Very Low Frequency Testing (VLF Testing) is an effective method for evaluating the aging of motor insulation. It simulates voltage stress in motor operation by applying an ultra-low frequency (usually 0.1Hz or less) AC voltage to detect defects or aging of the insulation system. The following is a detailed interpretation of how ultra-low frequency testing evaluates motor insulation aging.
1. Test principle
The ultra-low frequency test uses low frequency AC voltage to apply electric field stress to the motor windings to simulate operating conditions.
The state of the insulation system can be judged by monitoring the current, voltage waveform, or partial discharge signal during the test.
2. Evaluate indicators
Current change: Insulation aging will lead to an increase in leakage current, and the degree of insulation aging can be judged by monitoring the change of current in the test.
Partial discharge: Insulation aging is often accompanied by partial discharge phenomenon; VLF test can detect the strength and frequency of the discharge signal.
Voltage waveform: Insulation defects can cause voltage waveform distortion, and the insulation state can be assessed by analyzing the waveform change.
3. Test procedure
Preparation: Disconnect the motor power supply and ensure safe grounding, select the appropriate test voltage and frequency.
Apply voltage: Gradually increase the ultra-low frequency voltage to a preset value (usually 1.5 to 2 times the rated voltage) and hold it for a certain time (usually 15 to 30 minutes).
Monitoring and analysis: Record the current, voltage waveform or partial discharge signal during the test, and analyze the insulation status.
4. Evaluate the test results
Normal insulation: stable current, no distortion of voltage waveform, no obvious partial discharge signal.
Slight aging: The current is slightly increased, the voltage waveform is slightly distorted, and the partial discharge signal is weak.
Severe aging: the current increases significantly, the voltage waveform is seriously distorted, and the partial discharge signal is strong.
Through the VLF test, the aging degree of motor insulation can be comprehensively evaluated, providing a scientific basis for the maintenance and replacement of the motor, thereby extending the life of the equipment and reducing the operation risk.