induction motor
An alternating current motor in which currents in the secondary wiring of the rotor are created by induction from the magnetic field of the primary winding of the stator.
induction motor: AC power into rotating shaft, no brushes needed
An induction motor converts alternating current directly into mechanical rotation by using electromagnetic induction, requiring no electrical contacts between the stationary stator and the spinning rotor. The stator winding, energized by AC mains, creates a rotating magnetic field; this field cuts through the rotor's conductive bars (usually aluminum or copper) and induces currents in them. Those induced currents generate their own magnetic field, which reacts against the stator field to produce torque. The rotor never quite reaches the speed of the rotating field, a slip typically ranging from 2 to 5 percent at full load, depending on motor design.
Two main construction types dominate industrial use. The squirrel-cage rotor, by far the most common, consists of copper or aluminum bars embedded in slots around the rotor core and short-circuited at the ends by conducting rings; it is robust, cheap, and requires no maintenance. The wound rotor has actual winding coils and slip rings, allowing external resistance to be inserted into the rotor circuit for better starting torque and speed control, though this design costs more and needs brush maintenance.
Why induction motors prevail in industry
Induction motors dominate because they are simple, reliable, and cheap. Unlike DC motors, they have no commutator or brushes to wear out, and the squirrel-cage type has essentially no brushes at all. They start reliably on AC mains without elaborate control gear, and they tolerate mechanical shock and dirty environments. Three-phase AC motors deliver steady power and require no pulsing supply management. Standard frame sizes and ratings mean replacement parts are everywhere, and electric motors are built to match these conventions.
Starting current is a known trade-off: across-the-line starting draws 5 to 7 times full-load current for several seconds, which stresses the supply and mechanical components. Soft starters or variable-frequency drives (VFDs) smooth this, but add cost and complexity. At light load, induction motors operate less efficiently than loaded; slippage means they always consume power even when barely running.
The name reflects how the motor works: current in the rotor is induced by the stator's changing magnetic field, not delivered to it through electrical contacts. This induction principle, explained by Faraday's law, was the breakthrough that made AC motors practical. Today, induction motors range from fractional horsepower (a few watts) in household appliances to thousands of kilowatts in industrial pumps, fans, and compressors, driving about two-thirds of all mechanical motion in manufacturing.