Calculating Flux from a Permanent Magnet

Discussion in 'General Electronics Chat' started by nickw1881, Jan 30, 2011.

Dec 25, 2009
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I am hoping someone here has experience with permanent magnets. I have a theory on how to size a permanent magnet for an alternator I am designing, and I want someone to confirm that my technique will yield a result that is useful enough to go through the expense of building a prototype. My goal is to find the minimum thickness of Neodymium magnet that will produce the needed voltage at the RPM I plan to run.

First, permanent magnets as I understand them: Permanent magnets have a 'Br' rating, which is the amount of flux that would flow if this magnet were part of a magnetic circuit with 0 Reluctance. They also have an Hc rating, which defines the opposing magnetic field intensity that would result in 0 flux in that same magnetic circuit. If Br is plotted as a point on the vertical axis, and Hc as a point on the negative horizontal axis, then the curve between them is the B-H curve.

The B-H curve (most of it) is drawn by reducing the net flux in the circuit by A) Adding an air gap that will store potential energy as a magnetic field, or B) Using an electromagnet to create an H-field that opposes the one from the magnet. It is my understanding that a magnetic circuit is roughly analogous to an electric circuit, in that Kirchoff's loop law can be applied to both.

****If I add an air gap that has a reluctance of 10 Ampturn/Tesla, and there is 1 Tesla flowing through the circuit from the permanent magnet, then is that air gap the equivalent to using a coil to supply 10 amp turns opposing the permanent magnet flux? If the air gap and coil were swapped, would the total flux in the circuit remain the same: 1 Tesla?****

By choosing my air gap and core material, I can know the reluctance of my magnetic circuit. I will take that reluctance, multiply it with the chosen flux density (flux needed to produce rated voltage@rpm) to get H opposing. Br is constant, no matter how thick or thin the magnet is, and since Hc is rated per unit length, I should be able to scale the H axis of the B-H curve to find the correct magnet thickness. It should be similar to how I would do per-unit calculations or normalized filter design in other EE calculations.

2. thatoneguy AAC Fanatic!

Feb 19, 2009
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You would need to contact the manufacturer to get the Gauss ratings of the various magnets you are looking at.

Usually, you can generate a much larger magnetic field with an electromagnet than you can with a permanent magnet. Automobile alternators, for example, use a stator winding instead of permanent magnets. This also allows the output voltage to be varied by changing the current in the field coils. The downside to this is you need some power to start the action, after that, it is regulated.