The Cooling of Electric Machines

The elevation in temperature of the windings of any electric machine due to the internal heat losses is usually the dominating factor in limiting the rating of the machine. This limitation found in the heating, is primarily due to the effects of high temperatures on the various types of insulation used in such machines. The American Institute of Electrical Engineers Standards specify the maximum permissible operating temperatures for the various grades of insulation. The temperature of the windings in the machines then, must be kept below this maximum permissible temperature. The heat losses produced in a machine are the results of electrical and mechanical losses. These losses must be transferred from the source of generation to the cooling medium. The first step of the heat flow path is by conduction through the copper, iron or insulation to the ventilating or cooling surfaces. The second step in the transfer of the heat may be called the liberation or dissipation of the heat from these surfaces to the cooling medium. This portion of the heat flow may be divided into two or more stages by the use of an intermediate cooling medium, such as oil, in an oil-insulated transformer. This paper deals with some of the more common types of ventilating and cooling systems used in the electric machines and particularly with the laws governing the flow of heat from the cooling surfaces.