Electronic – Primary current zero at no load? Transformers

circuit analysistransformer

I'm doing a revision on transformers and I read on a textbook that when there is no load connected, current in the primary winding is 0.

How is this possible? I mean we are still connecting a wire to a voltage source. We have the basic inductor circuit.

Best Answer

Look at the transformer model you have in your textbook. Or also here (taken from wikipedia).

In an ideal transformer, the magnetizing reactance Xm is infinite, the core losses are zero, i.e. Rc = infinite (by the way, Rp and Xp, X's and R's are 0 in an ideal transformer, but this information is not useful here). Therefore, since no current flows on the secondary (open circuit), from the transformer equations you get also no current flowing on the primary (Is = 0). Since Rc and Xc = infinite, Io = 0. Hence Ip = 0 too.

In practice, Xc and Rc are not infinite, (in particular at least Xc, since the number of turns is finite, as well as the magnetic permettitivity of the core) therefore you'll have a small current under even no load.

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