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

[DAN_IN_MN]

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]

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.

 

[Ray C]

Please read Post 14 and on from this thread for the correct answer: http://www.hobby-machinist.com/showthread.php/17359-Purchasing-a-Monarch-10EE-Round-Dial

Ray

 

[DAN_IN_MN]

Please read Post 14 and on from this thread for the correct answer: http://www.hobby-machinist.com/showthread.php/17359-Purchasing-a-Monarch-10EE-Round-Dial

Ray

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]

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]

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]

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]

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]

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]

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