METHOD AND APPARATUS FOR MEASURING PHYSICAL QUANTITY

摘要

PURPOSE:To suppress the operational error corresponding to the frequency variation of physical quantity by making the moment of time for stating the sampling of input signal to be measured correspond to an allowable variation rate of rated frequency and setting a plurality of specific phase angles of rated period. CONSTITUTION:An AC voltage from a transformer (PT) 1 is fed through an amplifying circuit 2 to a sample hold circuit 3 where it is converted into a rectangular wave which is then fed to an A/D converter 4 and subjected to A/D conversion based on an instruction from a central processing unit (CPU) 5 before being fed to the CPU 5 and stored temporarily therein. An AC voltage from the PT 1 is also fed to a zero-cross circuit 6 and a rectangular wave synchronized with the zero-cross timing is fed to the CPU 5. When the error rate of effective value with respect to the phase angle for every variation of rated frequency is simulated for 32 samples, the error is minimized at the phase angles of 40+ or -10 deg. and 130+ or -10 deg. of rated period. It is also true for the phase angles of 220+ or -10 deg. and 330+ or -10 deg.. Operational error due to frequency variation can be suppressed by limiting the phase for starting the sampling.