by Chris Woodford. Last updated: January 10, 2012.
Do you know how electric motors work? The answer is probably yes and no! Although many of us have learned how a basic motor works, from simple science books and web pages such as this, almost all the motors we use everyday—in everything from vacuum cleaners to food blenders—don't actually work that way at all. What the books teach us about are simple direct current (DC) motors, which have a loop of wire spinning between the poles of a permanent magnet; in real life, the majority of high-power motors use alternating current (AC) and work in a completely different way: they're what we call induction motors and they make very ingenious use of a magnetic field that rotates. Let's take a closer look!
Photo: An everyday AC induction motor with its case and rotor removed, showing the copper windings of the coils that make up the stator (the static, non-moving part of the motor). These coils are designed to produce a rotating magnetic field, which turns the rotor (the moving part of the motor) in the space between them. Photo by David Parsons courtesy of US DOE/NREL.
How does an ordinary DC motor work?
The simple motors you see explained in science books are based on a piece of wire bent into a rectangular loop, which is suspended between the poles of a magnet. (Physicists would call this a current-carrying conductor sitting in a magnetic field.) When you hook up a wire like this to a battery, a direct current (DC) flows through it, producing a temporary magnetic field all around it. This temporary field repels the original field from the permanent magnet, causing the wire to flip over. Normally the wire would stop at that point and then flip back again, but if we use an ingenious, rotating connection called a commutator, we can make the current reverse every time the wire flips over, and that means the wire will keep rotating in the same direction for as long as the current keeps flowing. That's the essence of the simple DC electric motor, which was conceived in the 1820s by Michael Faraday and turned into a practical invention about a decade later by William Sturgeon. (You'll find more detail in our introductory article on electric motors.)
Before we move on to AC motors, let's quickly summarize what's going on here. In a DC motor, the magnet (and its magnetic field) is fixed in place and forms the outside, static part of the motor (the stator), while a coil of wire carrying the electric current forms the rotating part of the motor (the rotor). The magnetic field comes from the stator, which is a permanent magnet, while you feed the electric power to the coil that makes up the rotor. The interaction between the permanent magnetic field of the stator and the temporary magnetic field produced by the rotor is what makes the motor spin.
Artwork: A DC electric motor is based on a loop of wire turning around inside the fixed magnetic field produced by a permanent magnet. The commutator (a split ring) and brushes (carbon contacts to the commutator) reverse the electric current every time the wire turns over, which keeps it rotating in the same direction.
How does an AC motor work?
Unlike toys and flashlights, most homes, offices, factories, and other buildings aren't powered by little batteries: they're not supplied with DC current, but with alternating current (AC), which reverses its direction about 50 times per second (with a frequency of 50 Hz). If you want to run a motor from your household AC electricity supply, instead of from a DC battery, you need a different design of motor.
In an AC motor, there's a ring of electromagnets arranged around the outside (making up the stator), which are designed to produce a rotating magnetic field. Inside the stator, there's a solid metal axle, a loop of wire, a coil, a squirrel cage made of metal bars and interconnections (like the rotating cages people sometimes get to amuse pet mice), or some other freely rotating metal part that can conduct electricity. Unlike in a DC motor, where you send power to the inner rotor, in an AC motor you send power to the outer coils that make up the stator. The coils are energized in pairs, in sequence, producing a magnetic field that rotates around the outside of the motor.
How does this rotating field make the motor move? Remember that the rotor, suspended inside the magnetic field, is an electrical conductor. The magnetic field is constantly changing (because it's rotating) so, according to the laws of electromagnetism (Faraday's law, to be precise), the magnetic field produces (or induces, to use Faraday's own term) an electric current inside the rotor. If the conductor is a ring or a wire, the current flows around it in a loop. If the conductor is simply a solid piece of metal, eddy currents swirl around it instead. Either way, the induced current produces its own magnetic field and, according to another law of electromagnetism (Lenz's law) tries to stop whatever it is that causes it—the rotating magnetic field—by rotating as well. (You can think of the rotor frantically trying to "catch up" with the rotating magnetic field in an effort to eliminate the difference in motion between them.) Electromagnetic induction is the key to why a motor like this spins—and that's why it's called an induction motor.