Evaluation of Power Drive System Efficiency Calculation Methods for HVAC Applications

Power Drive Systems (PDSs) consisting of motors and variable frequency drives (VFDs) are used for many applications, including for powering heating, ventilation, and air-conditioning (HVAC) equipment components, such as fans and pumps. Many of these end-use products are sold by component manufacturers without the PDSs. Various standards organizations and regulatory bodies have considered and/or adopted approaches for rating product performance that allow manufacturers to use predictions of efficiency for typical PDSs to determine wire-to-flow medium (e.g., air or water) efficiency, thus avoiding the need to rate performance for every combination that might be sold. To provide information to customers that is accurate but does not overpredict product efficiency, test procedures should ensure that PDS efficiency prediction methods do not overestimate actual PDS performance, while avoiding large underestimates that unfairly penalize products.

The IOUs analyzed data from test programs supported by the IOUs as well as test data collected by Motor Coalition manufacturers. The analysis compared measured data with the available PDS efficiency prediction tools, with a focus on operating conditions relevant for fans and pumps. The evaluated PDS efficiency prediction methods included the following:

• AMCA 207:2017 method: The method prescribed in Air Movement and Control Association (“AMCA”) Standard 207-2017

• Adjusted AMCA method: Calculations using the same equation structure, but with revised coefficients (“Adjusted AMCA:2024”) as provided by AMCA in its 2024 comments responding to the U.S. Department of Energy’s Energy Conservation Standard Notice of Proposed Rulemaking for Fans (“DOE Fans NOPR”).

• DOE Pump Motor method: Calculations included in the DOE test procedure for Pumps as codified in 10 CFR 431, Subpart Y, Appendix A.

• International Electrochemical Commission (IEC) method: Calculations described in IEC Standard 61800-9-2:2023 and Technical Specification 60034-21:2021

• Extended IEC method: Calculations that extend the IEC method to a broader range of applicable motor speeds.

The research found the following results:

• The AMCA 207:2017 method is accurate for a PDS on average, but overpredicts VFD losses and underpredicts motor losses.

• The AMCA 207:2017 method is susceptible to PDS efficiency overprediction in the 20 hp and higher range.

• The Adjusted AMCA:2024, Extended IEC, and DOE Pump Motors methods rarely overpredict efficiency, but often underpredict efficiency. 

• The Adjusted AMCA:2024 method improves separate motor and VFD predictions compared to AMCA 207:2017. 

As a next step, the authors recommend the IOUs provide recommendations for updated coefficients for Adjusted AMCA:2024 which will (1) eliminate the overprediction for mid-level and larger hp levels, (2) reduce the underprediction in the 1-10 hp range, and (3) more accurately balance the VFD and motor losses according to lab measurements