Inherent overheating protection of D-C aircraft motors

IN 1925, introduction of thermostatic protection to domestic flatirons revolutionized the flatiron industry. In 1928, disk-type thermostats were built into a portable-tool motor to provide built-in thermal protection. Two or three years later, thermostats were used to provide built-in overload protection for single-phase induction motors. Since that time, a quiet revolution has been taking place in the fractional horsepower motor industry, in the form of an ever-increasing use of these built-in thermal protective devices. By 1939, the Underwriters' Laboratories had become interested, and in June of that year, after conferring with electrical manufacturers making and using such devices, the Underwriters' Laboratories wrote a standard for inherent overheating protective devices; this standard was incorporated as an appendix to the Underwriters' standard for industrial control equipment. If the present trend to thermal protection of motors continues, nearly all domestic fractional-horsepower motors will eventually be provided with inherent overheating protection.