# Effect on inductance in persence of permanent magnet

#### hubble

Joined Jun 17, 2009
22
Hi, let's say we have an iron cored stator coil having several turns and some current is allowed to flow through it. So, north and south poles are formed. How will the inductance of the coil vary when north or south pole of a permanent magnet is brought near the south and north pole of the stator coil?

Thanks.

#### wayneh

Joined Sep 9, 2010
17,405
It's complicated. The magnetic field lines of the stator core extend from the poles, out into the room, and eventually loop back to the opposite pole. The magnetic permeability of that path affects the inductance. When you place another magnet into that field, the field lines are disrupted and the permeability changes a little. This in turn would alter the inductance.

Your cases A and B imply you have monopoles, but I assume you mean a bar magnet in line with the field lines in A and opposite the field lines in B. So in A the permeability of space seen by the stator core is slightly increased whereas it is decreased in B.

I leave it to you to relate the impact of permeability on inductance.

#### hubble

Joined Jun 17, 2009
22
Thanks @wayneh for your quick response. Why does coil inductance increase in the first case and decrease in the second case? Here is the variation curve with respect to the above two arrangements:

As it is shown in the plot, inductance is decreasing in both the cases, although by different amount. Why is it so?

#### crutschow

Joined Mar 14, 2008
32,849
How are you measuring the inductance?

#### wayneh

Joined Sep 9, 2010
17,405
Just for background there is a reference on this site.

A picture of your apparatus might be useful. For now I can only speculate that the overall permeability goes up in case B, despite the opposing field of the external magnet. But it could all be artifact for all we know.

#### hubble

Joined Jun 17, 2009
22
Thanks all for your replies. This is not my setup. I have been trying to understand the working of sensorless brushless dc motor. For now, I am reading up on position estimation and starting methods of these sensorless dc motors. Here is the resource link:

I just want to understand how this method of position estimation works.
For that, I need to know how this coil inductance varies by bringing the permanent magnet in coil's close proximity.

Hope it helps.

#### alfacliff

Joined Dec 13, 2013
2,458
look up "magnetic amplifier" to see how a magnetic field affects inductors. your circuit shows no inductance, dc through coil will produce how much inductance? inductance is an ac effect, not dc.

#### BR-549

Joined Sep 22, 2013
4,928
Nature loves perpendiculars. Reorientate your magnet to 90 degrees of coil.

Try DC first. Then AC.

Along with distance....try rotation of magnet too.

#### GopherT

Joined Nov 23, 2012
8,009
Thanks @wayneh for your quick response. Why does coil inductance increase in the first case and decrease in the second case? Here is the variation curve with respect to the above two arrangements:View attachment 96275

As it is shown in the plot, inductance is decreasing in both the cases, although by different amount. Why is it so?
Look up Lenz's law