Working principle
A three-phase induction motor turns electrical energy into rotation without any electrical connection to its rotor. Three stator windings, spaced 120 electrical degrees apart and fed by a three-phase supply, produce a magnetic field that rotates at the synchronous speed. That travelling field cuts the short-circuited rotor bars, induces a voltage in them, and the resulting rotor current reacts with the field to develop torque. Because the whole process depends on induction rather than brushes or magnets, the machine is mechanically simple and extremely robust.
Slip and speed
Synchronous speed follows the supply frequency and pole count directly: 120 times the frequency divided by the number of poles. A 4-pole motor on a 50 Hz supply therefore has a rotating field at 1500 rpm. The rotor can never quite reach that speed, because at synchronism there would be no relative motion, no induced current and no torque. The small difference is called slip, and it is typically 2–5 % at rated load. Watch slip grow in the simulator as you add load.
Torque, current and starting
Starting current is the induction motor's most awkward characteristic: with the rotor at standstill the slip is 100 %, so a direct-on-line start draws roughly five to seven times rated current for a second or two. Torque rises with slip up to the breakdown (pull-out) point and collapses beyond it, which is why an overloaded motor stalls rather than simply slowing down. Star-delta starters, soft starters and variable-frequency drives all exist to reshape this starting behaviour.
Where it is used
Three-phase induction motors drive the majority of industrial machinery worldwide: pumps, fans, compressors, conveyors, mills and machine tools. With a variable-frequency drive maintaining a constant volts-per-hertz ratio, the same machine also delivers smooth adjustable speed, which is why it has largely displaced DC motors in general industry. Its low cost, high reliability and near-zero maintenance make it the default choice whenever a three-phase supply is available.