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# phase?

#1
09-17-04, 12:59 PM
William521
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phase?

I am totally lost when it comes to understanding that phase thing. I was hoping someone could help explain what kind of a measurement it is or what it measures etc.. I have read that it is expressed in degrees (360) and it has something to do with time sequence. Doesn't that me that a common household would have 2 phases running into its house because the 2 wires or circuits that come into a common house are different form each other in it's timing of which is usually 180 degrees or in other words half. Of course everyone says that a household only has one phase coming into it so I must be missing something. thanks

#2
09-17-04, 02:31 PM
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Join Date: Sep 2000
Location: United States
Posts: 18,497
It's mostly just terminology. What one person calls a phase is not necessarily how another person uses the same term. Power into a house is indeed just one phase. If you were to look the voltage curve between the two hot wires coming into the house, it would be a nice sine wave, with an RMS (root mean square, a fancy term for a special way to take an average) value of 240 volts. However, the power company also provides us with a grounded center tap off the transformer that provides that 240 volts. That allows us to get 120 volts too, and it keeps the voltage no more than 120 volts to ground of either of the two hot wires. This is done only to provide a safer power supply than the way it is done in the rest of the world. If you think about it, it's pretty ingenious.

If you look at the voltage curve between either hot wire and the ground, you would also see a sine wave with an RMS value of 120 volts. However, the sine wave from one hot to ground is 180 degrees out of phase with the sine wave from the other hot to ground. This is just a fancy way of saying that when one curve is up, the other curve is down. But it's really not "two-phase". You can't really take advantage of this in the same way that factories take advantage of the three phases in their power supply.

If it wasn't for multiwire circuits (aka shared neutral circuits), we wouldn't ever even use the word phase.

Some people like to think that the two 120-volt supplies are put together to make a 240 volt supply. However, I believe it more correct to think that the 240-volt supply is divided up to make two 120 supplies.

#3
09-17-04, 06:34 PM
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Join Date: Dec 2001
Location: New York
Posts: 1,365
John has done a good job in trying to describe a difficult concept. It is mostly terminology. A phase conductor is also called in slang, the hot or the energized wire. Your home has two of these, and many commercial occupancies have three of these phase conductors. In your home, the voltage for one phase conductor relative to ground as compared to the other is 180 degrees (when you look on an oscilloscope), as mentioned. The current, generally will also be 180 degrees apart as well.
Sometimes the current is offset as compared to the voltage (when you look on an oscilloscope), by varying amounts of degrees, and that is where you will hear the term power factor, as it is a calculated value indicating the relative position (when you look on an oscilloscope) of the voltage and current on a particular phase conductor.
If there is a specific question, ask, so we don't ramble about stuff you don't care about, but makes folks like me excited

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