How does an electric motor work?

The principle

Step 1: Magnets and poles. Every magnet has two ends, called poles: north (N) and south (S). Its pull loops out of one end and round into the other. Put two magnets together and opposite poles (N and S) pull towards each other, while the same poles (N and N, or S and S) push each other away.

Step 2: Current makes a magnet. Electric current is electricity flowing through a wire. Any wire carrying a current has a magnetic field circling around it; a compass needle next to it swings. Wind the wire into a coil, best around a piece of iron, and those fields add up: the coil becomes a magnet with its own N and S end. Switch the current off and the magnet is gone; reverse the current and N and S swap ends.

Step 3: Pull and push make it turn. Put such a coil on an axle between two fixed magnets. Each end of the coil is pulled towards the opposite pole and pushed away from the same pole, so the coil turns. But only until it lines up, N facing S: then the pulls point straight along the coil, it swings past a little and settles there, like a pendulum.

Step 4: Flip the current every half turn. The trick is to swap the coil’s N and S just as it lines up. In a simple (brushed) DC motor, a split ring called the commutator turns with the coil, and two brushes, carbon blocks that rub on it, feed in the current. Every half turn the ring halves swap brushes, the current through the coil reverses, its poles swap, and what was a pull to stop becomes a pull onwards. The coil keeps chasing, and the motor keeps spinning.

Step 5: Brushless, like in an EV. Most electric cars, drones and modern fans turn it round: the magnets sit on the spinning part (the rotor), and the coils sit still around the outside. Electronics switch the coils on in turn, so the pull moves round and the magnet rotor follows. With no brushes to wear out they last longer and can spin fast: the rear motor in a Tesla Model 3 gives its full power at about 17,000 to 19,000 rpm (revolutions per minute).

Step 6: Putting it together. Current makes a magnet, magnets pull and push, and switching the current at the right time keeps the rotor turning. Very little is lost on the way: a good EV motor turns about 90% or more of its electrical energy into motion. A petrol engine turns about 30% of its fuel energy into motion at best and the rest into heat, so the electric motor is roughly 3 times as efficient. (For the whole car, counting the charger, battery and gears, the U.S. Department of Energy gives over 77% for an EV against about 12 to 30% for a petrol car.)

Brushed or brushless?

• Brushed DC motor: Upside: simple and cheap, it runs straight from a battery with no electronics. Downside: the brushes rub and wear out, spark and limit the speed.
• Brushless motor (EVs, drones, cordless tools): Upside: no brushes to wear, more efficient, quiet, very high speeds. Downside: it needs control electronics to switch the coils.

Try it yourself: hold a compass next to a wire from a battery and tap the wire onto the battery for a moment: the needle jumps. (Only for a second, the wire gets hot.)