Abstract
Variable frequency drives (VFDs) are widely applied on induction motors to reduce motor electrical power under partial load conditions by adjusting both the frequency and voltage. For centrifugal fan, pump and compressor applications, a squared voltage/frequency ratio voltage control results in better motor energy performance than a linear ratio control and is recommended by VFD manufacturers. The motor efficiency model in EnergyPlus is regressed based on manufacturer motor data without VFDs by ignoring the VFD impacts while the motor efficiency model in AMCA standard 207 adopts a linear ratio control rather than a squared ratio control, thus these two models do not reflect the actual motor operating conditions and the inaccurate motor efficiency data may be used in energy simulation. The paper is to investigate these two motor efficiency models using AHRI certified motor data and an experiment conducted on a VFD-motor-belt-fan system. The investigation results show both the EnergyPlus and AMCA models cannot accurately calculate motor efficiency controlled by the squared ratio with VFDs. The EnergyPlus model underestimates the VFD-motor-belt drive system efficiency by 10–25% while the AMCA model underestimates the VFD-motor-belt drive system efficiency by 5–10% in a frequency range between 30 and 40 Hz.
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