VFD Motor Control for Overspeeding Industrial Fans
Once upon a time, direct-drive industrial fans could only go one speed. With the invention of the Variable Frequency Drive (VFD), industrial manufacturers gained a great degree of control and the ability to overspeed the fan for optimal operation in the application through which it is moving process air.
Why Overspeed an Industrial Fan?
When you overspeed a fan, you can gain more volume and/or pressure for your system. You might do that, for example, to maximize the output using up all of your available amperage on the motor.
Another reason could be that the fan is designed specifically to run in an overspeed condition. For example, if the motor is suitable to be run up to 2700RPM, even though it is 1800RPM on a standard 60Hz frequency, an application engineer could specify the fan to perform at 2250RPM using 36 brake horsepower, which would require a 40HP motor (or larger) being run at higher than standard (60Hz) frequency.
How Does a VFD Control Motor Speed?
A VFD motor controller can adjust the motor speed and thereby change the fan speed, allowing the fan user to adjust the airflow by either decreasing or increasing the motor’s speed. A system rarely operates at constant conditions, and flexibility often matters to the system’s user, along with cost savings from reduced power consumption.
What Speeds Can You Safely Achieve with VFD Motor Control?
Each country has a standard frequency. In the United States (US), that’s 60Hz. So in the US, if you’re using a VFD in your application, you have the ability to vary the speed of your fan anywhere from 3-60Hz. But there are many motors that can run safely over 60Hz if – and only if – you stay within the max safe speed of the fan.
Premium efficiency-rated motors can run over 3600RPM. How much higher can they run? It depends on the ratings from the motor brands, the size of the fan, and the horsepower. The lower the RPM the bigger the motor gets. For example, you can often run 300HP motors over 60Hz safely.
How to Use VFD Motor Control Safely
The motor and the fan work together. While many motors may be safe over your country standard, you have to be sure the fan is also safe at that speed. You should be able to get information easily on which fans can and cannot be run over 60Hz. To determine what’s possible, consult the National Electrical Manufacturers Association (NEMA) table for the brand of motor that you have, and reach out to your application engineer.
If the fan’s max safe speed is not rated to run higher than the standard, (60Hz in the US) don’t exceed it. Period. If your industrial fan’s max safe speed does exceed the standard, you can explore overspeeding through VFD motor control.
Again, consult the NEMA table and double-check with your fan application engineer to determine if the capacity is available to run over the rated frequency where your fan is installed. But always consult the max safe speed of the fan first.
Hear it from a Pro
To check on the max safe speed of your industrial fan and explore potential overspeed possibilities using VFD motor control, reach out and connect with one of our application engineers to discuss the details of your project.
For more information on troubleshooting and application system effects mentioned in this article, check out these additional pages:
- Choosing the Right Motor Speed for Your Industrial Fan
- Choices, Choices: Control Fan with VFD or Damper
- The Drive to Control Fan Speed
- Variable Frequency Drives
- Direct Drive Industrial Fans