Thermocouples and resistance coils for the determination of local temperatures in electrical machines

The paper discusses different devices for measuring temperatures in electrical machines and points out their various sources of error, and also the limitations of the uses of mercury and alcohol thermometers, thermocouples and resistance coils. Fluid thermometers can generally measure only surface temperatures, and temperatures of rotating parts must be measured after they come to rest, thereby introducing large chances of error due to equalization or rapid changes of temperature, slow rate of indication of thermometers, small contact between the bulb and the surface to be measured, emergent stem, etc. In the measurement of temperature by the electrical resistance method either the windings of the machine itself or fine wire coils placed in or about the windings are employed. In using the coils of the machine, the result depends upon the accuracy with which the coefficient of change of resistance of the copper with temperature is known. This method, of course, gives only the average temperature of the whole length of the winding and does not indicate the temperature of any part of the winding. Where coils of fine copper are used highly accurate measurements are possible, as the coefficient of temperature of the wire may be accurately known. The extent of the coil determines the extent to which the measurement is local. For accurate determinations, some sort of bridge for determining the resistance is required. The thermocouple is the most accurate device for measuring local temperatures, as the e.m.f. generated is a function of the difference in temperature between the junction of the wires of the couple and their free ends, and the temperature of the free ends can be accurately controlled. Thermocouples require the use of precision meters for reading their indications, and potentiometer arrangements may be applied to obtain any desired degree of accuracy. Ordinary thermometry is the simplest, quickest and least accurate method of temperature measurement, and usually applies only to surface conditions. The choice between the electrical methods of temperature measurement is largely determined by the ease of application in any particular case. The thermocouple will give the most rapid indications, although the resistance method is almost as rapid if the coil is of right proportions and is intimately applied to the parts whose temperature is to be measured.