Electric motors
by Chris Woodford. Last updated: June 5, 2011.
Flick a switch and get instant power—how our ancestors would have loved
electric motors! You can find them in everything from
electric trains to remote-controlled
cars—and you might be surprised how common they are. How many electric
motors are there in the room with you right now? There are probably two
in your computer for starters, one spinning your hard
drive around and another one powering the cooling fan. If
you're sitting in a bedroom, you'll find motors in hair dryers and many
toys; in the bathroom, they're in extractor fans, and electric shavers;
in the kitchen, motors are in just about every appliance from clothes washing machines and dishwashers to coffee grinders, microwaves, and electric can openers.
Electric motors have proved themselves to be among the greatest
inventions of all time. Let's pull some apart and find out how they
work!
Photo: Even small electric motors are surprisingly heavy.
That's because they're packed with tightly wound copper and heavy magnets.
This is the motor from an old electric lawn mower. The copper-colored thing toward the
front of the axle, with slits cut into it, is the commutator that keeps the motor
spinning in the same direction (as explained below).
Electricity, magnetism, and movement
The basic idea of an electric motor is really simple: you put electricity into it at one end and an
axle
(metal rod) rotates at the other end giving you the power to drive a
machine of some kind. How does this work in practice? Exactly how do
your
convert electricity into movement? To find the answer to that, we have
to go back in time almost 200 years.
Suppose you take a length of ordinary wire, make it into a big loop,
and lay it between the poles of a powerful, permanent horseshoe
magnet.
Now if you connect the two ends of the wire to a battery,
the wire will jump
up briefly. It's amazing when you see this for the first time. It's
just like magic! But there's a perfectly scientific
explanation. When an
electric current starts to creep along a wire, it creates a
magnetic field all around it. If you place the wire near a permanent
magnet, this temporary magnetic field interacts with the permanent
magnet's field. You'll know that two magnets placed near one another
either attract or repel. In the same way, the temporary magnetism
around the wire attracts or repels the permanent magnetism from the
magnet, and that's what causes the wire to jump.
How an electric motor works—in theory
Photo: An electrician repairs an electric motor
onboard an aircraft carrier.
The shiny metal he's using may look like gold,
but it's actually copper.
Although copper doesn't conduct electricity quite as well as gold,
it's much less expensive.
Photo by courtesy of US Navy.
The link between electricity, magnetism, and movement was originally
discovered in 1820 by French physicist André-Marie
Ampère
(1775–1867) and it's the basic science behind an electric motor. But if
we want to turn this amazing scientific discovery into a more practical
bit of technology to power our electric mowers and toothbrushes, we've got to take it a little bit further. The inventors who did that were Englishmen Michael Faraday (1791–1867)
and William Sturgeon (1783–1850) and American
Joseph Henry (1797–1878). Here's how they
arrived at their brilliant invention.
Suppose we bend our wire into a squarish, U-shaped loop so there are
effectively
two parallel wires running through the magnetic field. One of them
takes the electric current away from us through the wire and the other
one brings the current back again. Because the current flows in
opposite directions in the wires, Fleming's Left-Hand Rule tells us the
two wires will move in opposite directions. In other words, when we
switch on the electricity, one of the wires will move upward and the
other will move downward.
If the coil of wire could carry on moving like this, it would rotate
continuously—and we'd be well on the way to making an electric
motor. But that can't happen with our present setup: the wires will
quickly tangle up. Not only that, but if the coil could rotate far
enough,
something else would happen. Once the coil reached the vertical
position, it would flip over, so the electric current would
be flowing through it the opposite way. Now the forces on each
side of the coil would reverse. Instead of rotating continuously in the
same direction, it would move back in the direction it had just come!
Imagine an electric train with a motor like this: it would keep
shuffling back and forward on the spot without ever actually going
anywhere.
How an electric motor works—in practice
There are two ways to overcome this problem. One is to use a kind of
electric current that periodically reverses direction, which is known
as an alternating current (AC).
In the kind of small, battery-powered
motors we use around the home, a better solution is to add a component
called a commutator to the
ends of the coil. (Don't worry about the meaningless technical
name: this slightly old-fashioned word "commutation" is a bit like the
word "commute". It simply means to change back and forth in the same
way that commute means to travel back and forth.) The
commutator is simply a metal ring divided into two separate halves and
its job is to reverse the electric current in the coil each time the
coil rotates through half a turn. One end of the coil is attached to
each half of the commutator. The electric current from the battery
connects to the motor's electric terminals.
These feed electric power into the commutator through a pair of loose
connectors called brushes,
made
either from pieces of graphite (soft carbon similar to pencil
"lead") or thin lengths of springy metal,
which (as
the name suggests) "brush" against the commutator. With the
commutator in place, when electricity flows through the circuit, the
coil will rotate continuously in the same direction.
Artwork: Left: A simplified diagram of the parts in an electric
motor. Right: How it works in practice. Note how the commutator reverses the current each time the coil turns
halfway. This means the force on each side of the coil is always
pushing in the same direction, which keeps the coil rotating clockwise.
A simple, experimental motor such as this isn't capable of making
much power. We can increase the turning force (or torque)
that the
motor can create in three ways: either we can have a more
powerful permanent magnet, or we can increase the electric current
flowing through the wire, or we can make the coil so it has many
"turns" (loops) of very thin wire instead of one "turn" of thick wire.
In practice, a motor also has the permanent magnet curved in a
circular shape so it almost touches the coil of wire that rotates
inside it. The closer together the magnet and the coil, the
greater the force the motor can produce.
Inside a typical motor
Although we've described a number of different parts, you can think
of a motor as having just two essential
components:
- There's a permanent magnet (or magnets) around the edge of the motor
case that remains static, so it's called the stator
of a motor.
- Inside the stator, there's the coil, mounted on an axle that spins around at
high speed—and this is called the rotor. The
rotor also includes the commutator.
Photo: The main parts inside a medium-sized electric motor
from a coffee grinder. The gray magnet round the edge is the stator. The orange-colored coils
shown here link to the actual rotating coil inside the magnet
(which I've marked, but isn't clearly visible). Note also the slits in the commutator and the carbon brushes pushing
against it. Motors in such things as electric railroad trains are many times bigger
and more powerful than this, but essentially work the same way.
Other kinds of electric motors
In an ordinary motor, the rotor
spins inside the stator. In another ingenious motor design known as a linear motor, the stator is effectively
"unwrapped" and made it into a long continuous track so the rotor can
roll past it. Linear motors are used in such things as factory machines
and floating "maglev" railroads.
Another interesting design is the brushless DC (BLDC) motor. The stator and rotor effectively swap over, with multiple iron coils static at the center and the permanent magnet rotating around them. The commutator and brushes are replaced by an electronic circuit. Read more in our main article on hub motors.
Further reading
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