Direct drive and belt drive types. There are some key differences to consider between direct drive and belt drive fans. Characteristics of direct drive fans include: Low maintenance. There are no bearings to grease, or belts to adjust or replace. They are compact in design, meaning the impellers are mounted directly to the motor shaft, reducing the overall size. They work well with variable frequency drive, also called VFDs which are electrical devices that can adjust motor speed. In comparison, direct drive fans have less performance flexibility than belt drive fans. Without the integration of a VFD or speed controller a direct drive fan cannot be sped up or slowed down. A direct drive fan has limited motor access because the impeller is mounted on the motor shaft inside the housing, and it may be difficult to access for maintenance or replacement. Some key characteristics of belt drive fans include: Performance flexibility. It is one of the most important characteristics of a belt drive fan. Final system balancing can be achieved by adjusting the belt and pulley combination. Easy access. Belt drive fans offer easy access because the motor is external. Motors are commonly out of the airstream, which allows them to be used in high temperature or contaminated air applications. Standard speed replacement motors are easily available. However, belt drive fans do require maintenance, such as lubricating bearings. They are physically larger because the motors are external, so the fan takes up more space.