Two things we have found in the field with upblast fans and to a lesser degree, utility set (Cage) fans in grease exhaust systems since the introduction of the direct drive fans. This also applies to belt driven fans but not as critical with the motors being offset from the shaft hole. One issue is the amount of grease and other contamination inside the motor housing particularly coming up through the shaft hole between the fan wheel and the motor. The other is the heat on these rooftops plus moisture from the weather than can enter through various openings in the motor cap or even from the air intake. Both of these issues cause havoc with the electrical components inside the fan motor housing including any open motors and the Variable Frequency Drives (VFD). Remember that these VFD's are basically computers thus should be kept out of the weather and mounted inside the stores and not outside in the fan housing. All direct drive fans must have enclosed motors installed. This avoids issues from cleaning effluent, contamination the plugs up the motor air intakes, and also from weather related issues. Please see attached recent pictures showing what the inside of the motors housings look like in an upblast fan at a hamburger concept.
