| 摘要 |
<p>27 An input module and a sample and hold circuit cooperate to produce sample values representative of the instantaneous magnitudes of three-phase line voltages and line currents delivered to a load by a power distribution system. The sample values representative of the line voltages are input directly to a multiplexer. The sample values representative of the line currents are input to the multiplexer both directly and through ranging amplifiers. When the sample values of the line currents are below a predetermined magnitude, the sample values amplified by the ranging amplifiers are used, thus increasing the accuracy of the metering system. The sample values are input to an analog-to-digital converter where the sample values are digitized. A control circuit calculates a plurality of indica of the AC electric energy quantity delivered to the load from the digitized data. An output module is responsive to the control circuit for displaying, recording or transmitting the calculated indica. A timing circuit times out a predetermined sample timer interval. At the end of the interval the control circuit produces new sample values and resets the timing circuit. The timing circuit is constructed so as to recognize a plurality of failure modes of the control circuit and reset the control circuit in response thereto thus improving the reliability of the metering system.</p> |
