Hi Marek,
I'm no expert but I have been working in this area recently. I don't know how much experience you have.
With your Controller Card , you need to choose to use ONE of TWO methods to controller your Spindle speed.
Pulse Width Modulation ( PWM 0 to 100% which is a 12V peak to peak signal ) OR a 0-10 VDC signal. PWM on you card is a constant voltage but the mark/space ratio varies depending on requested speed. 0-10 V signal is just a variable voltage for the same.
Are you getting them confused? My problem was that my Card only had a PWM output and my VFD could only use 0 -20 mA signal or a 0-10 VDC signal.