Induction Motor Drive with Reduced Speed Range
Some mechanical loads, such as fans or centrifugal pumps, exhibit a strong dependence of the load torque on speed, so that a limited speed control range suffices for achieving the desired effect. The same is true for rotating converters employing a flywheel, Sect. 7.4; since the kinetic energy of the rotating masses varies with the square of the speed, there is little incentive in varying the speed by more than, say 20 or 25%. Similar situations exist if an electrical island grid with a fluctuating frequency, for instance a railway grid, is to be supplied from the public constant frequency line through a rotating converter, or conversely, when a variable speed generator is feeding power into the constant frequency grid, as may be the case with wind energy plants or pumped storage hydro sets; using variable speed there would improve the aerodynamic efficiency of the wind rotor with changing wind velocity or the hydraulic efficiency of a pump/turbine with varying head.
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