Magnetic Flux 1

[Clyde38]

I have drawings of magnets with physical dimensions, materials, tolerances, etc. How do I add a Magnetic Flux requirement (weber, volt second) so I can measure the magnet when is arrives and be sure that it is fully magnetized? I know how to add this to the drawing literally however where do I get the characteristics and tolerance for this? Should the magnet supplier give this information? Is there a way to calculate this value based on the material data sheet that is typically used with BH curves and other characteristics (see attached file)?

I have a Helmholtz coil and flux meter and I would like to make sure that the magnets are fully magnetized and are the right material.



Clyde Hancock
Design & analysis of electric motors and generators
www.motorconsultants.com

 

[MagMike]

I recommend using the magnetic moment. Magnetic moment is the product of pole strength and magnet length. Cullity’s book describes it well: ‘Although pole strength and length are uncertain quantities individually, their product is the magnetic moment which can be measured precisely’. The magnetic moment will be recognized by any reputable magnet supplier.

The units for magnetic moment are Volt-Second-Meter, Weber-Meter or Ampere-Meter²

Most magnetic modeling software packages have the capability of directly calculating the value for you. The equation for it is pretty simple though:

If your magnet dimensions are inches:
(Volume)*(2.54³)*(Br)*((Permeance coefficient + 1)/(Permeance coefficient + Recoil permeability))/100,000

If your magnet dimensions are millimeters:
(Volume)*(0.1³)*(Br)*((Permeance coefficient + 1)/(Permeance coefficient + Recoil permeability))/100,000

Br (remanence) should be Gauss.
The calculated value for both equations will have units of milliWeber-centimeter (mWbcm)

When I need to calculate Min/Max limits for a particular magnet part:
For minimum value: Model the part at minimum material condition and minimum Br
For maximum value: Model the part at maximum material condition and maximum Br
To be honest, most of the time I just need to calculate the minimum value.

 

[Clyde38]

MagMike,
Thanks!

Clyde Hancock
Design & analysis of electric motors and generators

http://www.motorconsultants.com

 

[EdStainless]

If we had a new part we would calculate the min value.
But then would test the first 50pcs or so in the HH coil and set the highest and lowest aside.
Then we would take a couple from each of these groups and actually run full testing on them.
This was to give us more confidence in setting a test min.
After all even if you 100% test there will be some that are out of range, it is all statistics.
Sometimes we would find undersaturation, but usually it spoke to a dimensional issue.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Clyde38]

Thanks Ed!

Clyde Hancock
Design & analysis of electric motors and generators

http://www.motorconsultants.com

 

[MagBen]

Mike offered a simple, practical approach to measure Br, but it is under quite a few conditions: 1. it is for high coercivity magnet, with close to rectangular/square hysteresis loop. 2. two poles, magnetized straight through, 3. consistent cross-section.

using HH coils, or a single search coil, you test moment for a fully open circuit, and so moment / volume = Bdi (intrinsic flux density). if recoil permeability = 1, e.g. Bd = Hd (in a perfect square loop), then Bdi = Br. In theory, Bdi<=Br.

 

[MagBen]

oops, Mike's formular is for Br, not Bdi. my apologies.