APPLICATION OF FINITE ELEMENT METHOD TO THE THERMAL ANALYSIS OF ELECTRICAL MACHINES

Abstract

The design of electrical machines involves essentially three major aspects, namely electro-magnetic, mechanical and thermal. Out of these three, considerable attention has hitherto been paid to the first two aspects and at present,electrical machines can be considered to be more or less perfect in these directions. The thermal performance of the machine happens to be still a strong factor deciding its final rating, since the technology for the thermal design is not yet fully developed, because of the fact that the problem becomes complex as far as electrical machines are concerned. The present work is an attempt to evolve a methodology for the thermal analysis of the electrical machines. The development of the s Finite Element Method during the last few years as a popular technique for the computer solution of complex engineering problems has resulted in its application to the analysis of engineering systems involving boundary-value problems. This method has been effectively used for the analysis of structural problems, electrostatic and electromagnetic fields, and heat-conduction problems. With particular reference to the thermal analysis of the electrical machines, very little application of this method seems to have been made. The present work makes use of the finite element method to predict the temperatures in different parts of the machine under steady-state. The application of the isoparametric elements is effectively made to represent the curved boundaries involved in

the three-dimensional geometry of the electrical machines. This also avoids the necessity of deriving explicit relations to arrive at the system matrices and forcing functions. Numerical integration technique has been applied to evaluate them. The usual practice in discretizing a region into finite elements is that the adjacent elements have common face over its whole entity. This condition unduly increases the total number of elements, since this number should necessarily be large in the regions where the temperature gradients are high or expected to be high. A modification is suggested in the present work to obviate this difficulty. This involves the technique of *Mesh- Grading , whereby the total number of elements is reduced, while the region with higher gradients still gets finely divided. To deal with the boundary conditions encountered in the field problems in general, a special element at the boundary, termed as 'incremental Element has been introduced in the present work. This is also applicable for the thermal problems. The transient thermal behaviour of an electrical machine is also important to ascertain its overload capacity, transient negative sequence heating of rotor surface due to faults etc. It assumes importance in designing motors intended for inter mittent loading. In view of this, the transient thermal analysis has also been carried out. The prediction of the temperature-distribution in different parts of the electrical machine at the design stage itself

enables the designer to arrive at a most satisfactory and economical design. But this requires a knowledge of fluid-flow in the machine besides heat-transfer phenomenon. Making use of the existing literature on these, the evaluation of the fluid flow and he at-transfer coefficients is attempted at. Computer programmes developed for implementing the above mentioned schemes are tested by taking up the analyses of some large electrical machines. The results of application of the programmes are compared with the factory-test results, wherever possible. Dealing with the design aspect, the effect on the ventilation and heat transfer capability of a machine, of some of the para meters like the angle of the fan blade, size of axial holes and radial ventilating ducts is studied. Two types of machines, one with closed air-circuit air cooling and the other with screenprotected drip-proof arrangement are considered to study the above effect. Keeping the results of this study in view, some design considerations are presented for the design of a proper ventilation scheme of an electrical machine. Summarizing the work done, some indications for the possible extensions to the present work are suggested for carrying them out in future.