Power Supplies for Adjustable Speed AC Drives
An important result of the preceding chapter is that the application of AC motors in continuous duty adjustable speed drives calls for static converters of adequate power, generating three-phase voltages of variable amplitude and frequency. This is necessary to maintain at all speeds a low rotor frequency, which is a precondition for acceptable overall efficiency of the drive. Converters of this type are available today employing thyristors, gate turn-off thyristors (GTO), or switched power transistors (MOSFET and IGBT), but the complexity and cost of the converter equipment and control still exceeds that of line-commutated converters of a similar rating. While part of the cost for the converter can be recovered by the reduced cost of the AC motor as compared to a DC machine of comparable rating, the overall cost of the AC drive may still be somewhat higher than that of a standard DC drive.
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