I got a call from a good customer of mine today regarding a slurry pump issue. This is a 16 hp 230V single phase submersible pump. It has a control panel with capacitors, circuit breaker, contactor, and a relay. The pump wouldn't start, and tripped the breaker upon trying to start.

I found that the coil on the start capacitor contactor was bad. So, I robbed the contactor from his spare control panel. When I engage the switch to start, the capacitors snap. Why???? When I left it running for about 3 seconds, one start capacitor started boiling.

So, I decided to install spare panel. Now the pump just hums and sometimes snaps at the run capacitors. Why??? It acts like a bad start circuit but all capacitors check out o.k. It is driving me crazy that I can't seem to figure out why this won't work.

Couple of thoughts.

Overheating start cap sounds like excessive current. Excessive current could be due to fried winding, shorted wiring, or jammed pump.

Have you ohmed out or meggered the motor leads to check for shorts to ground?

Can the pump be pulled to check for mechanical binding?

Yes, the pump was pulled and checked. It was free with no mechanical binding. I did ohm all the leads with no short to ground and phase to phase resistance. The strange thing is that it ran once until the caps started boiling. That is when I decided to switch to the spare identical panel.

Electrolytic caps do not like AC current. When the dielectric fails, they become resistors and the electrolyte usually boils and vents. I suspect that the start cap is failing because the starting circuit is remaining energized. That points to a centrifigal switch not opening up or an insulation failure between the start cap and motor frame or earth.

Requires megging...

Off hand it sure smells like the motor windings are shorted/ fused.

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Upon replacement have a 3-phase motor installed with a soft-starting VFD sized to the load -- and then some.

VFDs take line power and rectify it into DC... which is then used as a 'tank' of EM potential to synthesize AC -- with all kinds of solid state physics tricks.

This approach is THE current best practice...

Your conductors can be compromised every step down the hole. Blown capacitors in the starting winding circuit can roast it before the OCPD kicks in.

WRT cap failure: my experience has been that they fail in a fused and shorted state -- for what it's worth.

BTW, I have two 10 hp single phase motors... and they ALWAYS fail at the capacitors first. Fortunately, they are ONLY operated while attended.

You're not so lucky.

The owner took the pump in for repair. As it turns out, the pump had a seal blown and got oil into the windings. $3500 later, the pump is back in operation.