How can I stop AM radio interference?

We have a Direct Ventilation System with a small high volume 240v single ph variable speed drive in the ceiling.
Every afternoon when the air temp in the ceiling warms up from the sun on the roof my wife cannot listen to her favourite ABC AM radio station because of the interference that we traced to that variable speed fan. The only off switch is the power plug in the ceiling. We have tried numerous suppressors on the radio power cord and tried running the radio on battery only moving the radio to other circuits in the house all to no avail.
Perhaps there is a suppressor we could fit to the Variable Speed Fan power cord?
Any suggestions welcomed as my wife is driving me mad.