A major part of the power consumption in the low-wattage reciprocating compressors, used for household refrigeration units, is associated with the motor. A variable speed compressor as opposed to a single speed compressor is known to provide energy savings due to its ability to match the load demand from the pump more closely to the power delivered by the pump. The current article presents an overview of the design and optimization methodology for a variable-speed, three-phase PM BLDC motor. The design is evaluated for a four pole configuration with operating frequencies ranging from 66.66Hz all the way up to 150 Hz. A finite element tool to determine performance of the PM BLDC motor with different design parameters like magnet arc lengths and thickness, number of stator slots, Back EMF wave shape, skewing etc. is developed. This model is then used for obtaining dimensions for the most efficient motor design. A PMBLDC motor model based with-sensor and sensorless control strategy is developed for operating the motor at the desired speed while delivering the torque-demand to the pump over the desired speed range. The novel feature of the analytical model is how the results of the prior mentioned FE tool are leveraged. The back EMF wave shape is not idealized as a trapezoidal waveform. Instead the back EMF is calculated from an accurate FE model and this data is used to determine the generated voltage in the analytical model. This ensures a quick (relative to Finite Element simulation) yet accurate simulation. The algorithm is to be implemented on a dsPIC30F6010A microcontroller. Finally, load tests would be performed on a dynamometer to evaluate the performance of the motor over the entire speed and load ranges to ensure it conforms to the design and operational requirements.
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