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An inverter includes a DC power supply circuit, an inverter circuit having a plurality of switching elements and switching an output of the DC power supply circuit on the basis of a PWM signal to deliver a high-frequency voltage, a filter circuit converting the high-frequency voltage to a substantia

An inverter includes a DC power supply circuit, an inverter circuit having a plurality of switching elements and switching an output of the DC power supply circuit on the basis of a PWM signal to deliver a high-frequency voltage, a filter circuit converting the high-frequency voltage to a substantially sinusoidal AC voltage, a power detector detecting an effective or wattless power of the AC power, a phase angle calculator calculating a phase angle of current relative to voltage from the detected effective or wattless power, a phase detector detecting a leading or lagging state of the phase angle, and a controller decreasing a frequency of the output voltage when the phase detector detects the leading state of the phase angle, the controller increasing the frequency of the output voltage when the phase detector detects the lagging state of the phase angle.

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1. An inverter parallel operation system, comprising:a DC power supply circuit;a filter circuit:a plurality of AC output terminals;a PWM signal generator generating a PWM signal;an inverter circuit having a plurality of switching elements driven by the PWM signal; anda plurality of AC power supply u

1. An inverter parallel operation system, comprising:a DC power supply circuit;a filter circuit:a plurality of AC output terminals;a PWM signal generator generating a PWM signal;an inverter circuit having a plurality of switching elements driven by the PWM signal; anda plurality of AC power supply units that switch an output of the DC power supply circuit, based on the switching elements, to deliver a high-frequency voltage via the filter circuit to the respective AC output terminals, the AC power supply units connected and configured to supply AC power via the respective output terminals to a common load, each AC power supply unit comprising:a power detector configured to detect an effective power at an AC output terminal side and an apparent power corresponding to the effective power;a phase angle calculator that calculates a phase angle of current relative to voltage from the detected effective power and apparent power;a phase angle detector that detects a leading or lagging state of the phase angle calculated by the phase angle calculator; anda frequency controller that controls the switching elements to (a) decrease a frequency of the output voltage of the inverter circuit when the phase angle detector detects a leading phase angle, and (b) increase the frequency of the output voltage of the inverter circuit when the phase angle detector detects a lagging phase angle. 2. An inverter parallel operation system according to claim 1, wherein the decrease or increase in the frequency of the output voltage has values based on degrees of the leading and lagging angles, respectively. 3. An inverter parallel operation system according to claim 1, further comprising a wattless power calculator that calculates a wattless power based on the effective power, and wherein the frequency controller controls the switching elements so that the decrease or increase in the frequency of the output voltage has values based on a magnitude of the wattless power. 4. An inverter parallel operation system according to claim 1, wherein the power detector detects the effective power and the apparent power at least in a half cycle of a sine wave reference signal. 5. An inverter parallel operation system according to claim 1, further comprising an output voltage controller that increases the output voltage of the inverter circuit when the detected effective power is negative. 6. An inverter parallel operation system according to claim 1, further comprising an auxiliary controller that increases an output frequency of the inverter circuit when the detected effective power is negative. 7. An inverter parallel operation system according to claim 1, wherein the PWM signal generator includes:a storage device configured to store a number of sine wave reference data corresponding to a number of phase angles at least in a half cycle of an alternating current respectively;a sine wave reference signal generator to which the sine wave reference data are provided to generate a sine wave reference signal with a predetermined frequency;a PWM signal obtaining circuit that obtains the PWM signal based on the sine wave reference signal;an output current detector configured to detect a value of current produced by the inverter circuit;a corrected value calculator that calculates a corrected value on the basis of the current value detected by the output current detector; anda sine wave reference data corrector that reads from the storage device the first sine wave reference data corresponding to a current output timing and the second sine wave reference data leading the first sine wave reference data by 90 degrees and displaced from the first sine wave reference data by a phase angle,wherein the sine wave reference data corrector multiplies the read second sine wave reference data by the corrected value to produce corrected data, adds the corrected data to the first sine wave reference data corresponding to the current output timing to produce new sine wave reference dat a, causes the new sine wave reference data to be stored on the storage device so that the data thereon is renewed, and delivers the new sine wave reference data to the sine wave reference signal generator. 8. An inverter parallel operation system according to claim 7, wherein the output current detector detects the current value at least in a half cycle of the output voltage to calculate the effective value of the detected current. 9. An inverter parallel operation system according to claim 7, wherein the phase angle calculator calculates the effective power from sine wave reference data at least in a half cycle and the value detected by the output current detector to calculate the phase angle from the calculated effective power. 10. An inverter parallel operation system according to claim 7, wherein the corrected value calculator calculates the corrected value at least in a half cycle of the output voltage, the phase angle calculator calculates the phase angle at least in the half cycle of the output voltage, and the sine wave reference data corrector carries out control in a subsequent half cycle of the output voltage using the calculated corrected value and phase angle. 11. An inverter parallel operation system comprising:a DC power supply circuit;a filter circuit;a plurality of AC output terminals;a PWM signal generator generating a PWM signal;an inverter circuit having a plurality of switching elements driven by the PWM signal;a plurality of AC power supply units that switch an output of the DC power supply circuit, based on the switching elements, to deliver a high-frequency voltage via the filter circuit to the respective AC output terminals, the AC power supply units connected and configured to supply AC power via the respective output terminals to a common load, each AC power supply unit comprising:a power detector configured to detect a wattless power at an AC output terminal side and an apparent power corresponding to the wattless power;a phase angle calculator that calculates a phase angle of current relative to voltage from the detected wattless power and apparent power;a phase angle detector configured to detect a leading or lagging state of the phase angle calculated by the phase angle calculator; anda frequency controller that controls the switching elements to (a) decrease a frequency of the output voltage of the inverter circuit when the phase angle detector detects a leading phase angle, and (b) increase the frequency of the output voltage of the inverter circuit when the phase angle detector detects a lagging phase angle. 12. An inverter parallel operation system according to claim 11, further comprising a sine wave reference signal generator that generates a sine wave reference signal and an output current detector that detects a value of current produced by the inverter circuit, wherein the PWM signal is synthesized on the basis of the sine wave reference signal generated by the sine wave reference signal generator, and the power detector detects the wattless power on the basis of a detected value of an output current from the inverter circuit and a signal leading the sine wave reference signal by an electrical angle of 90 degrees. 13. An inverter parallel operation system according to claim 12, wherein the frequency controller controls the switching elements so that amounts of decrease and increase in the frequency of the output voltage have values according to a magnitude of the wattles power. 14. An inverter parallel operation system according to claim 11, wherein the PWM signal generator includes:a storage device storing a number of sine wave reference data corresponding to a number of phase angles at least in a half cycle of an alternating current respectively;a sine wave reference signal generator to which the sine wave reference data are provided to generate a sine wave reference signal with a predetermined frequency;a PWM signal obtaining circuit that obtains the PWM signal on the basis of the sin e wave reference signal;an output current detector configured to detect a value of current produced by the inverter circuit;a corrected value calculator that calculates a corrected value on the basis of the current value detected by the output current detector; anda sine wave reference data corrector that reads from the storage device the first sine wave reference data corresponding to a current output timing and the second sine wave reference data leading the first sine wave reference data by 90 degrees and displaced from the first sine wave reference data by a phase angle, that multiplies the read second sine wave reference data by the corrected value to produce corrected data, that adds the corrected data to the first sine wave reference data corresponding to the current output timing to produce new sine wave reference data, that causes the new sine wave reference data to be stored on the storage device so that the data thereon is renewed, and that delivers the new sine wave reference data to the sine wave reference signal generator. 15. An inverter parallel operation system according to claim 14, wherein the output current detector detects the current value at least in a half cycle of the output voltage to calculate the effective value of the detected current. 16. An inverter parallel operation system according to claim 14, wherein the corrected value calculator calculates the corrected value at least in a half cycle of the output voltage, the phase angle calculator calculates the phase angle at least in the half cycle of the output voltage, and the sine wave reference data corrector carries out control in a subsequent half cycle of the output voltage using the calculated corrected value and phase angle. 17. An inverter unit comprising:a DC power supply circuit;an AC output terminal;a PWM signal generator that generates a PWM signal;an inverter circuit having a plurality of switching elements driven by the PWM signal and switching an output of the DC power supply circuit by the switching elements to deliver a high-frequency voltage;a filter circuit provided between the AC output terminal and the inverter circuit that converts the high-frequency voltage to a substantially sinusoidal AC voltage;a power detector configured to detect an effective power at an AC output terminal side and an apparent power corresponding to the effective power;a phase angle calculator that calculates a phase angle of current relative to voltage from the detected effective power and apparent power;a phase angle detector configured to detect a leading or lagging state of the phase angle calculated by the phase angle calculator; anda frequency controller that controls the switching elements to (a) decrease a frequency of the output voltage of the inverter circuit when the phase angle detector detects a leading phase angle, and (b) increase the frequency of the output voltage of the inverter circuit when the phase angle detector detects a lagging phase angle,wherein the PWM signal generator includes:a storage device storing a number of sine wave reference data corresponding to a number of phase angles at least in a half cycle of an alternating current respectively;a sine wave reference signal generator to which the sine wave reference data are sequentially supplied to generate a sine wave reference signal with a predetermined frequency;a PWM signal obtaining circuit obtaining the PWM signal on the basis of the sine wave reference signal an output current detector detecting a value of current produced by the inverter circuit;a corrected value calculator calculating a corrected value on the basis of the current value detected by the output current detector; anda sine wave reference data corrector that reads from the storage device the first sine wave reference data corresponding to a current output timing and the second sine wave reference data leading the first sine wave reference data by 90 degrees and displaced from the first sine wave reference data by a phase angle, that multiplies the read second sine wave reference data by the corrected value to produce corrected data, that adds the corrected data to the first sine wave reference data corresponding to the current output timing to produce new sine wave reference data, that causes the new sine wave reference data to be stored on the storage device so that the data thereon is renewed, and that delivers the new sine wave reference data to the sine wave reference signal generator. 18. An inverter unit, comprising:a DC power supply circuit;an AC output terminal;a PWM signal generator that generates a PWM signal;an inverter circuit having a plurality of switching elements driven by the PWM signal and switching an output of the DC power supply circuit by the switching elements to deliver a high-frequency voltage;a filter circuit provided between the AC output terminal and the inverter circuit for converting the high-frequency voltage to a substantially sinusoidal AC voltage;a power detector configured to detect a wattless power at an AC output terminal side and an apparent power corresponding to the wattless power;a phase angle calculator that calculates a phase angle of current relative to voltage from the detected wattless power and apparent power;a phase angle detector configured to detect a leading or lagging state of the phase angle calculated by the phase angle calculator;a frequency controller that controls the switching elements to (a) decrease a frequency of the output voltage of the inverter circuit when the phase angle detector detects a leading phase angle, and (b) increase the frequency of the output voltage of the inverter circuit when the phase angle detector detects a lagging phase angle; anda sine wave reference signal generator that generates a sine wave reference signal; andan output current detector configured to detect a value of current produced by the inverter circuit,wherein the PWM signal is synthesized based on the sine wave reference signal generated by the sine wave reference signal generator, and the power detector detects the wattless power based on a detected value of an output current from the inverter circuit and a signal leading the sine wave reference signal by an electrical angle of 90 degrees.