Induction Motors
The workhorse of industry: a rotating field drags the rotor along with a little slip.
Three-phase stator windings create a rotating magnetic field at synchronous speed:
(4-pole motor at 50 Hz: 1500 rpm). The field induces currents in the rotor, which then experiences torque. The rotor must run slightly slower than — otherwise there would be no relative motion and no induced current.
Squirrel-cage rotors are rugged and nearly maintenance-free; slip-ring (wound) rotors allow external resistance for high starting torque.
Sub-topics (3)
Slip and Torque-Speed Characteristic
Slip measures how far the rotor lags the field; torque rises with slip up to a maximum (breakdown) torque.
Induction Motor Starting
DOL, star-delta, autotransformer and soft starters limit the high starting current.
Variable Frequency Drives
VFDs control induction motor speed by changing frequency and voltage together.
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