How To Control Fan Speed

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Wondering how to control fan speed? It’s the drive. Set up your operations for optimal efficiency using the right equipment for varying fan speed.
Controlling fan speed is as easy as having the right equipment and a few simple calculations. Senior Application Engineer Chet White demonstrates how to adjust fan speed based on the fan drive you’re using and the frequencies you need in this 3-minute video. Looking for more information? [e-mail] or visit our website at airprofan.com.
Full Transcript:
So how can you change the speed of your centrifugal fan? If you have a belt-driven fan like the one we’re standing next to, there are two primary methods you can use for changing the speed of your fan.
Your belt drive is set up to give you a constant running speed. Your motor is likely plugged into a 60 hz frequency if you’re in the United States. And so if you don’t have a variable frequency drive on the motor, your way of dealing with the speed change would be through your belt drive. If you do have your motor on a variable frequency drive, you can just adjust the frequency input in the motor to either go over 60 hz or under 60 hz to increase or decrease the speed.
So let’s say hypothetically that this fan is set up to run at 1200 RPM and this motor is an 1800 RPM motor. So originally, you would have a 2:3 sheave ratio. Your smaller sheave would be on a motor, and your larger sheave would be on the fan to reduce the speed of the fan to 1200 RPMs from an 1800 RPM-driven motor.
Let’s say you want to increase your speed from 1200 to 1500 RPMs and you want to do it through the belt drive. You would have to change your sheave ratio to be a 5:6 sheave ratio. So you’d have to go from a 2:3 to a 5:6, and so again your smaller of the 5:6 ratio would be on the motor and the larger of that 5:6 ratio would be on the fan, and that would increase your speed from 1200 RPM to 1500 RPM.
Let’s say you want to make that same change, but you want to do it through a variable frequency drive and you don’t want to change the belt drive. If you do it through a variable frequency drive, you need to change by the ratio of the new speed you want to get to from the speed that you were at originally. So you would have to increase your frequency by a 5:4 ratio.
So now that we’ve addressed how to change the speed on a belt drive, let’s head over and talk about how to do it on a direct drive fan.
So on this direct-drive arrangement 4 fan, your motor shaft is directly connected to the fan wheel, so the fan wheel will run at exactly the speed of the motor. And so if you don’t have a variable frequency drive, there’s no way to adjust this speed if you’re on a hard start or a soft start connection. But if you are connected through a variable frequency drive, you can adjust this speed either above 60 hz or below 60 hz by adjusting your variable frequency drive. So in this case, we have an 1800 RPM motor. Let’s say we wanted to go from 1800 RPM to 2000 RPM. You’d have to adjust it by a 10:9 ratio to go up from 1800 RPM to 2000 RPM.
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