by Chris Woodford. Last updated: November 13, 2014.
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.
Induction motors in practice
What controls the speed of an AC motor?
In synchronous AC motors, the rotor turns at exactly the same speed as the rotating magnetic field; in an induction motor, the rotor always turns at a lower speed than the field, making it an example of what's called an asynchronous AC motor. The theoretical speed of the rotor in an induction motor depends on the frequency of the AC supply and the number of coils that make up the stator and, with no load on the motor, comes close to the speed of the rotating magnetic field. In practice, the load on the motor (whatever it's driving) also plays a part—tending to slow the rotor down. The greater the load, the greater the "slip" between the speed of the rotating magnetic field and the actual speed of the rotor. To control the speed of an AC motor (make it go faster or slower), you have to increase or decrease the frequency of the AC supply using what's called a
variable-frequency drive. So when you adjust the speed of something like a factory machine, powered by an AC induction motor, you're really controlling a circuit that's turning the frequency of the current that drives the motor either up or down.
What's the "phase" of an AC motor?
We don't necessarily have to drive the rotor with four coils (two opposing pairs), as illustrated here. It's possible to build induction motors with all kinds of other arrangements of coils. The more coils you have, the more smoothly the motor will run. The number of separate electric currents energizing the coils independently, out of step, is known as the phase of the motor, so the design shown above is a two-phase motor (with two currents energizing four coils that operate out of step in two pairs). In a three-phase motor, we could have three coils arranged around the stator in a triangle, six evenly spaced coils (three pairs), or even 12 coils (three sets of four coils), with either one, two, or four coils switched on and off together by three separate, out-of-phase currents.
Advantages and disadvantages of induction motors
The biggest advantage of AC induction motors is their sheer simplicity. They have only one moving part, the
rotor, which makes them low-cost, quiet, long-lasting, and relatively trouble free. DC
motors, by contrast, have a commutator and carbon brushes that wear
out and need replacing from time to time. The friction between the brushes and
the commutator also makes DC motors relatively noisy (and sometimes even quite smelly).
Since the speed of an induction motor depends on the frequency of the alternating current that drives it, it turns at
a constant speed unless you use a variable-frequency drive; the speed of DC motors is much easier to control simply by turning the supply voltage up or down. Though relatively simple, induction motors can be fairly heavy and bulky because of their coil windings. Unlike DC motors, they can't be driven from batteries or any other source of DC power (solar panels, for example) without using an inverter (a device that turns DC into AC). That's because they need a changing magnetic field to turn the rotor.
Artwork: Electric motors are extremely efficient, typically converting about 85 percent of the incoming electrical energy into useful, outgoing mechanical work. Even so, there is still quite a bit of energy wasted as heat inside the windings—which is why motors can get extremely hot. Most industrial-strength AC motors have built-in cooling systems. There's a fan inside the case attached to the rotor shaft (at the opposite end of the axle that's driving whatever machine the motor is attached to), shown here in red. The fan sucks air into the motor, blowing it around the outside of the case past the heat ventilating fins. If you've ever wondered why electric motors have those ridges on the outside (as you can see in the top photo on this page), that's the reason: they're cooling the motor down.
Who invented the induction motor?
Nikola Tesla (1856–1943) was a Croatian-born physicist
and prolific inventor whose many amazing contributions to science and technology
have never been fully acknowledged. After he arrived in the United States at the age of 28, he began
working for the famous electrical pioneer Thomas Edison. But the two men fell out
disastrously and soon became bitter rivals. Tesla firmly believed
that alternating current (AC) was far superior to direct current (DC),
while Edison thought the opposite. With his partner George
Westinghouse, Tesla championed AC, while Edison was
determined to run the world on DC and dreamed up all kinds of
publicity stunts to prove that AC was too dangerous for widespread use
(inventing an electric chair, to prove that AC could be lethal, and
even electrocuting Topsy the elephant with AC to show how deadly and cruel it was). The battle between these two
very different visions of electric power is sometimes known as the War of the Currents.
Despite Edison's best (or worst) efforts, Tesla won the day and AC electricity now powers much
of the world. That's largely why many of the electric motors that
drive the appliances in our homes, factories, and offices are AC
induction motors, powered by rotating magnetic fields, which Nikola
Tesla designed in the 1880s (his patent, illustrated here, was granted in May 1888). An Italian physicist named
Galileo Ferraris independently came up
with the same idea at around the same time, but history has treated him even more cruelly than
Tesla and his name is now all but forgotten.
Artwork: Nikola Tesla's original design for the AC induction motor. It works in exactly the same way as the animation up above, with two blue and two red coils alternately energized by the generator over on the right. This artwork comes from Tesla's original patent deposited at the US Patent and Trademark Office, which you can read for yourself in the references below.