| 摘要 |
PURPOSE:To obtain an effective value of a fundamental wave without error even for frequency variation by sampling a wave at each of time points where one cycle is equally divided, and using an arithmetic mean of the sampled value and the sampled at the time point where the phase is shifted by 90 deg.. CONSTITUTION:A current flowing in a line 11 is detected by a CT 12, insulated by an auxiliary CT 13, level-converted, and a high frequency component is reduced through a low pass filter 1. In a sample-and-hold circuit 2, it is sampled and held at a predetermined time. The sampled value im digitally converted by an A/D converter 3 is stored in a memory 4. Values im stored in the memory 4 is used to obtain an arithmetic means of the sampled values im and the sum of the squares of the sampled values im shifted by 90 deg. therefrom by a CPU 5. Even if the frequency of a current flowing in the line 11 is varied, an effective value having less error can be obtained by using the sampled values shifted by 90 deg.. |
