Electromagnet design

1. Nov 8, 2009

likephysics

I was trying to get the numbers to design a 1T solenoidal electromagnet with a core of permeability >1000 (electric steel or Permalloy). I think the permeability saturates at some field strength(0.2T?). But I am not sure how to use the formula after saturation. should I just use u0 instead of u0ur?
How exactly is the field strength measured? Say for a solenoid and a button type permanent magnet. At what distance from the surface?
I am using the formula B=u0ur*n*I

2. Nov 8, 2009

Bob S

Here in thumbnail is the permeability curve for soft iron. 300 amp-turns per meter should push soft iron above 1 Tesla inside a long solenoid. The equation for the field inside a finite length solenoid is given in Smythe "Static and Dynamic Electricity" 3rd edition page 297.

Bob S

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• Iron_permeability3.jpg
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3. Nov 10, 2009

likephysics

Bobs, thanks. That makes sense. I just need to grab the static and dynamic electricity book.

4. Nov 11, 2009

Bob S

Smythe's exact solution for the on-axis B field in a finite-length solenoid is posted in the thumbnail.

Bob S

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5. Nov 11, 2009

likephysics

Thanks. Any idea how you measure the field of a permanent magnet?
I bought one of those neodymium magnets. They are supposed to be 1T. But I am not sure how to measure the field.

6. Nov 11, 2009

Bob S

I have used three ways. All are difficult.
1) Integrating coil. Wrap 100-200 turns of #40 wire into a coil with ID the same as the magnet OD. Count # turns. Build an integrating circuit with about 1 minute time constant. R series = 100 k, C feedback= 600 uF. Put a 10 meg bleed resistor in parallel with C. choose a low bias current, low voltage offset op amp.
2) Use a Hall Effect Sensor. Make good differential amplifier op amp to amplify signal. Calibration accuracy of sensor is only about +/- 20%
3) Proton resonance NMR. very accurate. Very difficult to set up.
Bob S.

7. Nov 11, 2009

likephysics

Proton NMR seems intriguing.
I tried (2), my hall effect sensor (from allegro) saturated at 0.4T when the sensor was close to the magnet (~2 inches). I know mag field drops as u go away from the magnet, but I don't know the relationship(1/r^2?).

8. Nov 12, 2009

Bob S

I am aware that the Allegro units saturate. I attach an Analog Devices datasheet that may not saturate, IF the offset adjustment allows for extending the linear range. The surface mount package is difficult to mount, however.
Proton magnetic resonance is difficult in inhomogeneous fields because it requires very small NMR samples, of the order of 1 mm cubed. You also need a grid-dip equivalent circuit (do you remember grid-dip resonance detecting circuits?) to detect resonance.
Bob S
[added] Here is a site that calculates axial field densities for any size cylindrical neodymium magnet.
http://www.magnetsales.com/Design/Calc_filles/FluxVsDistance-Disc.asp
Here is another calculator:
http://www.arnoldmagnetics.com/mtc/calc_gauss_cyl.htm [Broken]
Here is a third site:
http://www.dextermag.com/Calculation.aspx?mode=input&id=FieldCalcCylinder [Broken]
Bob S

Last edited by a moderator: May 4, 2017
9. Nov 12, 2009

Bob S

The two attached thumbnail drawings outline a voltage integrator circuit for measuring magnetic fields using the Faraday induction law.
Bob S

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