Insulation testing

Insulation testing is a key testing procedure to ensure the electrical safety and longevity of electric motors. Its purpose is the early detection of faults in wire insulation, which can lead to short circuits, failures, or even dangerous situations.

Testing principle and procedure

Insulation testing is performed by applying a high voltage to the insulation under test. Testing can be carried out using either direct current (DC) or alternating current (AC). Throughout the test, the contact of the test object is continuously monitored using the four-wire method to eliminate measurement errors due to contact issues.

Typical test objects and insulation paths

• Operational insulation: Testing the insulation between the lamination stack and the winding wire – standard for electric motors.
• Protective insulation: For double-insulated armatures, such as those used in double-insulated handheld power tools, additional testing is performed between the shaft and the winding wire as well as between the shaft and the lamination stack.
• Coil insulation: For stators with unconnected coils, the insulation between individual coils can be tested.

Monitoring and measured variables

During the entire measurement, both the test voltage and the leakage current are continuously monitored. If the preset limit for the leakage current is exceeded, the system automatically aborts the measurement to prevent damage to the device under test.

For AC testing, in addition to the RMS current, the peak currents are also recorded to reliably detect individual, short-term flashovers.

Evaluation and result presentation

The evaluation of the insulation test can be performed in two ways:

• Leakage current measurement: Direct monitoring of the current flowing through the insulation.
• Insulation resistance: Calculation of the resistance from the test voltage and leakage current.

Advantages of modern insulation testing

• High safety: Early detection of insulation faults minimizes failure risks and increases operational safety.
• Flexible testing methods: Choice between DC and AC testing, adaptable to different DUT types and standards.
• Automatic monitoring: Continuous monitoring of contact, test voltage, and leakage current for reliable and reproducible results.

Conclusion

High-voltage insulation testing is indispensable for quality assurance and safety of electrical machines. It enables early detection of insulation damage and makes a crucial contribution to the reliability and service life of the products.