Electrical Phases Basics...

[Rizorama]

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? or something else?
3. Are the 2 hot bus bars in my electrical panel in the same phase, or are they 180 degrees out of phase?
4. Would this be considered a 2-phase piece of equipment - I never hear the term 2-phase anywhere, so I am guessing the answer is no, but then what would it be classified as - single phase, or 3 phase?

I am confused! Please help!

Thanks!
Rizorama

 

[Nigel Goodwin]

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? or something else?
3. Are the 2 hot bus bars in my electrical panel in the same phase, or are they 180 degrees out of phase?
4. Would this be considered a 2-phase piece of equipment - I never hear the term 2-phase anywhere, so I am guessing the answer is no, but then what would it be classified as - single phase, or 3 phase?

In the UK we usually only have single phase supplies to domestic premises, but as it's 230V single phase we don't have your low voltage/high current problem. The mains in the street is all three phase, with alternate houses fed from alternate phases, to balance the load across the phases - which are (as you say) 120 degrees apart.

I've always assumed that the American 220V is two phase, with the two phases 180 degrees apart - but it always seems a clumsy idea, having two different supplies in a house?.

In the distant past, some parts of the UK used to have different wattage sockets, 15A for large items, 5A for small items, and even 2A for electric clocks. Now all sockets are 13A, with fuses in the plugs (for different rated items) and all plugs are three pin, although some two core moulded plugs have a plastic earth pin!.

Presumably the 110V is for safety?, but I understand people manage to get killed on that just the same?.

 

[loosewire]

Two of the three phases are used to get the 220v. They are 120 degrees out of phase. I don't know how you would know which two phases to use for a given application except that things should balance out as much as possible across all three phases.

 

[fredsm]

In the U.S., residence power is usually supplied by a
transformer with a center-tapped 230v secondary,
fed from one phase of a 3-phase overhead. This
allows larger loads to run from the 230v, and the
smaller loads (lighting, etc.) are balanced (hopefully)
across the secondary, with a common neutral, which
is grounded at the service entrance. There is also
a safety ground from the entrance panel, which
should carry NO current, except in the case of a fault.
I will not offer an opinion of one system's merits over
another, but it seems to work well here. Most small
appliances are 115v, with only the heavier loads
(> 20A) fed by 230v, always with an appropriate
socket which differs from the 115v/15A standard
outlet. In case it is not clear, the 115v lines are
anti-phase, as is the case in all center-tapped
transformers.
<als>

 

[stevez]

While some residential electric services may be fed from a three phase line there is only a single phase (high voltage) line that feeds the transformer to my house (and three others). It's possible that at some point the single high voltage line could be traced back to three-phases where each phase might serve a different housing tract.

Someone posted some info here a while back that helped me understand the evolution of power distribution systems in US and UK/Europe. I suspect that system in the US was selected to keep the maximum potential to ground/earth to 120 volts. What the best combination was at the time was likely quite debatable.

 

[Nigel Goodwin]

Someone posted some info here a while back that helped me understand the evolution of power distribution systems in US and UK/Europe. I suspect that system in the US was selected to keep the maximum potential to ground/earth to 120 volts. What the best combination was at the time was likely quite debatable.

So was AC or DC distribution :lol:

 

[Phasor]

[deleted - dual post]

 

[Phasor]

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? or something else?
3. Are the 2 hot bus bars in my electrical panel in the same phase, or are they 180 degrees out of phase?
4. Would this be considered a 2-phase piece of equipment - I never hear the term 2-phase anywhere, so I am guessing the answer is no, but then what would it be classified as - single phase, or 3 phase?
Rizorama

I have never been to the USA, so I cannot be certain, however I have had 2 different explanations of the LV distribution system there.

One explanation says that it uses a single phase transformer with a 240V centre-tapped secondary, with the centre earthed. This results in 2 phases of 120V, 180 degrees out of phase.

Another explanation I heard (recently) was that three single phase transformers (with 240V centre tapped windings) which are connected together in a delta-delta configuration, with the midpoint between A phase and B phase earthed. This would give you 120V from A or B phase to earth and 240V between any two phases (A, B or C), and 208V between C phase and earth.

I would say your '2 phase' equipment is usually referred to as "240V single phase".