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Electromagnetics: Transformers, Generators, Motors and Other AC Machinery. September 2017

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 of 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



Answers

The magnetic flux is the magnetic field. Flux usually refers to the portion of a magnetic field that passes through a particular imaginary surface, usually bounded by a circuit.

I have discovered that there is almost universal confusion and misunderstanding concerning electromagnetic induction. For example, some of the answers to your question that were published in N&V contain misinformation. Walter Lewin’s lectures were recommended. I watched one and found that he made some egregious conceptual errors: He maintained that one could measure two different voltages between the same two points of a circuit! He also conflates Faraday’s Law with one of Maxwell’s equations. Another answer mentioned moving magnetic field lines cutting through a wire and generating electricity. This does not happen. In addition, it seems that magnetic fields do not move. Look up Faraday’s Paradox in Wikipedia and Bruce DePalma’s N-Machine in YouTube.

Here is a quick lesson on induction. There are two distinct principles that are called induction. One is motional emf. It is just a direct application of the definition of the magnetic field. If a conductor moves through a magnetic field such that it cuts the magnetic field lines, a non-electrostatic emf is generated in the conductor. This is how generators work.

The second principle is that a time-varying magnetic field is accompanied by an electric field. This is formulated by one of Maxwell’s equations, sometimes called the Maxwell-Faraday Law. This is how alternators work. It is not the moving magnetic field lines produced by the primary winding of a transformer that induce a current in the secondary winding. It is the electric field produced by the changing magnetic field.

Faraday’s Law causes a lot of confusion because it seems to include both principles of induction, although it does not. In most circumstances it allows one to calculate the emf of a circuit using the second principle when, in actuality, the first principle is operative. The only difference in the equations of Faraday’s Law and the Maxwell-Faraday Law is that Faraday’s Law uses the ordinary derivative and the Maxwell-Faraday Law uses the partial derivative. The partial derivative eliminates the effect of any motion of the circuit. Faraday’s Law only applies to a circuit. The Maxwell-Faraday Law does not require a circuit or even a conductor.

Michael S. La Moreaux
Ann Arbor, MI

The magnetic field is a force field that exists around any current carrying conductor. When the wire is wound as a solenoid, the fields add and it acts like a magnet if the current is DC. If the current is AC, the north and south poles continuously reverse.

The force field is continuous; there are no lines but it is convenient to measure the density of the field in Webers per square meter or Gauss, depending on the system. The density of the field is sometimes called flux.

Russell Kincaid
Milford, NH

Magnetics can be really fascinating. I could sit here and type, but if you google left hand motor rule, it has nice diagrams to explain the functions. Motors are different so you need the right hand rule.

I was very confused as you are, but a instructor in the marines demonstrated the hand rules and it then made sense. Good luck and enjoy the hand rules.

Thomas Sides
Phoenix, AZ

My best attempt to explain a transformer: Any time you move a magnet near a wire, you generate electricity in the wire. Especially so if the magnetic field lines “cut through” the wire as they are moving.

The secondary (output) windings on the transformer are constantly being “cut” by the moving magnetic field from the primary, so they generate electricity. The job of the primary (input) windings is to set up the moving magnetic field in the core and in the secondary. The primary is acting as an electromagnet.

The “movement” results from the fact that the current in the primary is AC and constantly changing. A transformer will work on AC but not on DC. That’s it, fools rush in where angels fear to tread.

Chip Veres
Miami, FL

I am also struggling with the same thing. I recently found a series of great Electricity and Magnetism lectures that are easy on the math and have lots of practical demonstrations on youtube. These are by Walter Lewin at MIT and you can find them on youtube by searching for 8.02X.

The first lecture is at https://www.youtube.com/watch?v=x1-SibwIPM4&t=478s . Also, there are lots of other free video lectures available for free from MIT open courseware as well at http://ocw.mit.edu.

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