Thermal modeling for the motor in semi-hermetic screw refrigeration compressors under part-load conditions

In the semi-hermetic screw refrigeration compressor, the motor is mostly cooled down by suction refrigerant before it flows into the compressor suction port. To investigate cooling effects of the motor and performance of the compressor under part-load conditions, a lumped parameter model describing heat-transfer characteristics inside the motor and a part-load model of screw refrigeration compressor are developed and validated. Heating effects at compressor suction end and heat conduction from the male rotor screw to the motor shaft are taken into account. Nodes representing refrigerant inside the motor cooling passages are modeled separately. Temperature rise of refrigerant through the motor increases when the compressor load drops by either suction volume variation or speed variation. With drop of the compressor load, stator winding temperature goes up in the case of suction volume variation by slide valve motion, but it lows in the case of speed variation by frequency conversion.