Variable Frequency Synchronous Motor Drives
The speed of a synchronous motor with constant rotor excitation is determined by the stator frequency and the number of poles. As long as an efficient, variable frequency power supply was not available 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, apart from electric clocks, mainly high power drives, such as for compressors in the chemical industry. Another field of application exists in pumped storage power plants, where the synchronous generators are used as motors in periods of low demand for electrical power to drive pumps, feeding water into elevated reservoirs for later use during hours of peak demand. The type of motor is, of course, not a free choice in this instance 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. Problems with large synchronous motors operating on a constant frequency supply may be caused by the inherent oscillatory response and the required start-up procedure. Asynchronous starting at full or reduced line voltage with the help of the damper winding and the short circuited field winding as well as special starting motors are common practice; more recently, large synchronous machines are also started with variable frequency supplied from static inverters.
KeywordsCurrent Controller Induction Motor Synchronous Motor Synchronous Machine Rotor Flux
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