Some time ago, I saw a motor, which I don't understand. It is installed in a building in London, which used to be a pumping station, and is now a museum, The Kew Bridge Steam MuseumThe motor is used to drive a compressor, which supplies air for starting Diesel engines. The compressor is not the original one, but was installed by the museum; it was obtained from, if I remember correctly, a sewage works, where it served a similar function.
It is a three phase machine, running on the standard British supply voltage of 415V (pre recent change in nominal Voltage to 400) and the plate declares it to be an inducton motor. So far so good, but through the ventilation slots can be seen at one end of the shaft three slip rings. Why would an induction motor have slip rings? It's a relativly old machine, 1930s maybe, I can't remember the power rating, but from the size I would guess maybe 20HP.
The motor is started by a manual device, operated by a lever which swings in an arc, similar to the controller on an old style tram car. The lever cannot be swung over too quickly, as it has to be alternatly pushed in and pulled out, to clear a number of stops. Are the slip rings part of the starting system?