Why Can't Withstand Voltage Tests Be Repeated?

When a customer visited a factory, they raised the question of whether motor products can undergo repeated withstand voltage tests. This is a question that has been asked by many motor end-users as well.

The withstand voltage tests are the inspection of motor insulation performance, conducted during both the winding manufacturing process and the complete unit testing. The pass/fail criterion is whether breakdown occurs under specified conditions.

To ensure the motor's insulation performance meets requirements, in addition to selecting suitable electromagnetic wires and insulating materials, reliable process assurance is also essential - including protection during processing, appropriate tooling, effective varnish impregnation equipment, and suitable process parameters.

Taking high-voltage motor windings as an example, most motor manufacturers perform inter-turn and withstand voltage tests on each individual coil, carry out withstand voltage tests on the wound core before impregnation, and conduct withstand voltage tests on the complete machine during routine inspection testing. This leads us back to the customer's concern about repeated voltage testing.

Objectively speaking, this test is an irreversible, destructive test. Whether applied to a complete winding or an individual coil, repeated testing is not recommended unless it is necessary to identify a problem. When repeated testing must be conducted under special circumstances, the test voltage should be reduced in accordance with relevant standards to minimize damage to the insulation as much as possible.

The withstand voltage tester is an instrument used to measure dielectric strength. It can intuitively, accurately, quickly, and reliably test various electrical safety performance indicators of the object under test, including voltage withstand capability, breakdown voltage, and leakage current. Through the withstand voltage tester, problems can be identified and the compliance of insulation performance can be determined.

Evaluating the insulation's ability to withstand operating voltage or overvoltage.

Inspecting the quality of electrical equipment insulation during manufacturing or maintenance.

Eliminating insulation damage caused by raw materials, processing, or transportation, thereby reducing the product's early failure rate.

Verifying the compliance of electrical clearance and creepage distance of the insulation.

The best method for determining the test voltage is to set it according to the specifications required for the test. In general, the test voltage is set at twice the rated voltage plus 1000V. For example, if a product has a rated voltage of 380V, the test voltage would be: 2 × 380 + 1000 = 1760V. Of course, the test voltage may vary depending on the insulation class, and different product types may have different voltage requirements.

Withstand voltage testers on production lines are used very frequently. In particular, test leads and test fixtures are constantly being moved and handled, which can easily cause internal conductor breakage or open circuits - conditions that are generally difficult to detect. If any point in the circuit has an open connection, the high voltage output from the withstand voltage tester will not actually be applied to the device under test. All of these factors can cause the set high voltage to not be genuinely applied during the withstand voltage tests. Under such circumstances, the current flowing through the device under test is nearly zero. Since it does not exceed the upper limit setting of the tester, the instrument will indicate a pass result, deeming the insulation as acceptable. However, the test data under such conditions is not genuine. If the device under test happens to have insulation performance defects at that moment, a serious misjudgment will occur.