I have a 5-in-1 that is six years old, since I live in a hot, humid and dusty area the fan used to fail very often and I had to replace the dc motor frequently. Since, I got tired of the frequent replacements, I took one dead motor and sprayed copious amounts of WD40 through the holes on the dc motor and connected the terminals to 1.5V dc battery and allowed it run for sometime, initially you might need some push to start the motor. Repeated the process once again with WD40 to wash the dust out from the motor, after this I left the motor run for an hour and fixed it back in my 5-in-1 unit. Its now two months since I fixed it back and the fan is running fine.
The main problem here is dust which gets trapped inside the dc motor and the resultant friction stops the motor from running, if you can get rid of the dust from the motor it will work fine. This is where the WD40 comes in handy, you can also supplement the cleaning by blowing air through the holes. I used a small aquarium air pump to finally get rid of any excess WD40 from the motor.
Hope this helps