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An inverter type AC generator includes an inverter circuit converting a DC output into AC output of a predetermined frequency and supplying the AC output to a load via a load line in order to improve the quality of an AC output waveform at least in parallel operations. The inverter AC generator incl

An inverter type AC generator includes an inverter circuit converting a DC output into AC output of a predetermined frequency and supplying the AC output to a load via a load line in order to improve the quality of an AC output waveform at least in parallel operations. The inverter AC generator includes a zero-crossing detection circuit for detecting a timing of zero-crossings of an AC output voltage waveform on the output line. A controller generates a drive signal in synchronization with the timing of the detected zero-crossings, when a predetermined number of zero-crossings have been detected, and drives the inverter to perform a synchronized operation process.

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What is claimed is: 1. An inverter type AC generator for converting DC output from a DC power source generating unit into AC output of a predetermined frequency and capable of supplying the AC output to a load via an output line, comprising: a zero-crossing detection circuit configured to detect at

What is claimed is: 1. An inverter type AC generator for converting DC output from a DC power source generating unit into AC output of a predetermined frequency and capable of supplying the AC output to a load via an output line, comprising: a zero-crossing detection circuit configured to detect at least one zero-crossing of an AC output waveform on the output line between an AC generating unit and an external load, the AC generating unit comprising an inverter and electrically connected to an AC generator driven at least in part by a power supply unit; a controller configured to generate a drive signal in synchronization with a timing of the at least one zero-crossing detected by the zero-crossing detection circuit when the at least one detected zero-crossing meets a predetermined condition, the controller configured to drive the inverter via the drive signal to perform a synchronized operation process between the inverter type AC generator and a second inverter type AC generator electrically connectable to the inverter type AC generator; and a storage unit configured to store a plurality of preset frequencies and a plurality of control programs, the control programs configured to generate drive signals for the plurality of preset frequencies, wherein the controller calculates a frequency of the AC output based on the timing of the at least one zero-crossing detected by the zero-crossing detection circuit, the controller executing one of the plurality of control programs for one of the plurality of preset frequencies stored in the storage unit that corresponds to the calculated frequency to generate the drive signal. 2. The AC generator according to claim 1, wherein the controller discontinues the synchronized operation process when a variation in the frequency of the AC output exceeds a predetermined threshold value, the controller configured to execute a normal operation process to drive the inverter via a drive signal generated based on an internally generated timing signal. 3. The AC generator according to claim 1, wherein the controller executes the normal operation process to drive the inverter based on the internally generated timing signal when the zero-crossing detection circuit does not detect at least one zero-crossing. 4. The AC generator according to claim 1, wherein the predetermined condition is the number of detected zero-crossings. 5. The inverter type AC generator according to claim 1, wherein the second inverter type AC generator operates in parallel with the inverter type AC generator. 6. The inverter type AC generator according to claim 1, wherein the controller is configured to drive the inverter via the drive signal to perform a synchronized operation process, respectively, between a phase and frequency of an AC output waveform of the inverter type AC generator and a phase and frequency of the AC output waveform of the second inverter type AC generator. 7. An inverter type AC generator for converting DC output from a DC power source generating unit into AC output of a predetermined frequency and capable of supplying the AC output to a load via an output line, comprising: a zero-crossing detection circuit configured to detect at least one zero-crossing of an AC output waveform on the output line between an AC generating unit and an external load, the AC generating unit comprising an inverter and electrically connected to an AC generator driven at least in part by a power supply unit; and means for generating a drive signal synchronized with a timing of the at least one zero-crossing detected by the zero-crossing detection circuit when the at least one detected zero-crossing meets a predetermined condition, said means driving the inverter via the drive signal to perform a synchronized operation process between the inverter type AC generator and a second inverter type AC generator electrically connectable to the inverter type AC generator, wherein said means for generating the drive signal includes means for calculating a frequency of the AC output based on the timing of the at least one detected zero-crossing, said means for generating the drive signal also executing one of a plurality control programs for one of a plurality of preset frequencies corresponding to the calculated frequency to generate the drive signal, the plurality of preset frequencies and plurality of control programs stored in a storage unit. 8. The inverter type AC generator according to claim 7, wherein the second inverter type AC generator operates in parallel with the inverter type AC generator. 9. The inverter type AC generator according to claim 7, wherein the controller is configured to drive the inverter via the drive signal to perform a synchronized operation process, respectively, between a phase and frequency of an AC output waveform of the inverter type AC generator and a phase and frequency of the AC output waveform of the second inverter type AC generator. 10. A method for synchronizing the operation of an inverter type AC generator, comprising: monitoring a frequency of an AC output waveform, including detecting at least one zero-crossing in the AC output waveform on an output line between an AC generating unit comprising an inverter and an external load; calculating a frequency of the AC output based on the timing of the at least one detected zero-crossing; and generating a drive signal in synchronization with said frequency when a predetermined condition is met, wherein generating the drive signal comprises executing one of a plurality of control programs for one of a plurality of preset frequencies corresponding to the calculated frequency to generate the drive signal, the plurality of preset frequencies and plurality of control programs stored in a storage unit; and driving the inverter of the inverter type AC generator with said drive signal to perform a synchronized operation process between the inverter type AC generator and a second inverter type AC generator electrically connectable to the inverter type AC generator, wherein the drive signal is generated in synchronization with a timing of the at least one detected zero-crossing when the detection of the at least one zero-crossing meets the predetermined condition to perform a synchronization process between the inverter type AC generator and the second inverter type AC generator. 11. The method of claim 10, wherein the predetermined condition is the number of detected zero-crossings. 12. The method of claim 10, further comprising: discontinuing the synchronized operation process when a variation in the frequency of the AC output exceeds a predetermined threshold value; generating the drive signal based on an internally generated timing signal; and driving an inverter. 13. The method of claim 12, wherein the inverter is driven based on the internally generated timing signal when the number of detected zero-crossings is zero. 14. The method of claim 10, wherein the second inverter type AC generator operates in parallel with the inverter type AC generator. 15. The method of claim 10, wherein driving the inverter with said drive signal to perform a synchronized operation process includes driving the inverter to perform a synchronized operation process, respectively, between a phase and frequency of an AC output waveform of the inverter type AC generator and a phase and frequency of the AC output waveform of the second inverter type AC generator.