FAQ - Magnetics

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Richard Hull
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FAQ - Magnetics

Post by Richard Hull » Fri May 18, 2018 9:37 pm

There has been in recent months discussions revolving around electromagnets. Most of this has to do with possible systems for construction of cyclotrons. Cyclotrons can produce neutrons and accelerate deuterons. Both can lead to fusion in gifted amateur hands. However, there are other uses for electromagnets such as cloud chambers, accelerator separation of ions, etc.

A quick and, for the most part, ineffective pass at this subject is offered below. It was mostly prompted by flux measurement issues and a failure to understand magnetic circuits.

Detailed magnetic theory and the issues involved with permanent or DC electromagnets are one of the most understudied areas by technical people, forcing them to make general assumptions related to magnetic measurement and magnetic circuitry. That's right, a magnetic circuit! Look at the ohms laws for electrical circuitry versus the magnetic circuit laws.....They are, in effect, identical. (An exercise left to the student)

There is a slight difference in that in electricity, we look at resistance in ohms and bypass the term conductance, its reciprocal. In magnetics, we look at permeability,(magnetic conductance), and not its reciprocal, magnetic resistance, "reluctance", stated in "Rels". The Rel is virtually unknown, some consider it archaic, however electrical conductance in "mhos" is more commonly known and is still used.

Just as in electrical theory, in magnetic circuitry, the key is the path that magnetic lines follow from the original source of magnetism, (coil of wire or permanent magnet), back to that source as a "return path". Unlike electricity there is no loss of energy in a magnetic circuitry as it is not a power path, but a potential energy path. The losses are in flux or field lines per unit area. The electrical engineer is often looking at magnetic flux like electrical current. The analog is tempting and is good for general understanding only in magnetic circuits. Current in an electrical circuit is a sign real work is already being done. Flux in a magnetic circuit is not a flow of energy, work is not being done.

In the coil of an electromagnet, work is being done as it is creating a magnetic field in material where there was not one originally and heating the coil, as well.

Measuring magnetic field strength is like measuring neutrons. Both are less commonly followed paths for most technical people and just because you a have a neutron counter or a magnetic field strength meter, does not confer on you the ability to understand what you are measuring or how to apply and interpret information given by the instrument. A much deeper understanding of what you are dealing with at the material and theoretical level is needed.

I equate this with a person with a GM counter seeing a radioactive source read a 58 mrem/hr rate and another person with a different GM counter finding only a 20 mrem/hr rate from the exact, same source! Assuming each counter to have been recently calibrated by a national standards referenced facility, why the differential? Are they both right or both wrong or is only one reading really correct?? (Another exercise left to the student)

When I use the "exercise left to the student" phrase, it is a clue that we assume you to be inquisitive enough to become a student and seek out references as self-directed learning would force a naturally inquisitive person to do in order to find out causative agents and , thereby, "learn".

In the end, I more or less blame the manner in which magnetics is dealt with, If the Rel and magnetic resistance were emphasized in magnetic circuits we would see more wire,(electrical), in the metal magnetic circuit and more joints,(electrical), in the magnetic circuit which would cause a diminution of current(magnetic flux,Gauss), at the pole face. We are more trained to see and recognize resistance in circuits, where magnetic engineers are more on the look out for keeping the most permeability, (conductance), in their circuits.

Reluctance is discussed here and its relationship to electrical ohm's law.


Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: FAQ - Magnetics

Post by prestonbarrows » Sat May 19, 2018 2:39 am

To add to what Richard said, modeling permanent and electromagnets is dead simple in theory. Just plug and chug through Maxwell's equations from first principles.

FEMM is a great freeware 2D magnetics modeling package.

The trickiness is the mismatches between reality in the lab and the 'ideal' case in the model. Electromagnets are constructed of wires with a finite diameter, packing ratio, winding pitch etc. Permanent magnets can vary substantially, especially ones of poor quality or those exposed to external fields or high temperatures. If you are using a permeable core, it should dominate any of these inhomogeneities, but for air cores these can be significant especially for large conductor high current applications.

The ratio of permeability between air and, say, iron is on the order of tens of thousands of times. So, airgaps on the order of a thousandth of an inch (well under standard machining tolerances) can have significant effects in the magnetic circuit if one isn't careful. The exact material composition and work hardening effects can alter the permeability of stock off the shelf materials, nuts, and bolts significantly. Stainless steels are a classic bad actor here as they can vary in permeability from air to nearly pure steel depending on thier crystal structure (how the material was forged, machined, and cooled).

Permanenet magnets from a reputable dealer should have good enough info to recreate a decent model.
https://www.kjmagnetics.com/ is probably the best source for the hobbiest. Thier main drawback is they specialize in neodymnium which is not the best for high radiation and high temperature environments.

Radia is a similar package with 3D capability but a bit more flaky to use in my experience.
http://www.esrf.eu/Accelerators/Groups/ ... ware/Radia

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