coil inductance variation

[hubble86]

When a current carrying iron cored coil is brought near to a pole of a permanent magnet, how will the permanent magnet affect the inductance of the coil? Will the coil inductance increase/decrease in different arrangements like, south pole of coil with north pole of permanent magnet and south pole of coil with south pole of PM?

Thanks

 

[andre_luis]

I guess that what affects the inductance of the winded coil is not exactly the static magnectic flux crossing through it, but the magnetic path of the permanent magnet itself, therefore I believe that the orientation would not affect the inductance displacement.

 

[hubble86]

Here are two different arrangements with the corresponding inductance variation plot:



But I am not able to understand it completely.

 

[andre_luis]

I don't know where from you took this picture, but looks like not representing a real-world case, on which would be expected a symmetrical variation of he inductance along each pole. Seems like the inducing magnetic flow is saturating the core (?).

 

[jiripolivka]

The exact answer to your question depends on two things: the first is core material, the second is the current frequency.

At DC where iron cores are used, coil inductance is not important. Only ampere-turns that define electromagnet strength or pulling power like in relays, solenoid actuators, etc.

At AC and higher frequency, the coil inductance becomes important in circuits where the coil is connected. Iron cores are no more used due to their loss but iron-powder, ferrite and other core materials are used for induction coils.

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Inductance variation by a DC magnetization or a permanent magnet is seen in the AC case but generally unwanted.
In the switched DC/DC and AC/DC power supplies the DC magnetization problem with coil inductance has been solved by special ferrite core materials that reduce DC magnetization effect on inductance, or even can use it as an additional advantage in that application.

 

[BradtheRad]

Just for the fun of it, I have been experimenting with a coil and magnet (per diagram in post #3).

The coil has a metal tube going through the center. I run AC through the coil at various frequencies (from my sinewave generator).

I bring the magnet close to either end, slowly. The magnetic force by itself does not influence the waveform. North or south pole makes no difference.

But when the metal mass get close enough, it causes a change in amplitude. My deduction is that the magnet's metal mass changes the Henry value. I can touch the magnet anywhere on the inductor, and the change in amplitude is about the same.

I also tried moving the magnet quickly. This causes voltage swings in the coil, which are normal and expected, as a result of generator action.

 

[chuckey]

In the diagram, when the rotor tip is a N, its field is aiding the solenoid's field, more lines of flux and a slight amount of core saturation. When its S, the fields oppose so less flux and less core saturation.
Frank

 

[BradtheRad]

My magnets are neodymium, among the strongest available. They are disc shaped in several sizes. I stack them a few inches high, which creates extreme magnetic force.

The effect is strongest when I bring the large size magnets close to the coil. This indicates the effect is from metal mass, not magnetic force.

From my measurements, the coil admits greater AC current when the magnets are closest. I believe this indicates a drop in effective Henry value, although my intuitive sense would have expected there to be a rise in Henry value instead.

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Now I see post #7 mentions saturation. I would need to experiment some more, to send sufficient current through the coil, to drive it to saturation. Then I would watch whether the waveform amplitude is influenced by magnets.

 

[FvM]

The setup description in post #1 is vague in several regards. Most of all, which quantity does it consider as "inductance" , how is it measured?

Basically, by replacing air by ferromagnetic material in a magnetic circuit, you'll increase the inductance, at least for low frequencies. Core saturation might counteract. Also if you measure the coil AC impedance at a certain frequency, you don't see pure inductance, also eddy currents and other loss terms.

All in all, it can be calculated what happens in a well defined magnetic circuit, but it can be hardly predicted what happens in the partly unknown setup.

 

[hubble86]

Thanks all for your replies. I don't know how to respond. 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.

 

[andre_luis]

The picture shown in post #3 has not a steady variation, but seems like somewhat damped. Once OP mentions that coil was wound around an iron core that being a ferromagnetic material is able to "store" a magnectic field, that picture is likely depicting some initial condition, on which any residual magnetism wasn't previously present there.

 

[FvM]

O.K. I see that the plotted inductance is a well defined quantity within the scope of the paper, inverse rate of stator current change. The method is utilizing the non-linear stator magnetization characteristic. Suggest to read the paper thoroughly.

The underlying problem is to determine the rotor position while it's not or only slowly rotating, so e.m.f. can't be yet detected.