What are the causes of heating of DC generators?

The losses in a dc machine cause the temperature of the parts to rise until the difference in temperature between their surfaces and the cooling air is great enough to dissipate the heat generated.

Permissible measured temperature rises of the parts are limited by the maximum “hot-spot” temperature that the insulation can withstand and still have reasonable life. The maximum surface temperatures are fixed by the temperature gradient through the insulation from the hot spot to the surface.

The IEEE Insulation Standards have established the limiting hot-spot temperatures for systems of insulation. The American National Standards Institute Standard C50.4 for dc machines gives typical gradients for those systems, listing acceptable surface and average copper temperature rises above specified ambient-air temperatures for various machine enclosures and duty cycles.

Typical values are 40°C for Class A systems, 60°C for Class B, and 80°C rise for Class F systems on armature coils.

Class H systems usually contain silicones and are seldom used on medium and large dc machines. Silicone vapors can cause greatly accelerated brush wear at the commutator and severe sparking, particularly on enclosed machines.

Heating of End Connections of Armature Windings. Small machines often have “solid” end windings banded down on insulated “shelf”-type coil supports. Larger machines are more heavily loaded per unit volume and usually have narrow coil supports, air spaces between the end turns, and ventilating air scouring both the top and bottom surfaces of the coil extensions.

With this construction, the approximate allowable product of ampere conductors per inch of outer circumference times the amperes per square inch for various rotor velocities is shown in Fig. 8-51 for a 40°C rise on the end turns.

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