Insulation Testing and Judgment of High-voltage Motors in Steel Industry
Steel is the food of industry and the most widely used metal material by humans. It has a wide range of applications in various fields of social production and life. Steel plants are also the industry with the largest use of various motors, and they involve many supporting factories, such as mining plants, coking coal plants, chemical plants, power plants, sintering, blast furnaces, steelmaking, power generation, etc., all of which require the use of high-voltage motors. Effective and scientific management of motor operation, especially insulation testing and maintenance before motor operation, is crucial.
Qualified insulation resistance is an important condition for determining whether a motor can be started, especially for high-voltage motors. If the motor is forcibly started without meeting the specified insulation resistance standard, it will be damaged and the resulting losses will be enormous. Corresponding provisions have been made in various electrical regulations or experimental standards for determining the insulation resistance of motors..
When various equipment of steel enterprises enter the trial and operation stage. As the main power equipment of the device, the operation of high-voltage motors is crucial. In order to meet the process requirements, it is necessary to regularly test the insulation of the motor in daily work to ensure that the motor can be started at any time to meet the process operation needs. In the production and operation of enterprises, 3kV, 6kV, and 10kV are commonly used high-voltage motor models. However, high-voltage motors repeatedly experience insulation failure before power transmission. Due to the lack of unified judgment standards, different test results often occur, which makes it difficult for the operating personnel to accurately and timely judge whether power should be transmitted, resulting in passive work. Sikcon's high-voltage motor insulation automatic detection has been used in steel plants and some industry standards and requirements. Here are some relevant standards and information summarized for everyone.
1. Insulation resistance regulations for AC motor windings
1.1. According to the "Electrical Equipment Handover Test Standard" for electrical equipment installation engineering, when measuring insulation resistance, the voltage level of the megohmmeter should be used. Unless otherwise specified in this standard, the following provisions should be followed:
Electrical equipment or circuits below 100V shall use a 250V 50MΩ or higher megohmmeter;
Electrical equipment or circuits below 500V to 100V shall use a 500V 100M Ω or higher megohmmeter;
Electrical equipment or circuits below 3kV to 500V shall use a 1kV 2000M Ω or higher megohmmeter; Electrical equipment or circuits below 10000V to 3kV shall use a 2.5kV 10000MΩ or higher megohmmeter;
Electrical equipment or circuits of 10kV and above shall be equipped with a 2.5kV or 5kV 10000M Ω or above megohmmeter.
When used for residual current measurement, the short-circuit current of the megohmmeter should not be less than 2mA.
1.2 Regulations for Maintenance and Inspection of Electric Motors: Determination of Winding Insulation Resistance
For low-voltage motors, a 500-1000V megohmmeter is generally used. Its insulation resistance should not be lower than 0.5 MΩ; For motors with replaced windings, the resistance should not be less than 5MΩ.
For high-voltage motors, a 2500V megohmmeter is generally used. The insulation resistance should not be less than 1MΩ per kilovolt.
To determine whether the insulation of the winding is damp, the insulation resistance can be measured using megaohms. When the absorption ratio R60/R15 is ≥ 1.3, it indicates that the motor is not damp.
1.3 According to the electrical testing regulations: winding insulation resistance
For rated voltages below 1kV, the insulation resistance at room temperature should not be less than 0.5 megohms;
For rated voltages above 1kV, before AC withstand voltage, the insulation resistance of the stator winding near the operating temperature should not be lower than 1M Ω/KV; Before being put into operation, the insulation resistance (including cables) at room temperature should not be lower than 1MΩ/kV.
2. Insulation testing methods for electric motors
By measuring insulation resistance, insulation breakdown, severe thermal aging, local or overall moisture, dirt, and insulation oil cracking can be detected. Measuring insulation resistance can only apply a test voltage lower than the critical voltage of the insulator. Therefore, different rated voltage and range megohmmeters should be selected according to the equipment of different voltage levels.
2.1 Measurement of Insulation Resistance and Absorption Ratio
2.1.1 Disconnect all external connection wires of the device and discharge them fully.
2.1.2 Clean the dirt on the surface of the test sample.
2.1.3 Wiring: Connect the ground terminal E of the megohmmeter to the ground terminal of the test sample, connect the live terminal L of the megohmmeter to the measuring terminal of the test sample, and connect the shielded terminal G of the megohmmeter to the shielded area of the test sample.
Before testing, the live wire should not be connected to the test sample temporarily, and the megohmmeter should be checked. Drive the megohmmeter to the rated speed (120r/min), and the reading on the megohmmeter should be "∞".
2.1.5 Connect the live wire to the measuring end of the test sample, drive the megohmmeter to the rated speed, wait for the pointer to stabilize, and read the insulation resistance value.
When measuring the absorption ratio, first drive the megohm to the rated speed. When the pointer points to "∞", immediately connect the test sample and start recording the time. Read the insulation resistance values at 15 seconds and 60 seconds respectively, and calculate the absorption ratio R60/R15.
After reading the value, disconnect the live wire first, and then stop the megohmmeter to avoid damaging the megohmmeter due to capacitor reverse charging.
When adding equipotential shielding wiring to the test object, attention should be paid to the shielding ring on the test object, which should be close to the live wire end and far away from the grounding part to reduce surface leakage of the shielding to the ground and avoid overload of the megohmmeter.
2.1.9 Record the temperature of the test sample body.
Measure and manually calculate the insulation resistance and absorption ratio of high-voltage motors using a megohmmeter as mentioned above. But if the factory uses SKIM100H high-voltage insulation monitoring instrument, it only needs to directly read the insulation resistance value and absorption ratio displayed on the automatic detection instrument to determine.
2.2 Effects of Various Factors on Insulation Resistance
2.2.1 Effects of Humidity
The absorption of moisture or the formation of a water film on the surface of insulation will significantly reduce the insulation resistance, and the surface of dirt will be more severe.
2.2.2 The Influence of Temperature
Temperature changes have a significant impact on insulation resistance, and it is necessary to record the temperature of the insulation material during testing in order to convert it to the same temperature for comparison.
2.2.3 Effects of Discharge Time
When the discharge is insufficient, the false phenomenon of increased insulation resistance and decreased absorption ratio can be caused by the influence of residual charge.
2.3 Analysis and judgment
The insulation resistance value of equipment is not only affected by the aging, moisture, and local injuries of the motor, but also has a significant relationship with changes in external environmental factors. Therefore, a comprehensive judgment of the resistance value is necessary to draw the correct conclusion. When making judgments, follow the following principles:
2.3.1 The measured insulation resistance value should be equal to or greater than the standard value specified in the regulations.
2.3.2 For ease of comparison, the insulation resistance value should be converted to the resistance value at the same temperature and compared with the historical data of the equipment. The comparison result should not show a significant decrease or significant difference, otherwise attention should be paid. If necessary, inspection and confirmation should be carried out to identify the cause of poor insulation.
2.3.3 For equipment with relatively large capacitance (such as transformers, capacitors, cables, etc.), the insulation condition is mainly judged based on the absorption ratio. If there is a significant decrease, it indicates that the insulation is damp or the oil quality is severely deteriorated.
3. Insulation judgment criteria for high-voltage motors
3.1 Maintenance and repair procedures for electric motors: For high-voltage electric motors, a 2500V megohmmeter is generally used. The insulation resistance should not be less than 1M Ω per kilovolt (without specified temperature conditions)
3.2 Electrical Test Regulations: For rated voltages above 1000V, before AC withstand voltage, the insulation resistance of the stator winding near the operating temperature should not be less than 1 M Ω/KV; Before being put into operation, the insulation resistance (including cables) at room temperature should not be lower than 1M Ω/KV.
3.3 Other regulations: It is also stipulated that the insulation of motors above 1kV should be compared under standard conditions of 75℃ and not less than 1MΩ/KV.
The above standard regulations are very clear. Taking the insulation testing application of 10kV high-voltage motors as an example, there are several key words here - standard temperature insulation resistance, room temperature insulation resistance, operating temperature insulation resistance, and thermal state.
Standard temperature insulation resistance - refers to the insulation value of the test sample at 75 ℃
Room temperature insulation resistance - refers to the insulation value of the test sample under normal temperature conditions (generally 10℃ -40℃)
Operating temperature insulation resistance - refers to the insulation resistance of equipment under normal operating temperature conditions
Hot state—— Generally understood as the insulation value of the test sample under 55℃ conditions
3.4 In order to ensure the safe operation of high-voltage motors, we must determine the insulation condition of the 10KV motor winding by adding the minimum insulation resistance value under standard temperature conditions (75℃) and the absorption ratio.
Table 1 Reference Table for Insulation Resistance Conversion and Judgment of 10kV High Voltage Motors
| No. |
Ambient Temperature（℃） |
75℃Insulation resistance standard |
Insulation resistance standard |
| 1 |
5 |
≥10 MΩ |
≥1280 MΩ |
| 2 |
10 |
≥10 MΩ |
≥905MΩ |
| 3 |
15 |
≥10 MΩ |
≥640MΩ |
| 4 |
20 |
≥10 MΩ |
≥453MΩ |
| 5 |
25 |
≥10 MΩ |
≥320MΩ |
| 6 |
30 |
≥10 MΩ |
≥226MΩ |
| 7 |
35 |
≥10 MΩ |
≥160MΩ |
| 8 |
40 |
≥10 MΩ |
≥113MΩ |
From the calculation results, it can be found that the variation of the minimum allowable value of insulation resistance under different temperature conditions is regular, that is, for every 5℃ difference in temperature, the minimum allowable value of insulation resistance varies by 1.5 times, and for every 10℃ difference in temperature, the minimum allowable value of insulation resistance varies by 2 times. The higher the temperature, the lower the resistance value. By mastering this law, we can quickly calculate the minimum allowable value of insulation resistance at a certain temperature after remembering a typical value. For example, if Rt is 10MΩ at t=75℃, then Rt at t=65℃ is twice that of 10MΩ, which is 20MΩ.
4. Insulation Testing Regulations for 10kV High-voltage Motors
Through comprehensive analysis, it is found that there are many factors that affect the insulation testing of high-voltage motors. In order to unify standards and based on years of practical experience in operation and management, the following regulations are made for the insulation testing and judgment of 10KV high-voltage motors:
4.1 Conduct insulation testing on 10kV high-voltage motors using a megohmmeter. Before testing, the cable must be discharged to ground for 1 minute using a dedicated discharge wire. During testing, one end of the red probe is connected to the FLUKE1550B shaking meter "+", and the other end is connected to the motor cable side; One end of the black probe is connected to the shaking meter "-", and the other end is connected to the grounding copper bar, without a shielded wire.
4.2 When conducting insulation tests on 10kV high-voltage motors, the outdoor ambient temperature must be recorded. The outdoor ambient temperature should be determined by referring to the bearing temperature of the motor. If there is no temperature display, the bearing temperature of the motor in the area that has not been turned on can be used as a reference.
5 Conclusion
After establishing unified regulations for insulation testing and judgment of high-voltage motors, the on-duty personnel can strictly follow the requirements for testing and judgment, which not only avoids the occurrence of insulation resistance failure caused by improper testing methods, but also enables accurate comprehensive judgment of test results. The insulation testing of high-voltage motors through a megohmmeter is labor-intensive and tedious. If Sikcon’s high-voltage insulation monitoring device is used to achieve real-time monitoring of insulation resistance values, absorption ratios, and other data, it will greatly reduce the workload. If interested companies require online or offline high-voltage intelligent insulation monitoring devices, they can contact Sikcon company for customization.
