If the circuit is meant to be run on a 20 amp fuse, do not put a put a higher rated fuse in it's place! There's a reason it has a 20 amp fuse and not something larger. John is pointing you in the right direction though on the motor. The motor may be starting to go bad, or just needs a drop or two of oil in the bearings. Unfortunately, some motors don't have bearings that can be lubed. There may also be a direct short or a possible grounded spot in the wiring. As John said, check to make sure nothing is impeading the rotation of the blower. May be a varmit dead and dried up or who knows what else. If nothing there, then try unhooking the wiring to the motor and see if it still blows the fuse. If it does, there's a wiring issue. If it doesn't blow the fuse, the motor is then the guilty party.