Single Phase

  1. Hi,
    I am struggling with finding info on Electrical Phases. I understand that Single Phase is simply one ac signal, and I understand that 3 phase is 3 independent ac signals, all of which are exactly 120 degrees out of phase. My struggle comes in when I think about 220 volt dryer hookup in my house. There are 3 legs...2 'hots' and a ground. The 2 hots are each 110 volts.
    1. But are they in phase or out of phase? I think they are out of phase in order to get 220 volts between the hot legs, right?

    2. Are they 180 degrees out of phase?

    3.What makes them 180 degress out of phase?
     
  2. jcsd
  3. vk6kro

    vk6kro 4,059
    Science Advisor

    What you have in your house is nothing to do with the 3 phase high voltage power distribution system.

    Out in the street somewhere there is a transformer that converts the voltage between two phases of the high voltage power supply to a centre-tapped 220 volt output from a single winding.

    The opposite ends of this are out of phase with each other by 180 degrees. When one end swings positive relative to the centre tap, the other swings negative.

    Other appliances and lighting in your house use the centre tap (as a neutral) and one of the outside legs (as an active) of this winding to get 110 volts.

    This system is fairly unique to North America. Most other countries bring one of three phases or all three phases into the house at about 230 volts. High power (at about 400 volts) is available between the phases. Three phase motors are more efficient than single phase ones and self starting, too
     
  4. sophiecentaur

    sophiecentaur 13,771
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    Gold Member

    I think this is mostly a terminology problem.
    Two wires will just have an alternating voltage (say 50Hz) between them. You might call this a 'single phase system'. Another two wires may have another 50Hz AC between them. These two signals may not be in phase with each other; there is a 'phase difference'. You can have three pairs of wires with 120 degrees of phase difference between them. Let's make the amplitudes of the voltage on all three pairs the same. If you take the appropriate wire of each pair and join them together, you will have a 'three phase' system. The common point is called the Neutral. Somewhere in the system, this is usually connected to an 'Earth' . If you take the three pairs and connect them to equal loads then very little current (zero) will flow into the Earth connection because the neutral currents will add up (in phase and amplitude) to zero at the common point. There will be a voltage between any two of the phases which is √3 times the voltage on one pair. This is the Vector Difference between the two so-called Phasors. The three 'live', physical connections are referred to as 'phases' - which could be viewed as an unfortunate choice of name.

    If you have just two pairs carrying AC and the signals are in phase, you can connect one of each pair to a common point and you will get Zero Volts difference, connecting one way round and twice the voltage, connecting the other way. Again, this is the vector difference between the two signals.

    The US domestic system (above) seems very bizarre, to me, but it is just the consequence of the way the system grew and the fact that someone chose 110V as the operating voltage. Cables would need to be so thick for supplying large loads that 'they' had to introduce this strange 'two phase' system which can deliver four times the power with the same cable thickness by supplying twice the voltage as an option.
    (Power = V2/R).

    It has caused a lot of confusion - especially to non US observers! - but it serves a purpose and allows a lower (and a bit safer) operating voltage for most domestic equipment.

    It is the three phase system that is used, worldwide, for generation, distribution and large motors and it is this system which is most elegant and useful. The other system just gives you twice the volts.
     
  5. These two phases from the high voltage power supply are they 120 degrees out of phase of each other before they hit the transformer? what changes them to 180 degrees out of phase?
     
  6. sophiecentaur

    sophiecentaur 13,771
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    Gold Member

    The voltage between two wires can't be "out of phase" it is just an alternating voltage between them. You can only compare the phases of two signals. If you refer the voltages of two of the three phase supply to Earth, then they will be 120 degrees out of phase. But, using one as a reference, the other will just have an alternating voltage. The magnitude of that voltage is given by the trigonometry of the Phasors.
    It's a matter of terminology.
     
  7. A typical house in the US uses this setup:

    One of the power company's three-phase legs comes to an individual house. Power companies try to distribute the legs equally so that they each have the same load but that's a different story. If I remember correctly, a leg may be around 4kV at distribution level.

    The leg is connected to center-tapped transformer at the house. The primary side is a single coil that connects to the distribution voltage. The secondary side of the transformer is a center-tapped coil and the tap is connected to the house's ground which ties it to 0V. The center-tapped coil is arranged so that the ends of the secondary coil are +120V on one side and -120V on the other side with respect to the grounded center-tap. Note, I don't mean that the voltages are literally positive or negative, instead they are out of phase by 180 degrees.

    Smaller appliances can connect across one of the split 120V legs to ground. Larger appliances can connect across both 120V legs, and because they are out of phase by 180 degrees, they will get 240V.

    Go look inside the circuit breaker box (carefully, after opening the main switch outside your house at the main box under the meter). You'll see that the box has three buses. The center bus is the 0V ground an the sides are the +/-120V legs. Look at how the big appliance lines and small appliance lines are connected.

    ETA: Scratch that last part about looking inside the breaker box. Don't try this at home kids. (Are you a kid?)
     
    Last edited: Apr 16, 2010
  8. mheslep

    mheslep 3,489
    Gold Member

    Nice summary.
     
  9. berkeman

    Staff: Mentor

    Agreed. Lots of good info in this thread, so I highlighted it. It's in Engineering Highlights now. Thanks folks. :smile:
     
  10. sophiecentaur

    sophiecentaur 13,771
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    Fame at last chaps!
     
  11. vk6kro

    vk6kro 4,059
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    Good move.

    Variations on this question come up all the time.
     
  12. Everyone else is saying they are out of phase by 180 degrees... are you saying this becuase they are only 180 degrees out of phase relative to the center tap?
     
  13. Here you say 2 phases of the high voltage power supply are connected to the transformer and further down the post someone quotes that one leg is attached to the transformer (assuming the other side is hooked up to earth). Which one is correct or can it be done either way? what would be the point of tapping off 2 phases of the highvoltage instead of just the one?
     
  14. vk6kro

    vk6kro 4,059
    Science Advisor

    It doesn't make much difference. In the US, as I understand it, the high voltage distribution system does not include a neutral. In other countries it does.

    You can take the voltage between phases to the primary of the transformer or you can take the voltage from one phase to neutral to the primary of the transformer.

    [​IMG]
     
  15. sophiecentaur

    sophiecentaur 13,771
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    Gold Member

    If you connect between two phases you will get 'root three' times the voltage.
    But either the so called Star or Delta connections can be used. In the UK, it is more common, afaik, to have a three phase HV transformer to produce three low voltage , single phase supplies with a common Neutral. These are then distributed around a neighbourhood, houses being supplied with phase 1,2,3,1,2,3,1,2,3, alternately as you go down the road. It is discouraged to connect electrical supplies from a house to the adjacent house as this will involve the existence of 400V between points in the same premises and this is a potential hazard if you are not prepared for it.
     
  16. sophiecentaur

    sophiecentaur 13,771
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    I am, perhaps, being a bit picky but you can only have a phase difference between TWO signals. A signal must be the PD between two points. If you consider the phase of the voltages between the centre tap and one leg then the other leg, then they will be in antiphase. I think that's what you mean and I agree.

    If you consider the potentials of the two legs relative to some other reference, then there could be a different value of phase difference - but this is not a likely scenario unless you happen to be a power engineer waving a phase meter around in a substation and connecting, willy-nilly to all possible terminals in sight!
     
  17. That's exactly it. By the way, you hooked the house up with a delta source in your drawing. I'm pretty sure that delta is used for long distance transmission, because you only need three wires, but residential lines are in a wye configuration for safety (wye can be grounded). There's a delta-wye transformation somewhere in between long distance transmission and residential distribution.

    Delta has no ground. The hot wires serve as both the send and return path of current. If the loads are balanced, there is no return current at all. Note that we're talking about current in the frequency domain here. If the loads become unbalanced, the voltages become unbalanced as the currents return through neighboring wires. In wye, the current always go out through hot wires and returns through the neutral. That means that each leg can draw whatever current it wants without disturbing the other legs.

    The advantage of delta is that it only requires three wires as opposed to wye's 4 wires. The advantage of wye is that it can survive being unbalanced. Wye can be safer too because you can ground it. That's why both are used.

    Deakn, maybe you could look at a diagram for a delta-wye transformer to get an idea of delta and wye are related. A "leg" is simply a sampling of one voltage difference.

    Let's circle back around to your original question. I think there was a bit of initial confusion because the US system is totally different from European systems. The three-phase system is not important inside a US home. You won't have to deal with any three-phase power unless you work at an industrial park that uses lots of juice. You were right about the phases of the two buses being out of phase by 180 degrees. Vk's drawing shows how the transformer accomplishes that.
     
  18. In that picture drawing can i get more info explained on how the two phases that are taking from the powerplant that are 120 degrees out of phase relative to neutral are then changed to 180 degrees out of phase
     
  19. vk6kro

    vk6kro 4,059
    Science Advisor

    The voltage between phases goes right back to the generator at the power station, where three voltages are produced by a rotating machine.
    They are then connected in a delta pattern for transmission. They pass through transformers to get different voltages, but always retain their original phase timing.

    There is a very real, normal, (and deadly) AC voltage between any two phases and each phase is displaced 120 degrees from the other phase.

    The voltage applied to the primary of the transformer is a perfectly normal AC voltage.

    The other phase to phase voltages are the same, but occur 1/3rd of a cycle earlier or later.

    A neutral may be introduced at one of the transformers or at the generating machine, but it does not affect the voltage between phases.
     
  20. sophiecentaur

    sophiecentaur 13,771
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    you seem set on not understanding this. The phase difference between the two supply phases is IRRELEVANT. There is no 'phase' difference between two of the company wires. There is just an alternating voltage. One signal, which cannot have a phase difference with itself.
     
  21. This is confusing to many people, even those who have been working with single- and three-phase systems for several years. The thing to remember is that a simple single-phase transformer (with one primary coil and one secondary coil) takes one sine wave (on the primary) and duplicates it on the secondary (typically, at a smaller amplitude). The primary and secondary waveforms are in phase with each other since the secondary is an induced copy of the primary (the secondary increases as the primary increases and it decreases as the primary decreases).

    If you were to place another identical secondary coil (one with the same number of turns of wire in it) into the transformer so that it induces voltage from the same primary, it too would be in phase with the primary, and therefore with the first secondary. So now we would have 2 coils, each producing the same voltage, in phase with each other. Let's assume these coils are both inducing a voltage of 120 VAC each.

    Now, if you were to reverse the leads of only one of the secondary coils, it would be 180 degrees out of phase with respect to the other. Both coils are still producing the same voltage, only now one coil's voltage would be increasing as the other decreases.

    In either case, if you connect the coils end-to-end, their voltages add; in the case where they are in phase with each other the voltage measured across the 2 coils would be 120VAC + 120VAC = 240VAC. In the case where the coils are out of phase with each other (and therefore, so are their voltages), the voltage measured across the 2 coils would cancel out (120VAC + -120VAC = 0VAC).

    When you use a center-tapped transformer (like the one drawn in vk6kro's post), you are basically splitting the secondary coil into 2 equal parts (there are an equal number of turns of wire on either side of the center tap), and in effect, you have 2 coils producing the same sine wave at half the amplitude (voltage). The confusion stems from the fact that the center tap is your ground reference. Looking at the left half of the split coil, you have the right side of the coil grounded. But, looking at the right half of the split coil, you have the left side of the coil grounded. This is just like swapping the leads of one of two secondary coils in relation to the first ... it changes your perspective.
     
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