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Author Topic: Two Phase???  (Read 13635 times)

Steve M Smith

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Re: Two Phase???
« Reply #10 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.
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Cailen Waddell

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Re: Two Phase???
« Reply #11 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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Cailen Waddell

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Re: Two Phase???
« Reply #12 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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Matthew Knischewsky

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Re: Two Phase???
« Reply #13 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.
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Stephen Swaffer

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Re: Two Phase???
« Reply #14 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.
« Last Edit: February 03, 2016, 10:21:58 am by Stephen Swaffer »
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Steve Swaffer

Steve M Smith

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Two Phase???
« Reply #15 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.
« Last Edit: February 03, 2016, 03:12:20 pm by Steve M Smith »
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Jonathan Johnson

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Re: Two Phase???
« Reply #16 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:
  • Primary: 7200V (or other) single phase; end taps connected to hot/live/phase and grounded neutral*
  • Secondary: 120/240V center tapped; end taps live/hot; center tap grounded neutral

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.
« Last Edit: February 03, 2016, 12:58:09 pm by Jonathan Johnson »
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Jonathan Johnson

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Re: Two Phase???
« Reply #17 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.
« Last Edit: February 03, 2016, 01:11:53 pm by Jonathan Johnson »
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Stephen Kirby

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Re: Two Phase???
« Reply #18 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.
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Jeff Bankston

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Re: Two Phase???
« Reply #19 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.
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ProSoundWeb Community

Re: Two Phase???
« Reply #19 on: February 03, 2016, 02:24:56 pm »


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