Performance assessment of variable frequency drives in heating, ventilation, and air-conditioning systems

Variable frequency drives (VFDs) are widely applied on the induction motors in heating, ventilation and air-conditioning (HVAC) systems to reduce energy consumption and improve power factor (PF) in commercial buildings. On the other hand, VFDs not only consume energy but also create harmonic distortion, which may interfere with sensitive electronic equipment, cause additional motor energy losses, and worsen PF. These negative impacts are more often concealed by the significant energy reduction. The purpose of this paper is to comprehensively assess those positive and negative impacts of VFDs in actual HVAC systems. Experiments were conducted on three HVAC systems to quantify the energy reduction, PF improvement, voltage and current total harmonic distortions (THDs), and system efficiency degradation introduced by VFDs. The experiment results reveal that the VFDs significantly reduced the system input power by 85% at 50% rated frequency and increased the PF to 0.95 at 95% rated frequency with the voltage THDs always less than 1%. However, the current THDs increased to 90% at 50% rated frequency, which worsens the improved PF to 0.7. Moreover, the VFDs degraded the system efficiency by 3.5% at the rated frequency and by more percentage, potentially due to increased current THDs, at partial frequencies

Efficiency degradation detection for VFD-motor-pump systems

Electric motor-driven fan, pump, and compressor systems accounted for 27% of the electricity usage in commercial sectors in United States in 2013. Therefore, even small efficiency improvements will produce very large energy savings. In fact, the efficiency of these systems deteriorates over their lifetime. The efficiency of motors could degrade by up to 5% while the efficiency of pumps could degrade by up to 25%. Since variable frequency drives (VFDs) are widely applied on electric motors, the efficiency degradation is more often concealed by the significant energy reduction introduced by VFDs. The purpose of this paper is to develop and demonstrate an approach, which can effectively detect the efficiency degradation of VFD-motor-pump systems by simply measuring the pump water flow rate and head as well as the system input power. First, the ideal system efficiency was modeled by combining the pump, motor, and VFD efficiencies and applied to correlate the pump water flow rate and head to the ideal system input power. Then, an experiment was conducted on a ten-year-old VFD-motor-pump system to quantify the efficiency degradation by comparing the measured and ideal system input power. The results of the experiment reveal that the efficiency of the studied system degraded by 19%

Development of a virtual pump water flow meter using power derived from comprehensive energy loss analysis

Water flow rates are key controlled variables in chilled and hot water systems. The water flow rate through a pump can be virtually determined using a measured pump head and input power. Available input power can be either motor input power obtained from the connected variable-frequency drive or variable-frequency drive input power measured by a power meter. The harmonics created by the variable-frequency drive induce additional energy loss in the motor and consequently degrade the motor efficiency. The objective of this article is to evaluate a virtual pump water flow meter using either the variable-frequency drive input power or motor input power with calibrated pump, motor and variable-frequency drive efficiencies along with the consideration of additional harmonic energy loss. First, the water flow correlation with the measured pump head and available power input is discussed, then the calibration procedure is introduced to identify three efficiencies, and finally the experiment is conducted to develop and validate a virtual pump water flow meter on a pump motor variable-frequency drive system. The experimental results show that the water flow rates determined by the virtual water flow meter agrees well with the results measured by a physical water flow meter with the coefficient of determination of 0.91and 0.97, respectively