Melted Neff extractor hood fan motor

My Neff oven hood(D8990 NO GB) recently fused at the circuit board(...the lights work OK with a new fuse, but the plug fuse now blows as the motor has shorted).  The hood has had a history of intermittent humming and vibrating with faint electrical smell when on standby(i.e. not active). The fan motor had lately taken to jamming after a short run. I dismantling the lot after it fused and found that the plastic moldings inside the fan motor had melted and the earth and neutral shorted. Any ideas what could cause this ?...the unit has had very light use in my flat and has not been subjected to extreme heat. Any ideas on the intermittent humming ? I don't want to replace the fan motor only to find I've to replace the circuit board too !   Regards Dave G