Fan Drive Types

Fan drives can be reduced to two basic types: indirect drives (belt drives) and direct drives. Both types have strengths and weaknesses that should be considered carefully during selection. For a summary of the differences between direct and indirect drives, see Table C in the Appendix.

Indirect (Belt) Drives

Indirect drives, or belt drives, are traditionally the most common drive type employed in commercial air-moving equipment. These arrangements generally consist of an intermediary link between a driving motor and a driven fan, usually in the form of a drive belt. In order to facilitate adaptability in application, the motor and fan each have drive pulleys whose pitch may be adjusted to obtain desired fan speeds.

Belt drives have high flexibility in their fan operating speeds, since the fan rpm can be changed easily by adjusting the belts, sheaves and pulleys. Belt drives tend to require much less complex but more frequent maintenance than direct drives, as the maintenance generally pertains to changing belts or lubricating bearings. Indirect drive motors are capable of higher service factors than those of direct drives, since the indirect drive motors operate at their nominal frequencies (where cooling effects are at their highest) while direct drive motors do not.

The main drawback of indirect drives is the drive losses they entail. Generally equal to 3 to 5%, these efficiency losses negatively affect the brake horsepower (bhp) required, demanding higher bhp of the motor than is required by the fan. Indirect drives can also see higher first costs, possibly longer airway lengths, and higher vibration levels than their direct drive counterparts unless optimized as part of overall equipment design.

Direct Drives

Direct drives are simply fans coupled directly to their driving motor. This is usually accomplished by way of a common shaft, or by a mechanical coupling. As a result of the direct coupling, direct drives have no need for belts, sheaves or pulleys between the motor and the fan. This has both advantages and disadvantages.

With larger fans, the additional lateral stresses placed on the motor bearings may require a special motor or for the fan to be mounted on a separate shaft with its own bearings.

First costs and equipment savings may be realized by not requiring belts, pulleys or sheaves. These savings may be offset by the need for ancillary equipment to vary the fan motor speed. Direct driven fans will rotate at the same shaft speed as their driving motor. In almost all applications, the centrifugal fan will not be able to provide the project-specific airflow conditions at the motor’s nominal speed. The motor’s frequency must be altered to achieve the correct fan rpm. This is normally accomplished with a variable frequency drive (VFD).

Variable frequency drives are very commonly used for variable air volume (VAV) applications. Variable speed fans are the preferred choice from a power standpoint, since they take advantage of the the relationship between power and speed, namely, that a reduction in speed will result in an exponential decrease in power consumption. For example, decreasing speed by 10% will decrease horsepower by 33%. Variable frequency drives do have an efficiency cost due to electrical losses of about 2% within the drive. Serviceability may be a constraint for a direct-driven fan. Service to the driving motor often necessitates removal of the fan assembly, or at least alternate support within the unit. In some units, this may pose a significant problem as the clearances required are not always provided.

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