The Service Factor Rating of Arc-Welding Generators and Transformers

The primary purpose of rating an electrical machine is to give the user an idea of what he may expect from the unit which he purchases. It is the purpose of this paper to propose a method of rating arc-welding generators and transformers which will give a truer picture of their working ability than is afforded by the present one-hour rating. Such a method of rating will be beneficial both to the customer who buys and uses arc welders and to the manufacturers who produce and market the units. The user will be better able to select a machine which will do the job to be performed, and at the same time he will be assured that he is not purchasing a machine unduly large for his requirements. The present custom is to give welding transformers and generators a one-hour rating, specifying the current they can deliver for a one-hour period, starting cold, without exceeding the permissible temperature rise. The plan here proposed is to give them a current rating indicative of their normal operating capacity, or short-time welding ability; and an additional service factor rating, indicative of their continuous current capacity, as limited by thermal considerations. All electric apparatus has these two major limitations on its output, one a ``size'' limit expressed by breakdown torque, commutation limit, or voltage drop; and the other a thermal limit expressed by the degrees temperature rise permissible for the type of insulation used.