On Shortening the Numerical Transient in Time-Stepping Finite Element Analysis of Induction Motor Under Inter-Turn Short Circuit Fault Faults

An induction motor inter-turn short circuit fault usually occurs due to the stator winding insulation failure and spreads from only a few turns in its incipient stage, to a higher number of turns. The Time-Stepping Finite Element (TS-FE) analysis is amongst the most preferred techniques for modeling induction motors under inter-turn short circuit fault. However, such a technique suffers from lengthy numerical transients, before convergence to steady-state solutions are reached. In this paper, the computational burden due to such a lengthy numerical transient in TS-FE simulation of such an induction motor under inter-turn short circuit faults, is mitigated. This is accomplished through the so-called “Virtual Blocked Rotor (VBR)” approach. Also, using the results obtained from the VBR-based TS-FE analysis of such a faulty induction motor, an inter-turn fault index is extracted using a Phase Locked Loop (PLL) assisted Magnetic Field Pendulous Oscillation (MFPO) swing angle. This modified MFPO does not need monitoring of the phase voltages to extract the fault related swing angle (or fault index).

I. Introduction

Induction motors are the workhorse of the industry, especially due to their cost-effectiveness. However, they are susceptible to different faults. The stator winding inter-turn short circuit fault is one of the most critical faults that should be identified as fast as possible, before it spreads and cause catastrophic damage. Induction motor modeling under fault conditions is a very important step towards designing an effective fault diagnostic system. An analytical technique known as the Winding Function Approach (WFA) was used in [1] to models an induction motor under inter-turn short circuit faults. The WFA is based on the coupled magnetic theory [1] and uses differential equations to describe the motor performance in the case of fault presence. In this technique, the motor inductances are obtained through the magnetic stored energy in the airgap and used in the differential equations. The Magnetic Equivalent Circuit (MEC) technique was utilized in [2] for induction motor modeling under fault. This includes both stator winding inter-turn fault and broken rotor bar faults. The MEC technique is an analytical approach that allows the incorporation of the deep-bar effect, as well as the space-harmonics due to the discrete winding distributions, rotor and stator slotting, as well as the saturation-induced saliency in the rotor and stator cores. The TS-FE method [3] is a numerical approach for analyzing induction motors while including detailed geometrical, Electric and magnetic saturation characteristics of the motors. All these details mean more computationally demanding models which take longer to be solved. The TSFE method was used in [3], [4] for numerical analysis of an induction motor under inter-turn short circuit fault. A Time-Stepping Coupled Finite Element-State Space (TSCFE-SS) Model was introduced in [5], [6] for modeling induction motors coupled to adjustable speed drives through the use of TS-FE analysis, in conjunction with a phase-flux linkage-based State Space (SS) model. The induction motor inductances were calculated by means of an Energy Perturbation (EP) approach withing the TSFE analysis. The reason behind using an EP approach instead of the WFA is the benefits that the EP approach of fers such as taking into account the space harmonics, the load-dependent saturation, and the magnetic saturation-induced so-called rotor “apparent saliency The SS model represents the external switching circuit and imports the calculated inductances from the TSFE analysis in order to solve the SS equations and calculate the torque and current values and feed them back to the TSFE analysis to complete one full iteration. Such an iteration is repeated as many times as needed to reach convergence in terms of current and inductance values.