Page:Popular Science Monthly Volume 92.djvu/332

 316

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.

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��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.

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