2 Phases of a 3 Phase System

[Sahib]

phase is a property of a signal and does not need reference

Yes in the sense that the phase of a signal is measured with respect to another signal such as voltage signal measured with respect to current signal.

But if either signal is measured independently, it should have an explicit reference for phase : time.

 

[JFletcher]

Could you have a motor that uses just two phases, like AB and BC without using the 3rd, AC, phase? Nevermind why you'd want to do this, just asking if it is possible.

 

[ActionDave]

Could you have a motor that uses just two phases, like AB and BC without using the 3rd, AC, phase? Nevermind why you'd want to do this, just asking if it is possible.

Yes. That would be a single phase motor.

Never mind. I misread the question.

 

[GoldDigger]

Could you have a motor that uses just two phases, like AB and BC without using the 3rd, AC, phase? Nevermind why you'd want to do this, just asking if it is possible.

Sure. But it will not develop uniform torque over the full rotation. That is one of the advantages of a three phase motor.
It would still self start with additional hardware.

What you describe would be geometrically equivalent to a full three phase motor with one winding open. This would be different from a three phase motor with one input wire open, which make it a single phase motor with severe performance issues.

I suspect that if all winding details were equal the current in the two windings might not be equal.

 

[Sahib]

The phase of a signal is measured with respect to another signal such as voltage signal measured with respect to current signal.

Remember Lissajous figures.

 

[Dennis Alwon]

so if a single phase system is derived from two legs of a 3 phase 208/120 system are these legs 120 degrees apart? I have heard single phase is 0 degrees, 180, and 120. :?

 

[Sahib]

Use an oscilloscope and find for yourself.

I vote 120 (for voltages)

However the currents are 180 degrees apart.

 

[Dennis Alwon]

Use an oscilloscope and find for yourself.

I vote 120 (for voltages)

However the currents are 180 degrees apart.

So you think everyone owns an oscilloscope...:happyno:

 

[Sahib]

Not a big one; a hand-held variety ( such as a spectrum analyser in the bag of an electrician) will do.:happyyes:

 

[Dennis Alwon]

Not a big one; a hand-held variety ( such as a spectrum analyser in the bag of an electrician) will do.:happyyes:

:happysad::happysad::happysad: Sorry not a standard tool for electricians that I know. I am sure some have them

 

[Fitzdrew516]

Most anyone that has ever asked the question the OP did gets explanations way beyond their comprehension and don't know what to ask to help with what they want to understand on a somewhat simpler level.

It has been asked before sometimes in different ways but basically "what is a phase?" and with all the minds that participate here there is no single consensus on that definition that I can recall ever seeing.

I wouldn't say this topic is beyond my comprehension. I'm aware of what 3ph is and how it is derived and the math behind it. I've sat through all the theory classes and changed my i's to j's many times just like most here have (actually programmed it in my calculator that way ). I think I posted this Friday morning and then had my PM come tell me a job had to go out the door that day.... SURPRISE! So needless to say I got a little busy and I don't typically check this when I'm out of the office (weekends).

After reading through all the posts I'd have to agree that this rabbit trailed quite a bit. However, I'm ok with that. I like watching people's minds stir up discussion; it makes for a better learning process.

 

[mivey]

so if a single phase system is derived from two legs of a 3 phase 208/120 system are these legs 120 degrees apart? I have heard single phase is 0 degrees, 180, and 120. :?

I'll add another but specify the signal(s) you are talking about first.

There are two degree separations one might consider:

For the 120 volt signals, the two signals are 120 degrees apart if we take the smaller lag angle or 240 degrees apart if we take the larger lag angle (normally we reference the smaller angle because it makes more sense, especially when the third leg is present). So, we normally say two phase positions are present at any instant in time (see phase explanation below) and they have a 120 degree difference.

For the 208 voltage, there is only one unique signal of that magnitude so there is no other equal magnitude signal to use as an "apart" reference. We normally compare them in equal magnitude groups, and we also separate signals by unique signal point pairs (i.e. the same two signal reference points used together only once for the process of identifying signals). We could compare the 208 signal with the 120 signal but unless we are using them together we normally don't. So, with only the 208 volt signal, there is only one phase position present at any instant in time.

How many signals?:

Unless we use all three signals together, we normally use the 120 or 208 signals singly (separately) and thus we normally call them single-phase signals and loads. The utility industry calls the 120/208 3-wire a "network" system to distinguish it from a standard single-phase system.

When we use the signals together for a single load, we then consider how many signals we have (rarely done for less than three signals but there are some particular devices that use two signals). Thus, most loads are single, three, or multiples of three "phase" loads.



Phase:

Keep in mind that for these sinusoidal signals, "phase" is a location on a waveform measured relative to a reference point on the waveform. Uses of "phase" to indicate the number of signals, or to indicate which leg, etc. are all slang.

To measure "phase difference" (the "apart") between signals, we can use a common reference position on separate signals (like the minimums for example). If these wave positions occur at different times we have different phase positions (a phase difference). We can also look at any given instant in time and determine this difference in terms of degree separation. Since there are 360 degrees of phase in each waveform, we can measure this position difference at any instant.

 

[kwired]

Could you have a motor that uses just two phases, like AB and BC without using the 3rd, AC, phase? Nevermind why you'd want to do this, just asking if it is possible.

I believe that is one way to operate a true 2 phase motor. Doesn't necessarily work as efficiently as true 2 phase but you do get phase difference and rotation out of it where you can't do this with any single phase source at all without additional capacitors or inductors to create some phase shift.

 

[Besoeker]

I know there has to be a logical answer for this, but I can't seem to find it. I've been searching the internet, but the amount of misinformation that's out there is astounding. Here's the question -

Why is a feed that utilizes (2) phases off a 3ph system always called "single phase"? For example - You have a 208/120v 3ph, 4w panelboard that feeds a piece of equipment that is 208v single phase, but it uses (2) of the phases to do so. Why is this not called "2 phase"?

Thanks,
-Drew

Because it is single phase.

Measure the voltage across the two terminals of the load and there is only one voltage phase to be measured.

 

[Dennis Alwon]

I'll add another but specify the signal(s) you are talking about first.

Now I know why I am confused...

So what about a single phase 240V system delivered to a residence. It is single phase but we get 240V between the legs. Is that the same situation as you stated or do the phase angle change.

 

[kwired]

Because it is single phase.

Measure the voltage across the two terminals of the load and there is only one voltage phase to be measured.

I like this answer the best. Doesn't matter that it originates from a three phase source or even some hexahase or higher number of "phases" source, what is available for use can only be used as single phase in the absence of some kind of phase conversion component.

 

[Smart $]

Use an oscilloscope and find for yourself.

I vote 120 (for voltages)

However the currents are 180 degrees apart.

The voltage (singular) has only one phase.

To measure more than one [nominal] phase voltage requires more than the two 'lines' referenced.

 

[mivey]

​

Now I know why I am confused...

Sorry about that. Pictures might help but I'm not at a location where I can do that right now. I'll try with text.

So what about a single phase 240V system delivered to a residence. It is single phase but we get 240V between the legs. Is that the same situation as you stated or do the phase angle change.

It works the same way for any set of signals.

It works the same way for 3-wire 120/170 quadrature (the traditional "two-phase") as well except the phase angles change to 90 degrees insead of the 120 degrees we have with the 3-wire 120/208 network.

It works the same way with 3-wire 120/240 single-phase except the phase angles change to 180 degrees instead of the 120 degrees we get with the 3-wire 120/208 network.

For all three systems, I have used leg voltages with a common reference (the neutral) to define the signals, which is a common definition for measuring a common rise/fall between signal points (I'll put in a numerical example below if you want to use a different signal definition reference).

These would have the voltages L1 & L2 with a N common point. The voltage signals are L1 minus N for the first and L2 minus N for the second. Keep in mind that you can define the signals as L1 minus N for the first and N minus L2 for the second (you will change the phase differences between the defined signals).

120/208 volt numerical examples:

For the leg voltages:
L1-N = 120@0d = L1 (N is signal reference)
L3-N = 120@120d = L3 (N is signal reference)

At any point in time, L3 leads L1 by 120 degrees (or L1 leads L3 by 240 degrees). At any instant, a reference location (like the maximum or minimum or rising zero crossing, falling zero crossing, etc.) on signal L3-N will reach a value that the L1-N signal location will not reach until some time later (5.56 ms later for a 60 Hz signal). This separation at any instant can be measured as a phase difference of 120 degrees (it takes 360 degrees of phase change for the signal to complete a cycle).

However, since we normally only use either L1 or L3 separately for a device, we normally think of this as single-phase as we only serve single phase loads with the 120 volt signals. But we could serve a load that uses the 120 degree phase difference the same as we can serve a two-phase load that uses the 90 degree difference from a two-phase source. We would not call the 120 degree case "two-phase" as that name is reserved for the 90 degree case.

For the leg-leg 208 voltage we take the difference across the two windings (note that the sum would give us 120 volts at 60 degrees)
L1-L3 = (L1-N)-(L3-N) = 120@0d - 120@120d = 208@-30d

Note that the L1 signal leads this signal by 30 degrees, not that we normally compare them.

Only one signal of this 208 volt magnitude means only one phase position is present at any instant.



There is another thing to keep in mind and that is that we can use something different as the reference other than "N" for both.

120/208 volt numerical examples for the different signal definition:

For the leg voltages:
L1-N = 120@0d = L1N (N is signal reference)
N-L3 = 120@300d = NL3 (L3 is signal reference)

At any point in time, L1N leads NL3 by 60 degrees (or NL3 leads L1N by 300 degrees). At any instant, a reference location (like the maximum or minimum or rising zero crossing, falling zero crossing, etc.) on signal L1N will reach a value that the NL3 signal location will not reach until some time later (2.78 ms later for a 60 Hz signal). This separation at any instant can be measured as a phase difference of 60 degrees (it takes 360 degrees of phase change for the signal to complete a cycle).

However, since we normally only use either L1N or NL3 separately for a device, we normally think of this as single-phase as we only serve single phase loads with the 120 volt signals. But we could serve a load that uses the 60 degree phase difference the same as we can serve a two-phase load that uses the 90 degree difference from a two-phase source. We would not call the 60 degree case "two-phase" as that name is reserved for the 90 degree case.

For the leg-leg 208 voltage, we can now take the sum across the two windings instead of the difference (note that the difference would give us 120 volts at 60 degrees):
L1N+NL3 = (L1-N)+(N-L3) = 120@0d + 120@300d = 208@-30d

Note that the L1 signal leads this signal by 30 degrees, not that we normally compare them.

Again, only one signal of this 208 volt magnitude means only one phase position is present at any instant.



Finally:

You might wonder if we can physically add and subtract these winding voltages the same way we do with math and the answer is yes, and we do exactly that in real world applications.

I can go through the same numerical examples for 120/240 or 120/170 but it works the same way for any set of signals.

 

[mivey]

Because it is single phase.

Measure the voltage across the two terminals of the load and there is only one voltage phase to be measured.

For the 208 volts: yes; one signal means only one phase. But even with the two 120 volt signals, they would normally only be used one at a time with 120 volt single phase loads so the whole thing would tend to get thrown in the single-phase bin because only single-phase loads are served.

 

[mivey]

phase is a property of a signal and does not need reference

It has a phase at any given instant but to quantify the phase value we need a reference, rising zero crossing, falling zero crossing, maximum, minimum, etc.
A voltage signal is defined relative to some reference. A current signal is defined relative to some node or cross section.