Because of the widely used brushless DC motors (BDCMs) in high-efficiency variable-speed compressors, many sensorless controls with square-wave operation had been developed in the literatures. Recently, more and more low-production-cost BDCM-type compressors with position-dependent load torque, such as rotary compressors, are applied to the residential products, which lead to the importance of low-speed performance of sensorless control. The authors address the low-speed performance comparisons between two common back-EMF detecting circuits (BEDCs). Without using the actual refrigerant system, a Motor-Generator set coupled with a disk and copper cylinder are also built up to serve as the experimental position-dependent load torque. From the simulated and experimental results, one of the BEDCs is better than the other one for the low-speed application of position-dependent load torque.