How electricity turns into motion, which motors cars use, and what is inside one
An electric motor turns electrical energy into rotation that drives the wheels. There are no explosions in it: current in copper windings creates a magnetic field that travels around the fixed outer part, the stator, and pulls the inner part, the rotor, along with it. The faster the field travels, the faster the rotor spins. The same machine also works in reverse: when the wheels turn the rotor, it makes electricity. A simple design and few moving parts are why it needs so little servicing.
Where this page stops
This page is about the motor itself. The inverter that feeds it and the battery are explained in Electrical, the reduction gear in Transmission, and the coolant circuits in Cooling.
All of them chase a rotating magnetic field; the main difference is how the rotor gets its magnetism
Strong magnets are built into the rotor, so it is always magnetised and simply follows the stator's field in step. This is the most common type in today's electric cars and hybrids: compact, powerful and very efficient. Its weak spot is the magnets themselves, which use costly rare-earth metals.
Components
The rotor has no magnets, only a cage of metal bars. The stator's moving field induces currents in the bars, and those currents turn the rotor into a magnet. It always runs slightly behind the field. It is tough and needs no rare earths, but it is usually a little less efficient. With the power off it spins freely, with no magnetic drag, which suits a second axle used only when needed.
Components
Instead of permanent magnets, the rotor carries its own windings. A small direct current turns them into electromagnets, so the rotor's magnetism can be adjusted while driving. It needs no rare-earth magnets, which makes it an increasingly popular alternative. In exchange it is usually larger and heavier, and it needs a way to feed current into a spinning rotor: brushes or a contactless transfer.
Components
The other types are shaped like a can, with the rotor inside the stator. Here the rotor and stator are flat discs side by side, and the magnetic field runs along the shaft, so the motor is very short. That is a difference in shape, not in how the rotor is magnetised: most use permanent magnets. It is still rare and expensive, and appears mainly in high-performance cars.
Components
When you lift off the accelerator or brake, the wheels keep turning the rotor, and the inverter makes the motor work as a generator. Slowing the car now produces current that goes back into the battery. This is regenerative braking: part of the energy that brakes would turn into heat is saved, and the brake pads wear more slowly.
From none at all to the only source of drive
No electric motor drives the wheels; the engine does that. The car still has dozens of small electric motors for other jobs: the starter, window lifts, wipers, the fuel pump and fans. They work on the same physics at far lower power, from the 12-volt system.
The electric motor works alongside the engine and can sit on the belt, between the engine and gearbox, inside the gearbox or on the rear axle. Power ranges from about 10–20 kW in a mild hybrid, which only assists, to well over 100 kW in some plug-in hybrids that can drive on electricity alone.
The motor is the car's only source of drive, usually about 100–300 kW. One motor drives the front or rear axle; a second on the other axle gives all-wheel drive with no shaft between them. Some performance cars have three or four, and motors built into the wheels exist but are rare.
Shown on a permanent-magnet motor, the most common type
Click a node to see its description in the cards below
The fixed outer ring of the motor. It is a stack of thin steel sheets, each a fraction of a millimetre thick and insulated from the next, because a solid block would waste energy heating itself. Slots around the inside hold the windings.
Three groups of copper coils, one for each phase. Fed in turn by the inverter, they create the magnetic field that travels around the stator. Many modern motors use flat hairpin-shaped bars instead of round wire, because they pack in more copper.
The spinning part inside the stator, separated from it by a gap of about a millimetre. It never touches the stator: the magnetic field carries all the force across that gap. Many motors reach 15,000–20,000 rpm, so the rotor must be precisely balanced and strong.
Blocks of neodymium alloy set into slots in the rotor, with north and south poles alternating around it. A little dysprosium or terbium helps them stay strong when hot. These metals are costly and mined in few places, which is why makers look for motors that do without them.
The shaft carries the rotor's torque out of the motor, and a bearing at each end holds it exactly in the centre. They are among the few parts that physically wear. Some motors add a grounding ring, so stray currents do not pass through the bearings and damage them.
Tells the inverter exactly where the rotor is at every moment, because the current has to be switched at the right instant to keep pulling it forward. Without this signal, smooth and precise control would be lost. A common design is the resolver, a small contactless sensor on the end of the shaft.
Three thick conductors bring alternating current from the inverter. In electric cars the high-voltage system usually runs at 400 or 800 volts. High-voltage cables are orange by convention, so they are easy to recognise. Do not touch them: only trained technicians work on this part of the car.
Heat limits how long a motor can give full power. Coolant runs through channels in the casing, a jacket around the stator, and many motors also spray oil directly onto the windings. Keeping the motor cool also protects the magnets, which weaken if they get too hot.
A rigid casing, usually aluminium, that holds the stator and bearings in perfect alignment and helps carry heat away. In most electric cars the motor, reduction gear and often the inverter share one casing. Together they are called the drive unit.
A motor spins too fast for the wheels, so a fixed gear slows the rotation about 8–12 times and multiplies the torque by the same amount. This is usually the only gear an electric car has, because the motor pulls well across its whole speed range.