Overspeeding Fans with VFD's

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Feel the need for speed? Variable frequency drives make it possible to vary the speed - and in some cases over speed - on a direct drive industrial fan motor. Sales Manager Chet White shares what’s possible and what to watch for in this two-minute video.

Full Transcript:

This is an arrangement 4 fan. The motor is hooked up directly to the wheel. So this technology, before the invention of the variable frequency drive (VFD), could only run at the standard frequency in your country, wherever it is that you’re installing your fan. So in this case, this is the USA, this is a 60-hertz motor. It’s rated for 3520 RPM. So, before the VFD, that’s the speed that you were locked into when you received a fan like this. Today, not so much.

If you’re using a VFD in your application, you have the ability to vary the speed of your fan. Anywhere from 3-60 hertz often. Is it safe to run my motor above 60 hertz? If the fan is not rated to run higher than 60 hertz - if the max safe speed of the wheel does not exceed 60 hertz - don’t run over 60 hertz. But there are many motors that actually can be run safely over 60 hertz. If you consult your NEMA table for the brand of motor that you have or you reach out to your application engineer, you should be able to get information on which fans can and cannot be run over 60 hertz.

Some motors up to 50 horsepower, if they’re premium efficiency rated, can be run over 3600 RPM. How much higher can they run? You have to consult the table and see how fast they’re rated for. But some of them are rated for up to 5400 RPM, 5000 RPM, 4500 RPM. The number gets closer to 3600 RPM, the bigger the motor gets. At 1800 RPM, the horsepower even gets larger. You can run 300 horsepower motors often over 60 hertz safely.

Again, consult the NEMA table, go to your fan application engineer, you may find that there’s a lot more capacity to run over the rated frequency where your fan is installed. But always consult the max safe speed of the fan first.

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