Hi,

i had the same problem over the last couple of days and spend a lot of time reading the VFD manual an testing different settings
I bought a new 1.5kW Spindle and a used Toshiba VSF9S-2015PL-WP on Ebay. This VFD has many more options than the usually accomaning ones, so it took me a while to get it working.

The important parameters where:
- upper frequncy limit (UL) and maximum frequency (FH): set to 400Hz.
- V/Fi control mode setting (Pt): set to constant. So a linear ramp up of the voltage to the max ramp up frequncy.
- torque boost (uB) to 0% for now. So no extra voltage for start-up and high loads.
- set base frequency (uL) also to 400Hz.

The last one was the most important. The default base frequency was set to 50 Hz and with that the max voltage was reached at 50Hz.
I think i will try to reduce this later on to get a bit more torque at lower RPM. But only after improving my cooling setup (which is non existant for testing) :).

I hope this helps