Insulation evaluation techniques for motors used on heavy duty portable tools

A great deal of work has been done, and standards have been developed for the testing of enameled magnet wire in air (1), and for the evaluation of complete insulation systems (2) for induction motors, transformers, and similar electrical equipment. These techniques apparently give excellent correlation with field results when applied to equipment having stationary windings and operating at temperatures which do not exceed the ratings of the material used. When an attempt is made to use these techniques in evaluating materials for portable tool motors, however, the results do not correlate with the field experience. The most frequent source of motor burnout in a portable tool motor is turn-to-turn shorts in the armature. Virtually all portable tool motors are series wound for AC or DC operation. No-load speeds are in the vicinity of 20,000 tor 30,000 RPM. The rotating armature windings are random wound on high speed winding machines, resulting in many high pressure crossover points in the armature conductors. Armature temperatures can momentarily greatly exceed the temperatures for which the materials are rated. Class "C" temperatures (over 220°C) are possible. During these short periods of over-temperature, the windings are subjected to the high no-load speeds, vibration, and voltage, which, combined, can cause insulation breakdown in turn-to-turn shorts. On the other hand, portable tool motors have a short life expectancy in the neighborhood of 500 to 1500 hours, as contrasted with the expected life of 20,000 or more hours on induction equipment. The vast differences between conventional induction motors for stationary equipment and series motors for portable tools may be attributed to the portability requirement of the latter. In order to develop the necessary power to perform the required work, with a minimum weight, portable tool motors must be designed with electrical and mechanical components worked to the limits of their capabilities. Having less mass, these motors are subjected to more rapid temperature rises, when overloaded, than larger stationary motors of the same power. Furthermore, there are no standard loads for portable tools, the loading being literally in the hands of the operator. Variations in the materials cut, the sharpness of the tool, and other unpredictable conditions, assure the motor designer that any load from idle to stall is a possible condition of operation for his portable tool motor. For the above reasons, selection of an insulation system for portable tool motors is quite a different problem from that encountered in other motor design work. Because we frequently encounter high temperatures for short periods of operation on portable tool motors, we are especially interested in knowing what happens to an insulating material when exposed to temperatures higher than its rating for short periods of time, and at the same time subjected to high centrifugal forces and vibrations. It is this specific area of insulation study which requires a specialized approach to testing and evaluation of materials for series motors in heavy duty portable tools. R. B. Carmer and E. W. Daszewski, of Essex Wire Corporation (3) have shown that insulating films on magnet wires, when subjected to extreme temperatures, exhibit characteristics which are not predictable by extrapolating the conventional straight-line life-temperature curves obtained by the twisted pair(1) method. Certain Class "A" (105°C) films show better endurance than certain Class "F" (155°F) films in the high temperature regions.