A single-phase three-wire watt-to-pulse frequency converter using simple PWM and its accuracy analysis

Many types of electronic watthour meters have been developed; however, they are complicated and expensive for household meters. Therefore, cutting cost of the meters has been required especially for the most popular single-phase three-wire (one-phase three-wire) system in Japan. For this purpose, the authors developed a very simple one-phase three-wire watt-to-pulse frequency converter (W-F converter) for the watthour meter. In this new converter, a general-purpose one-chip quad operational-amplifier (op-amp) IC is used as a main component of the converter, for lower initial cost without a custom IC. As the one-chip quad op-amp IC has four op-amps, two op-amps are used for two watt sensors, and the other two op-amps for a de current-to-pulse frequency converter (I-F converter). The watt sensor uses a multiplier using an approximate pulse-width modulation (APWM) with only a comparator with hysteresis and a CR circuit, replacing the conventional Miller integrator which needs an op-amp. Therefore, the total number of op-amps of the developed one-phase three-wire W-F converter is reduced to four, obtained by one one-chip IC. Although the APWM has nonlinearity, it is negligible without instability. As the converter developed does not need any special custom IC or component and is very simple, the total cost can be very low