From experience electrical workers use a "Megger" as a verification test to ensure a circuit is safe (normally above 2 MegaOhm) before energizing.
The test is carried out by attaching the test lead to the motor phase terminal and the common to the motor frame (ground),while the test is essential for safety the "Megger" can be used as a predictive tool.
Firstly, tests must align with common failure modes of a component, in this case the three phase motor windings in the region of the crown where multiple phases cross and the voltage differential is the maximum, or in the stator slot where different phases exist (usually each third slot).
The former is quite common as mechanical movement can fatique the insulation causing a phase to phase failure.
The test requires the removal of the terminal links and supply leads and conducting a phase to phase insulation resistance test. To ensure you have each winding isolated conduct a continuity test across each terminal winding, test only those which have open continuity else your reading will be 0 mega ohm.
u1 to u2, v1 to v2, w1 to w2 should have all should have continuity with the links removed.
A standard megger test will not identify a phase to phase fault, but will find a stator slot fault which is less common and likely to be late stage failure propagation.
Stator slot failure are common with rotor rub from bearing or housing problems. The crown test (Phase to phase) can propagate like a bearing fault therefore trending the result is recommended.
Typically a new motor without pre existing fault will test off scale or above100 megaOhm. Any fall or trending lower is a concern, and a rule thumb cutoff of 40 megaOhm for replacement.
The crown fault area is prone to failure due to high current starts (DOL or soft starter) inducing mechanical stress to the winding. Windings in the slot area are mechanically supported and less prone. Crowns are also subject to dV/dT steps from variable speed drives and reflective voltages from cables where the insulation is the thinnest. VSD rated motor have thicker wire insulation to combat this problem.
The less common stator slot fault can be trended but different test is recommend to be discussed in a further blog.
Remember: Meggering with the links in only tests for safety as the total three windings see the DC charge. DC motors can be tested the same but ensure stator & field windings are isolated and tested separately. If you get a lower than expected result visually inspect the terminal block, cracks, corrosion, debris, or burnt material that will cause a low result with the winding ok.
It is recommended to conduct this test on acceptance, usually the links are out on delivery.
Mark Gurney Gerant 3Phi Reliability Sarl