Speed control of Three Phase Induction Motors

 Speed control of Three Phase Induction Motors

The induction machine, when operating from mains is essentially a constant speed machine. 
Many industrial drives, typically for fan or pump applications, have typically constant speed requirements and hence the induction machine is ideally suited for these. 
However, the induction machine, especially the squirrel cage type, is quite rugged and has a simple construction. 
Therefore it is good candidate for variable speed applications if it can be achieved.

✓ Speed control by changing applied voltage.
✓  Rotor resistance control.
✓  Pole changing method.
✓  Stator frequency control
✓  Cascade control

The power drawn from the rotor terminals could be spent more usefully. Apart from using the heat generated in meaning full ways, the slip ring output could be connected to another induction machine. The stator of the second machine would carry slip frequency currents of

the first machine which would generate some useful mechanical power. A still better option

would be to mechanically couple the shafts of the two machines together. This sort of a

connection is called cascade connection and it gives some measure of speed control.


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