THERMAL MODELLING OF INDUCTION MACHINE USING THE LUMPED PARAMETER MODEL

Temperature rise is of much concern in the short and long term operations of induction machine, the most useful industrial work icon. This work examines induction machines mean temperatures at the different core parts of the machine. The system’s thermal network is developed, the algebraic and differential equations for the proposed models are solved so as to ascertain the thermal performances of the machine under steady and transient conditions. The lumped parameter thermal method is used to estimate the temperature rise in induction machine. This method is achieved using thermal resistances, thermal capacitances and power losses. To analyze the thermal process, the 7.5kW machine is divided geometrically into a number of lumped components, each component having a bulk thermal storage and heat generation and interconnections to adjacent components through a linear mesh of thermal impedances. The lumped parameters are derived entirely from dimensional information, the thermal properties of the materials used in the design, and constant heat transfer coefficients. The thermal circuit in steady-state condition consists of thermal resistances and heat sources connected between the components nodes while for transient analysis, the thermal capacitances were used additionally to take into account the change in internal energy of the body with time. In the course of the simulation using MATLAB, the response curves showing the predicted temperature rise for the induction machine core parts were obtained. To find out the effect of the decretization level on the symmetry, the two different thermal models, the SIM and the LIM models having eleven and thirteen nodes respectively were considered and the results from the two models were compared. The resulting predicted temperature values together with other results obtained in this work provide useful information to designers and industries on the thermal characteristics of the induction machine.

This thesis is concerned with the thermal modelling of the induction machine. With the increasing quest for miniaturization, energy conservation and efficiency, cost reduction, as well as the imperative to exploit easier and available topologies and materials, it becomes necessary to analyze the induction machine thermal circuit to the same tone as its electromagnetic design. This would help in achieving an early diagnosis of thermo-electrical faults in induction machines, leading to an extensively investigated task which pays back in cost and maintenance savings. Since failures in induction machines occur as a result of aging of the machine itself or from severe operating conditions then, monitoring the machine’s thermal condition becomes crucial so as to detect any fault at an early stage thereby eliminating catastrophic machine faults and avoidance of expensive maintenance costs. Faults in induction machines can be broadly classified into thermal faults, electrical faults and mechanical faults. Currently, stator electrical faults are mitigated by recent improvements in the design and manufacture of stator windings. However, in case of machine driven by switching power converters the machine is stressed by voltages including high harmonic contents. The latter option is becoming the standard for electric drives. A solution is the development of vastly improved thermal system cum insulation material. On the other side, cage rotor design is receiving slight modifications, apart from that, rotor bars breakage can be caused by thermal stress, electromagnetic forces, electromagnetic noise and vibration, centrifugal forces, environmental stress.

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