Know the Speed/Power/Torque Trade-Off for Your Application

Did you know that when you control and vary the operating speed of your industrial centrifugal fan or blower using a variable frequency drive (VFD), you’re affecting both horsepower (HP) and torque? It’s not always obvious, but it’s important to understand when you may be operating at reduced power or torque due to changes in speed because you always need a certain horsepower to drive the fan and move air through the type of pressure in your system.

A variable frequency drive, by definition, changes speed according to changes in frequency designated in hertz (Hz), or cycles per second. Horsepower and torque are each a function of motor speed. They have a generally inverse relationship, so when speed is lower, torque is higher, and when speed is higher, horsepower is higher. But knowing how and where the available horsepower and torque change requires a curve. We’ll build that out below.

VFD Operating Frequencies and Curves

Before we can determine the available percentage of your fan’s motor nameplate horsepower, we need to know the VFD operating frequency. In the United States, we most commonly have a 60-cycle frequency. A smaller motor can occasionally run at a higher frequency, up to about 120 hertz.

To plot the HP and torque curves, we measure frequency in hertz from zero to a maximum of 120 on the horizontal x-axis, and we measure torque and horsepower as a percentage from 0 to 100 on the vertical y-axis.

Calculating Horsepower Availability

Horsepower is an important element of industrial fan performance. HP availability is proportional to speed up to 60 Hz, where it reaches 100 percent and remains at 100 percent up to the maximum of 120 Hz. At less than 60 Hz, you don’t have the full motor horsepower available.

This is really important to understand. For example, if you’re operating an 1800 RPM motor with 100 horsepower available at 60 Hz and 1800 RPM, but you use a VFD to reduce the frequency to 30 Hz, you’re now operating at approximately 900 RPM with only 50 percent of available horsepower.

Calculating Torque Availability

Torque is most important at fan start-up when you need to take the heavy wheel on its shaft from rest to rotating. Once you get a fan wheel rotating, that initial torque isn’t needed anymore because the fan wheel is free spinning on the shaft and bearings.

You’ll see on the curve that torque is at 100 percent between zero and 60 Hz. Once the VFD motor reaches the target of 60 Hz, torque starts to tail off, dipping rapidly and flattening out at 50 percent of available torque at 120 Hz. With centrifugal fans, it’s rarely important to have maximum torque past your 60 Hz operating frequency.

Hear from an Application Engineer About HP and Torque with VFD

Chet White, Senior Application Engineer / Sales & Marketing Manager, plots the torque and horsepower curves against fan frequency in this detailed whiteboard video.

To ask questions, get more details, or discuss your application, reach out and connect with one of our application engineers.

Related Content

Can’t get enough? Several additional posts might interest you as you think about VFD, HP, Torque, and fan performance.

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