In particular starting torque. All motors need a push in the right direction to get them going. Often this is an artificial phase made by the starting cap. You really want 3 phase for the big motors. In fact the power company generates 3 phase power (sort of the reverse of a synchronous motor), barring electronic means it is hard to get otherwise.

It is indeed. The main advantage here is that you can combine the phases to get a higher voltage. Less current. A 120V dryer would take some hefty wiring.

Jeff

Not hard to get. A rotary phase converter will do it. Many woodworkers use a three-phase induction motor as a rotary phase converter to power big tools (often bought at auction from commercial installations).

Technically, it's not two phase at all.

On 1/15/2011 11:22 AM, snipped-for-privacy@att.bizzzzzzzzzzzz wrote:

I hadn't seen those before, using just a 3 phase idler motor. The Rotary Phase Converters I had see were more traditional, and more involved pieces of machinery (not to mention expensive). But it is not necessary for a woodworking motor to have perfectly balanced 3 phase. Clever and useful, nonetheless.

HVDC doesn't seem to have much impact in the US, but my rough understanding is that converting back is all solid state these days.

Call it what you want, but the two 120V lines are 180 out of phase relative to neutral. Certainly not useful for starting a motor. I'll not quibble over symantics.

Jeff

I hadn't seen those before, using just a 3 phase idler motor. The Rotary Phase Converters I had see were more traditional, and more involved pieces of machinery (not to mention expensive). But it is not necessary for a woodworking motor to have perfectly balanced 3 phase. Clever and useful, nonetheless.

HVDC doesn't seem to have much impact in the US, but my rough understanding is that converting back is all solid state these days.

Call it what you want, but the two 120V lines are 180 out of phase relative to neutral. Certainly not useful for starting a motor. I'll not quibble over symantics.

Jeff

No they, in fact, aren't. One is the negative of the other.

snipped-for-privacy@att.bizzzzzzzzzzzz wrote:
...

Which is the same as a time phase shift of pi radians.

To see so (in Matlab)

>> t=[0:2*pi/100:2*pi]; >> v=sin(t); >> plot(t,v,t,-v) >> hold on >> plot(t2,v,'rx') >>

The resulting plot is a complete sine wave over 0-2pi, the negative of that and the last (while starting at -pi) overlays the -v section from 0-pi identically (and will from there on out if extended the t2 axis.

There are two meanings of "phase" here which is the difficulty in common usage. The generation is indeed a single electrical phase; the two derived currents are out of phase (in time) with each other.

--

Which is the same as a time phase shift of pi radians.

To see so (in Matlab)

>> t=[0:2*pi/100:2*pi]; >> v=sin(t); >> plot(t,v,t,-v) >> hold on >> plot(t2,v,'rx') >>

The resulting plot is a complete sine wave over 0-2pi, the negative of that and the last (while starting at -pi) overlays the -v section from 0-pi identically (and will from there on out if extended the t2 axis.

There are two meanings of "phase" here which is the difficulty in common usage. The generation is indeed a single electrical phase; the two derived currents are out of phase (in time) with each other.

--

Matlab is wrong.

snipped-for-privacy@att.bizzzzzzzzzzzz wrote:

...

Not unless

sin(pi–t) = sin t

cos(pi–t) = –cos t

are no longer identities... :(

--

...

Not unless

sin(pi–t) = sin t

cos(pi–t) = –cos t

are no longer identities... :(

--

Ok, your understanding of what Matlab is telling you is wrong.

snipped-for-privacy@att.bizzzzzzzzzzzz wrote:

How thinketh thou so? That's what's happening in the transformer by the location of the two taps -- taking the voltage at two differing points along the (single) sinusoidal waveform at the same point in time is the same thing as a phase shift of one relative to the other.

Again, as noted above it's the confusion between the two meanings of "phase" -- the (single) electrical generation phase and the phase shift along that sinusoidal waveform for the two individual voltages.

--

How thinketh thou so? That's what's happening in the transformer by the location of the two taps -- taking the voltage at two differing points along the (single) sinusoidal waveform at the same point in time is the same thing as a phase shift of one relative to the other.

Again, as noted above it's the confusion between the two meanings of "phase" -- the (single) electrical generation phase and the phase shift along that sinusoidal waveform for the two individual voltages.

--

Math <> engineering

Nope. Define CT as zero. The signals at each end are the same but opposite sign.

THen why are you using Matlab as your source?

snipped-for-privacy@att.bizzzzzzzzzzzz wrote:
...

Of course, because one "leads" the other by pi radians...

Not "source", simply a demonstration of how the phase shift leads to the apparent negation of a sine wave.

--

Of course, because one "leads" the other by pi radians...

Not "source", simply a demonstration of how the phase shift leads to the apparent negation of a sine wave.

--

Wrong, obviously.

Complete bullshit.

snipped-for-privacy@att.bizzzzzzzzzzzz wrote:
...

...

I didn't see the above mind-boggler earlier... :(

It (application of math) is pretty much the definition of engineering...

--

...

I didn't see the above mind-boggler earlier... :(

It (application of math) is pretty much the definition of engineering...

--

David Nebenzahl wrote:

It's been awhile since I asked any silly questions. Probably at least two minutes so I'm behind on my quota. Isn't the distinction also about timing? There is no time with three phase power when the voltage is zero on all three lines. There is in single phase. Did that happen with the old two phase systems?

It's been awhile since I asked any silly questions. Probably at least two minutes so I'm behind on my quota. Isn't the distinction also about timing? There is no time with three phase power when the voltage is zero on all three lines. There is in single phase. Did that happen with the old two phase systems?

My understanding is that the ancient two phase had a phase difference of 90 deg, so there would also not be any time when there is no voltage difference between the conductors.

I have to join dpb, David, and Jeff in saying that krw is totally wrong.

To address some of the specifics:

krw: "No they,(2 hots in 240V service) in fact, aren't. One is the negative of the other. "

One is the negative of the other and it's also 180deg out of phase with the other. Take a sine wave centered around zero, shift it by 180deg and it becomes the negative of the other.

From dpb: "There are two meanings of "phase" here which is the difficulty in common usage. The generation is indeed a single electrical phase; the two derived currents are out of phase (in time) with each other. "

I said about the same thing many posts ago. Is the 240V service in use today generally referred to as two phase? No. Is it the same as the ancient two phase system that was referred to that way? No, because there the phases differed by 90deg. What is the phase relationship between the two hots in today's 240V service? They have a phase difference of 180deg.

Matlab is wrong. "

Explain then how that is. If I have 3 signals that are 120 deg out of phase, krw agrees that consists of 3 phases. Apparently he agrees that the ancient 2 phase system, where the phase difference was 90deg, consists of 2 phases. So, why exactly if we have a system where there are two conductors, one of which is 180deg out of phase with the other, does it no longer qualify as consisting of 2 phases?

It's merely a special case of phase difference, where they happen to be 180deg out of phase and one is the direct opposite of the other. Try this mental experiment. Take two identical signals that are in phase, ie the phase difference is zero. Start adding phase shift, 1 deg at a time. Do you not now have a system with two phases? Continue until you have 180 deg of phase shift. Why now are there not still two phases?

Answer: In fact there are, it's just a special case where one can now be called the opposite or negative of the other.

krw: Wrong, obviously. "

Actually it's absolutely correct. I also don't understand the comment:

"Math <> engineering"

A core component of electrical engineering is math. Most of the courses are heavily math oriented. And the math engineers use is the same as the math everyone else uses. And the phase relationship of two signals would be described the same by anyone using math.

krw: "Nope. Define CT as zero. The signals at each end are the same but opposite sign"

Geez, opposite sign is exactly how you get 180deg phase difference.

Complete bullshit. "

I'm a degreed Electrical Engineer, and it isn't BS to me. If you take a signal centered around zero volts, and another one in the same system that is also centered around zero volts, but is 180 deg out of phase and they share that common zero volt referrence, then:

One leads the other by 180deg One lags the other by 180deg One is the opposite of the other One is the negative of the other. You have two phases

That is exactly what you have with a 240V service. Is it generally referred to as a two phase system? No. Probably because it can be generated off of ONE phase coming from the power plant via a center tap transformer. But how I generate it matters not a wit. KRW, ask yourself this. You say you're an engineer. Here's a simple test:

1 - I have a graph of what we all have been describing, what I outlined above. Two sine waves on a graph, both centered around the zero voltage axis, both from the same system. One is shifted by 90 deg from the other. How many phases are there in this system?

2 - Same graph, but now one is shifted by 180 deg from the other. How many phases in this system?

3- Same graph, but now I have 3 sine waves, one shifted by 90, one by 180. How many phases in this system?

My answer, and I think the answer from all other 3 who disagree with you would be:

two two three

snipped-for-privacy@optonline.net wrote:

Nobody I know would call 120/240 2-phase. You wouldn't buy a single core transformer and specify whether it is in-phase or 180 degrees out of phase. You don't get multiple phases out of a single transformer. If you ask for a 2-phase transformer you will completely confuse the transformer rep.

Analysis of real multiphase electricity commonly uses phasor analysis, using SQR(-1).

A simple 120/240V system is single phase with the math handled with***trivial*** plus and minus signs. "2-phases" confuses trivial math.
Calling it 2-phase confuses communication with anyone who understands
multiphase power systems.

If 120/240 is 2-phase, then single phase has no particularly useful meaning.

If you tell a utility you want a 120/240V 2-phase service what will they say? That is after they stop laughing.

-------------------- I never heard 120/240 called split phase either. A wiki article says it is. But the article also says 120/240 "it is sometimes incorrectly referred to as 'two phase'"

Nobody I know would call 120/240 2-phase. You wouldn't buy a single core transformer and specify whether it is in-phase or 180 degrees out of phase. You don't get multiple phases out of a single transformer. If you ask for a 2-phase transformer you will completely confuse the transformer rep.

Analysis of real multiphase electricity commonly uses phasor analysis, using SQR(-1).

A simple 120/240V system is single phase with the math handled with

If 120/240 is 2-phase, then single phase has no particularly useful meaning.

If you tell a utility you want a 120/240V 2-phase service what will they say? That is after they stop laughing.

-------------------- I never heard 120/240 called split phase either. A wiki article says it is. But the article also says 120/240 "it is sometimes incorrectly referred to as 'two phase'"

--

bud--

bud--

Whether 2 phases confuses anyone or not has no bearing on the fact that there are two phases. I could describe many physical processes by either very simple terms or varying degrees of complexity. When looking at electrical waveforms, that trivial plus and minus sign can equate to being described as 180deg out of phase.

I noticed you didn't specifically refute any of the statements:

I never said to call it 2-phase power, nor do I recall anyone else here really doing so. A couple of us have said consistently that while you can view a 240V service as having two phases that are 180 deg out of phase with each other, that terminology is not commonly used to refer to the actual service.

Let's get back to basics. The definition of phase and it's use in electrical engineering goes to the very roots of the discipline. IT doesn't depend on what terms people commonly call something. It doesn't depend on how the phases are generated. IT's not limited to only AC power systems. I can take any linear system that has a sine wave going into it and ask how many shifted sine waves of voltage are present in that system and what are their relationship to each other. I can ask a student to plot the voltages at various points in the system. Let's say I have a box with 3 wires coming out. Between A and C, there is one sine wave. Between B and C, there is an identical sine wave, but it's shifted by 90deg. I ask, what is the relationship between them and how many different phases are present.

What would your answer be?

Now the same experiment, but with the sine waves shifted by 180deg instead of 90. Is the correct answer that it is now simply a plus and minus, trivial issue and there is only one phase present?

Again, neither I nor anyone else I believe, has called it "2 phase service". What common terms are used and what things really are, are two different things. If I told probably 1/4 of Americans that my son is a homosapien, I'd get the same reaction because they don't even know what it means. It doesn't make it untrue or not technically correct.

Unfortunately, I'm heading out for a few days. I'll have to pick back up on this later on.

snipped-for-privacy@optonline.net wrote:

When the "phases" come from a single phase source (the utility transformer), and one of the "phases" is the negative of the other, calling them 2 phases makes no particular sense.

When I connect my 120-to-120V isolation transformer (for repairing equipment) to one of the "phases" is the secondary the "A" phase or the "B" phase?

This, of course, comes from a multi-phase source.

The voltages come from a single phase source (the service transformer primary is 2 wire). You get 2 phases from a single phase source?

The voltage on the secondary 2 wire combinations from that transformer are locked into either a positive or negative relation to other 2 wire combinations. When calculating currents in a 120/240V system with resistive loads you use plus and minus signs. Calling it 2-phase adds nothing useful and is not useful in calculations.

In the more general phasor calculations (which handle capacitance and inductance), a 120/240V system is represented as +120 and -120V (both "real").

Calling 120/240V "2-phase" makes as much sense as calling the old Edison 3-wire DC system 2-phase (at zero frequency).

Have a good trip.

When the "phases" come from a single phase source (the utility transformer), and one of the "phases" is the negative of the other, calling them 2 phases makes no particular sense.

When I connect my 120-to-120V isolation transformer (for repairing equipment) to one of the "phases" is the secondary the "A" phase or the "B" phase?

This, of course, comes from a multi-phase source.

The voltages come from a single phase source (the service transformer primary is 2 wire). You get 2 phases from a single phase source?

The voltage on the secondary 2 wire combinations from that transformer are locked into either a positive or negative relation to other 2 wire combinations. When calculating currents in a 120/240V system with resistive loads you use plus and minus signs. Calling it 2-phase adds nothing useful and is not useful in calculations.

In the more general phasor calculations (which handle capacitance and inductance), a 120/240V system is represented as +120 and -120V (both "real").

Calling 120/240V "2-phase" makes as much sense as calling the old Edison 3-wire DC system 2-phase (at zero frequency).

Have a good trip.

--

bud--

bud--

On 1/18/2011 9:49 AM bud-- spake thus:

I guess I'd have to call that question a red herring.

In the case of a transformer such as you describe, presumably with no center tap, then yes, there's only one phase. Only one set of conductors.

We're talking about something different: a center-tapped transformer, such as the utility company uses to deliver what's typically called "split-phase" power (i.e., 120-0-120).

There, you***do*** have two phases.

The main objector in this discussion rejects this, apparently because they don't consider the "inverse" of a phase (meaning a set of conductors that's 180° out of phase with another set), to be a separate phase. But it is.

It's just that this is not commonly***called*** "2-phase power": that
refers to something else, specifically that obsolete system with one
phase 90° to the other that's been described here.

Again: the output of a center-tapped transformer, whatever its use, is in fact 2 distinct and separate phases. But for some reason, it's not called that.

Now, I look forward to***your*** comments on my comments ...

I guess I'd have to call that question a red herring.

In the case of a transformer such as you describe, presumably with no center tap, then yes, there's only one phase. Only one set of conductors.

We're talking about something different: a center-tapped transformer, such as the utility company uses to deliver what's typically called "split-phase" power (i.e., 120-0-120).

There, you

The main objector in this discussion rejects this, apparently because they don't consider the "inverse" of a phase (meaning a set of conductors that's 180° out of phase with another set), to be a separate phase. But it is.

It's just that this is not commonly

Again: the output of a center-tapped transformer, whatever its use, is in fact 2 distinct and separate phases. But for some reason, it's not called that.

Now, I look forward to

--

Comment on quaint Usenet customs, from Usenet:

To me, the *plonk...* reminds me of the old man at the public hearing

Comment on quaint Usenet customs, from Usenet:

To me, the *plonk...* reminds me of the old man at the public hearing

Click to see the full signature.

David Nebenzahl wrote:

It is a minor illustration that "2 phases" is not useful.

It makes no sense to say you get 2 phases out of what is obviously a single-phase utility transformer.

From the wikipedia article http://en.wikipedia.org/wiki/Split_phase "it is sometimes incorrectly referred to as 'two phase'." (The article also suggests split phase is not the best name because of confusion with split-phase motors - which do start on 2-phases. I have never heard "split-phase" used for a 120/240V service.)

You can invent your own language. Where is any reasonable source that says a single phase transformer has 2 phases.

Not according to wikipedia. Find a transformer manufacturer that says their single phase transformer has secondaries that are 2 phases.

Not***commonly*** called 2-phase?
It is not ***ever*** called 2-phase. It is not a term used by any utility or
manufacturer.
It is a "single-phase" service, transformer, meter can, panel....

Wow, progress. You are right - "it's not called that."

It is not called that because it makes no sense. Voltages clearly comes from a single-phase transformer. The voltages of any secondary windings are locked into plus or minus relationships. The secondary is trivially understood with plus and minus signs. Calculations use plus and minus signs (not "phases"). Where inductance and capacitance are involved phasor analysis is used. A 120/240V service is characterized as +120 and -120.

It is not called that because it has no practical usefulness. (Except to cause confusion.)

It is a minor illustration that "2 phases" is not useful.

It makes no sense to say you get 2 phases out of what is obviously a single-phase utility transformer.

From the wikipedia article http://en.wikipedia.org/wiki/Split_phase "it is sometimes incorrectly referred to as 'two phase'." (The article also suggests split phase is not the best name because of confusion with split-phase motors - which do start on 2-phases. I have never heard "split-phase" used for a 120/240V service.)

You can invent your own language. Where is any reasonable source that says a single phase transformer has 2 phases.

Not according to wikipedia. Find a transformer manufacturer that says their single phase transformer has secondaries that are 2 phases.

Not

Wow, progress. You are right - "it's not called that."

It is not called that because it makes no sense. Voltages clearly comes from a single-phase transformer. The voltages of any secondary windings are locked into plus or minus relationships. The secondary is trivially understood with plus and minus signs. Calculations use plus and minus signs (not "phases"). Where inductance and capacitance are involved phasor analysis is used. A 120/240V service is characterized as +120 and -120.

It is not called that because it has no practical usefulness. (Except to cause confusion.)

--

bud--

bud--

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