Episode 12 of “Fun with Drives” focuses on testing electric motors and what to pay special attention to during a high-voltage AC test.
When performing an insulation resistance test, the first thing you need to define—in addition to the test voltage—is the leakage current. This current determines whether a motor is “alive” or “dead.” If these leakage currents occur, the systems “beep.” That means: motor “dead.”
Is that simple?
Not quite…
Depending on the motor size, varying levels of reactive currents flow during the measurement, which can significantly affect the results.
If I apply the same test specifications (simplified: “must withstand 2.2 kV”) to a 200W motor as I do to a 7kW motor, then it’s quite possible that the 7kW motor will “fail” even though it’s actually in good condition (i.e., a classic “false negative” result). The key here is to define the leakage currents appropriately.
Leakage currents of 3–5 mA are typical for smaller motors, while significantly higher values are common for larger motors. It’s always important that the leakage current remains constant and does not…
There are other test criteria for motors as well, such as partial discharge levels or behavior when a pulse voltage is applied, etc.
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