Influence of Temperature on Large Commutator Operation

The paper discusses the operating characteristics of large commutators. It is pointed out that many cases of blackened and burned bars are caused initially by a slight roughness which causes irregular commutation and a slight sparking which when once started gradually becomes worse. The requirements of the surface of a high-speed commutator are discussed, showing that radial variations of the order of one ten-thousandth of an inch between adjacent bars may give serious trouble. Some causes of roughness are discussed, showing that temperature is a major factor, but that redesigning the commutator to reduce the temperature does not necessarily cure the trouble, and may actually give poorer performance. Some methods of commutator testing are described with typical records showing the performance of various types of construction. The question of safe temperature limits is then discussed. It is maintained that the question of the permissible temperature limits is entirely unlike that of insulated windings, since the materials used are not injured by temperatures considerably above the present limits. The permissible limit of a commutator is determined by its mechanical construction. Low temperature limits may lead to illogical designs, since large heat dissipating surfaces are required which may give higher stresses and larger expansion effects, which may be more harmful than higher temperatures. A temperature limit based on operating characteristics would be more satisfactory.