Cooling of Electric Machines

the removal from electric machines of the heat liberated as a result of magnetic, electric, and other losses. The maximum permissible heating is determined by the heat resistance of the materials—insulation, solder, and lubricant—used in the machine. The most efficient method of heat removal is to cool the heated parts of the machine with a circulating intermediary substance that may be air, various gases (hydrogen, carbon dioxide, helium), or a liquid (transformer oil, water, chlorinated biphenyl).

Air cooling is sufficiently effective in most cases and is the simplest and easiest method. The principal air-cooling systems are (1) natural cooling without the forced circulation of the air around the heated parts; (2) cooling with air from the surrounding space through forced ventilation provided by one or more independent fans or by a single fan mounted on the shaft of the machine (internal self-ventilation); and (3) cooling of an electric machine of an enclosed, or airtight, design, where air circulation in the housing is maintained by an independent fan or by internal self-ventilation. The first cooling system is used in low-power (up to several hundred watts) enclosed or open machines that do not require intensive cooling. The second system is used chiefly in low- and medium-power machines. The third system is used in high- and medium-power machines and also when the air surrounding an electric machine has been heated to a high temperature, contains explosive gases, or contains acid vapors that corrode the insulation.

Special gases are used for cooling electric machines where the power consumption for air ventilation is very great, such as high-speed electric motors, turbogenerators, and synchronous compensators. When a hydrogen cooling system is used, the possibility of the hydrogen mixing with air and forming a dangerously explosive mixture must be avoided. If such a danger exists but air cooling is nevertheless undesirable, as with high-power electric motors located in dangerously explosive places having poor ventilation, a cooling medium such as carbon dioxide or helium is used.

Water is used to cool the stators of high-frequency electric machines, the bearings of high-power electric motors, and the step bearings of generators. Chlorinated biphenyl is used if there is a danger of the water freezing. The windings of heavy duty transformers are cooled by circulating oil.