I have been reading about the working of transformers at "no load" condition, that is when no load is connected to secondary side of the transformer. I don't understand why the transformer still draws a little current called "no load current." I read about it on different websites and in books and found that it is to set up magnetic flux in the system.

Does drawing the current make the system stable or anything else, mustn't there be a requirement just like water has to go down a hill so that it can lower its potential energy?


Does drawing the current make the system stable or anything else

It's called magnetization current and basically, with the secondary unloaded, you can regard the primary as a big inductor and the resulting current that flows is the current defined by the primary inductance (aka magnetization inductance): -

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\$L_M\$ is needed to induce a voltage into the secondary.

Of course you could use more primary turns (to make the primary inductance higher) and make this current smaller but, then it's a trade off between copper losses (more turns) vs "no-load" current (fewer turns).

Have you ever read "Goldilocks and the three bears"?

  • \$\begingroup\$ I gotta ask, what is the Goldilocks reference about? ;-) \$\endgroup\$ – relayman357 Jun 2 '20 at 17:12
  • 2
    \$\begingroup\$ @relayman357 The goldilocks principle - getting the number of turns just right. \$\endgroup\$ – Andy aka Jun 2 '20 at 18:51
  • \$\begingroup\$ @Apollo - does this give you the answer to your question - if you have queries then add a comment below. \$\endgroup\$ – Andy aka Jun 4 '20 at 9:45

The core flux is a feedback_system.

The core subtracts flux generated by Secondary currents from flux generated by Primary currents.

When there is no Secondary current, there is no Secondary flux, and all the core flux is flux generated by the Primary current.

Why is this important?

The transformer is modeled with some impedance Zp = (Rp + Lp) in series with the Ideal Transformer. The series impedance is one half of a Voltage Divider, with the other half being the Core Voltage.

If the Core Voltage drops a small amount, the voltage across the Zp increases and the current from the external voltage source will increase, to increase the Primary flux and increase the Primary voltage.

Summary: the transformer is modeled as a feedback_system, which attempts to sustain a constant voltage across the Primary.

When Secondary loads are connected and Secondary currents are drawn, the Primary voltage is forced (by the feedback) to drop, and more current is supplied by the external energy source. The additional energy is being delivered to the Secondary load.


Remember this is a model.

There are some copper wires, wound around an iron/steel core.

We use all those components in the model, to accurately explain how the Primary and Secondary will interact, and be able to transfer energy.

Imagine the excitement of Charles Steinmetz or George Westinghouse, as they came to understand transformer operation in the 1850s.

  • \$\begingroup\$ Did you mean "when there is no Secondary current, there is no Secondary leakage flux..."? Because there is certainly mutual flux cutting the secondary core. I like your feedback description. Primary amp-turns balances with secondary amp-turns via the mechanism you describe - and the core mutual flux stays substantially the same (controlled by the voltage across the magnetizing branch) as the difference between N1I1 and N2I2. \$\endgroup\$ – relayman357 Jun 2 '20 at 17:10

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