The conduction intervals of power semiconductors in static converters connected to the AC mains are dictated by the AC line voltages. Accurate and reliable synchronization signals must therefore be generated, despite the significant distortion that can be present in the AC voltage. The paper proposes a novel method for obtaining accurate zero-crossing signals, based on the use of switched-capacitor filters. The characteristic frequency of such filters depend upon the frequency of an external control signal. Therefore, this frequency can be accurately set and made to track the input frequency variations. The proposed scheme is presented and analyzed. Design criteria are derived. Experimental results demonstrate the superiority of the method over conventional filtering techniques.
The standard implementation of multiple loops in electrical drives is the cascade scheme, usually with an inner current loop and an outer speed loop. The parallel loop structure offers some interesting features and advantages. Implementation however requires a proper solution to the problem of transfer between loops. The paper proposes and analyzes a digital implementation that allows fast and accurate switching from one control loop to the other. Problems such as those caused by the presence of ripple and noise on the feedback signals are addressed. Experimental results obtained on a micro-controller based DC servo-drive indicate the excellent performance achieved with this scheme.
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