Inverter suitable for use with portable AC power supply unit

Abstract

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.

We claim:  
     
         1 . An inverter comprising: 
 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.    
     
     
         2 . An inverter according to  claim 1 , wherein the power detector detects the effective power of the AC output, the phase angle calculator calculates the phase angle of the current relative to the voltage from the effective power detected by the power detector, and the controller decreases the frequency of the output voltage according to the phase angle when the phase detector detects the leading state of the phase angle.  
     
     
         3 . An inverter according to  claim 1 , wherein the power detector detects the effective power of the AC output, which further comprises a wattless power calculator calculating a wattless power on the basis of the effective power detected by the power detector, and wherein the controller decreases the frequency of the output voltage according to the wattless power when the phase detector detects the leading state of the phase angle, the controller increasing the frequency of the output voltage according to the wattless power when the phase detector detects the lagging state of the phase angle.  
     
     
         4 . An inverter according to  claim 2 , wherein the power detector detects the effective power at least in a half cycle of a sine wave reference signal.  
     
     
         5 . An inverter according to  claim 1 , wherein the PWM signal is produced on the basis of a sine wave reference signal, the power detector detects the wattless power on the basis of a detected value of an output current and a signal leading the sine wave reference signal by an electrical angle of 90 degrees, and the controller decreases the frequency of the output voltage according to the wattless power when the phase detector detects the leading state of the phase angle, the controller increasing the frequency of the output voltage according to the wattless power when the phase detector detects the lagging state of the phase angle.  
     
     
         6 . An inverter according to  claim 2 , further comprising an output voltage controller increasing the output voltage when the detected effective power is negative.  
     
     
         7 . An inverter according to  claim 2 , further comprising an auxiliary controller increasing the frequency of the output voltage when the detected effective power is negative.  
     
     
         8 . An inverter according to  claim 1 , further comprising: 
 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 circuit generating the PWM signal on the basis of the sine wave reference signal to operate the inverter circuit;    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; and    a sine wave reference data corrector reading 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, the sine wave reference data corrector multiplying the read second sine wave reference data by the corrected value to produce corrected data, the sine wave reference data corrector adding the corrected data to the first sine wave reference data corresponding to the current output timing to produce new sine wave reference data, the sine wave reference data corrector causing the new sine wave reference data to be stored on the storage device so that data thereon is renewed, and delivering the new sine wave reference data to the sine wave reference signal generator.    
     
     
         9 . An inverter according to  claim 8 , wherein the output current detector detects the current value at least in a half cycle of the output voltage to calculate the effective value.  
     
     
         10 . An inverter according to  claim 8 , 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.  
     
     
         11 . An inverter according to  claim 8 , 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.