Fan Motors Size Selection

The key indicator for determining fan motor size is fan brake horsepower (BHP), which represents the force needed to brake, or stop, the motor. This is the minimum amount of power needed to operate the fan.

Your fan manufacturer will always specify brake horsepower. But there’s another factor you need to take into account before sizing your fan motor. First, you need to determine whether your fan will be belt-driven or direct-driven.

Selecting Motor Size Based on Fan Drive Arrangement and Brake Horsepower

Fan drive arrangement is an important factor in motor size selection because of horsepower losses associated with different arrangements. If you were to specify the motor size to exactly match the fan’s brake horsepower, the motor may not produce the horsepower you need.

Direct-Driven Fan Motor Selection

A direct-driven fan is directly connected to the motor – either by the wheel mounting to the motor shaft or by the fan shaft directly coupled to the motor shaft. This results in little-to-no horsepower losses. So in this case, you can size your motor to match or exceed the specified fan BHP.

Belt-Driven Fan Motor Selection

With a belt-driven fan, you need to pick a motor size that exceeds the fan BHP. A V-belt drive typically results in a 3-6% horsepower loss. So, if your application calls for a certain brake horsepower at the wheel, you need to add horsepower to your motor size  to achieve the actual required BHP at the wheel of the fan. For a higher horsepower application, we would typically add about three percent, and for a lower horsepower application we would add up to six percent to the motor size above the fan BHP.

Fan Efficiency and Brake Horsepower

The BHP-based specifications above are for the minimum horsepower required to drive the fan. While it can be safe to go above the specified size, be sure to consider fan efficiency in your specification. According to the report Improving Fan System Performance: A Sourcebook for Industry, developed by the U.S. Department of Energy’s (DOE) Industrial Technologies Program and the Air Movement and Control Association International, Inc. (AMCA), a DOE Allied Partner:

“An important aspect of a fan performance curve is the best efficiency point (BEP), where a fan operates most cost-effectively in terms of both energy efficiency and maintenance considerations. Operating a fan near its BEP improves its performance and reduces wear, allowing longer intervals between repairs. Moving a fan’s operating point away from its BEP increases bearing loads and noise. “

Hear it from the Application Engineer

Senior Application Engineer Chet White explains what we need to know about how to determine the correct motor size for your belt-driven or direct-driven fan application in this 1½ minute video.

When you’re ready to start your project, reach out and connect with one of our application engineers to discuss the details of your specification.

Related Content on Industrial Fan Applications

There are many factors to consider in every industrial fan application. We’ve seen it all. Here are two more articles that might be of interest as you think about your application:

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