current flow

[jeninchugiak]

Electrons leaving a power supply are always trying to return to the same power supply; they are not trying to go into earth. My question: is there a neutral conductor that goes all the way back to the power supply, from a single phase & three phase transformer?

 

[al hildenbrand]

Think of the definition that you gave.

The transformer at the customer's premises is going to be supplied by a substation transformer. So the secondary of the sub xfmr is the "supply" that the current is returning to.

If the customer's premises xfmr is wired in a delta on the primary side, there will be no neutral.

If the customer's premises xfmr is wired in a wye on the primary side, there is a neutral and it will go to the substation transformer secondary.

Like as not, there will be a couple more transformers, until the transformer outside the power plant that is a step up transformer.

The generator current only goes to the primary side of the step up transformer and then returns to the generator.

 

[winnie]

For current to flow, you must have a closed circuit. This means a loop that starts at one transformer terminal, goes through your load, and goes back to another transformer terminal. A single closed circuit is the minimum required for current flow (ignoring capacitance).

A neutral need not be present, the only requirement is a closed circuit.

A neutral is transformer terminal that is at the vector zero voltage of a set of transformer terminals. It could be the center tap of a single phase secondary, or it could be the common tap of several coils in a wye connected transformer or bank, or any of a number of other possibilities. A neutral conductor is connected to this transformer terminal, and may be part of a circuit carrying current. But the neutral, even if present, is not necessarily used.

One of the transformer terminals may be connected to 'earth' in order to create a grounded system. This terminal is connected to the 'grounded conductor'. A grounded conductor is not required, but is almost universal for electrical supply. The grounded conductor need not be the neutral, although if a service supplies a neutral, code requires that the neutral be used as the grounded conductor. If the service does not supply a neutral, then one of the other circuit conductors may serve as the grounded conductor.

-Jon

 

[winnie]

Just read Al's reply, and realized that I misread the original question.

The transformer secondary need not be connected to the primary. It simply isn't required by the physics. A closed circuit connecting one transformer terminal to the other is all that is necessary for current to flow.

However it is utility practise to connect the neutral of the transformer secondary circuit (if present) to the neutral of the transformer primary circuit (if present), and to connect both of these to Earth. Thus it is very likely that there will be a continuous conductive path (not a complete circuit) all the way back to the generator. This continuous conductive path is not necessary for current flow.

-Jon

 

[charlie b]

First, the ?power supply? to which the electrons are trying to return can be a utility?s generator, but that is generally not the case for most of what we work with. At most buildings, the ?power supply? to which the electrons are trying to return is the service transformer?s secondary windings, or the secondary windings of a transformer within the building, or perhaps a back-up diesel generator, or perhaps a UPS.

Secondly, you don?t need a neutral, in order to have a path for electrons to make their way back to the source. They may leave on a Phase A wire, and return on a Phase B wire.

 

[LarryFine]

I wasn't sure whether Jen was asking about primary or secondary circuits, but this applies either way:

The only reason earth is involved at all, and why we get shocks when we find ourselves between energized conductors and earth, is because one of the circuit conductors is intentionally grounded.

Likewise, the reason electricity wants to use earth (between the source and the point in question) as a conductive path is because, again, one of the circuit conductors is intentionally grounded at the source.

If you intentionally ground a circuit conductor at any point, any other grounded surface can potentionally allow the earth (or other conductive material) between them to become a parallel conductive pathway.

If we provide the intentional pathway (your "neutral conductor") with a low-enough resistance, the voltage between those points, and thus the resultant current, is rendered negligible.