How to test an induction motor?

Surge and hipot testing can be potentially destructive. But, so can a Megger test on a motor which is in very poor condition. When testing a motor, tests are performed starting with the low voltage tests and moving on to the higher voltage tests. When a motor fails any test, the more rigorous tests are not performed, so no further damage is done. If a motor fails a resistance test or a Megger test, you do not perform the hipot and surge tests.

Surge and hipot testing are performed in motor shops and in motor manufacturing plants because they are the only tests which guarantee that a motor insulation system is satisfactory for enduring real world stresses at operating voltage such as voltage transients. The same is true in field testing. These tests detect insulation issues which already exist. They do not damage insulation which is in good condition. Many insulation issues (90 - 95% in my experience) such as moisture and surface contamination are detectable with Megger and low voltage tests. But, if we perform only Megger and low voltage tests, we will continue to have 5 to 10% field failures. Medium and high voltage motors are especially likely to fail if only the low voltage tests are performed. And these are often the most critical applications. I have seen many instance where motors failed very soon after a shutdown when only Megger and low voltage tests were performed. We had an instance here in Colorado very recently at a power plant where PdMA equipment was used to test a critical 4160V motor during a shutdown. The motor insulation failed very soon after restarting. In my opinion, PdMA is doing a disservice to industry by claiming that their off-line tests provide adequate testing of insulation. PdMA should add surge and hipot capability to your products if you want to have adequate insulation test capability.

It is true that surge and hipot are destructive tests in that they cause a bit more damage to an already failing insulation system. That is why these tests are performed only during a shutdown when you can remedy the problem. So when faced with the question of performing the hipot and surge tests and having higher confidence in the condition of a motor or not performing these tests and having greater potential for unexpected downtime, I recommend performing the surge and hipot tests. Isn't that why we call it predictive maintenance?