Abstract :
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. These two limits are usually quite independent of each other, so that no single number, such as a one-hour rating, can fully describe the usability of the apparatus. The proposed service factor rating gives both of these limits, and therefore gives the user the data for applying the apparatus to a variety of duty cycles. In conclusion, it is suggested that welding generators and transformers be designed for a service-factor rating of 75 per cent, since this corresponds best to typical welding duty cycles. It is also proposed that t- e standard AIEE values of temperature rise by resistance for continuous rated machines, 60 degrees centigrade for class A, or 80 degrees centigrade for class B insulation, be recognized as the limiting values in continuous operation at the service-factor rating. Rating for Welding Service Before proposing a method of rating, it is advisable to consider the purpose of rating. “The rating of a machine or other equipment is a set of performance characteristics which are subject to verification by test under specified conditions and which by mutual agreement between buyer and seller may serve as a basis of specifications and contracts covering the purchase and sale of the machine or other equipment. The rating is used by the manufacturer in the design and fabrication of the equipment and is used by the purchaser as the basis for application; therefore, the terms in which the rating is stated should be definite, easily verified, of such a nature as to permit intelligent use of the equipment, and as far as possible inclusive of usual conditions as found in practice.”1
