Insulation resistance

The resistance, R, is a dimensionally dependent (i.e., extensive) property and is related to the intrinsic resistivity of the material described in the next section. 

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The Bathythermograph. The thermistor sensing probe of a disposable bathythermograph is coated with parylene. This instmment is used to chart the ocean water temperature as a function of depth. Parylene provides the needed insulation resistance and is thin and uniform enough to permit a rapid and accurate response to the temperature of the surrounding salt water (64). 

The most important electrical properties of insulation are dielectric strength, insulation resistance, dielectric constant, and power factor. Corona resistance, although not stricdy an electrical property, is usually considered also (10). 

For each specific appHcation of a mbber compound as an iasulating material, there is a minimum value of resistivity below which it does not function satisfactorily. In addition, iasulating compounds are required to withstand the effect of water, moist atmosphere, or heat without their resistivity values falling below a satisfactory level. Insulation resistance measurements frequently serve as useful control tests to detect impurities and manufactuting defects ia mbber products. 

Fillers. These are used to reduce cost in flexible PVC compounds. It is also possible to improve specific properties such as insulation resistance, yellowing in sunlight, scuff resistance, and heat deformation with the use of fillers (qv). Typical filler types used in PVC are calcium carbonate, clays, siHca, titanium dioxide, and carbon black. 

Grade XXXPC is similar in electrical properties to Grade XXXP and suitable for punching at lower temperatures than Grade XXXP. This grade is recommended for apphcations requiring high insulation resistance and low dielectric losses under severe humidity conditions. 

Conformal coatings are protective coatings appHed to circuit board assembHes. They protect the interconnect conductors, solder joints, components, and the board itself they reduce permeabiHty to moisture, hostile chemical vapors, and solvents in the coating. Use of conformal coatings eliminates dendritic growth between conductors, conductor bridging from moisture condensation, and reduction in insulation resistance by water absorption. 

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. 

Whenever a motor is installed in a humid atmosphere and is switched on after a long shutdown, insulation resistance must be checked before energizing the motor. As a precaution, insulation resistance must be checked before a restart after a long shutdown, even in temperate conditions. If the insulation level is found to be below the recommended level as shown in equation (9.1) it must be made up as noted below. 

The winding insulation resistance to be used for comparison with the recommended minimum value (/, ) is the observed insulation resistance obtained by applying a d.c. voltage to the entire windings for one minute, corrected to 40°C. In practice, motors having insulation resistance readings as high as ten to a hundred times the minimum recommended value R , are not uncommon. 

Figure 9.4 Variation of insulation resistance with time for class B insulation...

Figure 9.5(a) Typical values of 1- and 10-minute insulation resistances during the drying process of a class B insulated winding of a large machine... 

Figure 9.5(b) Approximate temperature coefficient of insulation resistance of rotating machines... 

The insulation resistance thus measured at different intervals during the process of heating up will represent the insulation resistance of the windings at that temperature. Before plotting the curves, these test values must be collected to one reference temperature, say, 40°C, to maintain coherence in the test results. 

To correct the value of insulation resistance at 40°C the following equation may be used ... 

The temperature coefficient curve is given in Figure 9.5(b). This is plotted on the assumption that the insulation resistance doubles for each I8°C reduction in temperature (above the dew point). 

It is recommended to test eacli phase to ground separately with the other phases also grounded. This is because the insulation resistance of a complete winding to ground does not provide a check of the insulation condition between the windings. 

It is observed that the insulation resistance of one phase with the other two phases grounded is approximately twice that of the entire winding. Therefore, when the three phases are tested separately. the observed alue of the resistance of each phase is divided by two to obtain the actual insulation resistance. 

Insulation resistance of the motor windings between phases and phase to ground should be checked and made up in the event of deficiency, according to Section 9.5. 

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 tests for insulation resistance and dielectric loss factor should, however, be carried out on a completed machine also with formed coils to establish reference data for field tests, as noted in Section 9.6. However, these tests on a completed machine with formed coils do not monitor the process quality of insulation. 

Recommended practice for. testing insulation resistance of rotating machinery. 

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