On Shortening the Numerical Transient in Time-Stepping Finite Element Analysis of Induction Motors: Method Implementation

Abstract

The phenomenon of a lengthy numerical transient response is a common event in Time-Stepping Finite Element (TS-FE) modeling of induction motors. In this paper, the aim is to substantially reduce the length of the transient response time in time-stepping finite element (TS-FE) simulation of induction motors. This goal is pursued by means of two different techniques introduced in this paper. In the first technique, an initial guess of the magnitudes and distribution of the rotor bar currents under steady-state operation is obtained using the motor's T-equivalent circuit. These initial values of rotor bar currents are injected in the TS-FE model of the case-study induction motor as boundary (or initial) forcing functions to “nudge” the numerical solution towards steady-state converged results. An improvement, however limited, in shortening of the transient response time in the TS-FE simulation of such an induction motor is observed, for both voltage and current source excitation scenarios. The second technique introduced in this paper explores the frequency domain FE eddy-current models to extract the initial permeabilities and the skin effect in the rotor bars associated with the operating point. A so-called “virtual blocked rotor” technique in both an FE eddy-current solver, and in the time domain TS-FE solver is developed. The “virtual blocked rotor” in the FE frequency domain solver gives an approximation of initial conditions for the TS-FE transient analysis performed in the time domain. The “well-determined” initial permeabilities are imported to the time domain “virtual blocked rotor” model by linking the results of the FE eddy-current solver to the TS-FE solver. Significant reduction of the transient response time was achieved, and this approach effectively and substantially reduced both the transient TS-FE response time and central processing unit (CPU) times to convergence.