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I have a question with respect to the current flow in step-up and step-down transformers.

AC transformers work only when an alternating current flows through the primary so as to induce current in the secondary.

So for all the different transformers located in different areas leading right up to our homes, there must be an AC current flowing through them at all times right? Does this also mean that there is AC current continuously present in the distribution lines at all instants?

Also, is the return path for these AC currents through the transformers one of the phases of the three phase power lines thereby not needing an earth ground?

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  • \$\begingroup\$ Voltage is induced not current (faraday). \$\endgroup\$ – Andy aka Jul 25 '19 at 16:47
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AC transformers work only when an alternating current flows through the primary so as to induce current in the secondary.

Yes. Although we would be more correct in saying 'induces a voltage in the secondary'. When a load is connected to the secondary, a current can flow.

So for all the different transformers located in different areas leading right up to our homes, their must be an AC current flowing through them at all times right? Does this also mean that there is AC current continuously present in the distribution lines at all instants?

Yes. Even if there are no end loads being supplied, the transformers need magnetising current, and the lines need a charging current.

Also, is the return path for these AC currents through the transformers, be one of the phases of the three phase power lines thereby not needing an earth ground?

Yes. Although most of the distribution is 3 phase, so all three conductors are used. It would be a mistake to think that one conductor was the 'return'.

A slight caveat, a neutral may be created in a delta-to-star transformer, and then this neutral can be used as a return for the current, for some light single phase loads. However, it's more usual to supply single phase loads from a transformer fed from two phases.

Earth ground is never (knowingly) used for a supply return conductor. It is only for safety duty during faults.

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  • \$\begingroup\$ Damn, feels good to finally get this right. Also, is the above true for the step up transformers present near the generating stations? i.e. one of the three phase acting as return path back to the generating station? \$\endgroup\$ – noorav Jul 25 '19 at 16:44
  • \$\begingroup\$ @noorav All three phases are simulataneously out and return, one isn't singled out as the 'return' \$\endgroup\$ – Neil_UK Jul 25 '19 at 16:47
  • \$\begingroup\$ for a DC circuit the wire that comes out of the applicance/load and goes back to the DC source is considered as the return wire and this path carries those electrons who've lost their "potential" and go to the source to get "charged" again. For an AC source, as the electrons don't actually move but rather just vibrate about their mean position, what does the return path in an AC circuit do? Does it just act as means for closing the circuit? \$\endgroup\$ – noorav Jul 25 '19 at 17:24
  • \$\begingroup\$ Forget about electrons. It causes too much confusion. Just think of charge and conventional current flow. All current flows in a circuit. It's a little like a chain drive: the drive runs continuously for DC or back and forward for AC. The effect is felt immediately at the driven cog wheel while the individual links travel relatively slowly (DC) or rock back and forward (AC). \$\endgroup\$ – Transistor Jul 25 '19 at 17:41
  • \$\begingroup\$ @noorav It's best to 'unthink', if you can, electrons losing their potential and getting charged again. Current flows in a circuit, both wires between source and load carry the same current, and can be considered equal in status. It's not necessary to have a ground, but if you do, you tend to use that as a zero reference. Some systems have one wire close in potential to ground, in which case this gets called the return, some have both conductors with a large potential to ground, this is often called line to line. The current flowing in the line has nothing to do with its potential to ground. \$\endgroup\$ – Neil_UK Jul 25 '19 at 17:43
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There will be current in the primary of a transformer. In the secondary, there will only be current flowing if there is a load. The primary of a transformer would draw current from the previous transformer's secondary.

Thus, yes there is current flowing to the last transformer primary.

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