Imagine you have a regular light switch and a dimmer switch for a light bulb. A regular switch only has “on” and “off,” while a dimmer lets you control how bright the light is. A VFD is like a dimmer switch for a fan motor – it lets you control the fan’s speed. Changing the fan’s speed with a VFD also changes the motor’s horsepower (its ability to do work) and torque (its turning power). Let’s break it down for centrifugal fans (the most common type).

Horsepower and Speed:

Interestingly, if you were to speed some smaller motors up beyond their normal speed (more than 60 Hz), they would still have their full horsepower available, as long as the motor can handle it.

Think of horsepower as how much air the fan can push. When the fan runs at its normal full speed (like the light switch fully on), the motor gives it 100% of its maximum horsepower. This normal speed is often linked to a frequency called 60 Hertz (Hz) in the US.

If you slow the fan down (like dimming the light) using the VFD, the horsepower available also goes down. It’s a direct relationship, meaning if you run the fan at half its normal speed (30 Hz), you only have about half the horsepower. So, a slower fan can move less air and might not be able to push it against as much resistance. Interestingly, if you were to speed some smaller motors up beyond their normal speed (more than 60 Hz), they would still have their full horsepower available, as long as the motor can handle it.

Torque and Speed:

If you run the fan faster than its normal speed (above 60 Hz), the turning power (torque) might start to decrease. However, for most fans, having the absolute maximum turning power at these very high speeds isn’t usually as important as having enough horsepower to move the air. Things like conveyor belts, which always have a load, need to worry more about maintaining high torque at all speeds.

Think of torque as the initial push needed to get the fan moving from a standstill. It’s like how much effort it takes to start pushing a heavy cart. When you start the fan from zero speed up to its normal full speed (60 Hz), the motor provides its maximum turning power (100% torque). This is good because fans need a strong initial push to get their heavy blades spinning.

Once the fan is running, it doesn’t need as much continuous turning power. If you run the fan faster than its normal speed (above 60 Hz), the turning power (torque) might start to decrease. However, for most fans, having the absolute maximum turning power at these very high speeds isn’t usually as important as having enough horsepower to move the air. Things like conveyor belts, which always have a load, need to worry more about maintaining high torque at all speeds.

In Simple Terms for Centrifugal Fans:

  • Slower fan speed = less horsepower available. This means less air moved or less pressure.
  • The motor gives its strongest push (maximum torque) when starting and running up to normal speed.
  • Going much faster than normal might reduce the turning power, but for fans, having enough overall power (horsepower) is usually the main thing to consider.

These relationships are important for choosing the right motor and VFD for a fan system and for understanding how changing the fan speed will affect its performance.

—————————————-

Our application engineers have extensive experience with a wide range of industrial applications. Contact our team today to discuss your project. We can help you select the best fan for your application.