Electric motors

Last updated: October 11, 2007.

They not be much to look at, but we couldn't live without electric motors like these. The top one is from a personal stereo: it converts the electrical energy in the batteries into kinetic (movement) energy that pulls the music tape through the mechanism. The bottom one is the power behind an electric toothbrush.

You can find motors similar to these 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's probably one in your computer, for starters, spinning your hard drive around, and another one powering your computer's 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 washing machines and dishwashers to coffee grinders, microwaves, and electric can openers. Electric motors have proved themselves to be one of the greatest inventions of all time. Let's pull some apart and find out how they work!

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.

Caption: Magnets—the technology behind motors! This artwork comes from NASA, with thanks.

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.

Making a motor

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 Englishman William Sturgeon (1783–1850) and his American counterpart Joseph Henry (1797–1878). Here's what they found...

Photo: An electrician repairs an electric motor onboard an aircraft carrier. By courtesy of US Navy.

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 upwards and the other will move downwards.

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 current 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. So 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.

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.

A simplified diagram of the parts in an electric motor. 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

Photo: The main parts inside a typical personal stereo motor. The motor should appear on your computer screen slightly bigger than actual size. Motors in such things as electric railroad trains are many times bigger and more powerful than this, but essentially work the same way. Note how the magnet is circular. The coils fill the space inside it completely.

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.
  

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.