There is more than one way to change the speed of your electric AC motor, and some of the most commonly used methods are eddy current drives and variable frequency drives (VFDs).
The Need for Adjusting Speed
Not all electric motor applications need to run at motor's standard speed. An application may need more torque (which requires a reduction in speed) or more additional speed (which in turns means a reduction in torque). A motors' speed really should match what the application needs, and when they do not, there are significant losses in efficiency. When you consider some experts' estimate that 65% of industrial power consumption comes from electric motors, you realize how much of an impact this can have on your overall plant efficiency and energy consumption.
However, if a motor is not a perfect match to the loading needs of the application, the speed and torque can be adjusted through the use of system devices (valves, dampers, etc.). That approach, however, is not ideal and still leads to wasted power and inefficiency -- which is something you normally try to avoid. An alternate, more effective solution is the use of eddy current drives or variable frequency drives (VFDs).