# Finding the flux in a PM driven magnetic circuit

Discussion in 'General Electronics Chat' started by nickw1881, Jan 27, 2011.

1. ### nickw1881 Thread Starter Member

Dec 25, 2009
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How can I find the flux in a permanent magnet driven magnetic circuit with a small air gap? Can someone point me to a decent tutorial that explains how to take the Hc and Br numbers, and calculate flux through a core and air gap?

2. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,854
968
http://www.magnetsales.com/Design/DesignG.htm

Scroll down and start reading. It has all the basics including formula. If any particular part of the process still stumps you then come back with more info and more detailed questions.

3. ### nickw1881 Thread Starter Member

Dec 25, 2009
31
0
So, if the magnet was surrounded by a perfect magnetic conductor, the flux from the north face and would be Br*A. In order to have 0 flux emanating from that face, I would need to wrap a portion of the magnetic conductor with wire and apply Hc*magnet_length amp-turns. I think I get that part.

What I don't get is how to compute the flux from a permanent magnet through an imperfect magnetic conductor. For example, a steel core of permeability u, area Ac with length Lc with an air gap of area Ag and length Lg. Using the Hc number in the B=uH formula gives WAY less flux than Br, even with super high permeability steel and no air gap. Since Br is a measured value, I must be doing something wrong. Apparently Br != uckt*Hc in a permanent magnet.

With an electromagnet this would be a piece of cake. Permanent magnets are very strange.

4. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,854
968
You need to compute the magnetic losses in the air gap and in the steel and subtract that from the flux-leaving roughly, the usable flux for doing work with.

5. ### nickw1881 Thread Starter Member

Dec 25, 2009
31
0
It seems like that is working backwards isn't it? Wouldn't I need to know the flux in the steel and air gap in order to know the field potential drop in those areas?

Or, wouldn't I need to know the field_potential_drop/unit_length * length in the core and air gap in order to compute the flux 'lost' through them?

Is the net flux in the magnetic circuit: Bnet= Br - Blost?

6. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,854
968
Some 'assumptions' necessarily need to be made in a true design approach. This design is figured from the equations and adjustments are made to original assumptions if some parameter from assumed design is exceeded.

Real world needs sometimes require a knowledgeable guess and some working backwards from the guess.

Math and Engineering go together but are applied quite differently to problems. School teaches us that all the information we need to solve the problem is present in the question. Sorry, reality begs to differ with that life lesson!