Enameled copper wire power frequency withstand voltage test method
In the electrical performance evaluation system of enameled copper wire, the Power Frequency Withstand Voltage Test is the most direct and critical factory inspection and type test item for judging the insulation integrity of the paint film.This test verifies that the paint film does not break down under electric field stress by applying a 50 Hz or 60 Hz power frequency AC voltage to the enameled wire sample to the specified value and maintaining the specified time, so as to ensure the long-term and reliable operation of the winding under rated working conditions and transient overvoltage.IEC 60851-5: 2008 + AMD1: 2011, GB/T 4074.5-2024, NEMA MW 1000-2018, JIS C 3202 and other international and major industrial country standards have made clear provisions on test methods, equipment parameters, sample preparation, and conformity determination.In this paper, we systematically sort out the enameled copper wire power frequency withstand voltage test methods from the ten dimensions of test purpose, standard system, sample preparation, equipment requirements, voltage grade, test process, qualified judgment, influencing factors, unqualified mode, and practical cases.
I. Test Purpose and Engineering Significance
The enameled copper wire bears the dual functions of current-carrying and insulation in windings such as motors, transformers, inductors, relays, and solenoid valves.Insulation failure will directly lead to short circuit between turns, short circuit between phases, ground breakdown, which will lead to equipment burning, shutdown and even safety accidents.The core purpose of power frequency withstand voltage test is to eliminate unqualified products with defects such as pinhole, particle, insufficient thickness, and eccentricity in the manufacturing process by controllable and repeatable overstress means, so as to prevent them from flowing into the downstream installation process.
Power frequency withstand voltage test is complemented by DC withstand voltage test, high frequency pulse test (Surge Test), and local discharge test (PD Test).The power frequency withstand voltage is biased against the insulation strength of the paint film body, the high frequency pulse test is biased against the weak spot of inter-turn insulation, and the local discharge test is biased against the starting point of Electrical Treeing in long-term operation.The four are combined to form a complete enameled wire insulation quality evaluation system.
II. Test standard system
2.1 IEC 60851-5 International Standard
IEC 60851-5: 2008 + AMD1: 2011 \”Winding wires – Test methods – Part 5: Electrical properties\” is the parent standard for winding line electrical performance testing, specifying five core electrical tests: Test 5 (resistance), Test 13 (breakdown voltage), Test 14 (insulation continuity), Test 19 (dielectric loss factor), and Test 23 (pinhole).Among them, the breakdown voltage of test 13 is the main method of power frequency withstand voltage test, which specifies the boost speed, holding time, electrode form, test temperature, environmental conditions, qualification judgment, etc.
IEC 60851-5 Key Equipment Parameters: Test transformer rated power ≥ 500 VA, AC sine waveform without distortion, frequency 50 Hz or 60 Hz; fault detection circuit operating at 5 mA or greater current; oven with forced air circulation for high temperature test; impedance meter accuracy ± 1% for medium loss factor measurement.
2.2 GB/T 4074.5 China GB
GB/T 4074.5-2024 \”Winding Line Test Method Part 5: Electrical Performance\” is equivalent to IEC 60851-5: 2008 + AMD1: 2011, which is the mandatory technical basis for the factory inspection and type test of enameled wires in China.GB/T 4074.5-2008 has been superseded by GB/T 4074.5-2024, which further refines the test conditions for 0.018-0.050 mm fine lines (Grade 3) and adds special test requirements for Class 220 enameled wires.
2.3 NEMA MW 1000-2018 US Standard
NEMA MW 1000-2018 Magnet
III. Sample Preparation
3.1 Mandrel Method
The round bar method is mainly used for enameled round wire testing with a diameter of 0.50-5.00 mm.The sample length is about 400 mm and is tightly wound 10-20 turns on a polished metal round bar (1-3 times the diameter of the enameled wire) with a specified tension (usually 1-5 N), leaving no gap between turns.The round rod is used as the high-voltage electrode, and the two ends of the enameled wire are shorted as the grounding electrode.This method simulates the bending stress state of the enameled wire at the end of the winding, and is particularly suitable for detecting the insulation weakness of the paint film under bending conditions.
3.2 Twist Pair Method
The twisting method is the most common test method for enameled circular wires of 0.018-1.60 mm.Take two 400 mm enameled wire samples first, twist them 8-12 turns (standard 8 turns) at a specified speed (30-60 rpm) on a special twister, apply pressure evenly to the direction, and the twisting length is about 125 mm.After twisting, the sample length is 400 mm, and both ends are short-circuited.The twisting method causes two enameled wires to form a metal-to-metal contact at 8-12 contact points, and the electric field is concentrated around the contact points, the most severe test of the insulation strength of the paint film.
3.3 Ball Electrode Method
The ball electrode method is suitable for flat wires, ribbon wires, thin film insulated wires, and samples that require a single click through evaluation.Two polished metal ball electrodes with a diameter of 10-25 mm are respectively pressed on the surface of the paint film, with a spacing of 0.5-2.0 mm, and a specified voltage is applied.The ball electrode method is mainly used in scientific research and failure analysis.
3.4 Pinhole Test Sample (Test 23 Specimen)
The pinhole test (Test 23) uses a continuous length enameled wire (usually 30 m) through a mercury electrode tank or a copper sulfate solution tank, and the test line speed is 30-50 m/min.Number of pinholes divided by 30
IV. Equipment Requirements
4.1 Test transformer
The test transformer is the core equipment for withstand voltage test, the main parameters are:
- * * Rated power ≥ 500 VA * * (mandatory for IEC 60851-5/GB/T 4074.5)
- * * Output voltage * *: 0-15 kV continuously adjustable (covers all Grade 1-3 voltage levels)
- * * Waveform distortion ≤ 5% * *, AC sine waveform without distortion is guaranteed
- * * Frequency * *: 50 Hz or 60 Hz, matched to grid
- * * Short circuit impedance ≤ 10% * * to avoid voltage drop during boosting
- * * Continuous working time ≥ 1 min * * (meets voltage holding time requirements)
4.2 Fault detection circuit
The fault detection circuit operates at a current of 5 mA or more.When the sample is broken down, the fault current exceeds the threshold, and the detection circuit cuts off the high-voltage output and alarms within 0.1 seconds.The fault detection sensitivity needs to be calibrated regularly, and the standard resistance simulates the breakdown current verification.
4.3 High Voltage Electrodes and Ground Electrodes
- * * Round rod method * *: polished metal round rod (stainless steel or chrome-plated copper), diameter error ± 0.01 mm
- * * Twisting method * *: two chucks of the twister, the chucks are spaced 200-300 mm apart
- * * Ball electrode method * *: 10-25 mm diameter polished metal ball
- * * High voltage leads * *: silicone rubber or polyethylene insulation, withstand voltage ≥ 1.5 times the test voltage
- * * Grounding * *: individual grounding resistance ≤ 4 Ω to avoid interference
4.4 Environmental control and security
Environmental conditions: temperature 23 ± 2 ℃ (room temperature test)/specified temperature (high temperature test); relative humidity 50 ± 5% RH; air pressure 86-106 kPa; oven temperature uniformity
V. Test voltage level
5.1 Grade 1/Grade 2/Grade 3 system
The enameled circular line is divided into three grades according to the film thickness: Grade 1 (thin film) minimum film thickness 0.018-0.058 mm; Grade 2 (thick film) minimum film thickness 0.030-0.087 mm; Grade 3 (extra thick film) minimum film thickness 0.048-0.130 mm.High-temperature test samples (180 °C, 200 °C, 220 °C) need to be measured at elevated temperatures, and the results may be 30-50% lower than the room temperature value.
5.2 Breakdown voltage level (based on IEC 60851-5 Grade 1 room temperature)
5.3 Boost speed and hold time
IEC 60851-5 specified step-up speed: Normal test 50-200 V/s continuous step-up; Rapid test 500 V/s (type test only); Step test 100 V per stage maintained for 1 min.10 seconds to the breakdown voltage is the more commonly used compromise speed.
Holding time: Keep 1 min after the type test reaches the specified voltage; keep the factory inspection for 10-30 s; if there is no breakdown within the holding time, it is qualified.
VI. Test Process
6.1 Preparation and Wiring
Preparation phase: check the status of the test equipment, connect the power supply, ground wire, high voltage lead; calibrate the test transformer (simulate breakdown current with standard resistance); calibrate the voltmeter (accuracy ± 2%); prepare the sample according to § 3 and adjust the state; wear insulating protective equipment.
Wiring phase: fix the sample to the electrode (round rod/twist/ball electrode method); connect the high voltage lead to the high voltage electrode; connect the ground lead to the ground electrode; close the safety fence door and confirm the interlock.
6.2 Boosting and Bucking
Step-up stage: Start the step-up, the voltage rises from 0 to 50-200 V/s; observe the current meter (normal leakage current \< 1 mA); maintain 1 min (type test) or 10-30 s (factory inspection) after reaching the specified voltage; if breakdown occurs within the holding time, the fault detection circuit acts, automatically shuts down and records the breakdown voltage.
Buck and Discharge: After the holding time is over, start the buck to 0; use the grounding discharge rod to contact the high-voltage electrode for 30 seconds; open the fence door, take out the sample and record the result.The complete test time of one specimen is about 2-3 minutes (calculated at 1500 V breakdown voltage, 100 V/s boost speed).
VII. Conformity Determination
7.1 Single point judgment
Qualified: The sample is kept at the specified voltage for 1 min without breakdown, and the leakage current is \< 5 mA.Unqualified: Breakdown occurs during holding time, or leakage current ≥ 5 mA.Suspicious: The sample showed obvious corona discharge but did not break down during the pressurization process, so it needs to be retested.
7.2 Batch determination
The representative samples (GB/T 4074.5 specification sampling number ≥ 5) need to be sampled for batch conformity determination, and all of them are deemed to be qualified for batch conformity; if one of them is unqualified, it is deemed to be unqualified for batch conformity, and whether it is necessary to expand the test is analyzed.Retest rules: abnormal discharge occurred during the boosting process is retested 1 time; boundary breakdown (voltage ≥ 95% breakdown) is retested 2 times; all retests are deemed as batch pass.
7.3 Data recording
The test data shall include: sample number, conductor diameter, film grade, thermal grade; test temperature, relative humidity; test voltage, hold time, boost speed; breakdown voltage (actual value)/pass judgment; test personnel, test date, equipment number.
VIII. Influencing factors
8.1 Temperature effects
The insulation strength of the paint film is significantly affected by the temperature.When the temperature rises from 23 °C to 155 °C, the breakdown voltage decreases by about 30-40%; when it rises to 200 °C, it decreases by about 50-60%.The high temperature test should be carried out at the specified temperature, and the results cannot be directly compared with the results at room temperature.Irregular sample pretreatment (\< 24 h conditioning or high temperature preheating \< 1 h) will result in large dispersion of results.
8.2 Humidity Effects
When the relative humidity rises from 30% to 80%, the breakdown voltage drops by about 10-20%.Under high humidity conditions, the surface of the paint film adsorbs moisture to form a surface conductive layer and reduce the corona voltage.The specification requires 50 ± 5% RH to eliminate the effects of humidity fluctuations.
8.3 Impact of Boost Speed
The breakdown voltage measured at the fast step-up (500 V/s) is 10-20% higher than the slow step-up (50 V/s). Due to the shortened observation time at the high voltage, the weak click-through is too late to develop.The specification requires a uniform boost speed to ensure comparability.
8.4 Effects of electrode contact and aging
Poor electrode contact and large contact resistance will lead to uneven voltage distribution and low results.In the twisting method, the number of twisting coils, the twisting pressure, and the spacing between turns all affect the results.Inappropriate storage conditions (high temperature, high humidity, ultraviolet exposure) after the enameled wire leaves the factory will degrade the performance of the paint film.The specification requires a storage temperature of ≤ 30 ℃, a relative humidity of ≤ 70%, protection from light, and a shelf life of ≤ 12 months.
IX. Analysis of Common Nonconforming Patterns and Causes
9.1 Insufficient pinhole and film thickness
Pinholes are tiny holes in the surface of the paint film that penetrate or are close to penetrating the entire thickness of the paint film.The pinhole may be caused by three types of factors: raw materials (microbubbles in the paint solution, impurities), processes (too fast painting speed, uneven baking temperature), and the environment (poor cleanliness).The pinhole test (Test 23) specifically detects such defects.
Insufficient film thickness will result in insufficient breakdown voltage.Common causes: too fast paint speed, low viscosity of lacquer, too high baking temperature (solvent volatilization too fast).It is necessary to use a paint film thickness gauge (caliper) to measure and verify the diameter of the conductor and the total diameter of the enameled wire respectively.
9.2 Paint film eccentricity and impurities
The paint film is unevenly distributed around the conductor, and the thickness of one side is lower than the specification.Common causes: unstable paint flow rate, mold wear, wire tension fluctuations.The eccentricity will result in the local electric field of the winding being concentrated and prone to breakdown.
The paint film is mixed with conductive impurities (metal chips, carbon particles) or insulating impurities (fluff, fibers).Metal impurities directly form conductive pathways, and insulating impurities form bubbles.It needs to be improved on cleanliness management (paint filtration, environmental class 100 cleanliness).
9.3 Poor bubbles and adhesion
Air bubbles inside or on the paint film, caused by excessive viscosity of the paint, excessive baking temperature, and insufficient volatilization of the solvent.Bubbles ionize under the action of an electric field, resulting in breakdown.
The paint film has poor adhesion to the copper matrix and peels off when bent or heated.Heat Shock Test + Jerk Tensile Test
X. Practical Cases
10.1 Case 1: Enameled wire for NEV drive motor Power frequency withstand voltage test
An NEV drive motor factory uses 1.0-1.6 mm enameled circular wire, Grade 2.According to IEC 60851-5 test 13, the twisting method is adopted, 8 turns of twisting, the specified breakdown voltage is ≥ 4.6 kV, the boost speed is 100 V/s, and the holding time is 1 min.
1000 pieces of batch test results: 995 pieces of breakdown voltage ≥ 5.0 kV (qualified), 3 pieces of breakdown voltage 3.8-4.5 kV (unqualified), 2 pieces of breakdown voltage 4.5-4.6 kV (boundary).Reason analysis for nonconformity: The paint film is eccentric, and the thickness of one side is lower than 30% of the specification.Take improvement measures: adjust the paint flow rate, repair the mold, and retest all 1000 pieces as qualified.
10.2 Case 2: Enameled Flat Line Power Frequency Withstand Voltage Test for Transformers
A transformer factory uses 1.00 × 4.00 mm enamelled flat wire, Grade 2.According to IEC 60851-5, the ball electrode method is adopted, the ball diameter is 10 mm, the spacing is 1.0 mm, and the specified breakdown voltage is ≥ 6.0 kV.
500-piece batch test results: 498 pieces of breakdown voltage ≥ 6.5 kV (qualified), 2 pieces of breakdown voltage 4.0-5.5 kV (unqualified).Reason for nonconformity analysis: The paint film contains metal impurities, and metal particles of 0.05 mm can be seen under the microscope.Improvements: Paint filtration accuracy increased from 25 μm to 5 μm, and environmental cleanliness increased from 1000 to 100.
10.3 Case 3: Power frequency withstand voltage test of fine enameled wire for inductors
An inductor factory uses 0.040 mm enameled circular wire, Grade 1.Twisting method according to IEC 60851-5, 8 turns twisting, specified breakdown voltage ≥ 0.45 kV.
300-piece batch test results: 298 pieces of breakdown voltage ≥ 0.50 kV (qualified), 2 pieces of breakdown voltage 0.30-0.40
Conclusion
Enameled copper wire power frequency withstand voltage test is the core link of winding wire insulation quality evaluation.The four standard systems of IEC 60851-5/GB/T 4074.5/NEMA MW 1000/JIS C 3202 are consistent in testing principles, and there are differences in sample preparation, equipment parameters, voltage levels, and conformity judgments.The accuracy and reliability of the test method depend on the standard implementation of the five elements of the test transformer capacity (≥ 500 VA), fault detection current (≥ 5 mA), boost speed (50-200 V/s), holding time (1 min type/10-30 s factory), and environmental conditions (23 ± 2 ℃, 50 ± 5% RH).
Enameled wire manufacturers should establish a complete power frequency withstand voltage test system, covering the four aspects of raw material inspection, process inspection, factory inspection, and type test, covering the three sample preparation methods of round rod method, twist method, and ball electrode method, covering the three core electrical properties of pinhole test, breakdown voltage, and insulation continuity.Test data should be traceable, analyzable, and improveable.
The LP factory provides enameled copper wires that comply with the four standard systems of IEC 60851-5/GB/T 4074.5/NEMA MW 1000-2018/JIS C 3202. All products pass the power frequency withstand voltage test and pinhole test 100% before leaving the factory, and the test data can be traced.With 30 years of export experience, we have provided enameled wires to customers in 50 + countries with complete quality records.
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