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.