Rotating Magnetic Fields, Explained

Rotating Magnetic Fields, Explained

If you made a motor out of a magnet, a wire coil, and some needles, you probably remember that motors and generators depend on a rotating magnetic field. Once you know how it works, the concept is pretty simple, but did you ever wonder who worked it all out to start with? Tesla figures into it, unsurprisingly. But what about Michael Dobrowolsky or Walter Bailey? Not common names to most people. [Learn Engineering] has a slick video covering the history and theory of rotating magnetic field machines, and you can watch it below.

Motors operated on direct current were not very practical at the time and caused a jerky motion. However, Tesla and another inventor named Ferraris realized that AC current could cause a rotating magnetic field without a moving commutator.

Tesla’s motor used two AC currents with a 90-degree phase difference produced by his two-phase generator. Ferraris used a single phase along with an inductor to create the phase difference. However, two-phase motors have limitations and Dobrowolsky’s three-phase design quickly replaced two-phase designs.

The video has many animations to help understand how motors work. It also explains why the magnetic poles of a motor winding appear opposite of the poles in a permanent magnet. Will you be ready to design the next super-efficient electric motor after watching this video? No. But you will have a better understanding of what really goes on inside an AC motor.

If you send electricity to a coil near a magnet, you can make a shaft rotate. If that shaft rotates a magnet and a coil, you get electricity. So, while not ideal, many generators can work as motors and vice versa. If you think the whole world runs on three-phase AC, you should read about the stubborn persistence of two-phase.