how do you meg out a motor. 75 hp triping out overloads and about every 6th or 7th time takes out 125 a. fuse. it is all ways b phase fuse
motor has been runing for 4 or 5 years so overloads and fuse are rated fine. motor turns hydrolic pump for demount mach. 480 v tap 9 lead went ground to L1 L2 L3 open any configuration of L1 L2 L3 to each other dead short
current is at 38 a when not under load will go up to 80.9 a when under heavy load it will run fine for 4 to 6 hours then trip over loads Greg the voltage is good at the motor i have not been there when it has triped so i dont know if the voltage drops when it trips but all the other machines are runing fine in the plant
Overloads can fail over time, but it doesn't explain the fuse. Motors don't always draw the same current on all phases, so B might be the high phase, or the B phase is failing to ground somewhere.
To test the starter, check the voltage from Line 1 to Terminal 1 and Line 2 to Terminal 2, etc. If you have a problem in the starter, one leg will read higher than the others. I would expect less than 1 volt, but comparing the readings on the three phases will give you an idea if something is out of balance. If you have excessive voltage drop on one leg, you can move your probes and narrow down the source of the problem. Then, try the disconnect.
More likely, though, something is changing on the load. Maybe a solenoid sticking?
You could try rolling the phases around one position and see if that changes what fuse blows. I would start with the input to the starter and if it moves, swap that back and roll the motor leads. Just be sure you keep the sequence the same.
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?