# Soft iron core

1. Apr 8, 2007

### momomo_mo

I would like to know what is the function of soft iron core?
Is there a difference in function when inserting soft iron core in inductor , motor , generator ?
Thank you

2. Apr 8, 2007

### lpfr

The function of an iron core is to produce a bigger magnetic field (as compared to the obtained without the core). For a long coil of length L and formed by N (big N) turns of wire and an intensity I, the magnetic field B is
$$B=\mu_r\mu_\circ{N\cdot I\over L}$$
Here $$\mu_\circ=4\pi10^{-7}$$ is the permeability of vacuum.
$$\mu_r$$ is the permeability of the core of the coil. If the core is just air, $$\mu_r=1$$, and you obtain the same B as in the vacuum. For almost all substances $$\mu_r \simeq 1$$. But for ferromagnetic ones (iron, nickel, cobalt, and a few more), $$\mu_r$$ can be very big, 10 000 or even 50 000. You get a magnetic field ten thousand times bigger!
"Soft" [iron] means that once you the current drops to zero, the iron do not stay magnetized. This is important in motors and the like because in alternating currents (and fields) "hard" iron has power losses.

3. Apr 9, 2007

### momomo_mo

I see.Thanks for your information provided.