Electricity - AC generation, supply and return question

[James MEng]

I am sure this is a set of naieve question first of all. However i'd appreciate someone taking the time to enlighten me, as my mind is boggled by this and has been for some days now. I have watched numerous videos on the subject of electricity but am still somewhat confused. My issue is taking the theory and translating it to a real world context

With AC power generation in a power station, I assume there are two terminals as in any circuit. AC as I understand it is the shifting of electrons either forwards or backwards through a wire. When users are 'consuming' electricity, I understand this to be a current of charge with some energy giving up that energy. So electrons must flow through the user. What is the user connected to which is at a lower potential difference than what's coming through the live wire? Surely not the wire in itself?

Is there two wires to our houses, one "supply" and one "return" through which the flow of current alternates? With just 1 wire supplying AC I cannot imagine how there is an infinite supply of electrons to the supply (if that makes sense, probably not).

This thread is similar but I couldn't answer my question from it:
https://www.physicsforums.com/showthread.php?t=491481

 

[Dale]

Hi James MEng, welcome to PF!

In most residential wiring (at least in the US) there are three terminals. One is "hot" meaning that its voltage is the one which goes up and down in alternating current. It carries current in normal situations. This is what you call "supply".

The second one is the "neutral" terminal. It is kept at the same voltage as ground (0 V), but it carries current in normal situations, but in the opposite direction of the "hot". This is what you call "return".

The third wire is the "ground". It has the same voltage as the "neutral", but it normally does not conduct current. It only conducts current when there is a failure, and its purpose is to conduct the current safely to ground and not through a person.

 

[Integral]

You might try looking at your power cord, many have 3 plugs as Dale speaks of above, but many have only 2. None have only 1. So 2 are necessary 3 is nice.

 

[James MEng]

Many thanks for welcome and responses.

Where is the neutral wire connected? i.e. does this then connect to a conductor which feeds back to the power station generator?

 

[Dale]

Many thanks for welcome and responses.

Where is the neutral wire connected? i.e. does this then connect to a conductor which feeds back to the power station generator?

Not directly to a generator in the power station, but to a transformer in the local power sub-station.

 

[James MEng]

I see. Then I assume the transformer will have a return circuit to the power station, on the opposite winding to the step down winding - as indicated here: http://en.wikipedia.org/wiki/File:Transformer_flux.gif?

 

[Nugatory]

Where is the neutral wire connected? i.e. does this then connect to a conductor which feeds back to the power station generator?

In American residential wiring, the neutral and the ground wires from an outlet inside the house are connected together at the main circuit breaker where the wiring enters the house, and these in turn are connected directly to ground and to a single ground wire back to the nearest transformer (the barrel-shaped round thing on a nearby pole, or the metal box sitting on a concrete pad somewhere in the neighborhood if you have underground service).

This arrangement ensures that the neutral and ground are always at zero potential relative to each other and the surroundings and that the hot wires are at the desired potential (usually +115 volts and -115 volts) relative to the ground.

 

[Nugatory]

I see. Then I assume the transformer will have a return circuit to the power station, on the opposite winding to the step down winding - as indicated here: http://en.wikipedia.org/wiki/File:Transformer_flux.gif?

No. It's a center-tapped transformer:

The very high voltage on the left is what comes from the power station and it doesn't have any neutral/return at all. The transformer steps that high voltage down to 230 volts, and by connecting the center of the secondary to the local ground and neutral the two ends of the secondary necessarily become +115 and -115 relative to the local ground.

Within the house, some of the 115 V circuits will be driven from one of the hot legs to the neutral, others will be driven from the other hot leg to neutral, and the 230V circuits will operate between the two hot legs with no neutral.

[Edit: I said 230/115; the picture says 240/120. It's the same thing, as the nominal voltage is 240/120 but household wiring and electrical devices are designed on the assumption that there may be some voltage drop, and anything above 110V at your receptacles should be considered normal].

 

[James MEng]

The domestic electrical wiring is making sense. Excuse my ignorance, but I must ask, how is the circuit completed from the generator at the power station? Is there not current flowing on the left of the transformer?

 

[davenn]

The domestic electrical wiring is making sense. Excuse my ignorance, but I must ask, how is the circuit completed from the generator at the power station? Is there not current flowing on the left of the transformer?

yes, of course there is current flowing in the primary ... there has to be ... the 2 wires out of the left side ( primary side) of the transformer go back to the generator at the power station. There is likely to be other step down transformers in between along the length of the transmission line.

just for example ( not exact figures) The power station generator may output at say 1000 Volts
this voltage is increased to sometimes as high as 300,000 Volts ( 300kV) really high voltage is used for long transmission lines to reduce power losses between the power station and the towns and cities. That 300kV is then stepped down to say the 7200 Volts as Nugatory showed in his drawing. This 7200V is the main grid power around the city. It is then stepped down again to feed feed individual suburbs/sections of the city for direct use by homes and businesses

cheers
Dave

 

[James MEng]

Nugatory & davenn - thank you, I will sleep well tonight!