A method of evaluating insulation systems for universal motors used to power portable tools

The requirement for the optimum portable electric tool motor is that it produces extremely high power output for its weight and size and that it holds up under the normal and abusive operation to which it is subjected during its expected life. To obtain the ultimate in this respect, full advantage must be taken of all applicable advancements in the state of the art. Widespread use has been made of new higher temperature materials to overcome the predominate short time failures caused by severe overloads, but in general the design standards for the allowable temperature rise at rated loads have remained unchanged. It is recognized that is some cases desired speed-torque characteristics, tool handle or case temperature may prevent deviating from the presently established limits; but in many cases, smaller long life motors using new insulating materials and operating at rated load at much higher temperatures are possible. A method of evaluating the insulation systems of portable electric tool motors which incorporate the exposure conditions peculiar to this type of application, and which covers the range of temperatures likely to be encountered under light to maximum "expected" overload conditions, is very much desired. Presented herein is a method of evaluation employing certain novel approaches developed by one manufacturer. While it was designed to meet the requirements of power tool motors, it is hoped that the solution suggested is of universal interest.