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Title: Two Phase???
Post by: Mike Sokol on February 02, 2016, 08:38:56 pm
I'm sensing some confusion on the part of our international visitors when we discuss single-phase vs. 3-phase distros. I've always hated that while we call typical USA 120/240-volt power "single phase", it is in fact TWO phases of power, 180 degrees apart. That's how all the math works for current, voltage, etc... When I've discussed this conundrum with my power station buddies they laugh and tell me I'm nuts, that 120/240 volt power only needs a single phase. I think that's because when they say "phase", they're referring to a single "wire phase" on a transmission line from the substation, which we then convert into two hot wires at 180 degrees of phase using a center-tapped transformer feeding a typical house or office. Of course, industrial buildings and theaters use all three of the power line "phases" that are 120 degrees apart, and thus that's obviously 3-phase. I usually end up referring this this single-phase 120/240-volt stuff as "Split Phase" when discussing wiring with my sound teams, but is there a better way? Or should we stand up for our right to call it what it is, 2-phase, and let the electricians mumble to themselves about our sanity?

Just trying to clarify, not obfuscate...

 
Title: Re: Two Phase???
Post by: David Buckley on February 02, 2016, 09:20:34 pm
I'm a big fan of the term "split phase", which is the nearest thing to a reasonable description of USA 110/220 eleccy.

Sometimes on here I get confused between a single phase distro, and a single phase distro, one having just one hot, the other having two hots.  I did see, in the UK, some years ago, a big single phase 110V distro intended to run of a genset for a visiting band, that was really was single phase with cams, just one hot.

Two phase is just wrong; there really are (or, more accurately, were) two phase systems and they were something else again.
Title: Re: Two Phase???
Post by: Geoff Doane on February 02, 2016, 10:21:04 pm
... Or should we stand up for our right to call it what it is, 2-phase, and let the electricians mumble to themselves about our sanity?

Just trying to clarify, not obfuscate...

 

An electrician once pointed out to me that it's not "2 phase" unless you're just using 2 phases of a 3-phase system.

It's probably best to call it what the electrician will understand, so that we get what we need when specifying a service for our sound or lighting gig.  Maybe even giving some extra or redundant information so that nobody makes ASSumptions.

For my typical needs, I find that asking for a "120/240V, 100A, 4-wire service.  2 hots, neutral and ground", gets me what I want and need.  Or alternately, a "range receptacle, breakered at 50A*, NEMA 14-50 type" for smaller services.  Don't confuse the issue (as I've seen in some recent threads here) by saying you need "100A of power" when you're talking about the 14-50 receptacle.  You may wind up with five 5-20 receptacles, each on its own breaker.  No good at all if you have some actual 208 or 240V loads, and you lose the benefit of having everything on a known common ground (your distro).

GTD

* In Canada and maybe parts of the US too, it's important to mention the 50A breaker.  Most of these are installed in homes, for electric ranges (stoves), and in that particular case they are allowed to use 8/3 (plus ground) wire and breaker it at 40A.  But if you want to make full use of the distro, spec 50A, and they'll have to use larger wire, which even if you're not using it to full capacity, will give you a stiffer supply, which is usually a good thing.
Title: Re: Two Phase???
Post by: TJ (Tom) Cornish on February 02, 2016, 10:22:03 pm
In my opinion, "two-phase" is an understandable and reasonable way to describe the load side.  How it's derived from the power company isn't particularly relevant to end users.

I seriously doubt anyone would confuse a modern usage of "two-phase" with a system that's been obsolete for 100 years.
Title: Re: Two Phase???
Post by: Jonathan Johnson on February 02, 2016, 11:27:11 pm
For my typical needs, I find that asking for a "120/240V, 100A, 4-wire service.  2 hots, neutral and ground", gets me what I want and need.  Or alternately, a "range receptacle, breakered at 50A*, NEMA 14-50 type" for smaller services.  Don't confuse the issue (as I've seen in some recent threads here) by saying you need "100A of power" when you're talking about the 14-50 receptacle.  You may wind up with five 5-20 receptacles, each on its own breaker.  No good at all if you have some actual 208 or 240V loads, and you lose the benefit of having everything on a known common ground (your distro).

It's good to specify the voltage (120/240V), as the NEMA 14-50 range plug *may* be used on two phases of three-phase wye service, providing 120/208V. Most ranges in North America are rated for both configurations. If you don't specify, you could end up with 120/208V in some commercial installations.
Title: Re: Two Phase???
Post by: Jeff Bankston on February 02, 2016, 11:29:09 pm
I'v always called it 2 phase. I have been made fun of. I dont care. I'm out of phase most of the time anyway. I cant hep it , I'm a drummer.
Title: Re: Two Phase???
Post by: Mike Sokol on February 03, 2016, 12:27:50 am
An electrician once pointed out to me that it's not "2 phase" unless you're just using 2 phases of a 3-phase system.
So is connecting a 50-amp "stove plug" to two legs of 3-phase, with the resultant 120/208-volt output, "Two Phase", while hooking it into two legs of 120/240-volt center-tapped transformer "Single Phase"? Seems like splitting hairs.

 Since I studied Electrical Engineering before I had any "Electrician" training, I tend to think of this as a different in phase shift (120 vs. 180 degrees) rather than how it's derived from the power company.

And yes, I know historically about 2-phase/90-degree power systems which are apparently still in use in some old electric railroads up north. From Wikipedia "As of 21st century, two-phase power was superseded with three phases and is not used in the industry. There remains, however, a two-phase commercial distribution system in Philadelphia, Pennsylvania; many buildings in Center City are permanently wired for two-phase[citation needed] and PECO (the local electric utility company) has continued the service. This type of service happens to exist in Hartford, Connecticut. It does serve a few buildings in that city."
Title: Re: Two Phase???
Post by: Steve M Smith on February 03, 2016, 02:55:15 am
I think I understand it now.  It could be confusing to us as all of our power distribution is three phase, so a single phase supply is just one leg of the three phase.

When I first heard the term two phase, I assumed it was two phases at ninety degrees rather than a centre tap on a single phase.

What I don't know though is how it is derived in the US.  Is it still from a transformer on part of the three phase distribution further down the line or do you run both single/two phase and three phase systems?


Steve.
Title: Re: Two Phase???
Post by: Mike Sokol on February 03, 2016, 06:38:50 am
What I don't know though is how it is derived in the US.  Is it still from a transformer on part of the three phase distribution further down the line or do you run both single/two phase and three phase systems?
In the US we only distribute 3-phase over any distance. Then at the point of use we either send in all three phases or a single phase. That single phase is typically 240 volts which is then split down to two hot legs with 120 volts each by a center-tapped transformer. So essentially, all small and medium size buildings in the US get either 3-phase 120/208 or single phase 120/240 service. We also use 3-phase 277/480 volt service for a lot of industrial plants to run motors, and many industries receive even higher voltages for industrial processes such as glass making, etc... I'm sure others here can chime in on special voltages needed by specific industries.
Title: Re: Two Phase???
Post by: Jeff Bankston on February 03, 2016, 06:44:50 am
I think I understand it now.  It could be confusing to us as all of our power distribution is three phase, so a single phase supply is just one leg of the three phase.

When I first heard the term two phase, I assumed it was two phases at ninety degrees rather than a centre tap on a single phase.

What I don't know though is how it is derived in the US.  Is it still from a transformer on part of the three phase distribution further down the line or do you run both single/two phase and three phase systems?


Steve.
single phase is one hot wire and a ground connected to a transformer. the transformer splits one hot wire into 2 hot wires. one phase goes in the transformer and two phases come out. single phase is used in the rural countryside. its cheap and is all thats needed. you can run any 240v equipment on it. my cousins farm is fed with one hot wire and a ground. thats all you see in the mississippi and california country side and most of rural america. heres a foto.
Title: Re: Two Phase???
Post by: Steve M Smith on February 03, 2016, 07:49:53 am
Much the same as we have then except for the centre tap to get two 110v supplies.

We tend to use large substation transformers at strategic places in the town and distribute three phases of 230v.  A house will get one plus neutral and a commercial premises will get all three plus neutral.

We only see those little pole mounted transformers near farms and other out of the way places.

Most in town distribution is underground now, but in the places where it is distributed by poles, there will be four wires.  Often the telephone lines share the poles with wires about 6' below power lines.


Steve.
Title: Re: Two Phase???
Post by: Cailen Waddell on February 03, 2016, 08:54:15 am

I'm sensing some confusion on the part of our international visitors when we discuss single-phase vs. 3-phase distros. I've always hated that while we call typical USA 120/240-volt power "single phase", it is in fact TWO phases of power, 180 degrees apart. That's how all the math works for current, voltage, etc... When I've discussed this conundrum with my power station buddies they laugh and tell me I'm nuts, that 120/240 volt power only needs a single phase. I think that's because when they say "phase", they're referring to a single "wire phase" on a transmission line from the substation, which we then convert into two hot wires at 180 degrees of phase using a center-tapped transformer feeding a typical house or office. Of course, industrial buildings and theaters use all three of the power line "phases" that are 120 degrees apart, and thus that's obviously 3-phase. I usually end up referring this this single-phase 120/240-volt stuff as "Split Phase" when discussing wiring with my sound teams, but is there a better way? Or should we stand up for our right to call it what it is, 2-phase, and let the electricians mumble to themselves about our sanity?

Just trying to clarify, not obfuscate...

 

I like calling it split phase with two hot wires, out of polarity with each other.  Seems more accurate than out of phase... 


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Title: Re: Two Phase???
Post by: Cailen Waddell on February 03, 2016, 08:59:05 am

single phase is one hot wire and a ground connected to a transformer. the transformer splits one hot wire into 2 hot wires. one phase goes in the transformer and two phases come out. single phase is used in the rural countryside. its cheap and is all thats needed. you can run any 240v equipment on it. my cousins farm is fed with one hot wire and a ground. thats all you see in the mississippi and california country side and most of rural america. heres a foto.

Nice graphics.... It's probably worth noting that primary voltage can vary.  For example, most of our town has primary at 13k volts or 12470 in older parts of town.  There are also some places that step the 12470 down to 2400 and then derive the split phase 240v from the 2400 volt lines...

Probably what is important is that we all know single phase goes into the transformer and then split phase, 120/240v comes out of the center tapped transformer with two hot legs out of polarity. 


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Title: Re: Two Phase???
Post by: Matthew Knischewsky on February 03, 2016, 09:57:33 am
should we stand up for our right to call it what it is, 2-phase, and let the electricians mumble to themselves about our sanity? 

We should call it what everyone else expects it to be called, single phase. Using common terms that electricians and other technicians know is fundamental to "speaking the language" when communicating with those people. Teaching a name other than single phase would be doing a disservice to the students IMHO.

"Split Phase" may be helpful to understand the fundamentals of how single phase power is distributed to beginners, but if you're asking for a tie in or a connector to be supplied at a venue you're asking for "single phase" or "three phase" and at what voltage- 120/240 volts or 120/208 volts here in North America.

We have to remember that not everyone we encounter in our line of work knows exactly what our requirements are. Using the term "2 phase" around an electrician who often works with electric motors may create confusion, obsolete distribution scheme or not. Use the terms that are established and everyone knows what they're talking about.
Title: Re: Two Phase???
Post by: Stephen Swaffer on February 03, 2016, 10:19:35 am
Steve,

I often see your distribution system duplicated in heavily built up areas over here-one bank of transformers feeding both 3 phase and single phase services.  The term the local POCO uses for this setup is "networked" which kind of threw me the first time they told me that I needed my meter setup for a networked service.  The only difference is the addition of a "fifth" terminal to the meter to provide a neutral connection for accurate metering-typical single phase meters have no neutral connection to the actual meter.

I agree that split-phase is a more accurate and precise description.  Even on the load side of a service there is a distinct technical difference between 3 phase and single/split phase distribution-particularly in industrial situations were the actual service is 480 V (some industrials use 4160 and even 13.8 kV as well-not my cup of tea though).  Single phase/and split phase is typically a center-tapped transformer.  Three phase is done with 3 single phase transformers (typically in one box).  For trouble shooting purposes, I think it best to keep this distinction clear.

From an electrician's viewpoint, I would want a voltage/current/phase spec and then a receptacle or termination spec.  So 240V/50A, single phase with an 14-50 receptacle will get you 2 hots/neutral/ground-but unless you specify I will supply you anywhere from 208-240 depending on the building supply.  If you insist on 240 in a building with 208 3 phase service, it will cost more because I will have to bring in a buck/boost transformer to get it there.  I would think it best not to design a system that is so close to the edge it will only run on 240-if you design at 80% load as you should, then 208 feeding universal power supplies should be fine.

When working around 3 phase, keep in mind that 220 V/30A/4-wire could be interpreted as 3 hots + a ground-which won't make your audio gear happy-though if ALL of your gear will run on 220 then you  would actually have more available capacity with the same number of wires.

You wouldn't believe how many people say "I need 220 available" and think that is all I need to know.
Title: Two Phase???
Post by: Steve M Smith on February 03, 2016, 10:36:10 am
Its much easier here.  It's either single phase or three phase.  All 230v from live to neutral... except that three phase is often refered to as 415v which is the phase to phase voltage.

No 230v with a centre tap to give two 115v feeds, no funny offset to give 208v.

Basically it's all 230v, but sometimes you have three of them!

The only other variable is the current rating.


Steve.
Title: Re: Two Phase???
Post by: Jonathan Johnson on February 03, 2016, 12:44:43 pm
In the US we only distribute 3-phase over any distance. Then at the point of use we either send in all three phases or a single phase. That single phase is typically 240 volts which is then split down to two hot legs with 120 volts each by a center-tapped transformer. So essentially, all small and medium size buildings in the US get either 3-phase 120/208 or single phase 120/240 service. We also use 3-phase 277/480 volt service for a lot of industrial plants to run motors, and many industries receive even higher voltages for industrial processes such as glass making, etc... I'm sure others here can chime in on special voltages needed by specific industries.

In the United States, for residential and light commercial service, the common configuration is single phase, with the utility step-down transformer configured this way:

Heavier commercial properties are typically served by 102/208V three phase wye service. (High-leg 120/208/240V delta service is fading.) The primaries of the transformers are connected to their respective hot/live/phase wires, and to a common grounded neutral point. The secondaries each provide 120V single phase (with a common grounded neutral point) and 208V between any two phases. To serve 240V single-phase loads, a step-up transformer is provided (as customer equipment), with the primary connected to two phases of the 120/208V service (208V primary), and the secondary providing a 120/240V center-tapped single-phase service. 120V loads may be connected to either the 120/208V service or the 120/240V service.

* Note that many transformers have multiple taps to allow for connection to different voltages.
Title: Re: Two Phase???
Post by: Jonathan Johnson on February 03, 2016, 12:57:35 pm
I'm sensing some confusion on the part of our international visitors when we discuss single-phase vs. 3-phase distros. I've always hated that while we call typical USA 120/240-volt power "single phase", it is in fact TWO phases of power, 180 degrees apart. That's how all the math works for current, voltage, etc... When I've discussed this conundrum with my power station buddies they laugh and tell me I'm nuts, that 120/240 volt power only needs a single phase. I think that's because when they say "phase", they're referring to a single "wire phase" on a transmission line from the substation, which we then convert into two hot wires at 180 degrees of phase using a center-tapped transformer feeding a typical house or office. Of course, industrial buildings and theaters use all three of the power line "phases" that are 120 degrees apart, and thus that's obviously 3-phase. I usually end up referring this this single-phase 120/240-volt stuff as "Split Phase" when discussing wiring with my sound teams, but is there a better way? Or should we stand up for our right to call it what it is, 2-phase, and let the electricians mumble to themselves about our sanity?

Are the legs of a 120/240V service in phase (single phase) or 180 degrees out-of-phase (two phase)?

If they are 180 degrees out-of-phase, they should be cancelling. If they are in phase, they should be additive.

Because the voltage is indeed additive, we can deduce that they are indeed the same phase, so it must be single phase, not two-phase.

(As a side note, tapping from two phases (120V each) of a three-phase service provides a single-phase (208V) waveform.)

Consider a single-phase supplying a transformer with one primary and two secondaries. If we wire the secondaries in series, we have effectively a center-tapped secondary with two voltages available; 120V (between either end tap and the center tap) and 240V (between the end taps). Imagine two D-cell batteries in series; between the ends of the two batteries we have 3V, but between the center point and either end we have only 1.5V. If we wire the secondaries in parallel, we have only 120V, but double the current capacity. Imagine two batteries with the + wired together, and the - wired together. Only 1.5V, but twice the current capacity.

Now if we swap the polarity of ONE of the secondaries, we now have true two-phase service. If we connect them in series, we get nothing; they cancel. Imagine two batteries with only the + connected together; there is no voltage measured between the two - terminals. If we connect them in parallel; bad things happen -- we have created a short circuit between the secondaries. Just as if we connected the + of one battery to the - of the other, and the - of the first to the + of the second.

I really don't think you want your electrician supplying 180-degree "two-phase" service. You would either have nothing at all, or a meltdown.

120/240V service is best called "split single phase." Also, the two legs are not inverse polarity; they are opposite poles of a single phase.

Quote
Just trying to clarify, not obfuscate...

Calling something it is not does not clarify; it obfuscates. If there is dissension regarding particular terminology, it may indicate a misuse of that terminology.
Title: Re: Two Phase???
Post by: Stephen Kirby on February 03, 2016, 12:59:12 pm
I also like the term split phase as it clarifies that there are two hot legs.  Which helps clarify the 50A/100A confusion some people get into with range plug/CS "single phase" distros.

The electronics manufacturing plants I work in are typically fed with 3ph 480.  Often there is also a 120/240 service as well for office equipment.  The manufacturing equipment is made all over the place and some really needs 208 while others really want 230/240.  So we get the 208 off one of the 480 lines with a transformer and the 240 off the single phase service.  The large soldering equipment and large robotic machines typically take the 480 although some soldering equipment needs 380 for which we need a local transformer.  Not much stuff wants 3ph 208 although once in a while there will be something with mid sized motors and that requirement.
Title: Re: Two Phase???
Post by: Jeff Bankston on February 03, 2016, 02:24:56 pm
Nice graphics.... It's probably worth noting that primary voltage can vary. 
Thanks....Yes the primary voltage is different for different areas. I was told by a lineman that worked on rural lines that the primary voltage in Mississippi is 25K and higher depending on how far the line has to run from the substation to the last farm house. In town it can be anywhere from 7500V and up. iirc here in Los Angeles its around 35K.
Title: Re: Two Phase???
Post by: Cailen Waddell on February 03, 2016, 02:33:28 pm

We should call it what everyone else expects it to be called, single phase. Using common terms that electricians and other technicians know is fundamental to "speaking the language" when communicating with those people. Teaching a name other than single phase would be doing a disservice to the students IMHO.

"Split Phase" may be helpful to understand the fundamentals of how single phase power is distributed to beginners, but if you're asking for a tie in or a connector to be supplied at a venue you're asking for "single phase" or "three phase" and at what voltage- 120/240 volts or 120/208 volts here in North America.

We have to remember that not everyone we encounter in our line of work knows exactly what our requirements are. Using the term "2 phase" around an electrician who often works with electric motors may create confusion, obsolete distribution scheme or not. Use the terms that are established and everyone knows what they're talking about.

I agree to an extent.  If you are going to a venue, I would prefer a site visit, but baring that would ask for a specific outlet.  Any venue big enough for camlock power usually has 3 phase in my experience. 

If I was speaking to an electrician I would ask for a single phase, 4 wire, 120/240v service.   

Or if I wanted 3 phase, a 3 phase, 5 wire, 120/208 service. 

And if I came across high leg delta, I'd probably just leave. 


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Title: Re: Two Phase???
Post by: Stephen Swaffer on February 03, 2016, 02:53:14 pm
 

And if I came across high leg delta, I'd probably just leave. 

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And where is the fun in that?  You have let smoke out now and then just to keep things interesting!

Actually, many (all?) high leg or wild leg services are really variations of a split phase service-they just add an additional transformer and create a 3rd phase.  So, the right way to deal with it it to use the 2 "normal" legs and the neutral just like you would use a single/split phase service. Leave the wild leg alone for anything other than 3 phase motors just to be safe.  IMO, this is a really good argument against the "2 phase" terminology.  Someone might think since single phase uses 1 transformer, and 3 phase uses 3 that if they see 2 transformers you have "2 phase" and that could lead to an interesting result.
Title: Re: Two Phase???
Post by: Cailen Waddell on February 03, 2016, 02:56:20 pm
Are the legs of a 120/240V service in phase (single phase) or 180 degrees out-of-phase (two phase)?

If they are 180 degrees out-of-phase, they should be cancelling. If they are in phase, they should be additive.

Because the voltage is indeed additive, we can deduce that they are indeed the same phase, so it must be single phase, not two-phase.

(As a side note, tapping from two phases (120V each) of a three-phase service provides a single-phase (208V) waveform.)

Consider a single-phase supplying a transformer with one primary and two secondaries. If we wire the secondaries in series, we have effectively a center-tapped secondary with two voltages available; 120V (between either end tap and the center tap) and 240V (between the end taps). Imagine two D-cell batteries in series; between the ends of the two batteries we have 3V, but between the center point and either end we have only 1.5V. If we wire the secondaries in parallel, we have only 120V, but double the current capacity. Imagine two batteries with the + wired together, and the - wired together. Only 1.5V, but twice the current capacity.

Now if we swap the polarity of ONE of the secondaries, we now have true two-phase service. If we connect them in series, we get nothing; they cancel. Imagine two batteries with only the + connected together; there is no voltage measured between the two - terminals. If we connect them in parallel; bad things happen -- we have created a short circuit between the secondaries. Just as if we connected the + of one battery to the - of the other, and the - of the first to the + of the second.

I really don't think you want your electrician supplying 180-degree "two-phase" service. You would either have nothing at all, or a meltdown.

120/240V service is best called "split single phase." Also, the two legs are not inverse polarity; they are opposite poles of a single phase.

Calling something it is not does not clarify; it obfuscates. If there is dissension regarding particular terminology, it may indicate a misuse of that terminology.

I'm not sure I follow what you are saying, so let me state it the way I understand it, and perhaps we are saying it the same way.  Many people think the legs are out of polarity, and they are relative to the neutral, because of how we measure

I'm going to reference this handy graphic I found that shows it as I understands it:
http://ep.yimg.com/ty/cdn/yhst-14463325294384/Electric-240-Volts

If I place a probe of an o-scope on the neutral of a 120/240v single phase outlet, and the other probe on one hot leg, I get the blue trace,
If I then compare to the other hot leg, the legs will appear to be 180 degrees out of polarity and I get the red trace.
If I place the leads on both hot legs, i get the black trace.

The reason the red trace and blue trace appear to be out of polarity is because we are reversing the o scope leads by keeping one lead on the center tap.  To really measure you would not do this.  If I put scope lead 1 on a hot leg, and two on neutral, and then put scope lead 1 on neutral and two on the other hot leg, the legs would appear to be in phase.

Are we saying the same thing?
Title: Re: Two Phase???
Post by: Mike Sokol on February 03, 2016, 06:32:16 pm
Interesting that there's so much passion over this.  ;D

I thought I would poke this with a stick since I'm brushing up on my Phasor Diagrams, and note that the reason there's no neutral current in a 120/240 volt system with balanced loads is that the two legs are 180 degrees out of phase with each other: http://www.electronics-tutorials.ws/accircuits/phasors.html

Yup, if you measure the two hot legs with a scope, one leg will be swinging positive with respect to neutral, while the other leg will be swinging negative. To know why you just have to mentally rotate the one leg up and see that one secondary coil is wound clockwise and other is wound counter-clockwise. Of course these two legs are locked together at 180 degrees out of phase. To me this is just like a really high-voltage balanced output in an audio system.

Yeah, I'm not going to play games with electricians who can blow up my gear. I'll ask for single-phase 120/240 volt service of appropriate amperage, or 3-phase, 5-wire service with camlocks. 
Title: Re: Two Phase???
Post by: Jonathan Johnson on February 03, 2016, 07:07:53 pm
I'm not sure I follow what you are saying, so let me state it the way I understand it, and perhaps we are saying it the same way.  Many people think the legs are out of polarity, and they are relative to the neutral, because of how we measure

I'm going to reference this handy graphic I found that shows it as I understands it:
http://ep.yimg.com/ty/cdn/yhst-14463325294384/Electric-240-Volts

If I place a probe of an o-scope on the neutral of a 120/240v single phase outlet, and the other probe on one hot leg, I get the blue trace,
If I then compare to the other hot leg, the legs will appear to be 180 degrees out of polarity and I get the red trace.
If I place the leads on both hot legs, i get the black trace.

The reason the red trace and blue trace appear to be out of polarity is because we are reversing the o scope leads by keeping one lead on the center tap.  To really measure you would not do this.  If I put scope lead 1 on a hot leg, and two on neutral, and then put scope lead 1 on neutral and two on the other hot leg, the legs would appear to be in phase.

Are we saying the same thing?

I think we are. If you use the same lead (lead 1) on the neutral and you measure each hot leg with the other lead (lead 2), you are effectively inverting the polarity on your second measurement. Your second example (moving lead 1 from hot1 to neutral, and lead 2 from neutral to hot2) maintains proper polarity.

To rephrase it, connect lead 1 to hot1 and lead 2 to neutral. Consider this measurement "left to right." If you connect lead 1 to hot2, you are now measuring right to left (inverse polarity). (Connect lead 1 to neutral, and lead 2 to hot2 and you are again measuring left to right.) Now if you connect lead 1 to hot1 and lead 2 to hot2 you are measuring both secondary coils in the same direction (polarity).

When you attach a 240V load to a 120/240V center-tapped transformer, you are not connecting to two 120V coils wired in series with opposite polarity; you are connecting to two 120V coils wired in series with the same polarity. They are not 180 degrees out of phase.

(This would be better with drawings.)
Title: Re: Two Phase???
Post by: Cailen Waddell on February 03, 2016, 07:11:22 pm

I think we are. If you use the same lead (lead 1) on the neutral and you measure each hot leg with the other lead (lead 2), you are effectively inverting the polarity on your second measurement. Your second example (moving lead 1 from hot1 to neutral, and lead 2 from neutral to hot2) maintains proper polarity.

To rephrase it, connect lead 1 to hot1 and lead 2 to neutral. Consider this measurement "left to right." If you connect lead 1 to hot2, you are now measuring right to left (inverse polarity). (Connect lead 1 to neutral, and lead 2 to hot2 and you are again measuring left to right.) Now if you connect lead 1 to hot1 and lead 2 to hot2 you are measuring both secondary coils in the same direction (polarity).

When you attach a 240V load to a 120/240V center-tapped transformer, you are not connecting to two 120V coils wired in series with opposite polarity; you are connecting to two 120V coils wired in series with the same polarity. They are not 180 degrees out of phase.

(This would be better with drawings.)

Thanks to all - while Before now I understood the result, I have been misstating some of how the transformer windings and such worked to produce that result before now...  This discussion has helped clarify how I describe it. 


Sent from my iPhone using Tapatalk
Title: Re: Two Phase???
Post by: Jonathan Johnson on February 03, 2016, 08:08:51 pm
Typically when we measure 120V circuits, we reference to the center-tapped neutral (probably because that's the way we've always done it). When we measure 240V circuits, we don't reference to neutral. With a voltmeter, there's no benefit to observing polarity. The benefit comes when we use a scope and can actually compare waveforms.

If the center tap is our reference, then for a 120/240V circuit, the phase angle between the end taps (+120V and -120V) is indeed 180 degrees (showing two cancelling waveforms in our scope). But when one end tap is our reference, the phase angle between neutral (+120V) and the other end tap (+120V) is zero so we have two additive waveforms summing to 240V.

In practice, we can't have two 120V "phases" of opposite polarity adding up to 240V. It just doesn't work. Opposite polarity cancels, just like it does in loudspeakers. They have to be the same polarity, and if there is no phase angle, they will add to 240V.

I haven't practiced the math for three-phase enough to answer what you'll see, but I suspect that measured one way you'll have two 120V waveforms that add to 208V, and if you measure the other way you'll have two 120V waveforms that add to either 88V or 32V.

The moral of the story is that when you use the neutral as the base reference for scope measurements, you can set yourself up for confusion if you fail to consider the polarity of your connections.
Title: Re: Two Phase???
Post by: Jeff Bankston on February 03, 2016, 09:34:43 pm
Steve,

I often see your distribution system duplicated in heavily built up areas over here-one bank of transformers feeding both 3 phase and single phase services.  The term the local POCO uses for this setup is "networked" which kind of threw me the first time they told me that I needed my meter setup for a networked service.  The only difference is the addition of a "fifth" terminal to the meter to provide a neutral connection for accurate metering-typical single phase meters have no neutral connection to the actual meter.

I agree that split-phase is a more accurate and precise description.  Even on the load side of a service there is a distinct technical difference between 3 phase and single/split phase distribution-particularly in industrial situations were the actual service is 480 V (some industrials use 4160 and even 13.8 kV as well-not my cup of tea though).  Single phase/and split phase is typically a center-tapped transformer.  Three phase is done with 3 single phase transformers (typically in one box).  For trouble shooting purposes, I think it best to keep this distinction clear.

From an electrician's viewpoint, I would want a voltage/current/phase spec and then a receptacle or termination spec.  So 240V/50A, single phase with an 14-50 receptacle will get you 2 hots/neutral/ground-but unless you specify I will supply you anywhere from 208-240 depending on the building supply.  If you insist on 240 in a building with 208 3 phase service, it will cost more because I will have to bring in a buck/boost transformer to get it there.  I would think it best not to design a system that is so close to the edge it will only run on 240-if you design at 80% load as you should, then 208 feeding universal power supplies should be fine.

When working around 3 phase, keep in mind that 220 V/30A/4-wire could be interpreted as 3 hots + a ground-which won't make your audio gear happy-though if ALL of your gear will run on 220 then you  would actually have more available capacity with the same number of wires.

You wouldn't believe how many people say "I need 220 available" and think that is all I need to know.
220V 3 phase 4 wire is 3 hots and a ground. 220V 5 wire is 3 hots , a neutral , and a ground but its written this way > 120/220(230,240) 3 phase 5 wire. whenever you have a neutral with 2 phases the voltage is written like this VVV/VVV the V stands for the numbers.
Title: Re: Two Phase???
Post by: Jeff Bankston on February 03, 2016, 09:41:03 pm


Yeah, I'm not going to play games with electricians who can blow up my gear. 
or do this > http://www.youtube.com/watch?v=QdnwLzrYe9c
Title: Re: Two Phase???
Post by: David Buckley on February 03, 2016, 11:48:36 pm
Just going to throw in here that there is the American way, and the European way.  For everywhere except Norway :)
Title: Re: Two Phase???
Post by: Tom Bourke on February 04, 2016, 12:59:25 am
Actually, many (all?) high leg or wild leg services are really variations of a split phase service-they just add an additional transformer and create a 3rd phase.  So, the right way to deal with it it to use the 2 "normal" legs and the neutral just like you would use a single/split phase service. Leave the wild leg alone for anything other than 3 phase motors just to be safe.  IMO, this is a really good argument against the "2 phase" terminology.  Someone might think since single phase uses 1 transformer, and 3 phase uses 3 that if they see 2 transformers you have "2 phase" and that could lead to an interesting result.
No, high leg delta or wild leg delta, is not just a transformer off a split phase service for a 3rd leg.  It is 3 transformers off of a 3 phase service, one of witch has a center tapped secondary for 120 volt loads. Leg to leg is 240V.  Neutral to the wild leg is 208 and can typically handle 10% or less of the total system capacity.

All modern systems are 3 phase if you go far enough up stream.  There is the possibility of a utility company using 2 transformers to service an area from a 3 phase system.  It is a cost savings measure. The trade off is the cost savings of a transformer vs system capacity of the installed transmission lines.  If the area grows to the point of needing more capacity they can simply add the last transformer.
Title: Re: Two Phase???
Post by: John Sulek on February 04, 2016, 04:05:16 am
Just going to throw in here that there is the American way, and the European way.  For everywhere except Norway :)
+1
And parts of Spain.
Title: Re: Two Phase???
Post by: Stephen Swaffer on February 04, 2016, 08:23:59 am
No, high leg delta or wild leg delta, is not just a transformer off a split phase service for a 3rd leg.  It is 3 transformers off of a 3 phase service, one of witch has a center tapped secondary for 120 volt loads. Leg to leg is 240V.  Neutral to the wild leg is 208 and can typically handle 10% or less of the total system capacity.

All modern systems are 3 phase if you go far enough up stream.  There is the possibility of a utility company using 2 transformers to service an area from a 3 phase system.  It is a cost savings measure. The trade off is the cost savings of a transformer vs system capacity of the installed transmission lines.  If the area grows to the point of needing more capacity they can simply add the last transformer.

Tom, I have personally seen wild leg delta implemented with 2 transformers-my understanding is that that is a cost savings measure on the part of the POCO.  I have no doubt they use 2 transformers on a 3 phase system at times as well-but my experience is based on actual physical observation of both the install and working with it inside the building.

In either case, ignoring the high leg and using the 120/240 phases with their neutral should still give the same result as a normal single/split phase service since the neutral is not common to the high leg.  I would not expect triplen harmonics to create the same issue on this type of system.
Title: Re: Two Phase???
Post by: Tom Bourke on February 04, 2016, 09:46:24 am
Tom, I have personally seen wild leg delta implemented with 2 transformers-my understanding is that that is a cost savings measure on the part of the POCO.  I have no doubt they use 2 transformers on a 3 phase system at times as well-but my experience is based on actual physical observation of both the install and working with it inside the building.
For true 3 phase you need 120 Deg between the legs.  You can't create that with just a transformer.  In the case where you saw wild leg delta with just 2 transformers it was most likely a main transformer for the split-phase loads and a second smaller one for the 3rd leg in a V formation.  It would still have been 3 phase from the POCO.

I had to look up the proper term, it is Open Delta.  For more info:
http://forums.mikeholt.com/showthread.php?t=144067

Title: Re: Two Phase???
Post by: Frank DeWitt on February 04, 2016, 10:30:32 am
Well,  I'm glad we got that settled.

I think Mike is just going through a phase.
Title: Re: Two Phase???
Post by: Mike Sokol on February 04, 2016, 10:32:21 am
Well,  I'm glad we got that settled.

I think Mike is just going through a phase.

Yup... :o
Title: Re: Two Phase???
Post by: Stephen Swaffer on February 04, 2016, 04:15:11 pm
For true 3 phase you need 120 Deg between the legs.  You can't create that with just a transformer.  In the case where you saw wild leg delta with just 2 transformers it was most likely a main transformer for the split-phase loads and a second smaller one for the 3rd leg in a V formation.  It would still have been 3 phase from the POCO.

I had to look up the proper term, it is Open Delta.  For more info:
http://forums.mikeholt.com/showthread.php?t=144067

You are right-it is run off a different phase.  Still my point being that the single phase/split phase transformer is the same as a single phase-so it can be used in the same way.  I made the assumption that open delta was the most common form because the primary reason for using a high leg scenario is to save the cost of a transformer. On the service distribution side it is nothing but a headache.
Title: Re: Two Phase???
Post by: Frank Koenig on February 05, 2016, 02:36:51 am
I'm a little late to the party but can't pass up the opportunity to argue about nomenclature. With all respect, I object to both "split-phase" and "two-phase" as names for the funky center-tapped system we use in North America. The problem is that both terms already refer to something else and, aside from just dulling the language, borrowing a term -- incorrectly in someone else's view -- can make one sound ignorant. Like calling the rear hatch on a hatch back passenger car a "lift gate", which all sound guys know refers to that toe crushing appliance on the back of a truck. I'm sure there are better examples.

Split-phase is a type of single-phase induction motor that uses a starting winding (without the benefit of a phase-shifting capacitor) to provide the necessary field rotation to get going. Once the motor is spinning the starting winding is disconnected by a centrifugal switch.

Two-phase is also preempted in the world of motors but a bit more obscure. Two-phase motors are induction motors where two windings are driven in quadrature (90 degrees out of phase). These were used as servo motors at a time but, I believe, have largely been supplanted by DC motors of various kinds (brushless, brushfull, permanent magnet, etc.).

So what to call it? How about "Edison system" or "125/250" or "3-pole (4-wire) single-phase" like in the plug catalogs? Thinking about it, 3-pole single phase sounds pretty good and is only a little awkward.

Best,

--Frank
Title: Re: Two Phase???
Post by: Steve M Smith on February 05, 2016, 02:43:48 am
I think Mike is just going through a phase.
Is that like the guitarist who goes to the doctor because his voice is sounding strange, who gets told "don't worry, it's just a phaser you're going through"?


Steve.
Title: Re: Two Phase???
Post by: Mike Sokol on February 05, 2016, 06:54:32 am
Is that like the guitarist who goes to the doctor because his voice is sounding strange, who gets told "don't worry, it's just a phaser you're going through"?
I was pretty sure this topic would bring out both the best and worst comments.  ;D
Title: Re: Two Phase???
Post by: Mike Sokol on February 05, 2016, 03:41:58 pm
All modern systems are 3 phase if you go far enough up stream.
If you want to make your head hurt, try getting it wrapped around something called a Scott-T Transformer invented back in 1890. This gadget would convert 2-phase/180-degree power to 3-phase/120-degree power or visa-versa without any rotating parts. https://en.wikipedia.org/wiki/Scott-T_transformer

Title: Re: Two Phase???
Post by: Jeff Bankston on February 05, 2016, 04:01:24 pm
If you want to make your head hurt, try getting it wrapped around something called a Scott-T Transformer invented back in 1890. This gadget would convert 2-phase/180-degree power to 3-phase/120-degree power or visa-versa without any rotating parts. https://en.wikipedia.org/wiki/Scott-T_transformer
the result of having too much time on ones hand and gettin ideas
Title: Re: Two Phase???
Post by: David Buckley on February 05, 2016, 08:03:44 pm
If you want to make your head hurt
In the head hertz department (sorry!), its not widely appreciated that a delta/wye transformer gives a phase shifted output, which is, I suspect, a clue to how that Scott transformer thing I'm not even going to look at works.
Title: Re: Two Phase???
Post by: Scott Holtzman on February 06, 2016, 02:46:48 am
In the head hertz department (sorry!), its not widely appreciated that a delta/wye transformer gives a phase shifted output, which is, I suspect, a clue to how that Scott transformer thing I'm not even going to look at works.

ELI the ICEman - Same theory different application
Title: Re: Two Phase???
Post by: Jeff Bankston on February 06, 2016, 06:49:29 am
I bet you can't guess wye I like Delta !
Title: Re: Two Phase???
Post by: Mike Sokol on February 06, 2016, 07:25:34 am
I bet you can't guess wye I like Delta !

Like I said... I was sure this topic would bring out the best AND the worst comments...  :o
Title: Re: Two Phase???
Post by: Jonathan Johnson on February 07, 2016, 11:15:33 pm
Like I said... I was sure this topic would bring out the best AND the worst comments...  :o

Yes, it's a two-phased* thread.

*(spoken with a soft S)
Title: Re: Two Phase???
Post by: Mike Sokol on February 20, 2016, 07:32:25 am
According to Pat Brown, it's called "Split-Phase".

http://www.prosoundweb.com/article/why_power_matters_beyond_amplifiers_to_the_big_picture/

Title: Re: Two Phase???
Post by: Stephen Swaffer on February 20, 2016, 05:05:23 pm
Personally, I feel like this is an accurate and descriptive term.  Phase to phase voltage is typically 208 (220 nominally),   , so when you only have one phase available it is 220 (240 actually)-but it is "split" into 2 110/120 volt legs.  I understand that the term is often used with motors and that the mechanism involved there is different-but I don't think it leads to confusion.

I wish that article at least included the acknowledgement that 3 phase and split phase are different enough that there can be different considerations depending on the supply.  Obviously, going into depth on the difference would be well beyond the scope of the article.
Title: Re: Two Phase???
Post by: David Buckley on February 20, 2016, 05:07:47 pm
Can we call it settled then?  Split phase???
Title: Re: Two Phase???
Post by: Jeff Bankston on February 20, 2016, 05:14:04 pm
Can we call it settled then?  Split phase???
How about "half of 220" or "one short of a balanced line"
Title: Re: Two Phase???
Post by: Mike Sokol on February 20, 2016, 05:19:32 pm
How about "half of 220" or "one short of a balanced line"

Half Phase? Half Pint? Half Time? Half & Half?  :o
Title: Re: Two Phase???
Post by: Jeff Bankston on February 20, 2016, 05:29:28 pm
Half Phase? Half Pint? Half Time? Half & Half?  :o
Half & Half ? Coffee time ! I will plug the coffee warmer into a single pole outlet.
Title: Re: Two Phase???
Post by: Mike Sokol on March 03, 2016, 09:44:23 pm
Hey, they argue about 2-phase/split-phase even on Mike Holt's website.

http://forums.mikeholt.com/showthread.php?t=174950
Title: Re: Two Phase???
Post by: Stephen Swaffer on March 04, 2016, 02:19:00 am
To keep things interesting-I recently installed a 3 phase panel in a new home.  I then subfed two single phase panels from the 3 phase panel-each single phase panel uses 2 phases from the 3 phase panel.

Technically, these are different and have different potential issues (from harmonics) than a true single phase/split phase panel.

Not that the distinction makes much difference to most people-especially those that will do 120 volt wiring, but are afraid to do "220" wiring because it is more dangerous.