Electromagnetic Field Analysis of Canned Motor for Hermetic Compressor using 2D Finite Element Method Combined with 3D Method.

Abstract

Owing to the regulation of Freon gas in environmental problem, ammonia is recently revalued as refrigerant of compressors. A canned motor was developed as a reliable driver of a hermetic-type ammonia compressor. The canned motor has a thin, metallic can inserted in the airgap of a squirrel-cage induction motor to prevent ammonia from entering the stator and damaging the primary windings.As the can is conductive, eddy current and electromagnetic force are generated in it. To know these phenomena correctly, the finite element analysis is necessary because the equivalent circuit approach is inadequate. As end positions of the can have strong effect on eddy current in the can, a 3D analysis is needed to calculate electric field accurately. Furthermore, a nonlinear analysis with fine mesh in the airgap is required to calculate electromagnetic force distribution. In this case, large amount of computer memory and calculation time will be needed if a full 3D analysis is carried out.In this paper, we present a nonlinear 2D analysis combined with 3D analysis which can calculate both eddy current and electromagnetic force distributions accurately with small computer resources.The analytical results show the good agreement with experimental data. Moreover, we obtain new information which is instructive for the design of canned motors.