LearnTronics
History
The Induction Motor
A rotating magnetic field eliminates the commutator
Direct-current motors had already shown how electricity could produce continuous rotation, but their commutators and brushes were mechanical switching parts that wore, sparked, and required maintenance.
Alternating current suggested another possibility: instead of mechanically switching current in the rotor, could the magnetic field itself be made to rotate?
It is several alternating currents arranged so their magnetic fields form one rotating field.
The rotating magnetic field — 1880s
During the 1880s, Galileo Ferraris in Italy and Nikola Tesla in the United States independently developed rotating-field AC motor ideas. Ferraris demonstrated rotating magnetic fields and two-phase motors. Tesla filed important AC motor and polyphase-system patents in 1887; major patents were granted in 1888.
How a polyphase stator creates rotation
Place several stator windings around a motor and feed them with AC currents that do not reach their peaks at the same instant. Each winding produces a magnetic field that rises and falls at a different time. Together those fields form a magnetic pattern that appears to rotate around the stator.
The rotor current is induced
In an induction motor, the rotor does not need a commutator to receive switched current. The moving stator field cuts the rotor conductors and induces current in them. Those rotor currents produce their own magnetic field, and the interaction of the fields produces torque.
- AC creates the rotating stator field;
- the rotating field induces current in the rotor;
- the induced rotor current creates rotor magnetism;
- the magnetic interaction produces torque.
Why the rotor runs slightly slower
If the rotor caught up perfectly with the rotating magnetic field, there would be no relative motion between the field and rotor conductors. With no relative motion, little or no rotor voltage would be induced.
The rotor therefore normally runs a little slower than the field. The difference is called slip.
Measurement trail 19 — synchronous speed
The speed of the rotating magnetic field depends on supply frequency and the number of magnetic poles:
where Ns is synchronous speed in RPM, f is frequency in hertz, and P is the number of poles.
A four-pole motor on 60 Hz
A real induction motor under load might run somewhat below 1800 RPM because it needs slip to produce torque.
A remarkably durable machine
The cage-type induction motor can have a rotor with no brushes, no commutator, and no external electrical connection at all. That simplicity is one reason induction motors became extraordinarily common in pumps, fans, machine tools, compressors, conveyors, and countless industrial systems.
Meanwhile, wireless needs a detector
Hertz could make and detect radio waves in a laboratory, but the receiver spark was weak and inconvenient. The next device in our sequence is one of the first practical radio detectors: the coherer.