Motor Branch Circuit Design

[iceworm]

...There is probably more to the story on your pulp mill. High inertia - maybe too many starts in too short of time period was a bigger problem then a little overloading.

Yeah, I didn't explain that part. Figured everyone would guess. If the goof had not changed the overloads, the motor would have continued as it had been four trips/week.

But by setting the overloads down, the motor tripped under the normal non-tripping overload. The consecuetive starts heat up the motor much faster than just running slightly overloaded. And of course, the crew was not willing to shut down production to cool the motor. Again the point: You can't fix the bad design by changing the overloads. The bad design? The motor was too small. I went back with a 100hp 1.15. Probably cost twice what the original motor did. But it did not trip with the overloads set to 140%fla.

For the one you describe: I suspect the operators can easily burn the motors up. All they have to do is push the equipment to the limits, keep resetting the tripped overloads, whoops smoke came out. Changing the overloads won't fix that.

However you can design around this - but I'll bet a cup of yuppie coffee neither the owners nor operators will tolerate the solution. Install an AB E3+, enable the motor thermal model. When the overload trips and there is not sufficient headroom in the thermal model for a start - it will not allow reset until the thermal model cools sufficiently.

the worm