| 摘要 |
PROBLEM TO BE SOLVED: To lessen an error in an effective value and a range of change even if there is a change in the system frequency, by finding a mean effective value using a present effective value and a specific effective value, and then finding the range of change using the mean effective value. SOLUTION: In an effective value calculating section 1, square values of instantaneous values are calculated by a square calculating circuit 11 and these values are successively stored in memories 12, 13, a square value of an effective value is calculated by an adding circuit 14, and then an effective value is calculated by a square root calculating circuit 15. In a mean calculating section 2, the effective value calculated by the square root calculating circuit 15 is stored in a memory 16 as an effective value obtained at 90 deg. before, and this value and an effective value at the present are added by an adding circuit 17 and the added value is divided by 2 by a division circuit 18 to find an effective value obtained at 90 deg. before and a mean value of it. In a range-of-change calculating section 3, the mean value calculated by the division circuit 18 is stored in a memory 19 as a mean effective value obtained at three cycles before, this value is subtracted from an effective value at the present by a subtraction circuit 20, an absolute value of the calculation result is calculated by an absolute value circuit 21, and that value is outputted as a change. |
