The cooling of electrical machines

Abstract

This paper is intended to serve the dual purpose of recording the present-day attitude towards cooling electrical machines and of describing tests carried out to fill the gaps left by the hitherto published data.In a simple air-cooled machine the temperature-rise of the hot spot is built up in three stages' —θt = θg + θs + θmwhere θt = hot-spot temperature-nse,θg = temperature-difference between the hot spot and the dissipating surfaces over which the air flows,θs = temperature-difference between dissipating surface and cooling air, andθm = temperature-rise of cooling airIt is believed that this three-term equation is the key to rational design of machines in regard to their cooling, and in the paper the subject is studied on a basis of this equation. Although some engineers now deal with temperature problems on these lines, the older practices, of which an example is the rating of field coils according to the watts dissipated per square inch of surface, are still the standard in many quarters The first part of the paper describes the limitations met with when approaching the problem of cooling. The standard permissible temperatures in Great Britain, Germany and America arc examined, and the restrictions due to various types of enclosure are shown.The merits and disadvantages of radial, axial, and other forms of ventilation are then discussed in relation to the different classes of machines now being manufactured, and the broader problems of cooling are reviewed. Following this, the subject is gone into in detail, and the internal flow of heat in the different classes of apparatus is investigated. The results of various experiments to improve the heat flow by reducing temperature gradients are given. The ability of various types of ducts and cooling surfaces to transfer heat to air is discussed, and comments on predetermining the temperature-rise of a machine are included. The flow of air through the various paths met with in electrical apparatus is also investigated, and the results of certain experiments on this subject arc tabulated.The design of fans for electrical machines is next considered, and the special features introduced by the combination of the fan and the electrical machine are dealt with. The paper concludes with some notes on the progress in cooling during the last 30 years, and summarizes the present-day position.