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William K.
User Rank
Platinum
The powerless pump
William K.   12/17/2011 8:19:48 AM
Actually, the pump being driven by flow was my initial guess. Of course, I have a bit of experience with controls for pumping systems.

I had an interesting diagnostics challenge a while back that involved a large pump driven by a large motor. It was in a hydraulic power unit that we had loaned a customer while we were repairing their failed power unit.

The complaint was simple: the pump will not start. Because the customer was an hours drive away, I first asked a few questions, then asked them to have their electricial check the 3 100 amp fuses in the 480 volt power feed circuit. He reported that one 100 amp fuse had failed, I suggested that he replace it and try starting the system again, He replaced the fuse and the replacement fuse failed as soon as they tried to start the motor. So now I had to head over to the customers site and find the problem. My approach was first to do a complete inspection, since that sort of short circuit shaould provide some evidence of itself. But all of the visible wiring looked good. Next, I did a resistance check at the motor terminals of the starter-overload assembly, which showed an open circuit in one phase. That was a good clue. I opened the connection box on the side of the motor and found that, because it had been assembled with the splice pressing against the cover of the box, the one connection had slowly cold-flow, penetrated the tape wrapping and contacted the box cover, short circuiting the phase to ground and evaporating part of the connection. The repair was simple, which was to cut off the damaged end, install a split-bolt splice instead of the lug and bolt splice, and tape the new connection. Then I was careful to position the wires in the splice box so that they did not press against the cover, and replaced the cover. After installing a new 100 Amp fuse, the systm started and ran correctly.

Note that I did switch off the service to the system before I started working on it.

Rob Spiegel
User Rank
Blogger
Re: Weird
Rob Spiegel   12/16/2011 5:24:38 PM
NO RATINGS

I agree, Chuck. That's what makes these Sherlock so much fun. And it's always the super simple answer that wins -- like the magnet that isn’t magnetized or the resistor what was marked incorrectly.


Charles Murray
User Rank
Blogger
Weird
Charles Murray   12/16/2011 5:21:27 PM
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My first reaction would have been that the breaker had gone haywire. Pump running backwards? It woudn't have crossed my mind.

Rob Spiegel
User Rank
Blogger
Re: Looks can be deceiving
Rob Spiegel   12/16/2011 5:14:46 PM
NO RATINGS
Yes, good observation, Tim. Jennifer is right about how these Sherlock postings show some excellent deductive logic. It's not named "Sherlock" for nothing. What I find most impressive is that many of the solutions arise when the Sherlock involved refuses to make any assumptions.

Noswad
User Rank
Gold
Re: Looks can be deceiving
Noswad   12/16/2011 4:16:57 PM
NO RATINGS
Elementary my dear Watson.

Jennifer Campbell
User Rank
Gold
Re: Looks can be deceiving
Jennifer Campbell   12/16/2011 2:54:47 PM
NO RATINGS
That's good advice Tim, and is generally the case on many of our Sherlock Ohms stories.

Tim
User Rank
Platinum
Looks can be deceiving
Tim   12/16/2011 6:29:20 AM
NO RATINGS
Good job double checking something that your eyes and ears were telling you.  Sometimes it is good to take a step back and ask if what you are seeing is the whole situation.

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