I had this 3/4HP centrifugal pump running off of a float switch. From some reason I'd go through pump motors regularly. Like, every three months. Another production line with the same setup has never had the pump burn out. When I autopsied the motors they appeared to have been single-phased to death. I replaced contactors several times, once or twice the contacts were welded together. When the motor died it would blow the fuses in the disconnect instantly upon startup. Never did it blow the overloads, despite the fact that I had turned the overloads down to half the FLA value. I tested the switch and it was working properly (on and off, pretty straight-forward).
Anyway, I thought water might have been a contributing factor because it's a wet area, so I added a 'doghouse' over the pump, and eventually bought a washdown-duty pump, but that wasn't it. I finally figured it out. I'm sure many of you out there already know what the problem was, but for the rest of us, here is what was going on:
The float switch might have been functioning properly from an electrical perspective, but it wasn't working right mechanically. It was rubbing internally and sometimes it would kick off 'early' and at these times it would be kicking off after only two seconds or so, meaning that the contactor was trying to disconnect under full inrush current - a load almost three times higher than it was rated to handle. It was arcing internally.

Don't you just _love_ intermittent problems?

Right now I've bypassed the switch and set the pump to run all the time. It seems happy with that (it's a self-priming centrifugal, and they're tough to kill). I'm considering adding a relay that would make sure the pump stays on for 10 seconds minimum when it gets triggered... either that or I'll run a signal wire from the main control panel for the line over to the starter and have it run continuously but only while the line is running. That's actually my favourite option, right now.

Anyway, that was my adventure with a float switch, where a $40 switch cost me 4 motors, three contactors, an overload unit, and a bag-full of fuses. I haven't added it up... I'm thinking maybe I'm not going to. Ignorance _can_ be bliss!

Mike