Testing of transformer sheet steel

The quality of steel depends not only on the chemical composition, but also upon the treatment of it during the process of manufacture, and it is essential that samples of the various melts be tested for losses, etc. Due to the hardening effect of shearing or punching, which increases the losses, it is advisable to anneal test samples after punching. From an industrial standpoint the aim is to obtain a test which will give accurate results in the shortest time and using the minimum amount of steel. There are many methods which will give very accurate results, but most of them are slow and therefore impractical for commercial work. The oldest and best known method for determining losses in steel is by the use of the ballistic galvanometer, but the slow and tedious process of plotting a curve of induction and magnetizing force and of obtaining the area of the resulting hysteresis loop is not rapid enough for factory testing; furthermore, it gives no idea of the eddy current losses. Professor Ewing´s apparatus for obtaining hysteresis losses offers a purely mechanical determination, the sample of steel being revolved between the poles of a permanent magnet and the torque being read directly by means of a pointer moving over a graduated scale. Its principal advantage is its rapidity of test, small amount of material used, and simplicity of operation. The eddy current losses cannot be obtained, and the samples are so narrow that a large percentage is affected by the cutting; also the range of densities for which it may be used are very limited. Alternating current methods using voltmeter and wattmeter are universally recognized as the most rapid and accurate methods of testing transformer steel. By a single observation at one density and frequency, the total watts lost in the sample due to eddy current and hysteresis are obtained. A sine wave and a generator of fairly large capacity should be used. Voltage control must never be obtained by variable resistance or induct- nce in the magnetizing circuit. Some of the best known alternating current methods are Kapps´ yoke test and the Epstein test which was designed by the hysteresis committee of the German Association of Electrical Engineers. The Epstein method has recently been modified by the Bureau of Standards so that less material is necessary. The so-called “ring test” is also employed a great deal at the present time. It has the advantage of practically no leakage and that but small magnetizing current is required. There are many factors which affect the results of the above method, such as pressure, insulation and temperature, and great care should therefore be taken in making the determinations.