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Electric Power Engineering

Questions of phase by stevenal
Posted: 24 Sep 03 (Edited 23 May 05)

These questions come up often enough, that I thought I'd do an FAQ. What's single phase, what's two phase, what's three phase, how does one tell? The definitions I use come from the IEEE.

Quote (IEEE Std C57.12.80Ö-2002):


single-phase circuit: An alternating-current circuit consisting of two or three intentionally interrelated conductors that enter (or leave) a delimited region at two or three terminals of entry. If the circuit consists of two conductors, it is intended to be so energized that, in the steady state, the voltage between the two terminals of entry is an alternating voltage. If the circuit consists of three conductors, it is intended to be so energized that, in steady state, the alternating voltages between any two terminals of entry have the same period and are in phase or in phase opposition.

two-phase circuit: A polyphase circuit of three, four, or five distinct conductors intended to be so energized that in the steady state the alternating voltages between two selected pairs of terminals of entry, other than the neutral terminal when one exists, have the same periods, are equal in amplitude, and have a phase difference of 90 degrees. When the circuit consists of five conductors, but not otherwise, one of them is a neutral conductor.

NOTEùA two-phase circuit as defined here does not conform to the general pattern of polyphase circuits. Actually, a two-phase, four-wire or five-wire circuit could more properly be called a four-phase circuit, but the term two-phase is in common usage. A two-phase three-wire circuit is essentially a special case, as it does not conform to the general pattern of other polyphase circuits.

three-phase circuit: A three-phase circuit is a combination of circuits energized by alternating electromotive forces that differ in phase by one third of a cycle, that is, 120 degrees.

NOTEùIn practice, the phases may vary several degrees from the specified angle.

six-phase circuit: A combination of circuits energized by alternating electromotive forces that differ in phase by one-sixth of a cycle, that is, 60 degrees.

NOTEùIn practice, the phases may vary several degrees from the specified angle.


The note following the definition of two-phase is true. Polyphase of course excludes single phase, so of the four systems defined, two of them follow this general rule. Not much of a rule.

The most common question seems to involve why it would be improper to call the three-wire single phase circuit two phase. We could then call the 90 degree thing four phase, and all would be right with the world. Take a pie, slice it into x symmetrical wedges that all reach the center point and you have an x-phase system. In my opinion, the most important reason for differentiating the two-wire and three-wire single phase from polyphase systems is this: a single phase circuit, center tapped or not, cannot produce the rotating magnetic field needed to start an ordinary induction motor. The field simply alternates with the applied voltage frequency. With 180 degree separation, no clear direction is established. Polyphase phase systems provide this rotating magnetic field because of the smaller angles. Single phase motors exist, but they use added tricks to get them started.

Once you've accepted the IEEE definitions, there don't seem to be any rules except: "the IEEE says it is x-phase." If there is a formula that involves counting transformer cores or core legs or windings or ungrounded conductors, or voltage phasors; I havenÆt seen it. If you have a formula to test, here are some things to consider: Three phase can be transformed using two or three single phase transformers. A Scott T connected transformer can supply single or two phase from a three phase source. A six phase circuit can be supplied from a three phase source using three center tapped single phase transformers. Rearranging the secondary connections on the last example can change it to three phase.

If I were king, I'd take the pie slice rule above, and, if even, divide by two. Just seems wrong to have two phasors of a collinear pair both counting toward the total. They forgot to ask me.

Related questions involve tapping three phase circuits. A single line to neutral tap provides a single phase circuit. A line to line tap is also single phase. A tap involving two lines and neutral is an open wye or V circuit. (ANSI C12.1-2001)

Another wrinkle is monocyclic power. Start with a single phase winding with a center tap. Extend from the center tap a second teaser winding at 90 degrees from the first, at 1/4th the voltage. Used primarily for single phase loads with the occasional motor load. The presence of the teaser provides a rotating field that can start a motor, but the main source of power to run the motor comes from the main single phase winding. This was Steinmetz's and GE's short term answer to the Tesla and Westinghouse patents on three phase.  

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