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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