Hi,
measuring the output voltage of a VFD is very problematic. It is not, as you might believe, a sine wave. It is in fact a PWM modulated square wave and without at the very least an oscilloscope
you'll not get any useful info from it, and ideally you need a high voltage differential probe. Any attempt to apply an earth reference on the output will be met with a shower of sparks wrecking your scope, your
VFD and if your lucky both of them!!

Measuring the output with a multimeter is just not going to cut it.

What most people don't realise is that the VFD should have a V/F characteristic. That is to say the at low frequency the voltage must also be low and only at high frequency will the output voltage approach
the maximum (determined by the input voltage of the VFD). VFDs are programmed from the factory to be applied to a 50Hz or 60Hz motor. Such induction motors would be 220VAC output at 50-60Hz,
but your 400Hz motor will need be only 220 x 50/400=27.5VAC at 50Hz. If you hooked up this VFD to your motor you will cause a fault, which may, at worst destroy the VFD or the spindle or both Don't do it.

Most people program the max frequency to be 400Hz....fine, but what they do not do is program the V/F curve properly. I don't know how many spindles or VFDs have been blown up as a result.
Do not attempt to run the spindle until you are 100% that you have understood and correctly programmed the V/F characteristic correctly.

Craig