THE APPLICATION OF THE FINITE ELEMENT METHOD IN DESIGN OF ELECTRIC MOTORS

摘要

The classical approach to design of electric motors is based on the concept of simplified magnetic circuit analysis. This approach fails in today's design situation due to utilization of new materials and new designs. Application of sophisticated numerical methods becomes inevitable. Some problems arising in the application of the finite element method in design of electric motors are discussed in this paper. Electric motors are always part of a system and their behaviour within the system, which is characterized by integral parameters (torques and reactances), should be known at the design stage. Computation of these parameters from a finite element magnetic field solution is described here. The computation of torque/angle characteristics from finite element field solution by application of two basic approaches: - global virtual work method and cubic spline interpolation technique, - Maxwell stress tensor integration, is described. The applicability of both approaches is illustrated by computation of the torque in a switched reluctance motor and the advantages of virtual work approach are emphasised. The main problem in application of the finite element method to computation of magnetic fields in electric motors is that the field sources (currents) and load angle are unknown. External environment (terminal voltage and mechanical load) are known, and the magnetic field solution is iterated until the external constraints are satisfied. To avoid finite element mesh rotation an iterative process was implemented in which only the fundamental harmonic of the lumped stator winding distribution is taken into consideration instead of the three-phase winding excitation. The direct and quadrature reactances are computed from the finite element magnetic field solution utilizing flux linkage and stored energy approaches. The procedure is illustrated by computation of the reactances of a permanent magnet synchronous electric motor. [Vol. 9, No. 2 (1994), Special Issue on The Numerical Computation of Low Frequency Electromagnetic Fields, pp 30-36]