Induction Motor

An induction motor, also called an asynchronous motor, is an electric machine in which the rotating magnetic field of the stationary windings induces electrical currents in the rotor, and those currents in turn create the rotor's own magnetism. It exists because it offers a remarkably simple and rugged way to convert electrical energy into rotational motion without any permanent magnets, brushes or rare-earth metals. First demonstrated by Nikola Tesla and Galileo Ferraris in the 1880s, it became the workhorse of industry, and it returned to prominence in passenger cars when Tesla chose it for the original Model S.

The principle rests on electromagnetic induction. Three-phase alternating current fed to the stator windings produces a magnetic field that sweeps around the motor at a speed set by the supply frequency. The rotor, typically a 'squirrel cage' of aluminium or copper bars short-circuited at each end, sits inside this field. Because the rotor turns slightly slower than the field, the field lines cut across the bars and induce currents in them; those induced currents generate their own field, which the stator field drags around. This gap in speed between field and rotor is called slip, and it is essential: with no slip there would be no induced current and no torque.

For the driver and the vehicle, the induction motor's appeal is durability and cost. With no magnets to demagnetise and a rotor that is little more than a casting, it tolerates heat and high speeds well and is cheap to build from abundant materials. A further practical benefit is that when the inverter stops energising the stator, the rotor carries no magnetism of its own and so produces almost no drag. The motor can freewheel freely, which avoids the parasitic losses a permanent-magnet machine suffers when spinning unpowered.

That freewheeling trait explains why induction motors frequently appear on the front axle of dual-motor, all-wheel-drive electric cars. The car cruises on a more efficient permanent-magnet motor at the rear and calls on the induction motor only for extra traction or acceleration, letting it spin with negligible loss the rest of the time. Variants differ mainly in rotor material and winding design, with copper-rotor versions sacrificing a little cost for better efficiency.

The chief drawback is efficiency at light, steady loads. Sustaining the rotor field requires continuously inducing currents, which produces resistive heating in the rotor, so an induction motor is generally a few percentage points less efficient than a comparable permanent-magnet motor in everyday gentle driving. It can also be marginally larger and heavier for a given output. Its behaviour is governed entirely by the inverter, which shapes the frequency and amplitude of the supplied current, so the two components must be considered together as a single drive system rather than in isolation.