The speed of a normal 3-phase induction motor is a function of the frequency of the supply voltage. Changing the speed of such a motor hence requires building a 3-phase power frequency convertor. The driver can be realised using power mosfets (or IGTB’s) capable of handling high voltages and fast switching speeds. The generated frequency can be programmed in a small PIC controller and even in a fast Basic Stamp.
Note that at lower than normal frequencies, the voltage should be decreased proportionally. If you forget this, the motor may overheat and eventually even burn out. (See note at the bottom of this paragraph). The circuits shown here serve mere educational purposes (although they do work!) and are not always the most suitable nor safest sollution.
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