Page:Electrical Engineering Volume 2.djvu/17



3541. The output of a constant-potential dynamo is limited by two factors: the carrying capacity of the armature conductors, and the distortion of the field by armature reaction. The evidence of a machine having reached the limit in the first of the above cases is the development of serious heat in the armature; distortion of the field is indicated by excessive sparking at the commutator.

3542. The question of sparking must be considered fully in the design of a dynamo; for it is not merely a matter of shifting the brushes which should be relied upon to prevent sparking, nor yet an excessive strength of field, as this is wasteful of energy. In a well-designed dynamo, sparking should not occur within the limits of the working range, which may include an overload above the rated output of 15 or 20 per cent.

3543. The heat that is being continually generated in the machine, owing to the resistance which the conductors present to the passage of current, is given off from the surface of the armature and of the whole machine to the surrounding air. This giving off of heat can occur only when the dynamo is hotter than the air, for if two bodies are equally hot, one can not give any heat to the other. Conversely, the greater the difference in temperature between two bodies, such as a dynamo armature and the