Your OP doesn't indicate that you tested for insulation breakdown.
It's a very good bet that you've got a short across one of your windings, perhaps two.
When connected delta ( a pretty good bet ) B fuse feeds two windings.
If those windings are shorted ( just so ) the motor will draw excessive amps across the B fuse -- yet still run. The key thing is such a short must be somewhat close to the taps. Then these two legs produce low back-EMF and draw too much juice.
Meggers test for insulation breakdown AND low resistance.
Go back and re-test both.
None of your comments provide enough information: what are the resistance values across the windings -- what is the breakdown voltage?
