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  1. #1
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    Default Can someone explain the difference between phases and poles??

    I am still confused about the meaning of "phase" in the context of typical North American household wiring. Is anyone able to explain the difference to me?

    My understanding is that a typical household is wired with a "Single-phase, 3-wire, common neutral" power supply from the utility company. The power coming from the high voltage line at the power pole is a single phase from the distribution grid. The transformer serving each household steps the power down to 240 volts, which is split into 2 wires carrying 120 volts each, and a center-tap neutral off the transformer. These are the three wires coming into our main distribution panel. Thus the red and the black hot wires are actually different poles of the same phase.

    Electrical discussions sometimes say you can share a common neutral between different phases, since the neutral wire sees the load at different times in the cycle (120 degrees out of phase). My understanding is that this is only true in a 2-phase power supply. However, hooking up a common neutral to circuits fed by a black and red wire in a typical household would be dangerous since in fact the neutral is now carrying 240 volts from the same phase.

    Is my understanding of this correct, or am I confused somewhere? I keep reading stuff, and the terminology does not always seem to be clear.....

  2. #2
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    Thumbs up Re: Can someone explain the difference between phases and poles??

    This is why old neighborhoods in the US have one transformer outside the house.

  3. #3
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    Default Re: Can someone explain the difference between phases and poles??

    Quote Originally Posted by ohmy View Post
    This is why old neighborhoods in the US have an old dumpster outside the house.
    Ummm Yup!.............

  4. #4
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    Default Re: Can someone explain the difference between phases and poles??

    Quote Originally Posted by ohmy View Post
    This is why old neighborhoods in the US have one transformer outside the house.
    What????????

    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

  5. #5
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    Default Re: Can someone explain the difference between phases and poles??

    ***, I just read all 9 pages of this thread. I wish I had seen it sooner because the original poster and some of the original responders probably have lost interest in this thread.

    I work for a distribution transformer manufacturer. We make single phase pole, single phase pad, three phase pose and three phase pad transformers for the coop market mainly but we do have some municipalities.

    My main function is QA and training. My company sends me out to lineman classes on occasion to teach transformer fundamentals.

    Single phase poles can have one or two bushings on top for the primary connection. One bushing pots are the most common. The two bushing pots are mainly for transformers that will be put in three phase banks with a primary delta connection.

    The secondary of the transformer may be connected internally in phase with the primary or out of phase with the primary. The secondary bushings are labeled X1, X2 and X3, X2 is grounded and used as the return/neutral. Those connected in phase are called subtractive wound, those connected out of phase are additive wound. This is only important if three pots are being used in a three phase bank, they all have to be the same or the three secondary phases won't be 120 degrees apart.

    Almost all utility systems today have a neutral wire. It not only provides a common ground reference for all the transformers on the grid, it provides a ground for all the telephones. Otherwise telephones, which are fed by 48vdc could float to a much higher and dangerous voltage.

    Three phase transformers have three single phase coils inside them. The primary's can be connected in either a Wye or Delta configuration, same with the secondaries. The Y-Y (Wye primary, Wye secondary) is the most common, D-Y second most common. Delta secondaries are a real small part of our production and a D-D is so rare that we only see a couple a year.

    The 480Y/277 is the most common secondary as this is what most large machines use. The 208Y/120 is second most common. We see some 480D secondaries but 240/120 delta secondaries are more common.

    Delta's are not inherently more dangerous that Wye configurations. Both still use a fourth wire for ground. Either configuration that doesn't have a ground is equally dangerous. Yes there is an ungrounded three wire Wye configuration, but a ground wire is still provided for safety.

  6. #6
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    Default Re: Can someone explain the difference between phases and poles??

    Hey Canuk:

    Thanks for the reply. The discussion you referenced is what got me thinking about this topic...I've read it over several times, trying to figure this out.

    I am still slightly confused...how can the different feed legs be at 90 degrees to each other when they both come from the same phase?? How does that happen? I guess that's where I'm getting hung up.

    Am I correct in that the power produced by the utility at the generating station is typically 3-phase power, with each phase at 120 degrees to each other? By the time it gets to the transformer on the pole outside our house, we are only seeing one phase...the other phases have gone to a different part of the grid. Is this true?

    Also, I assume that household circuits sharing a neutral must therefore be on adjacent breakers, so that each circuit gets power from a different leg. If the breakers were spaced 2 apart, then the neutral would be carrying the full 240 volts. This would be dangerous. Is this correct?

    Any further explainations from the forum would be much appreciated.

  7. #7
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    Default Re: Can someone explain the difference between phases and poles??

    Quote Originally Posted by rickpantel View Post
    If the breakers were spaced 2 apart, then the neutral would be carrying the full 240 volts. This would be dangerous. Is this correct?

    Any further explainations from the forum would be much appreciated.
    well for one thing it wouldn't be regular split single phase and the wires especially the netural would have to be much bigger. i don't think this is allowed with less than 1 awg.

  8. #8
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    Default Re: Can someone explain the difference between phases and poles??

    ]3-wire AC 240 volts 60 cycle is still single phase 240 ac just split phase - from the top of a peak to the bottom of a valley the total is 240 but. picture one 120vac side being the top arcs and going flat between for half of the cycle at neutral or zero in the middle and the other 120vac half being the bottom valleys being flat in the middle. it is 120 from the middle to the top of the peak wave and 120 from the middle to the bottom valley of the other half of the spilt wave. 90 degrees would be a real two-phase system we dont have that we have single split phase each split is in-phase so between positive peak and netural is 120 v and negative and netural is 120 v, we're on one phase and that single line is 180 degrees relationship off the netural axis, not two-phase that are 90 degress off peak to peaks (canuk made mistake) or three-phase are three phases each 120 off on the timing cycle of peak).

    so you prolly still have single phase just split to two poles (north/positive south/negative?) the top half or A side and the bottom half or B side. between the two splits rides current on the neutral not voltage. does that make sense?

    not what you'd see on a scope but think of it kind of like this, blue and orange are each split.

    the other way to think of it is kind of like speed walking not running heal to toe like a drunk test on a balance beam or a striped line. as you move forward you transfer your weight from the left foot to the right foot as you make continuous forward motion but until you reach total weight transfer from your left foot to your right foot you do not lift your leftt foot to move forward, and vice versa. unlike running where you spring off each foot and jump to the next foot. if you think of split single phase power like speed walking 120 steps a second (or sixty left foot forward steps and sixty right foot steps) and the weight transfer between steps of your body and its relationship to the ground as neutral that is kind of like how the two splits or legs of the split phase single phase works. if you look at the graphic and think of the center zero as your body and the path you are going in time and turn the graphic sideways and think of the positive half as your left leg and the negative side as your right leg speed walking on a balance beam (or a stripped line) it might make more sense?
    Last edited by Blue RidgeParkway; 09-08-2008 at 12:34 PM. Reason: put in a graphic for explanation.

  9. #9
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    Default Re: Can someone explain the difference between phases and poles??

    Transformer
    Primary side Secondary side.
    The poles are the three legs of the secondary side of the trans former. The top leg is line 1 (hot) the bottom is line 2 (hot) the center is the common. All legs have voltage and current flow but the center is tied to ground, that references it to ground so you see no voltage difference between it and ground. As the current is flowing out of the top leg it is flowing in the bottom leg so the current flow is in opposite directions on the two legs then reverses direction.This happens 60 times per second. Measuring between the top leg and the bottom leg you would read 240 volts RMS. By using the center tap (common) and the top leg for some of the loads you only measure the current flow and voltage for the top leg in this case 120 volts RMS. The current flow goes out the top leg and in the common then reverses.The voltage goes from 0 to 120 volts RMS to 0 then to -120 volts RMS. That is as referenced to ground.

    Any single leg of the secondary could be connected to ground and you would get the close to the same results. If the top leg was tied to ground instead of the center you would read 240 volts RMS from the top leg to the bottom leg and 120 volts RMS from the center to either of the other two legs. The only difference would be reading to ground (the reference point). In this case you would read o volts from the top leg to ground, 120 volts RMS from the center tap to ground, the difference would be reading from the bottom pole to ground which in this case would be 240 volts RMS.
    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

  10. #10
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    Default Re: Can someone explain the difference between phases and poles??

    Thanks for the GREAT answers!!!...lots of information here. It will take me a while to understand this stuff. Not easy for a layperson! Thanks again to the forum!

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