# Under open-circuit conditions, does a transformer behave more like a resistor, capacitor, or inductor?

I have the below schematic showing the equivalent circuit of a transformer under open-circuit conditions. Will the transformer behave like a resistor, inductor, or capacitor? And will this behaviour vary depending on the values of the open circuit current, open circuit voltage, etc?

I notice that we have a resistor in series with an inductive reactance, which is then in series with a parallel combination of a resistor (representing core losses), and a magnetising inductive reactance.

So far, I have ruled out the transformer behaving as a capacitor. I am thinking that perhaps the transformer is behaving as an inductor? Although, I do not have a great understanding of why this is, it's really just a wild guess.

I appreciate your time and help,

thank you. ## 3 Answers

Inductors and transformers are constructed similarly, except that a transformer has multiple windings on the same core and an inductor has only one. If you are only using one winding of a transformer, it will behave exactly as an inductor.

For most transformers, Rc and Xm would be negligible because they represent core losses, which are kept to a minimum to allow the transformer to operate efficiently.

If you neglect those nodes in the circuit, the circuit would become an ideal inductor and a resistor in series. It would behave like a real inductor with some nonzero internal resistance.

Go with what Thor has said. When it comes to testing, beware of using transformers in open loop conditions expecting them to work as inductors. Transformers are constructed to work with minimal magnetizing current. Using them as inductors will usually saturate the core and you would see a short circuit

Your schematic tells you the answer.

The input impedance is dominated by the higher primary Inductance so that Xl >> Xm and Rl << Rm so it can be approximated by Zin=Rl+jXL.

For a transformer with a primary L = 1 H at 240 Vrms the primary current is ~ 0.75A with 4H it reduces by 4.