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Motors go green
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09/01/2006
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Energy efficiency is an established figure of merit for domestic appliances, as well as in building services systems. However, the induction motors traditionally employed in such applications usually run at a fixed speed derived from the line frequency and are simply cycled on and off to maintain – for example – a target temperature. This results in inefficient operation, and so the selected motor tends to be larger, heavier and more expensive than is really necessary to maintain the target conditions.
Varying the motor speed, on the other hand, allows the most efficient operating speed to be selected in normal running. Since speed control can boost efficiency by more than 30%, designers are keen to implement variable speed motor drives in next generation appliances.
Speed control challenges
High performance speed control of an induction motor is difficult to implement. Various open- and closed loop techniques have been established, but high dynamic torque control is difficult because the rotor currents cannot be measured and the rotor circuit has a large and varying time constant. On the other hand, this is quite straightforward to achieve with a synchronous motor once the rotor angular position is known.
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Author
Vanessa Knivett
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