I had a look though the Huanyang inverter manual and the only reference I can find to PWM is SPWM, which is used within the inverter to control output switching. If your arduino is putting out 5v PWM signal and you have connected it to VI it will be switching on and off like it is stupid. What software are you using on the arduino. If you are using GRBL v0.9 then D11 should put out a 0-5v analogue signal
https://github.com/grbl/grbl/wiki/Connecting-Grbl. I have the feeling that describing D11 output as PWM is in error.