The operating efficiencies of an indoor centrifugal blower and a hermetic reciprocating compressor, both having induction motors, were measured in a breadboard heat pump test facility. The indoor blower was tested using a six-step voltage source inverter (VSI) produced in the early 1980s and also using a motor-generator (M-G) (ideal induction motor drive) for blower motor drive frequencies ranging from 20 to 60 Hz. The reciprocating compressor was driven in separate tests by a VSI, a pulse-width modulated inverter (PWMI), and an M-G To observe the effect of each adjustable-speed drive on the efficiency of the compressor for drive frequencies ranging from 15 to 90 Hz.

Laboratory data indicated that the coefficient of performance (COP) of the heat pump and component efficiencies became increasingly sensitive to the magnitude of three-phase voltage input to the induction motors as drive frequency decreased from 60 to 15 Hz. A linear relation of volts per hertz yielded best compressor efficiency and thus best COP; however, for the indoor blower, the volts per hertz relationship was varied with speed for the best combination blowerand- blower-motor efficiency.

A VSI efficiency of only 85%, measured at 60-Hz drive frequency, caused the overall blower and drive system (blower motor and inverter) efficiency to drop 16o5% of efficiency measured with the blower driven by the M-G set. The isentropic efficiency of the compressor was observed at 15-Hz speed to be roughly 30% less than that measured with the ideal induction motor drive, due to the indirect effect of current harmonics on the compressor and compressor motor.

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