Older versions of core loss testers used higher values (the upper limit was set at 10 W/lb. or even higher) for “marginal” and “good” watts-loss per pound. Today, we recognize that the core loss should not exceed five watts/pound, and there should not be hot spots more than 15 degrees C hotter than ambient. Both of those considerations are critical. A half-inch diameter (12mm) hot spot will probably not raise the watts loss, but the localized hot spot can lead to premature winding failure. Both the core loss and hot spot temperature MUST be considered when evaluating a core prior to rewind. Eddy-current losses in a core vary as the square of the lamination thickness – or diameter of the hot spot. That means a ¼" (6mm) diameter hot spot generates about 80 times as much heat as a single lamination.

Figure 1 is a winding failure in a large synchronous alternator. Figure 2 is the same alternator with a failure in the same location. Although a core hot spot was identified, it was decided not to fix the hot spot due to an emergency required repair. The second failure occurred within an hour of operation. It is telling just how much the top sticks along two teeth have discolored due to heating from a core that was still “bad.”

Figure 1: This winding failure shorted laminations in the core.

Figure 2: This failure resulted from hot spots due to shorted laminations, which resulted from the first winding failure. This winding failed in under an hour of operation.

Categories: Technical topics, AC motors, Stators/windings, Laminations, Testing, Motor testing

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