VSD Calculator for Fans

Significant energy savings can be achieved in a fan system by reducing the fan rotational speed. Variable speed drives (VSD) are one of the primary devices used to control fan rotational speed. A VSD is a device that controls the rotational speed of motor-driven equipment. Variable frequency drives (VFDs), the most common type of VSDs, efficiently meet varying process requirements by adjusting the frequency and voltage of the power supplied to an AC motor to enable it to operate over a wide speed range. External sensors monitor flow and/or pressure and then transmit a signal to a controller that adjusts the frequency and speed to match process requirements.

This tool calculates the estimated energy and cost savings that would result from installing a VSD on a fan system. Required inputs include nameplate horsepower, efficiency, motor load, annual operating hours, fan type, and cost of electricity. Using these inputs and the duty cycle, the tool calculates the current energy use, potential energy use with a VSD, potential energy savings, and potential cost savings.

This calculator was developed to provide a very general estimation of potential energy savings and simple payback. Any final economic decisions should be based upon the use of more extensive analytical tools.


For the VSD Calculator for Fans, users will need to input the following:

  • Motor specifications such as nameplate horsepower, efficiency, and motor load
  • Annual operating hours
  • Existing flow control method and fan type
  • Cost of electricity
  • Duty cycle

Based on input, the VSD Calculator for Fans will estimate the current energy consumption, potential energy savings, and potential cost savings.

Limitations of the VSD Calculators for Fans
  • A variable speed drive (VSD) may not be appropriate for all applications. Potential risks include unstable operation, damage from operating at resonant speeds, and power quality issues. See the DOE publication Improving Fan System Performance: A Source Book for Industry for additional information and a fact sheet on controlling fans with variable loads.
  • This calculator may not provide accurate results for all situations, such as systems with high static head.
  • The calculator assumes the working fluid is air.
  • The calculator assumes the working fluid is incompressible.
  • The calculator assumes friction is negligible.
  • The calculator assumes the flow is steady.
Downloadable Version of the VSD Calculator for Fans

This version of the tool can be downloaded and installed on a local computer to run the tool. System requirements to use the tool include:

  • Microsoft Excel 2003 or newer
Download the offline version of the VSD Calculator for Fans

Additional Resources

This tool was created by the Bonneville Power Administration (BPA). For more information on BPA, visit their website at http://www.bpa.gov/energy/n/industrial/audit/.