Richard... Honestly, input frequency probably doesn't really matter, and here is the reason why....
VFDs work by rectifying the AC inputs so that they can create a high voltage DC bus inside the VFD. That bus has some big capacitors on it. Anyhow, the computer *generates* the three phase outputs for the motor from that DC bus. It controls speed by changing the output frequency and voltage, constantly monitoring these to manage the desired speed.
Herein lies part of the consideration when configuring the VFD (aka, programming it). Frequency is dv/dt, which is a nice way of mathematically saying "the change of voltage in time." The higher the frequency, the faster change in voltage. Now, the problem with that is that this is hard on the insulation of traditional motors if they are not built for VFD use. The net result is that you have minute arcs between windings. Over time, these can get worse and eventually result in a solid arc that burns out the motor. This is why you want to limit the frequency to something reasonable.
This, by the way, is why VFDs can be used for smaller HP motors. In essence, they are small phase converters but with a twist. They are not designed to have heaters or other devices that act as switches between their outputs and the motor. For this same reason, they cannot be used to run multiple motors that turn on and off the way you would use a traditional phase converter. The one exception is Phase Perfect. It is built on VFD principles, but it does this with a patented algorithm and technology.
Hopefully this helps explain a bit of how a VFD works and explain your question a bit.
