# Transformer primary

• posted

Another transformer question:

What prevents current overload on a primary side of a transformer when there is no current flow in the secondary windings? As I understand it, the current flow in the secondary opposes a flow of current in the primary. If there is no load in the secondary, isn't there basically an open circuit in the primary, or does the reactance of the primary prevent current flow until a secondary field exists in the secondary windings?

The Moon is Waning Gibbous (98% of Full)

• posted

Yes. See magnetising inductance.

Graham

• posted

The term there is counter electromotive force. the magnetic field generated from the voltage applied when reversed, counters the flow of current on the ac swing. Balances out till some of the flux field is used by the secondary load etc.

How well the transformer core is made determines how much magnetic flux is lost to ( eddy currents ) generated in the core material. Which ends up being heat. If you are going to try and build your own, get a good core. Thinner the slices on the core, will have fewer eddy currents, as in the core of a microwave transformer has fairly thick slices and gets hotter from my exp. with them.

• posted

Nope, (when dealing with AC) current flow int the primary opposes current flow in the primary, allowing a current in the secondary counteracts some of that and allows more in the primary.

Bye. Jasen

• posted

When the secondary has no load, the primary winding acts like a very large value inductor. It draws a small (relative to full load) inductive (current delayed almost 90 degrees relative to the applied voltage) current.

That is roughly backward. load current through the secondary increases the current through the primary.

Not an open circuit, but a high inductive impedance.

Here are the basic relationships.

1. The flux in the core is proportional to the total, instantaneous amperes wrapped around it (amperes times turns)
2. The voltage produced across each turn is proportional to the rate of change of the flux through the core.

A load current through the secondary does not oppose the primary current, it opposes the core flux generated by the primary (cancels some of the flux created by the primary current). The reduced rate of change of flux generates slightly less voltage across the primary windings, which holds back the primary current less, so the primary current rises.

But even with no secondary current, there must be a little primary current, because something must generate the core flux, so there can be a rate of change of that flux to generate the voltage that bucks most of the primary voltage, to keep the primary from looking like the resistance of a piece of wire across the applied voltage.

• posted

you just about answered your own question. (Yes). you don't have a complete open circuit but what ever the inductance is in the primary windings. placing a load on the secondary side will cause load effects on the primary side.. one neat thing i did years ago was to use a primary side of a xformer as part of a low freq oscillator and cause load effects on the secondary side to change the inductive value of the primary thus change the frequency of the oscillator.

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