The Motor-generator Today
Motor-generators have been replaced by semiconductor devices for some purposes. A popular use for MG sets was in elevators, where accurate speed control of the hoisting machine was required, but the impracticality of varying the frequency to a high power AC motor meant that the use of an MG set with a DC hoist motor was a near industry-standard solution. Modern solid state AC drives have seen elevator installations move to direct AC drive, which has more than a 50% efficiency gain over DC machines with an MG set. In industrial settings where harmonic cancellation, frequency conversion, or line isolation is needed, MG sets remain a popular solution.
A useful feature of the motor-generator is that they can handle large short-term overloads better than semiconductor devices of the same average load rating. Consider that the thermally current-limited components of a large semiconductor inverter are solid-state switches massing a few grams with a thermal time constant to their heat sinks of likely more than 100 ms, whereas the thermally current limited components of an MG are copper windings massing some hundreds of kilos which are intrinsically attached to their own large thermal mass. They also have inherently excellent resistance to electrostatic discharge (ESD).
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