The inherent overheating protection of single-phase motors

THE problem of controlling and restricting the temperatures of motor windings, is of vital importance to the motor manufacturer, the motor user, and the general public. The motor manufacturer is anxious to have winding temperatures restricted to a safe value, so that his products may have customer acceptance. The motor user is interested in winding temperatures because they affect motor life and maintenance cost. The general public is not conscious of any interest in the matter, but is certainly affected in all cases where excessive temperatures result in fire hazards. In the early days of the industry, motors did not have adequate protection against overheating, because suitable protective equipment was not available. Motor circuits were fused, but the fuses had to be large enough to accommodate the motor starting current and were, therefore, too large to properly restrict the running current or protect the windings. However, the lack of proper protection was partially offset by the special attention which was given motors by their owners and attendants. Summary Motor protectors are selected and applied so that they restrict the motor winding temperature to specified values at normal voltage and frequency and a room temperature of 40 °C. The accompanying table shows the probable increase (or decrease) in winding temperatures which may occur on account of abnormal voltages, frequencies, or ambient temperatures, or with restricted ventilation.