Electric motors

Hold two magnets against each other and you can feel it: an invisible force that pulls or pushes, without any touch. An electric motor turns exactly this force into rotation. It hums in toy cars, spins the drum of the washing machine, drives power drills, trains and electric cars. The trick behind it is surprisingly simple and has stayed the same for almost 200 years.

Attract and repel

Every magnet has a north pole and a south pole. Unlike poles attract, like poles repel. And now the key point: when current flows through a coil of wire, the coil itself becomes a magnet, an electromagnet. It has a huge advantage over a normal magnet: you can switch it on and off with the current, and even swap its poles by reversing the direction of the current.

The trick with the commutator

Inside the motor, a rotatable coil sits between two fixed magnets. When current flows, the coil becomes magnetic, is repelled by one pole and attracted by the other, and starts to turn. But after half a turn it would stop: the poles would line up nicely and the coil would stand still. This is where the commutator helps, a split metal ring on the axle. It reverses the direction of the current in the coil at exactly this moment. The coil's poles swap, the repulsion starts again, and the motor keeps turning and turning. In modern electric cars, electronics take over this switching, with no rubbing contacts at all. The principle stays exactly the same.

From toys to electric cars

The principle stays the same, only the size changes. The strength of a motor is stated in watts, the unit of power. The motor in a toy car delivers about 1 watt, the one in a power drill a few hundred watts, the one in an electric car over 100 kilowatts, which is 100000 watts. Electric motors are astonishingly efficient: large ones often turn more than 90 percent of the electricity into motion, tiny toy motors far less. A petrol engine manages only around 30 percent, the rest becomes heat. On top of that, an electric motor delivers its full pulling force straight from a standstill, which is why electric cars accelerate so strongly.

The generator, a motor in reverse

Now for the best part: the whole thing also works in reverse. If you spin the coil between the magnets from outside, a current appears in it. The motor becomes a generator. A bicycle dynamo is exactly that, and the giant generators in power plants and wind turbines work the same way. Electric cars use both directions: while driving, the motor is a motor, and while braking it becomes a generator and feeds the energy of motion back into the battery as electricity.