My spindle RPM slowly goes up and down. from 10000 up to 13000 and back to 10000 in 5 - 6 seconds period. Tachometer measurements coinside with VFD.
Set up: Mach3 - Huanyang VFD (110V, 2.2 kW) and 110 V 2.2 kW spindle.
If I set J1 jumper to VR position - RPM fluctuation disappear (some variation on the level of 100 RPM at set value of 15000 is tolerable)
With jumper in VI position (PWM control from RNR USB breakout board) RPM fluctuates within 20% of the set value.
There are also difference between set value and real average RPM value: At 10000 RPM set on Mach3 spindle RPM fluctuates around 12000 RPM.
I check PWM signal on VI terminal with oscillopscope - it is fairly clean.

Any help is highly appreciated.