VFD? to power aircraft non-motor/inductive load OR power conversion
This is very unlikely to work well.



Figure 1. PWM approximation to a sine wave. Source: VFDS.

VFDs use PWM from their internal DC bus which in turn is full-wave mains and will be close to \$ \sqrt 2 V_{RMS} \$. A motor can tolerate this as its inductance will act as a low-pass filter. Your microwave oven, more than likely, will not tolerate it.

You probably require a 400 Hz sine inverter for the task.