cheaper to operate.
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Offline pacoast

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Re: Three-phase vs. single-phase power
« Reply #2 on: September 13, 2009, 03:54:08 PM »
With apologies to Steve, a minor clarification. There is such a thing as two-phase power. In North America, it is obsolete but around the turn of the century, the largest generators in the world at Niagra Falls were two-phase, polyphasic AC systems. Current practice is to distribute single-phase (actually, split-phase) to most homes and full, three-phase power to industry. As Steve points out it can be crazy expensive to run three-phase power into locations where it does not currently exist. However, phase converters exist that can convert single-phase power to three-phase.

And in common parlance, two-phase power exists in Europe but refers to the distribution of only two of the three phases to farms and light commercial users.

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