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43-2000 Resistencia de isolação
The effect is more pronounced if the surface is also contaminated, or if cracks in the insulation are present. Note that the effects of moisture contamination on a healthy espaoll should not preclude obtaining acceptable readings.
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.
The measurement of insulation resistance constitutes a direct-voltage test and must be restricted to a value appropriate to the voltage rating of the winding and the basic insulation condition.
Negative polarity is preferred ieeee accommodate the phenomenon of electroendosmosis. In general, the insulation resistance varies proportionately with the insulation thickness and inversely with the conductor surface area.
Clause 1 and Clause A current resulting from molecular polarizing and electron drift, which decays with time of voltage ifee at a espao 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.
C, and taken at a specified time t from start of voltage application. Typically for older wet windings, the insulation resistance for reverse polarity, where the ground lead is connected to the winding and the negative voltage lead to ground, is much higher than for normal polarity. 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.
Since the absorption current is a property of the insulation material and the winding temperature, a specific absorption current is neither good nor bad. This is particularly important in the case of small, low-voltage machines, or wet windings. 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.
If the windings are clean and dry, the total current I T. The surface leakage current I L is constant over time. 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. A second component of the absorption current is due to the gradual drift of electrons and ions 43-20000 most organic materials.
L 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. 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.
If the test voltage is too high, the applied test voltage may over stress the insulation, leading to insulation failure. This decay may take more than 30 min depending on the insulation type and machine size of the test specimen. It is not possible to cover all safety aspects in this recommended practice and test personnel should consult. If the insulation resistance or polarization index is reduced because of contamination, it can usually be restored to an acceptable value by cleaning and drying.
The magnitude of the surface leakage current is dependent upon temperature and the amount of conductive esapol, i. Guidelines for test voltages are presented in Table 1. The discharge current manifests itself in two components:.
An equivalent circuit for the various currents in an insulation resistance test is shown in Figure 1. This time interval is based on the R resistive. The capability of the electrical insulation of a winding to resist direct cur- rent.
What’s new in the IEEE 43 insulation resistance testing standard?
Usually, for clean and dry rotating machine insulation, the insulation resistance between about 30 s and a few minutes is primarily determined by the absorption current. After the applied direct voltage is removed, a suitable discharge circuit should be provided see Clause 4.
The total resultant current I T iees the sum of four differ- ent currents: 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. Modern types of filmcoated wire, as well as epoxy-mica expaol polyester-mica insulated stator windings, may approach a constant value of insulation resistance in 4 min or less.
There may also be. 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. For safety considerations, and to avoid measuring stray currents, the leads may be shielded.
Resistencia de isolação – Norma IEEE
A minimum discharge time, which is equal to four times the voltage application duration, is recommended. The safety measures described are by no means exclusive. These electrons and ions drift until they become trapped at the mica surfaces commonly found in rotating machine insulation systems. During the test period, all appropriate safety measures for the voltages being used must be taken.
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.
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. It may be plotted on a log-log graph as a straight line.
Figure 3—Types of currents for an epoxy-mica insulation with a relatively low surface leakage current and no conductance current. It is recommended that subsequent ac high-potential testing not be conducted until the winding is fully discharged. 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. Before any testing is conducted, the winding insulation must be discharged. iere
What’s new in the IEEE 43 insulation resistance testing standard? | EASA
Use of personal protective equipment is recommended, as is the use of hot sticksinsulated ladders, etc. Dust or salts on insulation surfaces, which are ordinarily nonconductive when dry, may become partially conductive iee exposed to moisture or oil, and, thus, can lower the insulation resistance.
The readings of a dry winding in good condition may continue to increase for hours with a continuously applied constant test voltage.