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

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.
 
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
 
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
 
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.
 
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
 
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.
 
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.
 
<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. :))
 
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