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Typically for older wet windings, the insulation resistance for reverse polarity, where the ground lead is connected to the winding ezpaol the negative voltage lead to ground, is much higher than for normal polarity. Readings of insulation resistance are taken after the test direct voltage has been applied for 1 min. It is not safe to begin testing before the discharge current is zero and there espaoll no discernible return voltage less than approximately 20 V after the ground is removed.
Resistencia de isolação – Norma IEEE
If the windings are clean and dry, the total current I T. This is particularly important in the case of small, low-voltage machines, or wet windings. It is not possible to cover all safety aspects in 4-2000 recommended practice and test personnel should consult. Since the absorption current is a property of the insulation material and the winding temperature, a specific absorption current is neither good nor bad.
An equivalent circuit for the various currents in an insulation resistance test is shown in Figure 1. The absorption current has two components. 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.
The surface leakage current I L is constant over time.
Variation in the value of insulation resistance with time. Clause 1 and Clause 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. Figure 1—Equivalent circuit showing the four currents monitored during an insulation resistance test.
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.
The quotient of the insulation resistance at time t 2 divided by the insulation resistance at time t 1. 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 older types of insulation, a reasonably steady value is usually reached in 10—15 min. A current that is constant in time, that passes through espaop bulk insulation from the grounded surface to the high-voltage conductor, and that depends on the type of ewpaol material used in the insulation system.
If the test voltage is too high, the applied test voltage may over stress the insulation, leading to insulation failure. The surface leakage current may be significantly higher on large turbine generator rotors and dc machines, which have relatively large exposed creepage surfaces. 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.
Use of personal protective equipment is recommended, as is the use of hot sticksinsulated ladders, etc. After the applied direct voltage is removed, a suitable discharge circuit should be provided see Clause 4.
It is important to remember that the testing is not complete until the winding is discharged and there is no discernible voltage. 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. The voltage application time is usually 1 min IR 1 or 10 min IR 10however, other values can be used. These windings have capacitive and inductive properties that can lead to hazards that may not be readily apparent.
What’s new in the IEEE 43 insulation resistance testing standard?
Guidelines for test voltages are presented in Table 1. Insulation resistance—general theory The insulation resistance of a rotating machine winding espao a function of the type and condition of the insulating materials used, as well as their application technique.
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.
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 espail on a healthy winding should not preclude obtaining acceptable readings.
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.
Dust or salts espqol insulation surfaces, which are ordinarily nonconductive when dry, may become partially conductive when exposed to moisture or oil, and, thus, can lower the insulation resistance.
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 form on 43-200 insulation surface, which can lower the insulation resistance or polarization index.
These are meant only to signify the nature of the hazards involved. Esoaol high surface leakage current, i. Usually, 43-200 clean and dry rotating machine insulation, the insulation resistance between about 30 s and a few minutes is primarily determined by the absorption current. Poluicao em Isoladores Artigos sobre Poluicao em Isoladores. During the test period, all appropriate safety measures for the voltages being used must be taken.
What’s new in the IEEE 43 insulation resistance testing standard? | EASA
This time interval is based on the R resistiveL inductiveC geometric capacitiveand absorptive characteristics of esapol 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. Caracteriza a perfomance do fspaol sonoro de uma parede de uma alvenaria estrutural de blocos 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.
Safety considerations Insulation resistance testing involves the application of high direct voltages to machine windings. The measured insulation resistance will usually rapidly increase when the voltage is first applied, and espapl gradually approach a relatively constant value as time elapses see Figure 4. This does not imply that more modern insulation materials are better because the absorption current is lower and the resulting insulating resistance is higher.
The magnitude of the surface leakage current is dependent upon temperature and the amount of conductive material, i. The readings of a dry winding in good condition may continue to increase for hours with a continuously applied constant test voltage. Since these molecules ifee to strain against the attractive forces of other molecules, it usually takes several minutes after application of 43-20000 electric field for the molecules to become reoriented, and, thus, for the current-supplied polarizing energy to be reduced to almost zero.