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Author Topic: Two Phase???  (Read 13636 times)

Cailen Waddell

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Re: Two Phase???
« Reply #20 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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Stephen Swaffer

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Re: Two Phase???
« Reply #21 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.
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Steve Swaffer

Cailen Waddell

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Re: Two Phase???
« Reply #22 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?
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Mike Sokol

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Re: Two Phase???
« Reply #23 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. 
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Jonathan Johnson

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Re: Two Phase???
« Reply #24 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.)
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Cailen Waddell

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Re: Two Phase???
« Reply #25 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. 


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Jonathan Johnson

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Re: Two Phase???
« Reply #26 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.
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Jeff Bankston

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Re: Two Phase???
« Reply #27 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.
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Jeff Bankston

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Re: Two Phase???
« Reply #28 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
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David Buckley

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Re: Two Phase???
« Reply #29 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 :)
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Re: Two Phase???
« Reply #29 on: February 03, 2016, 11:48:36 pm »


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