Everything for Electronics

# Tech Forum

September 2017

### Electromagnetics: Transformers, Generators, Motors, and Other AC Machinery

I’m trying to make sense of everything coiled, but the only thing getting wound up is me! I thought I knew a bit about electromagnetics, but recently I’ve been trying to make sense of all these fields and flows.

What is the difference between the magnetic field and the flux? How does flux work in a transformer or a generator? Does anyone really understand Maxwell’s Equations?

So many textbooks dealing with electromagnetism speak in equations instead of English. I want to know HOW it all works, not just how to compute these things. Am I just reading the wrong books? Can you help me figure out what the flux is going on?

#9172
Taylor Street
Felton, CA

I have spent a lot of my life wondering about magnetics. Welcome to the club! Hopefully the information that I have is correct

Here are a few basics:

1. When an electron moves, it generates a magnetic field in addition to the electric field that's always there.  — Why?: Because
2.The field exists if the electron/electrons move in a wire, a stream (e.g. in a tube), or anything else.
3. Magnets 'work' because (if I have this right), the arrangement of their molecules is such that the electron orbits of the individual atoms are oriented such that the fields generated add together. The relative strength of a magnetic material is based on how well they are aligned. Magnet discussion usually describes magnetic domains within the material. (Magnetizing a material means that you apply a magnetic field to align the domains.)
4. Non magnetic materials don't feature this alignment. Electron orbits are in random orientation and the fields cancel.
5. Magnetic fields are constant for DC current and non moving magnets.
6. Time varying fields are generated from AC current and moving magnets.
7. Time varying fields do the following: A. If they pass through a wire they will cause the electrons to move — thus a transformer. B. If they encounter another magnetic field they will cause either an attractive or repulsive force — thus a motor. (This force is also present in static field interactions, but that won't make a motor.)
8. Coils of wire are used in transformers and motors because the fields add and compact devices can be made.
9. The field descriptions are always confusing. What I got out of it was that the B field is the description for the field you would find due to the current or magnetic material. The resultant field that you can measure depends on the material that the field is in.

Exmple: If you have a long solenoid, the field inside will be fairly constant if you're not too close to the ends. If you put a piece of ferrous material inside, there will be an increased field through the metal because it's easier for the 'flux' to go through this material. (This also affects the input current to some extent, in the same way that a a lower resistance load affects an electric circuit.)

10. There seems to be no end of magnetic units — just like farenheit, centigrade, and Kelvin for temperature.
11. Maxwell hopefully understood his equations, along with some other smart people.

The math that you see everywhere generally shows what fields you can expect due to different circumstances. It is very difficult, and I certainly don't understand it. Happily, computers are now available that are powerful enough to avoid a lot of it. They use a method called finite element analysis. Basically they calculate fields based on the sum of tiny elements at each point of interest in a field.

Harold Johnson
via email