Manufacturers of electric motors have for many years been faced with the problem that their motors can only run at certain almost fixed speeds determined by the number of poles in the motor.
As we learned on the previous page, the motor (or generator) slip in an asynchronous (induction) machine is usually very small for reasons of efficiency, so the rotational speed will vary with around 1 per cent between idle and full load.
The slip, however is a function of the (DC) resistance (measured in ohms) in the rotor windings of the generator. The higher resistance, the higher the slip. so one way of varying the slip is to vary the resistance in the rotor. In this way one may increase generator slip to e.g. 10 per cent.
On motors, this is usually done by having a wound rotor, i.e. a rotor with copper wire windings which are connected in a star , and connected with external variable resistors, plus an electronic control system to operate the resistors. The connection has usually been done with brushes and slip rings, which is a clear drawback over the elegantly simple technical design of an cage wound rotor machine. It also introduces parts which wear down in the generator, and thus the generator requires extra maintenance
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