RJ8214,
A filtering scheme that is commonly used is as follows:
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Connect a series resonant LC filter (tuned to the mains frequency) in series with the output to the transformer primary. Connect a parallel LC filter (again, tuned to the mains frequency on the output side of the series filter, and in parallel with the transformer primary.
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In theory, this sounds simple. However both the series capacitor and the series inductor must carry the 80 ampere load. This means that the inductor must be wound with 5 AWG wire (assuming a conservative 400 circular mils/ampere).
In addition, the capacitor(s) must have extermely low ESR (Equivalent Series Resistance) to keep the capacitor dissipation, and hence the capacitor temperature, within the capacitor rating.
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A more practical approach would be to generate a "stepped pseudo sine wave" and filter this waveform. For example, a wave form that has the following amplitudes will generate a pseudo sine wave for which all harmonics up through the ninth are absent (a symmetrical waveform contains no even harmonics):
0-30 degrees: .265 X the peak
30-60 degress: .735 X the peak
60-120 degrees: 1 X the peak
120-150 degrees: .735 X the peak
150-180 degrees: .265 X the peak
The negative half cycle would be a mirror image of the above.
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Another approach is to use a pulse width modulation scheme. See
www.tinaja.com/magsn01.asp for more details.
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Both these schemes will make your filtering requirements less stringent, since the bandwidth of the filters could be much wider (Lower Q).
regards,
Kral