Insulation polarization index

Motors rated 7500 kW and less must be considered suitable for dielectric tests or operation only when the polarization index or the value of the insulation resistance (at 40°C) is at least the minimum recommended values. 

Motors rated above 7500 kW must have both the polarization index and the insulation resistance above the minimum recommended values. 

The recommended minimum value of the polarization index for motors having class B or F insulation must be 2 when determined according to lEC 60034-18-1. 

These test results are then compared with similar test data obtained from the manufacturer on similar windings carried out during manufacture. If the manufacturer s original test results are available, the results obtained at site can be quickly compared and the condition of the insulation assessed easily and accurately. If the test facility to obtain test results at 1 minute and 10 minutes is not available, the results may also be obtained for 15 seconds and 60 seconds and a graph plotted as shown in Figure 9.5(a) to determine the polarization index. 

Check for winding insulation by polarization index (Section 9.5) and dissipation factor, tan S (Section 9.6)... 

By monitoring the insulation condition of the windings during maintenance, at least once a year, which can be carried out by measuring (a) the polarization index (Section 9.5.3) and (b) the dielectric loss factor, tan S (Section 9.6) and making up the insulation as in Section 9.5.2, when the condition of the insulation is acceptable and only its level is less than permissible. 

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). 

For the purpose of estimating the solubility of a solute it is necessary to have some measure of the polarity of a solute or a solvent. Based on Eqs. (1) and (2), a useful polarity index should be a measure of a material s intermolecular forces, Cn and C22-Table 1 contains a list of solvents that are typically used in liquid pharmaceutical formulations and three measures of solvent polarity. Each measure of solvent polarity, or polarity index, is based upon a different measure of a material s property. For example, dielectric constant is a measure of the electrical insulating properties of a solvent, solubility parameter is determined from the molar energy of vaporization, and... 

The dielectric constant of coal is strongly dependent on coal rank (van Krevelen, 1961 Speight, 1994, and reverences cited therein). For dry coals the minimum dielectric constant value is 3.5 and is observed at about 88% w/w carbon content in the bituminous coal range. The dielectric constant increases sharply and approaches 5.0 for both anthracite (92% carbon) and lignite (70% carbon). The Maxwell relation which equates the dielectric constant to the square of the refractive index for a polar insulators generally shows a large disparity even for strongly dried coal. 

Polarized light microscopy is frequently used for determining the asbestos content of bulk samples of insulation or other building materials (see, for example, NIOSH Method 9002 [NIOSH 1989] and OSHA method ID-191 [OSHA 1994]). This method also enables qualitative identification of asbestos types using morphology, color, and refiactive index. 

PAEK can be used as good electrical insulators in a wide range of environments and over a wide range of temperatures. The polar nature of the carbonyl group means that materials such as fluoropolymers have superior dielectric constants and dissipation factors. However, relative to fluoropolymers, PAEK insulation offers lower toxicity in fire situations and more resistant mechanical properties. PAEK can have a relatively low comparative tracking index which measures susceptibility to electrical breakdown on the surface of an insulator. Possibly this reflects the relatively easy degradation of PAEK to carbonaceous char. 

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