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The Normal Running Temperature 



of Electric Machines 



AN electric current flowing through a 

 l\ conductor tends to raise its tempera- 

 ture, especially where there is a resistance 

 offered. In electrical terms, this 

 is proportional to the square of the 

 current. In designing electrical 

 machines, this heating is kept 

 down as much as possible, since it 

 represents a waste of energy and 

 is apt to char the insulation and 

 cause damage to the machine. 



It is evident that the heating up 

 of a machine will take some time, 

 depending on the size and cooling 

 ability. The temperature will 

 rise until finally a value is reached 

 at which the temperature remains 

 constant. This is the normal run- 

 ning temperature of the machine. 

 For cotton insulated windings, it 

 should be below 85 deg. C. For 

 this reason, other materials, such 

 as asbestos, mica and enamel, 

 have been developed for purposes where the 

 heating effect is great. 



The method of procedure for the heat run 

 of the electrical generator is as follows: 

 Two or three thermometers are stuck to the 

 filed coils and core of the machine with putty. 

 Read the thermometers and record the 

 readings; then start the generator. Allow 

 the generator to feed normal current to a 

 water rheostat or feed it back into the line. 

 This latter method of using the current is in 

 general practice because of its economy. 

 Take thermometer readings every five 

 minutes for at least an hour and then shut 

 off the power and take armature core and 

 winding temperatures. Suppose the set of 

 readings is as follows: 



Popular Science Monthly 



Plot the data as given, with temperature 

 rise as ordinates and time as abscissas, and 

 the curve, as shown, will be obtained. By 

 studying the curve, it will be noticed that the 

 temperature of the machine rises rapidly at 

 first and then gradually approaches a 



5 10 15 M 25 10 55 « *5 50 55 60 



TIME (MINUTES) 



A curve obtained by plotting the data given with 

 temperature rise as ordinates and time as abscissas 



constant temperature. The reason for this 

 is that heat transfer between the machine 

 and the surrounding air will depend on the 

 difference in temperature between them. 

 At first, there is little difference in tempera- 

 ture between the machine and the air and 

 therefore, almost all the heat goes to warm up 

 the machine. But when the machine gets 

 warm, it dissipates the heat rapidly and 

 reaches a final temperature at which the heal 

 generated by the current equals the heat 

 dissipated. — Alex V. Polson. 



A New Use for Fogged Photographic 

 Plates 



SOMETIMES, by accident a photographic 

 plate is fogged by exposure to light 

 before it is exposed in a camera or before it 

 is developed. These plates can be used to 

 make a very soft negative from a harsh one. 

 The method is as follows: Place the fogged 

 plate in a printing frame with a negative, 

 film against film. The plate prints slowly 

 and makes a positive. In broad daylight it 

 is placed in the developer and the positive 

 disappears slowly, the plate gradually be- 

 coming a negative. After several experi- 

 ments, it was found out that the shorter the 

 printing, the harsher the negative became. 

 Long printing is best. — W. S. Standiford. 



