INDEX loss tangent

A d.c. insulation resistance test or polarization index reveals only the surface condition of the insulation and does not allow a realistic assessment of internal condition. Loss tangent values are true reflections of the insulation condition to detect moisture content, voids, cracks or general deterioration. The tan 5 versus test voltage curve may be drawn and compared with the original curve provided by the manufacturer, and inferences drawn regarding the condition of the insulation. The different starting tan lvalues will reveal the condition of the insulation in terms of amount of contamination, as noted in Table 10.4 (See lEE, Vol. 127, May 1980). 

The loss index of silicone elastomers made from polydimethyl siloxane, generally speaking, is low due to their low values of dielectric constant and loss tangent. The dielectric constant of polydimethylsiloxane is almost independent of the frequency, where as the tan 6 is highly dependent on the frequency in the micro-wave region (3, 4 ) At 3 x 109 Hz, a 1000 cs polydimethylsiloxane fluid has a tan 6 of about 0.0096 and a dielectric constant of 2.76. This gives a loss index of 0.0264, which puts polydimethyl siloxane in the poor heatability category. 

These results support the proposal that the loss tangent is an index of damping properties. Judging from the loss tangent, IPNs are useful damping materials at elevated temperatures. 

Even without an analytical expression to describe the shape of H, it is clear that increasing steepness of H in the transition zone as portrayed in Fig. 12-11 will be accompanied by a compression of the transition from rubberlike to glasslike consistency into a narrower region of logarithmic time scale. Plots of both transient and dynamic moduli and compliances, as exemplified in Chapter 2, rise and fall with steeper slopes. Perhaps the most sensitive index of the sharpness of the transition is the loss tangent, which is plotted in Fig. 12-12 for several prototypes the polyurethane rubber, poly( -octyl methacrylate), poly(vinyl acetate), and Hevea rubber. Here the frequency scale has been arbitrarily selected to make the maxima coincide. The sharpness in the loss maximum correlates with the slope of H in the transition zone. The shape emphasizes the failure of the modified Rouse theory to provide a detailed description of the properties in the transition zone, since it predicts tan 5 = 1 independent of frequency in this region. The drop in tan 5 at high... 

The PDMS samples under test were 375 and 960 pm thick sheets. Reference and sample measurements were conducted at room temperature with a nitrogen-purged chamber to avoid water vapor absorption Hnes. The index of refraction and the absorption coefficient for both thicknesses were determined with similar characteristics obtained. The refractive index and absorption coefficient for the 960-pm-thick PDMS sheet in the terahertz region are shown in Figure 14.16. The absorption coefficient of 20 cm at 1.0 THz compares well with that of 30 cm for polyimide [8]. Using these results, the dielectric constant and loss tangent... 

The ratio of the loss index to its relative permittivity, the tangent of its loss angle, 5 or the cotangent of its phase angle, 0. 

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