Yes all grounds are connected, my brushed fan works fine on the same code and connections.
Red goes to +12V
Black to common Ground
Yellow to pin8 of arduino
I posted the whole code so i can study it better than on the original Polish language forum in that small scrollingwindow.
That guy from the code uses analogwrite as well and timer1 library...
I tried my own code which works for the brushed fan but this Delta fan just won't slow down a bit. It has it's own chip onboard (i learned) for speedcontrol.
When i only connect ground and the yellow-pwm to arduino (red wire not connected) the fan won't spin.