Electric motor failure — A comparative study of its causes

Abstract

A broad comparative study of the causes of electric motor failure is difficult to accomplish. Motor manufacturers are involved in failure studies, as such, but only as far as is appropriate to satisfy warranty requirements. Once the warranty period has passed the motor manufacturer almost never is advised of the cause of failure and, as a result, an attempt by the manufacturer to continue a failure study beyond the warranty period would be extremely difficult and costly. Motor users, although exceedingly interested in maximum motor life, do not have the range of failures sufficient to determine trends and percentages that can be applied to a general motor population. Frequently the user does not particularly care why a motor fails, but rather only that it has failed. Two surveys have been conducted, however, dealing with motor failures. They were accomplished at a large motor repair facility in Southern California by two separate individuals 17 years apart. Each survey dealt with the same general geographical area, the same general climate, the same general cross section of industrial users, and the same general motor population profile. The results of each of these surveys demonstrates significant changes in the causes of electric motor failure. Consequently they yield considerable factual evidence about the field performance and usage of three-phase industrial electric motors. The first survey was concluded in 1950 and encompassed 8,000 to 10,000 motor failures. The motor population at that time was predominantly Class A rated. The insulation system was composed of cellulosics such as rag paper, capaco, and cotton. Formvar-type magnet wire films and phenolic insulating varnishes had matured to popular use. Motor duty was accommodated by a service factor of 1.2 or more. The copper and steel were "worked" conservatively. The effects of a 50 to 60 cycle frequency change (1945-1948) were still visible. The second survey was finished in 1967 and included 4,076 failures. The sample during this survey was mostly rated Class B with a 1.15 service factor or less. Polyester plastics and resins had come into vogue as Mylar® and Dacron cell insulation, polyester magnet wire films, and insulating varnishes. The encapsulated motor winding had come into popular approval by 1967. Epoxy systems in the form of composites, films, and liquids were gaining on the comfortable position formerly achieved by the polyesters. The results of these two surveys are offered as Table I.