Winding and motor insulation systems follow the Arrenius Equation: A chemical reaction rate doubles for every temperature increase of 10° C, since insulation systems are dielectrics they follow these rules. This means that a motors life decreases by 50% for every 10° C increase in motors temperature.
Many electric motors use thermal convection to keep the motor cool. The greater the surface area of contact, the greater heat transfer ability. Fins on the motor enclosure increase the surface area of the motor housing which increases the heat dissipation capabilities of the motor, thereby maximizing the life of the motors insulation systems.
Allowing build-up on the motors exterior limits the motors ability to adequately dissipate heat, dramatically reducing the life of the motors insulation system and the life of the motors. Frequent cleaning of the motors exterior will allow the motors to achieve its expected life.