Heating of railway motors in service and on test-floor runs

The rating given to a motor is the manufacturer's guarantee of the motor performance under the conditions given on the name plate. Assuming that this rating is entirely safe, then the successful functioning of the motor depends entirely upon the application engineer's analysis of the particular duty that the motor will be required to perform. Where the required motor output is practically constant the application is simple; however, in many cases the motor load is apt to be anything but constant, consisting of loads of all degrees of magnitude, and in such cases the economically correct application is especially difficult. The past improvements made in the motor design, mechanically and electrically, have resulted in greater importance of the motor-operating temperatures, in fact in the great majority of cases the motor rating is limited only by the motor temperature. It is obvious then that correct motor applications depend to a very great extent upon correct operating temperatures. Ratings such as the continuous, short-time, normal, and duty-cycle ratings give the performance of the motors under some particular conditions; however, the duty required of a great number of industrial and railway motors will not agree with any of the above ratings. Thus the application of motors to cranes, hoists, steel mills, and railways must be made with the knowledge of the motor's performance under one or more arbitrary conditions. In general the two ratings which should be known for motor application to such irregular duty are the continuous and a short-time rating. The time period of the short-time rating should not exceed one hour and in many cases a one-half hour run is preferable. The correct application of a motor requires a knowledge of the thermal conditions inside of the motor. Thus it is evident that the motor must be able to dissipate eventually all of the heat losses generated. On a continuous load the final rate at which the heat is transferred from the motor to the air will be equal to the rate of heat generation. The resulting temperature rise can be estimated with the physical conditions known. This is simply a problem in physics and involves the conditions of ventilation with the corresponding ventilating surfaces. To predetermine the internal temperatures requires a knowledge of the rate of heat flow along the various heat flow paths to the ventilating surfaces. Under irregular loads the temperatures are transient and are determined not only by the conditions of ventilation and rates of heat flow, but also by the motor's ability to store heat, which is proportional to the product of the motor's mass and specific heat. Hence the temperature rise of any part of a motor under any given load for some definite time is a function of the rate of heat flow from that part to the surrounding air and its thermal capacity. Thus from the known physical conditions the temperature rise of the motor can be predetermined and with certain assumptions a simple equation can be developed which will give an approximate value of the motor temperature under any given load conditions. When the constants of the theoretical equations are based upon tests (such as given by short-time and continuous rating) the above method of temperature predetermination will be sufficiently accurate for most practical purposes, and will make possible the calculation of the motor temperature rise under any duty cycle. The temperatures referred to are not only those temperatures obtained by thermometers upon the surfaces of the machines, but also the maximum internal temperatures, since it is the latter temperatures which first produce insulation failure. The temperatures obtained by thermometers bear no fixed relation to the maximum temperatures for all types of machines under various loads. In order to have something concrete to work upon the writer has taken up the heating and cooling characteristics of railway motors. A brief analysis of the heating at the standard ratings is given. The fundamental equations defining the temperature rise of a motor on a continuous or short-time load are developed, and finally the applications of these equations are made in several specific examples.