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Why is 220V called single phase when it has two phases?

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DAN_IN_MN

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#1
Something that I've been thinking about for a while. Why is 220V called single phase when it has two opposing phases? As I understand it, 3 phase has 3 60 hz 120 degrees opposed.
 

tigtorch

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#2
Another way to look at it is that 220 only has 2 wires (ignore ground for the moment) and the voltage between these wires is pure single phase (even though derived from 2 phases). Three phase, on the other hand has 3 wires and there is a pure, but DIFFERENT single phase between each of the three pairings of wires, thus 3 phase. In other words, it is impossible to have more than one phase with only 2 wires, i.e. there is only one pair to measure between.

If, on the other hand you actually used 220v with one leg to ground and then the other leg to ground you would, indeed have 2 phases but no application I am aware of actually uses 220v this way.
 

DAN_IN_MN

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The following applies to North American electrical systems....

Two phase is not the correct terminology and sometimes, the terms "split phase" or "split leg" is used. The 220 power that we are familiar with is what comes into our home service panels and it consists of one neutral and two "hot wires". Somewhere in the neighborhood, there is a transformer with high voltage coming into it. They put a winding next to the core and it induces 220 volts into the winding. The two wires from that winding are the two hot wires that enter your house. If you measure the voltage between those two hot wires, it will read 220 volts. In the middle of that winding, they put a "center-tap" which is just a wire that connect to the center of the winding. That wire is the neutral that is brought into your house. If you measure between the neutral wire and any hot wire, the voltage is 110 but, one with respect to the other is 180[SUP]o[/SUP] out of phase. Without a significant description of phasor mathematics and something called "complex numbers", it would be very hard for me to describe in other terms why the phase change occurs but, suffice it to say, it does indeed occur.

BTW, the terms 110, 120, 220 and 240 are a little confusing. 110 and 120 are the same really; it's just that the power company is allowed to vary the voltage under different circumstances and it could be somewhere in that range. The frequency (60 Hz in North America) will not change and if it ever does, there are serious, serious problems going on in at the generation facility -to the extent you will probably see a bright flash coming from the facility. -Basically, it does not vary and if it does, it happens simultaneously all through North America. The entire system is synchronized.

Ray
Thanks Ray!
 

Alan Douglas

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#5
Why is 220V called single phase when it has two opposing phases?
It's because the term "phase" is being used to describe two different things. 120V/240V is 3-wire single phase, not two phase. True two-phase is an antiquated system (Tesla's original multiphase motors were two phase) that has largely been supplanted by three-phase. It's difficult to explain in words without reference to diagrams, and math or at least vectors.

At the risk of confusing the issue, "one-ten" and "two-twenty" volts are figures of speech and have not been used for eighty or so years. The present standards are 120V and 240V and have been since the 1950s. 115V and 230V were standardized in 1928 and before that, there was a mix of "standard" voltages from 110V to 125V.

Historically Thomas Edison selected 100V for his lamps, and allowed 10% drop in the supply lines, so the generators produced 110V. That's where all the multiples of 11 came from (110, 220, 440, 550, 2200).

Early carbon-filament lamps could not be produced uniformly, and the lamps were selected at manufacture to suit a particular voltage. Various cities were encouraged to standardize on different voltages, from 110V to 125V, to create a market for all lamps produced. Over time, higher voltages predominated, and by 1919 115V overtook 110V in popularity. By 1926 110V lamps accounted for only 12% of demand (35% were 120V).

The present US standard is 120V +/-5% or +/-6V, or 114V to 126V. Power companies like to keep the voltage as high as possible to get the maximum use of their distribution network. The line voltage here has been 123V since 1960 or so.
 

Codered741

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The two legs of a single phase 240vac circuit, like the one found in most houses, are NOT out of phase with each other. They are in fact exactly IN phase with each other, meaning that there is only ONE phase. If the two legs were 180deg OUT of phase (technically two phases), measuring the voltage between the two legs would show 0v. This occurs because as the AC waveform cycles, the two voltages would be exactly opposing at all times (Leg 1 +120v, Leg 2 -120v), making the resultant voltage 0. However, because they are phased Exactly the same, measuring the voltage shows double the voltage(Leg 1 +120v, Leg 2 +120v), as the two voltages are added together.

Three phase legs are 120deg out of phase, phase to phase. Normally adding three legs together would add all the voltage together (360vac). However, the phase differential causes some of the voltage to be lost to some of the voltage being cancelled by the other legs drawing it back. The line to line voltage is calculated by multiplying the line to ground voltage by the square root of three. Eg. 120 x sqrt(3) = 207.84, or 208vac 3ph.

The phase shift is often created at the generator, but can be created by the use of capacitors to delay the waveform, as in a static converter.

-Cody
 

Ray C

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#7
Cody,

You might want to have a look at this. http://www.youtube.com/watch?v=o1ywY0JeMKw He shows the waveforms on an oscilloscope but conveniently side-steps the issue of why it happens -that's because it will take few years of math class to understand why... Also, please consider that two lines which are in-phase have zero potential difference and thus, the voltage would read zero.


Ray
 

tigtorch

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I beg to differ, Codered741 (I agree with RayC who posted just as I did). If both legs of the 220V circuit were "In Phase" then there would be NO voltage difference between them (i.e. they would both be doing the same thing and voltage between 2 voltages that are the same is ZERO). They are 180 degrees out of phase and thus the voltage between them is TWICE (220V) what the voltage of each to ground is (110V).

Actually many years ago I graduated as an BSEE. There was a lot of complicated math involved, especially with AC current including imaginary numbers. In this case, however, it is pretty intuitive. Just picture two wave forms going positive and negative around zero. If you take the difference between them (i.e. the voltage when measure between them) at ANY INSTANT, it is intuitivly obvious that there would be ZERO voltage between them if they were in phase.
 

Alan Douglas

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I think we all agree in practice, and the differences are semantic. 180 degrees is nothing more than a polarity reversal, so 0 and 180 are both "in phase" in the sense that they are "single phase."

"Two phase" is 90 degrees out, an entirely different animal from the "two phases" of 240V service. It comes down to using the term "phase" for different things.
 

comstock-friend

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As said previously, the correct designations for power company generated electrical power (USA) is 240 V and 120 V for our homes. The power company is allowed 5 percent high. We were in a building one block away from the Burbank, CA, powerplant. Our voltage was the 5% high, or 252 V. I was testing some hydraulic power units going overseas that had 50 hertz wound motors. They would have been very happy at 220 V, but the windings saturated at the plus toleranced voltage and they just sat there and hummed and got hot very quickly. If the voltage had been 253 V, the power company would have to come by and set the taps of the transformer on the pole down, but they were OK. We had to replace the light bulbs in that building much more frequently; I believe the power in the lights goes up by the 4th power or so of the voltage.

Your motor nameplates will say 115 V, 230 V or 460 V (new ones anyway). This is because the National Electrical Manufacturers Association (NEMA) did a survey of industrial plants and found that the actual voltage inside the equipment junction box was 115 V on a 120 V system, 230 V on the 240 V system and 460 V on the 480 V systems. So the equipment is designed for the nominal expected voltage. This applies to ratings for motors and at least electric heating elements of which I am aware.

John Friend
 

cdhknives

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#12
Lets make it simple.

US 110 and 220V household power has 60 peaks per second. Single phase. The only real difference is voltage level, though the wiring required is a little funky.

Three phase power has 180 peaks per second (3x single phase lines organized in 1/3 cycle increments, 3x60=180)

If you want a longer treatise on what that means/does/results, I can provide it (EE, working with industrial power and controls every day), but that's the basic answer. Suffice it to say 3 phase power provides much smoother torque both running and especially starting.
 

comstock-friend

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#13
Power is determined by the square of the voltage.
I misspoke, I was referring to life of the light bulbs, not the power, and it varies by the -16 power of the voltage, so increase in voltage is VERY detrimental to bulb life. And for some reason power below is listed as 1.6 instead of the square of voltage. I suspect that the resistance change in the bulb element as a function of temperature enters in to it.

From Wiki:
For a supply voltage V near the rated voltage of the lamp:

  • Light output is approximately proportional to V [SUP]3.4[/SUP]
  • Power consumption is approximately proportional to V [SUP]1.6[/SUP]
  • Lifetime is approximately proportional to V [SUP]−16[/SUP]
  • Color temperature is approximately proportional to V [SUP]0.42[/SUP][SUP][86][/SUP]

John Friend
 

Ray C

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#14
As a side-note... It seems that in the USA, we get to keep our incandescent light bulbs a little while longer... -They were going to be phased-out due to inefficiency. I'm very pleased but will still hold onto my stock pile as I use them as heaters in my gun safes, sandblasting unit and other similar things.

BTW: Yes, higher voltage shortens incandescent bulb life considerably. The bulbs used in "Exit" signs are typically rated for 150 volts and thus last much longer at 115 or 120. The real life killer is on/off cycles. The thermal shock of the tungsten going from room temperature to about 5000F is a really doozie on that thin piece of wire. The light bulbs in my safe typically last just under a year of constant operation -about 6 times longer than if cycled on/off.

Ray



I misspoke, I was referring to life of the light bulbs, not the power, and it varies by the -16 power of the voltage, so increase in voltage is VERY detrimental to bulb life. And for some reason power below is listed as 1.6 instead of the square of voltage. I suspect that the resistance change in the bulb element as a function of temperature enters in to it.

From Wiki:
For a supply voltage V near the rated voltage of the lamp:

  • Light output is approximately proportional to V [SUP]3.4[/SUP]
  • Power consumption is approximately proportional to V [SUP]1.6[/SUP]
  • Lifetime is approximately proportional to V [SUP]−16[/SUP]
  • Color temperature is approximately proportional to V [SUP]0.42[/SUP][SUP][86][/SUP]

John Friend
 

DAN_IN_MN

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#15
Excellent responses! Thanks guys!
 

Tony Wells

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#16
I've found the concept easier to understand by looking first at the 240v angle first. It is represented easily by a basic sine wave. P-P voltage is 240. This is taken from a single winding on the secondary of the pole pig. Of course, AC changes polarity at the 60 or 50 Hz cycles it is generated as. Striking an imaginary line horizontally through this sine wave gives a zero crossing point, at which the potential is zero relative to absolute ground (or near to it, for you nit-pickers out there :))) Being that the secondary of said pole pig is center-tapped, you can choose to use one end of the winding and that center-tap and you will not get the other half of the sine wave that falls below the zero line. That part of the cycle runs from zero volts to 120v for the other half of the secondary winding, while the first half simply sits and waits for the sine curve to come back, cross it, and climb back to 120v again. This simply repeated over and over. It's like you're really only getting a 50% duty cycle, as for half the time, each half of the secondary sits at zero. But from both ends of the secondary, you get the entire sine wave, so it runs from the zero line up to the peak of 120v, back to zero, then back to 120v on the opposite side of the zero line. That's why you can measure peak to peak and skip the neutral, which is represented by the zero line on the curve, and get 240v. It's sort of like a zero and a +120v on one side, and a -120v on the other side, but of course it's not steady state energy, so it doesn't stay at that potential. Well, not for long, anyway.

I hope I haven't muddied the water more. To me, it seems more accurate to consider 120v half wave, and 240 full wave single phase. Entirely different discussion from 3 phase.
 

tigtorch

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#17
Tony said it well. Here is a graphical representation of his explanation, along with a verbal one, all from http://www.phaseconverterinfo.com/phaseconverter_deltawye.htm :

240vac.jpg "Figure 2 represents 120/240V single-phase with two voltages 180 degrees apart with neutral and ground halfway between the two voltages. The voltage measured between the two legs is 240V, and since neutral is halfway between, the voltage from either leg to neutral would be 120V."

Threads like this tend to bring out us "nit pickers" but it is all in good fun. To sum it up, household 240v is a product of a single phase of the three phases generated by the power company. That being said, the two legs of the 240 volts are
are 180 degrees out of phase which, at least to me, means that they are not "in phase" .

One "nit" to pick with Tony: a 240vac circuit is 240 volts RMS and for a sine wave that is 0.707 times one side of the P-P voltage.So 240V RMS is equivalent to 339V peak or 679 V Peak to Peak.


240vac.jpg
 

Ray C

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#18
Guys,

This has been a topic of debate among the EE crowd for a long time. LOL: I remember being in the same debates 30 years ago... There are several definitions of what a "Phase" is -and quite honestly, I don't care. If looked at purely mathematically (my preferred method because terminology and semantics don't become roadblocks) there is no debate and the answer is loud and clear. I'm half tempted to re-derive the solution (it's going to take a while as I haven't done it in years) to show the phasor mathematics on this and in the calculations, you can see how a number goes from real to imaginary denoting the switch in phase.

Anyhow... The important thing is that we understand and by now, we should all be experts...


Ray


Tony said it well. Here is a graphical representation of his explanation, along with a verbal one, all from http://www.phaseconverterinfo.com/phaseconverter_deltawye.htm :

View attachment 59758 "Figure 2 represents 120/240V single-phase with two voltages 180 degrees apart with neutral and ground halfway between the two voltages. The voltage measured between the two legs is 240V, and since neutral is halfway between, the voltage from either leg to neutral would be 120V."

Threads like this tend to bring out us "nit pickers" but it is all in good fun. To sum it up, household 240v is a product of a single phase of the three phases generated by the power company. That being said, the two legs of the 240 volts are
are 180 degrees out of phase which, at least to me, means that they are not "in phase" .

One "nit" to pick with Tony: a 240vac circuit is 240 volts RMS and for a sine wave that is 0.707 times one side of the P-P voltage.So 240V RMS is equivalent to 339V peak or 679 V Peak to Peak.
 

oldgoaly

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#19
In my area there was 2 phase, but was on it's last legs in the 1960's just a few businesses went to the 1970's running it. Don't mean to confuse anyone.
 

Tony Wells

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#20
<snip>.

One "nit" to pick with Tony: a 240vac circuit is 240 volts RMS and for a sine wave that is 0.707 times one side of the P-P voltage.So 240V RMS is equivalent to 339V peak or 679 V Peak to Peak.
Oh, I know, but didn't think it germane to the discussion. My attempt was only to provide an illustration in graphic terms that hadn't been brought up yet. Plus we generally don't think in terms of RMS measurements. Not all of us even have true RMS meters. :))
 

DAN_IN_MN

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#21
Thanks! I think:thinking:! :whistle: LoL! As the eyes roll back into my head! Glad I could generate a topic of such great discussion! I got the basics of my question answered. On to the next item to ponder ......... whatever that may be!
 

Larry42

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#22
Our local power company stopped providing 240V 3 phase and went to all 208 or much higher. There are no "wild legs" (as there are on 240V systems) on a 208V 3 phase so you can use any leg to neutral as a 120V "house hold power." It helps the power company balance out the leg to leg power flow. Our shop has lots of European equipment. It runs fine on 60 cycle just 20% faster than "as designed" (50 cycle.) That makes quite a difference on the motors running on frequency converters (14,400rpm VS 12,000!). We have lots of 3 phase 380/400V transformers for those machines.
 

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#23
Simple the AC motor can not on just single phase. So second winding at 90 deg or 2 phase. A capacitor is use to form the two phase from the single phase


There is shaded pole AC motor but only use on very small motors
Something that I've been thinking about for a while. Why is 220V called single phase when it has two opposing phases? As I understand it, 3 phase has 3 60 hz 120 degrees opposed.
Sent from my SAMSUNG-SM-J320A using Tapatalk
 

Tony Wells

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#24
What I said up there is rubbish. No idea where my brain was.......out to lunch I suppose. Center tapping does NOT split a phase. Current flows between the windings in a turn:turn ratio and that is used to reduce, increase or at times make no change to the voltage. That's about it. Center tapping simply allows one winding to act as two, or even can be multitapped at different points yielding different ratios and therefore voltages.

I apologize for the brain-dead misinformation.
 

Ray C

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#25
OMG: Deja Vu... This is a really old thread. -So let's not waste this opportunity to dredge it up..

Please see the image below which was captured from this link: https://diy.stackexchange.com/questions/33602/why-do-240v-circuits-not-require-neutral


I have modified the original image to make it easier to understand. In the diagrams, a ground connection was added to the center-tap. In reality, this is what happens. I have also added + and - symbols in the top diagram. The ground leg by definition is considered to be the - (negative) leg. In the top diagram, if you were to put a bridge rectifier at the end of each 120 volt leg and measure the voltage with a DC volt meter without flipping the leads, one leg would read positive, the other would read negative. This is where the perceived phase shift comes into play ... because the voltage is reversed. If you were to draw this out using phasor diagrams, the representation is called a phase shift.



220CenterTap.JPG

Ray
 

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#26
The answer is buried in your original post in that the two legs of a 240-volt system are offset 180 degrees. The 240 is derived by averaging potential difference measurements taken from intervals across the positive waveform and that exact same point on the corresponding negative waveform - which is why you are getting 240 volts - that is the difference between the mountain and the gorge. In three phase the waveforms are staggered 120 degrees as you say and because there is less "dead" space between cycles, averaging waveform measurements result in a line-to-line voltage (phase to phase) voltage that is 1.73 x phase to neutral voltage.
 

JPMacG

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#27
It is a logical extrapolation from 3-phase to think that single phase 240V with neutral might be thought of as 2 phase. I have asked this same question myself (I am an electrical engineer working in the microwave antenna field). None of the explanations I was given were convincing to me. I think the real answer is that historically the electric power industry just decided to call it single phase.
 

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#28
Maybe this will help.

The power going to most residential service (In the US), is 1 leg of the three phase high voltage. That 1 leg, is then lowered by transformers into 2 120 volt, legs, and a neutral. If you look close at the transformer, it has 1 hot going in, and 2 half voltage hots coming out, and a ground in and out.

Here is a good page that explains it.



http://waterheatertimer.org/See-inside-main-breaker-box.html

There is such a monster as 2 phase, in some parts of Philadelphia, and a few other rural areas in the US. So you can run across 2 phase motors once in awhile.
 

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#29
What cleared this up in my head was looking at waveforms on an oscilloscope several years ago when I was cutting my teeth on electronics. I had seen some banter back and forth on electronics forums; someone was saying 3ph is out of phase by 120deg and single phase is out by 180 degrees, some disagreed, and I didn't know who to believe. I learn best hands-on, so I set out to resolve it myself.

Using an isolated oscilloscope (fluke scopemeter) I looked at the 3 phases of a 480V supply. Probe 1: ground to L1, clip to L2. Probe 2: ground to L2, clip to L3. Probe 3: ground to L3, clip to L1. I get 3 sine waves separated by 120 degrees, as expected.

Now I go to do the same thing on a 240V single phase supply. Probe 1: ground to L1, clip to L2. Probe 2... what do I do with probe 2?... ground to L2 and clip to L1? That's just monitoring the same thing as probe 1, but with inverted polarity. But whatever, let's try it... ground to L2 and probe to L1... aaand... It's "180 degrees out of phase" with what? ... with itself. There is only one phase, and saying it's "180 degrees out of phase" is a waste of your breath and everyone else's sanity.
 

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#30
The center tapped transformer is stupid and they should have just done 220V without the neutral, but we're stuck with it now. There was probably a good reason back in the day with insulation requirements and such. From the power companies' perspective, it's one phase of a 3 phase feed, so single phase. The center tapped neutral L1/L2 creating opposed waveforms doesn't matter to them.

Even better would be 3 phase 480V standard, but that's pushing it. :)
 
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