Basic Electric Motor Drive Alternatives
See also: Induction motor and synchronous machineAlternating-current electric motors can be run in fixed-speed operation determined by the number of stator pole pairs in the motor and the frequency of the alternating current supply.
AC motors can be made with one or more stator pole pairs the number of which determines the motor's synchronous or asynchronous speed, synchronous speed being defined as
where n is synchronous speed in RPM, f is frequency in Hertz and p is number of poles.
The number of such fixed-speed-operation speeds is constrained by cost as number of pole pairs increases. If many different speeds or continuously variable speeds are required, other methods are required.
Direct-current motors allow for changes of speed by adjusting the shunt field current. Another way of changing speed of a direct current motor is to change the voltage applied to the armature.
An adjustable speed drive might consist of an electric motor and controller that is used to adjust the motor's operating speed. The combination of a constant-speed motor and a continuously adjustable mechanical speed-changing device might also be called an adjustable speed drive. Power electronics based variable frequency drives are rapidly making older technology redundant.
Read more about this topic: Adjustable-speed Drive
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