3-D electro-magnetic analyses of a cage induction motor with rotor skew

Abstract

A b s t r a c t -The paper studied the performance characteristics of a 3-phase cage induction motor using 3-D finite element technique. Especially, the effects of rotor skew and the rotor end-rings and the distribution of the electro-magnetic field toward the axial direction, which have not been able to be analyzed accurately by 2-D analysis, were investigated. Since the 3-D analysis enabled 11s to analyze the continuity of the air gap flux density a t t h e core ends a n d t o take into consideration the rotor end-ring impedance, the motor parameters were calculated with high accuracy, compared with 2-D analysis. Firstly, we made the relation of the rotor end-ring current with the rotor b a r current clear, by analyzing the harmonics of the rotor bar current and the rotor end-ring current in the cases of motors without rotor skew and with rotor skew. Secondly, the torque and the rotor axial force were calculated from the distribution of the electro-magnetic field obtained by the 3-D analysis in the case of rotor skew. We also calculated the axial forces on the both end-rings in the case of rotor skew. The results obtained i n t h e p a p e r showed t h e importance of 3-D analysis for the design of the induction motors.

Calculation and effects of end-ring impedance in cage induction motors

“…There is much literature about calculating the bar resistance and leakage reactance when skin effect is present (where the current travels at the top of the bar and increases the resistance and decreases the leakage reactance). However there is little on end-ring calculation except for [3] and [5] to [8]. Reference [3] is used as the basis of the analytical model here although it is modified.…”

“…While several people have studied the rotor as a complete 3d simulation, the problem with this approach is that the mesh region has to be fine in the bar/end-ring join to get accurate current distribution and also there is no inter-bar current in the model. This was illustrated by Kometani et al [8] who carried out a full 3d solution on a skewed rotor but still obtained a 5.5 % error in the phase current. This work also showed the rms current in the end-ring which is not necessarily the correct component to illustrate; while the total rms current in the endring may not change greatly, since it is almost current fed, the impedance will change since the end-ring voltage is floating.…”

Analysis of Natural Frequency, Radial Force and Vibration of Induction Motors Fed by PWM Inverter

“…If the motor has skewed slots, we should use one of the multi-slice model, the coupled method of 2D and 3D models (Yamasaki, 1996) and the full 3D model (Kometani et al, 1996). The influence that the skewing has on the radial force and vibration is not taken into account in this paper.…”

Finite element analysis of end ring impedance in squirrel cage induction machines

“…For all mentioned reasons, the use of 3D finite element (FE) analysis is essential if an accurate model of an induction machine is to be developed [6], [7]. Once the rotor impedance is obtained in 3D FE, it can be further used in two-dimensional (2D) FE software or analytical tools for various studies, which require repetitive calculations, such as sensitivity analysis or optimization procedures.…”