GB/T 20833.4-2021Rotating electrical machines - The winding insulation - Part 4: Measurement of insulation resistance and polarization index (English PDF)

3.1 Rated voltage

For three-phase AC motors, it refers to the rated line voltage between the two phases; for single-phase motors, it refers to the rated line voltage; for DC motors and excitation motors, Refers to the rated DC voltage.

3.2 Insulation resistance

The electrical insulation capability of the winding against DC current. At a certain time (t) after the voltage is applied, the applied DC voltage is divided by the The quotient of the total DC current of the insulation.

Note 1. The voltage application time is usually 1min (Ri1) and 10min (Ri10), but other values can also be used. The unit of time t is stipulated. the unit of 1 to 10 is Minutes, the unit of 15 and above is seconds.

Note 2. Insulation resistance is sometimes abbreviated as IR.

3.3 Polarization index

The quotient of the insulation resistance measured at two different times. Usually, after applying DC voltage, t2 and t1 are 10min and 1min respectively This is an indicator of the insulation state.

Note: For other times, see Appendix A, A.2.

3.4 Polarization current

6.1 Factors affecting insulation resistance test

The polarization index should reach the minimum requirement (see 8.3 for the recommended value) before the withstand voltage test or put into operation.

6.1.1 Overview

Insulation resistance test results depend on environmental factors, mainly winding temperature and air humidity. The influence of winding temperature on insulation resistance can be Evaluate with empirical data or test measurements, which can be used to modify the insulation resistance test results at different temperatures (see 6.1.2).

The relative humidity of the air affects the surface leakage current. This effect is usually difficult to evaluate because the relative humidity of the air mainly depends on the air. Slightly disregarded.

6.2 Testing equipment

For the direct measurement method, the preferred device is an ohmmeter. When the reading of Ri1 is less than 5000M ohm, the digital high resistance meter should meet the following condition.

---Display. three digits;

---Accuracy. Reading accuracy ±5%, range accuracy ±5.

If there is no insulation resistance meter, the insulation resistance can be obtained by measuring voltage and current (indirect method). This method may use regulated DC Power supply, voltmeter, microammeter. In fact, the voltage output by the DC voltage source will fluctuate, causing the current to also change. ic(t)=C0dU0/ dt. Since the capacitance C0 of most high-voltage motors is very large, the DC power supply should have minimal stability and noise to ignore this effect.

According to the voltage and current readings, the insulation resistance is calculated according to formula (4). Where. Rit---Insulation resistance value at time t, in megaohms (M ohm);

U ---The measured voltage of the DC voltage source (Voltmeter reading), in volts (V);

It --- the measured current (ammeter reading) at time t, in microamperes (µA).

When measuring large insulation resistance values, in order to avoid the influence of the leakage current and capacitance of the test cable, it is advisable to use a protective instrument.

The whole set of equipment should raise the voltage to the test value within 5s.

6.3 Test product and test circuit

6.3.1 Overview

According to the test target and its design, different test circuits are used. In order to check the minimum recommended value of insulation resistance, the entire winding should be checked Test; In order to check the insulation problems between the windings of each phase and the windings, if the windings of each phase can be easily disconnected, it should be carried out phase by phase measuring. In order to achieve the test purpose, the same connection method should be used.

If possible, all external devices such as cables, switches, capacitors and current transformers should be disconnected from the windings. To record with Winding connection equipment.

For the insulation resistance test of the direct water-cooled winding, it is advisable to completely drain the water to make the internal circuit completely dry. Maybe some water-cooled windings are manufactured The supplier has provided a method for measuring insulation resistance without draining the cooling water. Generally, if the water is not excluded, the conductivity of the water should be less than the production plant Recommended value of home. In this case, the conductivity of water will greatly affect the insulation resistance value, so it can be estimated that PI=1, Ri10= 1M ohm.

In any case, untested winding components should be grounded to avoid adverse effects, such as compensation current or induced AC in the test loop Current.

6.3.2 Three-phase stator winding

6.3.2.1 Connection mode of integral winding to ground test

The windings of all phases are connected together, as shown in Figure 2.

Insulation test is an important part of motor calibration. It should be carried out before and after the test, and check with a high voltage resistance of about 100M ohm. Operation of equipment. This is especially important for battery-powered equipment.

6.7.2 Special procedures

In order to obtain more information about the insulation status, other test procedures can also be used, see Appendix A for details.

7.1 Overview

Insulation resistance and polarization index can be used in the following aspects, but not limited to.

---Assess whether a motor is suitable for withstand voltage test or put into operation, it can be based on the minimum estimation of insulation resistance and polarization index. Recommended value (see 7.2);

---As long as the test conditions of the controllable variables involved are the same, the historical data of the insulation resistance and polarization index of a given motor can be used An effective method for judging the change trend of its insulation state after several years (see 7.3);

---As long as the test conditions of the controllable variables involved are the same, compare the windings of each phase by testing the insulation resistance and polarization index or the same Insulation conditions of different motors designed.

Based on the above points, the factors that affect the test results should be fully considered when testing (see 7.4).

7.2 Feasibility of test and operation

Insulation resistance and polarization index can be used to evaluate the feasibility of the motor withstand voltage test and put into operation. Insulation resistance and poles of the motor The chemical index should be greater than the minimum recommended value (see Table 3 and Table 4). If the test value is less than the minimum recommended value, it is not advisable to carry out a withstand voltage test on the motor Or put into operation, unless there are design reasons or there are records of such lower values.

Insulation resistance can also be used to determine whether the winding is damaged in the withstand voltage test. If the insulation resistance value after the withstand voltage test is much smaller than the test Before the test, it indicates that the insulation may be damaged during the withstand voltage test, even if the test voltage does not drop rapidly due to the presence of high fault current.

If the test value is lower than the minimum recommended value due to contamination or excessive moisture, after eliminating the possibility of insulation defects, it can be cleaned and Dry measures improve the measurement results. The insulation resistance value can be used to monitor the effectiveness of the drying process.

If the insulation resistance Ri1 value at 40°C is greater than 5000M ohm, then the polarization index value may not be specified and may not be considered.

For shellac and asphalt-based insulation structures, an excessively high polarization index (such as greater than 8) indicates that the insulation may have been thermally aged and there is a risk of failure It will be very high. If the physical inspection (such as knocking on the insulation) finds that the insulation is dry and brittle, it is best not to clean the winding or conduct a voltage test.

If the motor is put back into operation, it may malfunction at any time.

7.3 Trend analysis of insulation conditions

If you can get the historical data of the motor insulation resistance (Ri1), comparing the current test results with the previous ones will help you understand the insulation status.

However, comparative tests with similar conditions are very important, namely winding temperature, voltage range, test duration and relative humidity (see 6.1). At the same time, use the same test equipment as much as possible. To compare the test results at different winding temperatures, the results should be corrected to The same reference temperature (see 6.1.2).

If Ri1 or PI drops sharply compared to the previous reading, it means that the surface is contaminated or damp. When the temperature increases (above 60°C), PI The value is low. In order to check the actual insulation condition, the second measurement should be performed above the dew point and below 40°C.

7.4 Comparison between different motors or different phases

If the Ri1 of the entire winding is less than 5000M ohm, the significant difference in Ri1 or PI between the same motors may indicate surface contamination and moisture Or the insulation is damaged. In this case, it is advisable to find out the cause of the discrepancy.

The characteristic polarization current IP in can cause significant changes in the insulation resistance values at 1 min and 10 min.

If there is a significant change in the winding temperature between the insulation resistance at 1 min and 10 min, the value should be corrected Calculate the polarization index after reaching the reference temperature of 40°C.

--- The winding is connected to a motor with an end anti-corona layer that is in contact with the current (see Appendix B, B.6 and Appendix C, C.3);

9.1 Winding in operation

The test report should provide at least test data and test conclusions. If it is used for trend analysis and comparison of insulation conditions, Result to be explained. The test report should include the following.

a) Identification information.

---Factory Name;

---Motor number.

b) Date of measurement. c) Environmental conditions.

---Ambient temperature;

---Winding temperature;

---Calculate relative temperature and ambient temperature of absolute humidity and dew point. d) Test equipment.

---Name, model;

---Manufacturer;

---serial number;

---The effective date of measurement.

e) Test circuit, instrument settings and test conditions.

---Test voltage and polarity;

---Test duration (if required).

f) Test the insulation characteristics under the measurement conditions and correct to the reference temperature.

--Insulation resistance at -1min;

--Insulation resistance at -10min (if applicable);

---Polarization index (if applicable);

---Temperature correction (if applicable).

g) Draw measurement curve (if applicable).

---The relationship between the measured current and time, the logarithmic coordinate is better;

---The relationship between the measured insulation resistance and time, the logarithmic coordinate is better.

h) Name of test personnel.

i) Record specific information.

j) Notes (such as differences from the standard test procedure, phenomena observed during the test, etc.).

It is advisable to save all test results in the form of original data for future reference.

According to the document type, independent testing and certification, or as part of the status assessment report, the report content can include corresponding additional information, such as Working mode, motor life, etc.

9.2 New winding

For brand new windings, the insulation resistance test can be used as a supplement to the withstand voltage test, and the insulation resistance is carried out for 1 min before and after the withstand voltage test. test.

Therefore, most of the information required by a complete test report will become part of the withstand voltage test protocol. Defined in 9.1 The required information may be sufficient.

A.1 Overview

In the insulation resistance test and polarization index calculation described in this section, the winding is understood through DC voltage and DC current in actual operation.

State is not the only way. In addition to measuring insulation resistance and polarization index, the new insulation tester provides other methods.

This appendix summarizes the current better methods for evaluating the winding condition of rotating electrical machines, including changing the test time, DC test voltage, and environmental conditions. Pieces etc.

A.2 Dielectric Absorption Ratio (DAR)

The polarization index (PI) is generally defined as the ratio of the insulation resistance (Ri10) at 10 min to the insulation resistance (Ri1) at 1 min. In the old For insulating structures (such as pitch-mica), it takes 10 minutes or even longer for the polarization current to decay to close to zero.

However, in low-voltage motors with scattered windings and field windings, the polarization current may decay to close to zero within 1 min. So some tests Candidates will calculate a derivative polarization index that is different from the traditional polarization index PI. The derived polarization index includes but is not limited to the calculation of formula (A.1) method. DAR=Ri1/Ri30 (A.1) Where. DAR---dielectric absorption ratio; Ri1 ---Insulation resistance reading when voltage is applied for 60s; Ri30 ---Insulation resistance reading when voltage is applied for 30s.

Figure A.1 illustrates the insulation resistance value of the random-wound stator winding measured within 1 min after the voltage is applied, and Ri1 is 4083M ohm.

Ri30 is 3745M ohm, and the obtained DAR is 1.09. Some insulation resistance testers have automatic test settings to perform DAR measurements. versus The difference between the traditional method is that the time for applying a DC voltage is shorter, so the time for the winding to ground is also shorter (see 6.6). Due to the use of With the insulation structure of mica, the polarization current will drop to 0 within 1 min. Therefore, the insulation resistance of a shorter time is selected for calculation, which can be greatly shortened.

Test time, and almost no loss of information on the degree of contamination or moisture of the winding. When testing Ri30, the test object should rise to the test within 1s Voltage. The limitations of using these derived polarization indices are.

--- There is no standard for the time interval for recording Ri values, and different institutions use different ratios (see 3.3);

---Different from the traditional polarization index, there is no consistent judgment standard for the derived polarization index.