The power frequency withstand voltage test of the motor is the core inspection item for testing the insulation strength of the motor. By applying a power frequency alternating current voltage higher than the rated voltage between the motor winding and the housing, it verifies the tolerance of the insulation system under overvoltage conditions. This test falls under the category of destructive tests and is a crucial step in the factory inspection of the motor, the acceptance after installation and commissioning, and the re-testing after major repairs.
Test voltage and time parameters
1. Determination of test voltage
The test voltage is determined based on the rated voltage level of the motor.
For low-voltage motors with a rated voltage of ≤ 1 kV, the calculation formula is 2Un + 1000 V, with a lower limit of 1500 V.
For high-voltage motors with a rated voltage of > 1 kV, the calculation formula is 2Un + 1000 V.
For example: A low-voltage motor with a rated voltage of 380 V, the test voltage is 2×380 + 1000 = 1760 V; a high-voltage motor with a rated voltage of 6 kV, the test voltage is 13 kV; for a 10 kV motor, it is 21 kV.
For micro-motors with a rated power of less than 1 kW, the test voltage can be appropriately reduced, generally using 2Un + 500 V.
2. Test duration
The standard stipulates that the test duration is 1 minute, with the starting point of the timing being the moment when the voltage reaches the specified value.
For 5 kW and below small-power motors produced in batches, to improve the detection efficiency, an alternative test scheme can be adopted:
1-second alternative scheme: The test voltage is raised to 120% of the standard value
Approximately 5-second alternative scheme: It can be equivalent to replacing the 1-minute standard test
3. Test frequency and waveform
The test voltage uses 50 Hz (or 60 Hz) sinusoidal alternating current voltage, with a waveform distortion rate not exceeding 5%.
Preconditions for the test
1. Insulation resistance pre-check
The power frequency withstand voltage test needs to be carried out after all non-destructive tests such as insulation resistance and absorption ratio are qualified.
For motors with a rated voltage of ≤ 1000 V: The insulation resistance at normal temperature should be no less than 0.5 MΩ
For motors with a rated voltage of ≥ 1000 V: Converted to the operating temperature, the stator winding should be no less than 1 MΩ/kV, and the rotor winding should be no less than 0.5 MΩ/kV
Absorption ratio: Motors with a rated voltage of 1000 V and above should not be lower than 1.2
The power frequency withstand voltage test has insulation destructive nature and will amplify the existing defects in the insulation. If the insulation resistance is significantly不合格， the withstand voltage test shall not be carried out.
2. Environmental conditions
The test environment temperature should be maintained at 23 ± 5 °C， with a relative humidity of ≤ 75% RH. If the tested motor has the possibility of condensation， it is necessary to complete 72-hour constant temperature dehumidification pre-treatment.
Standardized operation procedures
1. Safety protection setup
The withstand voltage test is a high-voltage operation， and complete safety measures need to be implemented：
Set up barriers in the test area and post “High Voltage Hazard” warning signs
There should be no less than 2 personnel for the test operation， and a dedicated supervisor should be set up
The personnel should wear complete insulating protective equipment
Before the test， confirm that the grounding connection is reliable， and the grounding is不合格， the test shall not be initiated
2. Wiring requirements
All non-tested windings， motor housings， cores， embedded temperature detection elements， etc.， that do not participate in the test are reliably grounded. When the neutral point of the three-phase windings cannot be separated， the three-phase windings are applied with the test voltage simultaneously. The high-voltage leads should remain suspended and shall not touch other objects.
3. Voltage rise operation
Start rising the voltage from the 1/3 to 1/2 position of the test voltage. The rising rate should not exceed 5% of the rated value per second. Uniform rising or segmented rising can be used； the time for the voltage to rise from half value to full value should be no less than 10 seconds.
4. Timing and monitoring
Start timing for 1 minute after the voltage reaches the specified value. During the test, the following key observations should be made: the state of the voltmeter pointer, the changes in the current meter values, whether the motor emits smoke or makes abnormal sounds.
5. Voltage Reduction and Discharge
After the test is completed, reduce the voltage uniformly to below 1/3 of the full value and then cut off the power supply. After fully discharging the tested winding, the wiring can be removed.
Test Equipment Configuration
The test device includes a test transformer, a voltage regulator, a control and protection device, and a measurement device. The technical requirements for each part are as follows:
High-voltage generator: Output alternating voltage of 0-10 kV (or higher), with an accuracy of ±1.5% FS
Test transformer: Capacity matched to the capacitance load of the tested motor, output sinusoidal waveform
Insulation resistance tester: Range 0.01 MΩ – 10 TΩ, optional test voltage 500 V/1000 V/2500 V
Leakage current meter: Resolution 1 μA, equipped with overcurrent automatic cut-off protection
Safety protection system: Includes an emergency stop button, access control, and grounding alarm device
When the winding capacitance of the tested motor is large, the capacity of the test transformer needs to be calculated accordingly to ensure the stability of the test voltage.
Result Determination and Abnormal Handling
1. Qualified Determination
Under the specified voltage and duration conditions, there is no breakdown, flashover, smoke emission, or abnormal sounds, and the increment of leakage current does not exceed the initial value by 15%. The test is judged as qualified.
2. Abnormal Handling
If any faults occur during the test process, such as significant fluctuations in the voltmeter, rapid increase in leakage current, smoke emission from insulation, or abnormal sounds, immediately reduce the voltage and cut off the power supply. After the winding is fully discharged, conduct fault troubleshooting.
3. Commissioning Conditions
For motors with qualified withstand voltage tests, the insulation resistance converted to the operating temperature should be no less than 1 MΩ/kV. Such motors can be directly put into operation without drying; internal operations are not allowed before commissioning.
Post time: Sep-09-2026