# Why is the Exciting Current zero in an ideal transformer?

An ideal transformer having infinite permeability core is supposed to have zero exciting current.

An ideal transformer has infinite permeability core, which means that all the flux in the universe would want to settle inside the core, which in turn would mean that the core has an infinite flux. The relationship to exciting current is given by :-

Now given that PHI --> Infinite & mu --> Infinite, wouldn't the mathematical proof of the above equation to prove that Ie is zero be of the form :-

My question:

1. Most books use the flux to be finite to prove that exciting current is zero, but fail to tell as to why they are considering the flux to be finite given that they started the concept by assuming an infinite permeability core meaning that the flux accommodated in the core would be infinite.

2. Is there any other proof for this mathematically, or is this the right one and I have got the concept wrong. If so where exactly?

• Zero current isn't very exciting. Commented Mar 17, 2018 at 0:09
• $\infty/\infty$ is an indeterminate form.
– τεκ
Commented Mar 17, 2018 at 0:49
• Anyways the flux is not infinite with infinite permeability. “All the flux in the world” is a finite value and besides, that approximation is based on considering the coil alone. External fields would change the flux.
– τεκ
Commented Mar 17, 2018 at 0:54

The big mistake in your question is that with infinite permeability, the inductance of the primary winding also becomes infinite and hence the magnetization current becomes zero. So no, there won’t be infinite flux in a core of infinite permeability.

• @ Andy you are delightfully able to cut to the heart of a question. Commented Mar 17, 2018 at 4:20