Your output stage has the LC arranged as a 2nd order low-pass filter. 80 or 90 or 100 Hz is a reasonable rolloff frequency to remove everything above 60 Hz. It's also possible to cause a kind of ringing or oscillating at that frequency especially if the load is high impedance. It's similar to a caution stated by amplifier builders, that is, always connect a load when operating this type of filter. Your schematic has resistive loads attached although the L:C ratio should be customized to the load. And 3-phase delta may raise its own complications.