Sashicage The polarization index is indicative of the slope of the characteristic curve see Figure 4 and can be used to assess the insulation condition see. Interpretation of insulation resistance measurements iewe machine windings and the recommended minimum values of polarization index and insulation resistance are described in Clause 1 and Clause 12, respectively. It is not possible to cover all safety aspects in this recommended practice and test personnel should consult. The capability of the electrical insulation of a winding to resist direct cur- rent. This time interval is based on the R resistiveL inductiveC geometric capacitiveand absorptive characteristics of the circuit during charging time of the application of the voltage and discharging elapsed time since the removal of the voltage source and subsequent grounding of the winding under test.
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Sall Figure 1—Equivalent circuit showing the four currents monitored during an insulation resistance test. A reversible current of comparatively high magnitude and short ieeee, which decays exponentially with time of voltage application, and which depends on the internal resistance of the measuring instrument and the geometric capacitance of the winding.
The surface leakage current I L is constant over time. Guidelines for test voltages are presented in Table 1. Note that the effects of moisture contamination on a healthy winding espoal not preclude obtaining acceptable readings.
The total resultant current I T is the sum of four differ- iee currents: In general, the insulation resistance varies proportionately with the insulation thickness and inversely with the conductor surface area. To provide greater accuracy around the 1 min point and allow the data to be plotted on log paper, it is also common to take iere at other intervals such as 15 s, 30 s, 45 s, 1 min, 1. It is the responsibility of the users of the test equipment to completely ascertain the possible hazards involved in the testing, to protect personnel from harm, and to eliminate the risk of damage to the equipment.
Clause 1 and Clause An equivalent circuit for the various currents in espaoo insulation resistance test is shown in Figure 1. Interpretation of insulation resistance measurements of machine windings and the recommended minimum values of polarization index and insulation resistance are described in Clause 1 and Clause 12, respectively. Figure 3—Types of currents for an epoxy-mica insulation with a relatively low surface leakage current and no conductance current. The polarization index is normally defined as the ratio of the 10 min resistance value IR 10 to the 1 min resistance value IR 1.
Parte 2 de 5 3. A second component of the absorption current is due to the gradual drift of electrons and ions ieeee most organic materials. The capability of the electrical insulation of a winding to resist direct cur- rent.
See Annex A for the use of other values. If the winding is wet or dirty, a low steady value will usually be reached 1 min or 2 min after the test voltage is applied. A current that is constant in time, that passes through the bulk insulation from the grounded surface to the high-voltage conductor, and that depends on espzol type of bonding material used in the insulation system.
Insulation resistance—general theory The insulation resistance of a rotating machine winding is a function of the type and condition of the insulating materials used, as well as their application technique.
There may also be. Comparing the change in insulation resistance or total current with the duration of the test voltage application may be useful in appraising the cleanliness and dryness of a winding.
These windings have capacitive and inductive properties that can lead to hazards that may not be readily apparent. Regardless of the cleanliness of the winding surface, if the winding temperature is at or below the dew point of the ambient air, a film of moisture may on the insulation surface, which can lower the insulation resistance or polarization index.
These electrons and ions drift until they become trapped at the mica surfaces commonly found in rotating machine insulation systems. A phenomenon occasionally observed, more often on older windings, when, in the presence of moisture, different insulation resistance values may be obtained when the polarity 43— the tester leads are reversed.
Readings of insulation resistance are taken after the test direct voltage has been applied for 1 min. The effect is more pronounced if the surface is also contaminated, or if cracks in the insulation are present. If times t 2 and t 1 are not specified, they are assumed to be 10 min and 1 min, respectively.
After completion of the test, the winding should be discharged through a suitable resistor, sized to limit the instantaneous current to 1 A.
Eespaol discharge current manifests itself in two components:. A minimum discharge time, which esppaol equal to four times the voltage application duration, is recommended. The safety measures described are by no means exclusive. A current resulting from molecular polarizing and electron drift, which decays with time of voltage application at a decreasing rate from a comparatively high initial value to nearly zero, and depends on the type and condition of the bonding material used in the insulation system.
Modern types of filmcoated wire, as well as epoxy-mica or polyester-mica insulated stator windings, may approach a constant value of insulation resistance in 4 min or less. Insulation resistance tests are usually conducted constant direct voltages of —10 0 V having negative polarity. The first component is due to the polarization of the impregnating materials because the organic molecules, such as epoxy, polyester, and asphalt, tend to change orientation in the presence of a direct electric field.
These are meant only to signify the nature of the hazards involved. During the test period, all appropriate safety measures for the voltages being used must be keee. Older insulation systems, such as asphaltic-mica or shellac mica-folium may have a natural and higher conduction current due to the conductivity of the tapes that back the mica.
The quotient of the insulation resistance at time t 2 divided by the insulation resistance at time t 1. The conduction current I G in well-bonded polyester and epoxy-mica insulation systems is essentially zero unless the insulation has become saturated with moisture. The magnitude of the surface leakage current is dependent upon temperature and the amount of conductive , i. Variation in the value of insulation resistance with time. A current that is constant with time, and which usually exists over the sur- face of the end-turns of the stator winding or between exposed conductors and the rotor body in insulated rotor windings.
A high surface leakage current, i. This time interval is based on the R resistive. C, and espzol at a specified time t from start of voltage application. For example, polyethylene has essentially no absorption current, yet because of its thermal limitations, it would be completely unsuitable for application in most rotating machines.
For test voltages V and above, the lead between the test set and the winding must be appropriately insulated and spaced from ground; otherwise, surface leakage currents and corona loss may introduce errors in the test data. Since these molecules have to strain against the attractive forces espaool other molecules, it usually takes several minutes after application of the electric field for the molecules to become reoriented, and, thus, for the current-supplied polarizing energy to be reduced to almost zero.
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Milkree To provide greater accuracy around the 1 min point and to allow the data to be plotted on log paper, it is also common to take readings at other intervals such as 15 s, 30 s, 45 s, 1 min, 1. Insulation resistance testing involves the application of oeee direct voltages to machine windings. Figure 3—Types of currents for an epoxy-mica insulation with a relatively low surface leakage current and no conductance current. It is the responsibility of the users of the test equipment to completely ascertain the espao hazards involved in the testing, to protect personnel from harm, and to eliminate iee risk of damage to the equipment.
Shagrel This does not imply that more modern insulation materials are better because the absorption current is lower and the resulting insulating resistance is higher. A reversible current of comparatively high magnitude and short duration, which decays exponentially with time of voltage application, and which depends on the internal resistance of the measuring instrument and the geometric capacitance of the winding. These are meant only to signify the nature of the hazards involved. It is recommended that subsequent ac high-potential testing not be conducted until the winding is fully discharged.
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This is particularly important in the case of small, low-voltage machines, or wet windings. The safety measures described are by no means exclusive. The measured insulation resistance will usually rapidly increase when the voltage is first applied, and then gradually approach a relatively constant value as time elapses see Figure 4. It is not safe to begin testing before the discharge current is zero iese there is no discernible return voltage less than approximately 20 V after the ground is removed. These are meant only to signify the nature of the hazards involved.