Variable Frequency Synchronous Motor Drives
The speed of a synchronous motor with DC excitation in the rotor is determined by the stator frequency and the number of poles. As long as efficient, variable frequency power supplies were unavailable this meant constant speed operation at fixed frequency. There are drive applications, where constant speed is desired or where the reactive power that can be generated with line- connected synchronous motors is an important feature; these are, besides electromechanical clocks, mainly high power drives, such as for piston compressors in the chemical industry. Another field of application exists in pumped storage power plants, where the synchronous generators serve as constant-speed-drives for pumps in periods of low demand for electrical power, feeding water into elevated reservoirs for later use during hours of peak demand. In this instance the type of motor is, of course, not a matter of choice but the synchronous machine is very well suited for this duty; it is, in fact, the only one that could be used at a power level of, possibly, several hundred MW.
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