Examples of system (motor plus inverter drive) efficiencies of two broad types of adjustable speed control are compared for predicted compressor and indoor blower load profiles. The two classifications are inverter-driven induction motors (IDIMs) and permanent-magnet electronically commutated motors (PM-ECMs). Reference sine-wave-driven induction motor (SWDIM) efficiencies are also given.

Available bench data in the ASHRAE literature (Lloyd 1982) on late 70’s IDIM compressor drives are compared to recent bench and application data obtained on IDIM and PM-ECM drives. The drive efficiencies are compared over a common set of predicted operating torque requirements for heating and cooling conditions. A modulating heat pump model was used to develop predicted reciprocating compressor torque / drive-frequency mappings and the expected operating torque ranges. The variation in modulating compressor torque requirements is analyzed and the major determining factors examined. Ways to adjust the given torque relationships for different compressor types and sizing strategies are also discussed.

Similar performance comparisons are made for modulating indoor blower drives. Modulating blower performance data on an early ’80s generation modulating heat pump with an IDIM drive (and SWDIM reference drive) were obtained from Miller (1988). These data are compared to bench data on recent IDIM and PM-ECM drives under similar torque conditions.

In both compressor and blower applications, the combined system efficiency of the PM-ECM drives is shown to be nearly equal to or higher than that of the reference SWDIM cases and significantly better than IDIMs available in the late ’70s. When compared to available data on more recent IDIMs, the PM-ECM efficiency advantage over IDIM compressors has been reduced 40 to 50% between half and nominal speed (3600 rpm) but still remains 14 to 9% higher, respectively. For blowers, the IDIM improvement has not been as great.

PM-ECM-driven blowers appear to be very competitive at present with IDIM blowers on the combined basis of efficiency, cost, and reliability. For the compressor PM-ECM drives, if progress continues on cost and reliability issues with minimal loss in efficiency, these drives will likely be a leading candidate for future-generation (5 to 10 year range) modulating heat pumps.

Units: Dual