The Drive to Control Fan Speed

How to Control Fan Speed

Controlling fan speed for optimal efficiency and performance is as easy as having the right equipment and making a few simple calculations. The ability to control fan speed rests on your motor drive. On direct-driven fan applications, the most common way to change speeds is with a variable frequency drive (VFD). On belt-driven fan applications, a VFD is also useful for simple speed changes. In the absence of a VFD, simple sheave ratio modifications should do the trick. And of course, you need to know the input frequency of the motor and a few simple ratios.

Controlling Fan Speed on a Belt-Driven Fan

With a belt-driven fan, there are two ways to control speed. One uses a variable frequency drive, and the other makes adjustments based on calculated sheave ratios.

Using a Variable Frequency Drive to Control Belt-Driven Fan Speed

Using a variable frequency drive means you can simply adjust the frequency input in the motor to the new frequency calculated by determining the VFD Adjustment Ratio based on where you are and where you want to be.

To control fan speed this way, first, calculate the VFD Adjustment Ratio:

Next, calculate the desired frequency:


Finally, program the VFD using the new Input Frequency.

Using Sheave Ratio Adjustments to Control Belt-Driven Fan Speed

If you’re using a belt-driven motor and don’t have a variable frequency drive, the way to make the speed change would be through adjustments to the sheaves in the belt-drive. Like the VFD calculation, this is done using ratios.

Calculate the Sheave Adjustment Ratio needed for the desired speed:

Using the calculated Sheave Adjustment Ratio, change the sizes of the sheaves accordingly, with the smaller sheave on the faster end and the larger sheave on the slower end. That means if you are reducing the speed of the fan driven by a faster motor, the larger sheave would be on the fan and the smaller sheave would be on the motor. If you are increasing the speed of the fan driven by a slower motor, the larger sheave would be on the motor and the smaller sheave would be on the fan.

Controlling Fan Speed on a Direct-Driven Fan

There’s only one common way to control speed in a direct-driven fan, and that is to use a VFD. Without the VFD, the fan will always run at the speed of the motor. The method for changing fan speed on a direct-drive fan using VFD is the same as outlined above for belt-drive fans. Determine the VFD Adjustment Ratio based on where you are and where you want to be, calculate the new frequency, and program it in.

Recap: How to Control Fan Speed

In a nutshell, here’s how to control fan speed:

  1. Use a Variable Frequency Drive (VFD) and/or a belt drive between the motor shaft and fan shaft
  2. Know your ratios (current to target RPM, motor to fan RPM)
  3. Adjust input frequency (with a VFD) or adjust sheaves to the calculated ratio (with a belt-drive motor)

Hear it from an Application Engineer

Senior Application Engineer Chet White demonstrates how to control fan speed in an industrial centrifugal fan in this 3-minute video.

When you’re ready to start your project or update your fan and blower maintenance program, reach out and connect with one of our application engineers to discuss the details of your project.

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For more information on fan speed or motor drives, check out these additional articles:

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