Meaning of "Phase"

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tfdchief

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I have a question I hope someone can help me with. I recently took a class on motors Art 430. Two phase motors came up. It got me thinking again about something I always struggle with. Single, Split, Two, Three Phase power. I know the common supply in this country is not 2 phase but split phase. But don't we call each of the supply wires in a split phase supply, a phase, when we put a meter on it? Can't there be a phase to phase arc? Am I just struggling with terminology? I just can not get this straight in my mind.
:confused:
 
Re: Meaning of "Phase"

It would help if you has a picture of phases as applied to transformers and motors the following URL has several diagrams .

http://electrical-contractor.net/ubb/Forum15/HTML/000053.html

The Term split has is a method of causing the Current of electromagnetic coil to reach a peak value (And thus its magnetic polarity) at a different reference time than that of a second electromagnet coil which is wound on the same pole piece. This is a method to create rotation. This can get rather in depth and certainly goes beyond the time available here.

I think you have some reading to do!!
 
Re: Meaning of "Phase"

The term "phase" is confusing in that it is used to identify many different things that are really similar, but still different. One use is for theory and the other is for application

For example. A phase conductor could be used to identify any conductors other than the neutral conductor. (Ungrounded verses grounded) At the same time, the collective system can be identified by its phase. (1?, 3?) Here's the confusion. A single phase system has one or two phase conductors. A 3-phase system has 1, 2, or 3 phase conductors, and a two phase system can have 2 or 4 phase conductors.

So, a typical 1? 3-wire, 120/240V system has two phase conductors but is still just single phase.

[ February 14, 2005, 01:01 PM: Message edited by: bphgravity ]
 
Re: Meaning of "Phase"

Yes, this is a problem of terminology/semantics. The conductors used to determine phase differences are referred to as phase conductors and this term is usually shortened to simply phase.

The term phase should refer to a time-domain difference. To have a single voltage difference you need to have at least two conductors.
So every (1) "pair" of hot/line/ungrounded conductors is a single phase, and every (3) pairs is three phase.

Most "phase" confusion arises from the bad habit of using the neutral on a 3 wire system as a voltage reference point. To be consistent we cannot change our "phase" reference points between 2 wire, 3 wire, "wild leg" delta, ungrounded delta, grounded delta, and wye systems. We must always reference either a neutral, which often doesn't exist, or the line conductors which always do exist.
 
Re: Meaning of "Phase"

Thanks! That helps. Anyone else have a different explanation?

So Jim, if the supply is 120 V only (single phase of course), isn't the reference point the neutral, 1 phase to neutral?
 
Re: Meaning of "Phase"

Power distribution is often 3 phase. The power company (POCO) actually generates 3 phase voltages 120 deg. out of phase by using a generator that has 3 windings physically 120 deg out of phase.

Those three phases are distributed around by at least 3 wires. But for 120/240V single phase services, only one of these three phases are used. It is placed across a primary transformer winding. Then the secondary winding is center tapped. That generates what I would call a "split phase". One time varying voltage is tapped to create two or more voltages out of phase. (some may argue since the voltages are 180 deg. out of phase, they are really in phase if you reverse the polarity of the wires).

If I remember right, a similar thing happens with single phase motors. A single time varying voltage is applied to a motor, and a tap on the motor winding along with a capacitor produces a voltage out of phase with the applied voltage. I think this is called a "split phase motor". (There may be other ways to generate the split voltage on these motors).

Steve

[ February 14, 2005, 04:50 PM: Message edited by: steve66 ]
 
Re: Meaning of "Phase"

?Voltage,? also known as ?potential difference,? is defined in terms of the energy required to move a given amount of charge from one point to another point. When we speak of ?line to line voltage,? it is really a measure of the energy needed to move charge from a point on one of the lines (call this one the ?reference point?) to a point on the other line (this is the point at which you are measuring voltage). We can also use ?ground,? meaning Planet Earth, as the reference point, and measure voltage from Earth to a point along a circuit. This would be the ?line to ground voltage.? We aren?t actually measuring ?line to neutral? voltage, though for practical purposes that would be the same as ?line to ground voltage.?

Jim was right in saying that the word ?phase,? as used in the context of the original question, is a matter of relationships with regard to time. ?Single phase? means that we are measuring voltage between two conductors, and that current flowing through one conductor must necessarily flow through the other. There is no difference in the points in time at which the two currents (or the two voltages) reach their peak values. By contrast, in a ?three phase? system, current that flows out of the source on a Phase A conductor might find its way back to the source along a Phase B conductor or along a Phase C conductor. For this to become possible, the voltages in the three conductors must necessarily reach their respective peaks at different points in time. That is the ?120 degrees? to which Steve66 has referred. On a ?two phase? system, the voltage in one phase to ground reaches its peak 90 degrees earlier than the voltage in the second phase to ground.
 
Re: Meaning of "Phase"

tfdchief,

Technically there is no neutral in a 2 wire 120V circuit. This circuit has a "hot" and a grounded conductor.
 
Re: Meaning of "Phase"

Tfdchief, here is a simple diagram of why the grounded conductor is not a "neutral" in a two wire circuit,

not_a_neutral.JPG


and why it is a neutral is a multi wire circuit.

unbalanced__neutral.JPG



Roger
 
Re: Meaning of "Phase"

The term "neutral" is so ingrained into the jargon of electricians and handymen too that it will always be used to denote the current carrying conductor which is "neutral", that is, it has no potential on it. The use of the simliar terms, "grounding" and "grounded" is confusing. I prefer "neutral".

I think the term "phase" to indicate the two hot conductors 120/240 system is improper. I prefer to talk of the hot conductors or the hot legs. I have been bitten enough to know they are hot.

[ February 15, 2005, 12:33 AM: Message edited by: rattus ]
 
Re: Meaning of "Phase"

Rattus, the grounded conductor doesn't always have to carry current, and when this is the case the grounded conductor is truly a "Neutral"

Roger
 
Re: Meaning of "Phase"

The use of the simliar terms, "grounding" and "grounded" is confusing. I prefer "neutral".
Click to expand...
There is a very distinct difference, in both meaning and function, between the "grounding" and "grounded" conductors in a system.

And, the term "neutral" is not an appropriate substitute for either one of them. As Roger stated, the grounded conductor is not always neutral, and the neutral is not always grounded.

Ed
 
Re: Meaning of "Phase"

Ed,

Yes, there is a difference, and that is all the more reason that such such similar terms should not be used to describe disparate things.

Now give me an example of an ungrounded neutral. If it is not grounded, it is not neutral anyway according to my understanding.

I am of course speaking of the common usage--not going by the book.
 
Re: Meaning of "Phase"

Now give me an example of an ungrounded neutral.
Click to expand...
Rattus, you got me there. I plead "brainfade".

Here, if any system incorporates a neutral conductor, that conductor is required to be grounded.
I presume the NEC does as well?

Ed
 
Re: Meaning of "Phase"

From tfdchief: Again, thanks for all the input. However, I think I have only succeeded in reinforcing the obscurity of the term ?phase?.

Roger and Jim,
I guess until we all use the same terminology, defined the same, it will always be difficult to talk about this stuff. I understand how single phase, 120/240 V power is derived (I think). However I don?t think that your ?simple diagram? explains why the grounded conductor is not a neutral. And Jim, aren?t the ?hot and grounded conductor? in a 2 wire 120V circuit the same 2 conductors as in a 3 wire 120/240 V supply, whether you call it a ?neutral? or not? I have never seen a 120 V supply to a structure that wasn?t taken from a typical 120/240 V transformer, also supplying other structures in the area with 120/240 V supplied power. I guess the power company just deals with the unbalanced current.
 
Re: Meaning of "Phase"

Originally posted by rattus: Now give me an example of an ungrounded neutral.
Click to expand...
Here?s one, though I admit it is an imprecise use of the terms. From my Navy days, I recall that the 120 volt on-board systems were not ?grounded,? meaning that there was no connection to the ship?s hull. The two wires going out to receptacle and lighting loads were described as ?hot? and ?neutral.? ;)
 
Re: Meaning of "Phase"

Roger,

True, but the neutral is designed to carry continuous current while the EGC (we hope) never carries current.

That is to say, the neutral conductor is current carrying wire with a neutral (earth) voltage. Now, I expect some purists to draw and quarter me for taking issue with the confusing terminology.
 
Re: Meaning of "Phase"

While voltage configurations may be different a "phase" is any conductor from which power is derived. A phase, based on system configuration will have a specific potential difference in relation to: other phases, neutral, grounded circuit conductor or to the grounding conductor where the neutral or grounded circuit conductor are bonded to ground.
 
Re: Meaning of "Phase"

Charlie B.,

There is the right way, the wrong way, and the Navy way. Right? I think in this case, we have a transfer of terms from land based installations which does not quite fit. Both wires are hot with respect to each other.

Now, I wonder why they didn't tie the "neutral" to the hull at the generator? It seems that such a system would present less of a shock hazard--maybe?
 
Re: Meaning of "Phase"

Yes, you are right. It would present less of a shock hazard. But they are more concerned with the ability of the system to continue to operate after sustaining battle damage.
 
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