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VFDs versus Motor Starters for Fan Control
The motor of a fan used in a commercial building is a substantial part of the cost of the fan. A variable frequency device (VFD) or motor starter can be used to protect that investment. Either device controls the electrical current to and from the motor, providing a fail-safe in case of a short circuit or overload. But a VFD provides additional value through its capability to adjust the motor’s speed. We have compared the two devices to help you determine which would be better for a given application.
How Motor Starters and VFDs Work
A motor starter allows for the motor to be either on or off in a controlled manner. It has a contactor switch that starts and stops power to the motor. Current flows to the motor when the contactor is closed. The motor starter’s overload relay monitors that current. If the motor is drawing too much current or overheating, the overload relay opens the motor starter control circuit to turn the motor off.
A VFD controls the speed of the fan by changing the power supply frequency. It can be digitally operated and allows for more precise modulation of a fan’s performance.
Cost and Performance Comparison
On average, the initial cost for a VFD is three times the initial cost of a motor starter. However, the VFD allows for other savings and benefits in addition to the motor protection that both devices provide.
- Modulation Capabilities: Fans with VFDs can modulate their speed up or down to meet changes in the space. Most VFDs have a modulating signal that is 0–10 volts, which is a common output of HVAC controllers and Building Management Systems (BMS). This enhances the fan’s ability to regulate humidity and temperature and helps maintain indoor air quality (IAQ).
- Simplified Systems: Because VFDs allow the same fan motor to run at a variety of speeds, they may make it possible to install only one fan for an application where two would have been needed. For example, when a store is open for business, it might need a fan to operate at 10,000 cfm. But when it is closed, it only needs a fan to run at 2,000 cfm. With a VFD, those can be the same fan.
- Reduced Maintenance Costs: A VFD has soft start capability, which reduces wear and tear on the fan system by smoothing out the acceleration and deceleration of the motor and reducing the inrush of electrical current when the fan starts. This saves on repair and replacement costs.
- Reduced Operating Costs: VFDs also reduce energy usage, saving on monthly energy bills.
VFDs and the Department of Energy
The Department of Energy (DOE) encourages the use of VFDs to make HVAC systems more efficient and reduce energy demand. Motors using VFDs also start and stop more smoothly, which further diminishes impact on the power grid. Rebates for entities that purchase VFDs are available through the DOE.
VFDs can also help a fan meet a DOE requirement for energy efficiency that it wouldn’t otherwise meet. For instance, by 2030 the Department of Energy will be regulating fans for compliance with the Fan Energy Index (FEI). FEI is a ratio that compares the energy performance of a fan to that of a baseline fan with an FEI of 1.00 at the same duty point. A higher FEI value means a fan is more efficient. Using a VFD to slow a fan down can raise its FEI rating and keep it in compliance with a given FEI requirement.
Application Examples
Deciding between a motor starter and VFD usually requires consideration of how and where the fan will be applied. For instance, take a sidewall propeller fan in a parking garage. In this application, there will be a high amount of CO from cars driving in and out. The airflow volume requirement for that fan might be 3,000 cfm. But during a sporting event when nearly every parking spot in the garage is full, the CO sensor might tell the fan it needs to exhaust 10,000 cfm. Does the parking garage owner buy two fans, a lower-CFM fan to run most of the time and a higher-CFM fan to run only during special events? Or do they add a VFD to the higher-CFM fan to allow it to vary its speed?
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Another application to consider is a warehouse that can vary in temperature. When it is below 60 degrees, the fan can be off. If it is above 75 degrees, the fan can run at 100%. If it is between 60 and 75 degrees, a VFD can make the fan run at a predetermined setting of 50% or 80% that maximizes comfort for the warehouse’s occupants.
In other applications, a motor starter might be sufficient. For instance, you might have a rooftop exhaust fan that moves the correct amount of air at full speed. In this case, you would not need a speed change, just protection and controllability.
Conclusion
The application, the fan horsepower, and the fan cost are deciding factors in whether to add a VFD, a motor starter, or no device to protect the fan’s motor at all. For a small ¾ hp fan, it might not be practical to invest in either technology to keep the motor running smoothly if the fan short-circuits or the power overloads. But for a larger fan (1 hp or greater), it is often worth it to add one of these devices that extends the motor’s life and prevents unexpected interruptions in airflow. Furthermore, a VFD that allows the fan to vary its speed can be an investment that pays for itself in energy savings, indoor air quality, and Department of Energy compliance.
Learn More
Find a Greenheck Representative: Find a representative in your area who is knowledgeable about options for protecting your fan’s motor.
Vari-Green Drives: The Greenheck Vari-Green® is a popular VFD solution with its own app and Bluetooth connectivity.
Greenheck Motor Starters: Browse Greenheck’s options for motor starters to manually control input power to a fan or HVAC system.
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