Heating.—The heat generated in the magnetizing coils is dissipated in three ways; by:
1. Induction; 2. Radiation; 3. Convection.
In the first instance, it passes through the copper and the insulation, either to the external surface, whence it passes off by radiation and convection into the air, or to the magnet core and yoke, which in turn conduct it away. In large multi-polar machines the masses of metal in the pole cores and frame are more efficient in dissipating heat than the external surface of the coil.
Ventilation.—Sometimes provision is made for ventilation of the field magnet coils as shown in fig. 246. Here the series and shunt coils are wound side by side, ample ventilation being provided lengthwise through and between the coils.
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