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A three-phase asynchronous machine (also called induction machine) can be used either as motor or as generator. It has a passive rotor, that is either is permanently (squirrel-cage rotor, cage rotor) or from case to case (dragging ring rotor) short-circuited . When used as a generator the rotor of the asynchronous machine can be inducted also with a divergent frequency (double induction asynchronous machine). Single-phasely operating asynchronous machines are capacitor motor, AC motor and gap pole motor.
Three-phase asynchronous machines are produced with performances up to several megawatt. The unique advantage over other electromotors is lack of commutator and brushes. Brushes wear out and produce spark ("brush fire") that disturbes the power mains with high-frequency oscillations. Asynchronous machines, however, show especially in the operation at a frequency converter or servo amplifiers with regenerated power, harmonic waves which react upon the mains.
The development of the asynchronous machine declines to work from Galileo Ferraris, 1885, Nikola Tesla, 1887, and Michail of Dolivo - Dobrowolsky, 1889, who built the first single cage rotor and double cage rotor later.
The asynchronous machine is the most used electromotor today. The technical development tries to integrate frequency inverters to save space in the cabinet. Furthermore temperature sensors are increasingly installed in the windings at high performances. This improves the motor protection.
The still relatively new servomotors are lighter and more compact comparatively and replace the asynchronous machine increasingly. This occurs particularly with positioning tasks less for the continuous operation.
more --> see Induction Machine - General