Autotransformer: Why does primary current go through coil?

[greypilgrim]

Hi.

Why is it that in an autotransformer apparently most of the primary current goes through the winding and not through the load?

 

[Baluncore]

Why is it that in an autotransformer apparently most of the primary current goes through the winding and not through the load?

The primary current circuit is a separate circuit to the secondary current circuit. It can be argued that no primary current flows in the secondary load.

When a transformer is used in a way that shares a terminal between the primary and secondary, there is often no need to duplicate the turns that are parallel in both windings. But the transformer can still be analysed as having independent windings.

The shared turns will have a current that is the sum of the independent primary and secondary currents.

 

[greypilgrim]

The primary current circuit is a separate circuit to the secondary current circuit. It can be argued that no primary current flows in the secondary load.

Why? To me it looks like the shared turns and the load are connected in parallel to the voltage source. If this were a DC circuit, there would be current in both, depending only on the resistances of coil and load.

 

[Baluncore]

To me it looks like the shared turns and the load are connected in parallel to the voltage source.

Without an annotated circuit diagram it is difficult to comment on what it looks like.
It is not a DC circuit, there is magnetic coupling between the primary and secondary.

 

[greypilgrim]

Well I'm just talking about the simplest circuit with AC source, autotransformer and load such as

It is not a DC circuit, there is magnetic coupling between the primary and secondary.

Sure. But how does magnetic coupling prevent any of the primary current going through the load?

 

[nsaspook]

This is what we have in this autotransformer configuration.

http://www.electronics-tutorials.ws/transformer/auto-transformer.html

When the primary current IP is flowing through the single winding in the direction of the arrow as shown, the secondary current, IS, flows in the opposite direction. Therefore, in the portion of the winding that generates the secondary voltage, VS the current flowing out of the winding is the difference of IP and IS.

 

[Baluncore]

The current in the primary circuit flows equally through both parts of the coil.
The secondary current flows only in the bottom part of the coil.
The current in the top part is therefore only primary current.
The current in the bottom part is the sum of primary and secondary currents.

Here is a diagram that shows the equivalent circuit. Note that there is only one connection between primary and secondary.