Variable Frequency Drives - are they worth it?

Variable Frequency Drives - are they worth it?

Beau McLeod, P.E., Chief Engineering PLLC
Running a 100 horsepower motor for a year at full speed can be costly. Installing a variable frequency drive (VFD) can result in big savings, with payback periods often measured in months, particularly when utility company rebates are considered. A VFD controls the speed of a motor by varying the frequency and voltage of its power supply. Motors required to meet varying demand by operating with a throttled-down output are good candidates for VFD operation. Ultimately, three things help determine if a VFD is right for the application:

1. Operating conditions and characteristics. For new installations, the motor needs to meet the minimum and maximum performance requirements of the system.  To determine the system parameters, determine if the motor will run for long periods of time, if the system requires running at different speeds, or if the application requires constant torque. If the answer to any one of these questions is no, then a VFD may not be suitable for the application.
2. Installation cost. As a rule of thumb, the typical cost of adding a VFD to an applicable system is between $200-$500 per horsepower. The cost of adding a VFD to any system is significantly higher than other starting methods[AS1] . The capital cost of a VFD can determine the length of the payback period.
3. Cost savings. VFDs enable longer motor life and reduced maintenance expenses. Also, utility companies often offer rebates for VFD installations. Make sure to consider these cost savings benefits when determining if a VFD is the right choice.

Let’s use an example to determine if a VFD is suitable for a certain application. The following table shows input power percentage vs. flow percentage controlled by a VFD and an internal vane damper.

Now, let’s assume that the fan specifications include the following:

• 100 HP motor nameplate
• 95% efficient motor
• 20k cfm
• 19.5” w.g.
• 2295 operating hrs./yr. @ $.12/kWh
• 30,000 hour VFD service life
• $35,000 VFD capital cost.
  

Using the Affinity Laws for VFDs, the chart below shows the amount of savings that can be achieved with a VFD when the load is reduced. Using the fan specifications above, the average annual energy costs can be calculated for both a VFD and an internal vane damper configuration. At 80% reduced load, using a VFD requires much less horsepower to achieve the same airflow rate than utilizing a vane damper. This can lead to significant energy savings and a longer lifespan for the equipment.   

To determine if a VFD is financially feasible for the application, a detailed financial analysis will need to be performed to calculate the payback period to cover the initial cost of the equipment. According to the example above, if the application only utilizes an 80% airflow load every year, the VFD will have an annual energy savings of $6,704.10. By dividing those savings from the capital cost of the VFD itself ($35,000), the result is an approximately 5-year payback period. If the VFD service life is 25,000 hours and the fan is operating 2,295 hrs./yr., then the lifetime of the VFD will be approximately 11 years. In this case, a VFD does make sense for this application and will receive ~$40,225 in VFD savings ($6,704.10 x 6 yrs.) after capital costs have been recuperated.

Even though VFDs are expensive, they can be a huge money saving device served in the right application. So, are VFDs worth it? Yes, in the right circumstances. Follow the exercises above to determine if a VFD makes sense for your application. If you need help with an application regarding fan sizing and performance, contact a professional engineer at Chief Engineering or visit our website at www.chiefengineering.us.

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References:
http://www.sustainableplant.com/assets/Baldor/Baldor-0411-Pump-Energy-Savings-with-VFDs.pdf
https://www.prismengineering.com/resources/calculators
http://www.vfds.org/variable-frequency-drive-for-constant-torque-loads-337002.html
https://library.automationdirect.com/vfd-or-not-vfd/
https://www.waterworld.com/articles/print/volume-24/issue-3/departments/pump-tips-techniques/when-should-variable-speed-drives-be-used-to-save-energy.html
https://www.buildings.com/article-details/articleid/9345/title/roi-from-vfds-cutting-costs-with-variable-frequency-drives
https://www.controleng.com/single-article/combine-variable-frequency-and-fixed-speed-drives-for-better-economy/adc588521448f4908e77aadbe45d61ef.html