A little late, but here it is anyway:
1) Create a PLC timer (TON) that is enabled when the fan is on. Later you can adjust the timer settings to get the animation desired. Start with 0 to 3 in full seconds.
2) You want this timer to automatically reset itself, so insert an XIO instruction pointed at the timer's .DN bit before the TON instruction.
3) Create an RSview tag that looks at the timer's .ACC bit.
4) Go to the fan object's animation properties and use the new tag to set the rotation. Adjust the angle settings as desired.
The timer will cycle from 0 to 3, and the fan object should rotate 90 degrees each second. If you have more than one fan, you can use a single timer constantly running for all. You could then use the visibility animation to hide the rotation for the fans that are not running.