Single Phase Transformer: How Does Secondary Load Change Magnetic Flux and Cause Primary Current?

[wwhitney]

A followup to https://xenforo.mikeholt.com/threads/transformer-basic-physics-question.2569242/#post-2771964 which was too wordy.

My understanding: if you energize an idealized single phase transformer with an open circuit on the secondary, you get the transformed voltage on the secondary, in phase with primary voltage. The only current flowing on the primary is the magnetizing current, and it lags the primary voltage by 90 degrees, like any inductor. Correct?

So now if you add a load on the secondary side, what happens, including to the magnetic flux? I understand the secondary circuit can treat the secondary coil as just a voltage source, and you can find the secondary current accordingly. How does that secondary current change the magnetic flux (if it does) and cause the corresponding transformed current on the primary? That transformed current just adds to the magnetizing current?

Thanks,
Wayne

 

[bwat]

I'll take a stab at it.

The current and flux are coupled. If you're ok getting to the point of the new secondary current when the load is added, then all that's needed is to understand is that secondary current then changes the flux in the transformer, and that flux then changes the current in the primary.

Yes, I would say it adds to the magnetizing current in the same way as if you didn't have a transformer but only an ideal reactor. All you have is 90deg shifted current. Then apply a resistive load to that same circuit. You'd have a total current from the resulting L and R connected "loads".

 

[winnie]

Come take a visit to idealized approximation land where transformers are perfect inductors, with no resistance or core loss. The transformer is supplied from a perfect voltage source. Be sure to wear breathing apparatus and bring a thruster; we will be in a frictionless vacuum.

In an ideal transformer, core flux depends only upon transformer characteristics and applied voltage/frequency. The voltage induced in the secondary is simply Ns:Np * primary voltage. Secondary current doesn't change the core flux at all. Any current in the secondary is perfectly matched by current in the primary, so that the net current is unchanged and flux stays the same. You can imagine that the secondary current itself creates a magnetic field which reduces the 'back emf' produced in the primary, allowing more primary current to flow.

In a real transformer secondary current is reflected as primary current, which passes through the primary resistance. This means that the voltage available for magnetizing is reduced and thus core flux goes down when secondary current increases.

-Jon

 

[GoldDigger]

The simple way I try to use to look at it is that in the ideal case the flux and therefore the magnetizing current will stay the same since the applied voltage and core magnetic properties are unchanged. This means that additional flux produced by current in the primary will exactly cancel the flux produced by the current in the secondary.
That says what happens, but not why it happens.

 

[wwhitney]

That says what happens, but not why it happens.

Thanks, I think I am understanding the what, but not the why. What is the causation chain? E.g. as winnie suggested, an infinitesimal increase in secondary current causes an increase in the magnetic flux which causes a reduction in back EMF on the pimary, which causes an increase in primary current, which causes a counteracting change in the magnetic flux, so the magnetic flux stays constant.

Or if causation is maybe indeterminate, what physics law(s) should I think of as governing the system?

Thanks,
Wayne

P.S. For this idealized transformer, it suffices to think of the flux as a scalar that is constant around the single loop of this single phase transformer core, correct?