Power Losses in Insulating Materials 



By E. T. HOCH 



Synopsis: It is shown that a satisfactory measure of power loss in a 

 dielectric is the product of phase angle and dielectric constant. Although 

 the dielectric constant need not be explicitly considered in the design of 

 condensers, it i? important in such cases as the design of apparatus panels, 

 and vacuum tube bases. The method used in measuring phase angle and 

 dielectric constant is discussed. — Editor. 



IN working with electrical circuits operating at very high frequen- 

 cies and moderately high voltages, such as radio transmitting 

 circuits, it is found that failure in the insulation is seldom due to 

 puncture or flashover as is usually the case at power frequencies, 

 but is generally due to excessive heating which, in turn, causes both 

 mechanical and chemical disintegration. As this heating is due 

 almost entirely to the energy losses occurring in the dielectric itself, 

 it is essential that the factors involved in the calculation of these 

 losses be well understood. 



In the past, various indices have been used as a measure of power 

 losses for the purpose of comparing different dielectrics. Of these, 

 power-factor, phase difference and watts per cubic centimeter prob- 

 ably are the most common. None of these, however, is very satis- 

 factory for this purpose since the first two give only part of the de- 

 sired information, and the last is not in any sense a property of the 

 material, as it is dependent on both the voltage gradient and the 

 frequency. 



However, it can be shown that the product of the phase difference 

 and the dielectric constant of a material is to a sufficient approxima- 

 tion an index of its power losses. Let us consider for a moment the 

 complete expression for dielectric loss. In any condenser the capacity 



C =^ aK 



where a is a constant depending on the geometrical dimensions, and 

 K is the dielectric constant. If a voltage, E, is applied to the con- 

 denser the power loss 



P = E I sin ^, 



where / is the current through the condenser and ^ is the phase 

 difference of the dielectric; sin ^ being the power factor. (Plate 



110 



