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Sherlock Ohms

If It's Not the Undervoltage, It’s the Overvoltage

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Rob Spiegel
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Good catch
Rob Spiegel   4/3/2013 9:50:37 AM
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Good catch on this Andrew. Sherlocks never know what the ultimate solution is going to be -- or even what the problem is in the first place.

Charles Murray
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Re: Good catch
Charles Murray   4/3/2013 6:36:08 PM
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I agree, Rob. It was a very good catch. It also reminds us that undervoltage issues can be just as insidious as overvoltage.

Rob Spiegel
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Re: Good catch
Rob Spiegel   4/3/2013 9:26:47 PM
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Yes Chuck. It's good to see these engineers ferret these problems out. 

DJP
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Re: Good catch
DJP   4/4/2013 9:48:41 AM
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This sort of reminds me of troubleshooting repeat failing of a Fusetron on one side of my AC compressor. The new  15 A Fusetron's would fail regularly while the old one on the other side of the 230 did not. I measured the voltage on the circuit: over 250 VAC. Ontario Hydro advises to conserve energy, then RAMS it down the line. Replacing the 15A Fusetron's with 20A fixed the problem. Although we are in a village (pop 700+) we must be getting rural high voltage that requires a bit of adaptation.

William K.
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Re: Good catch
William K.   4/4/2013 6:37:42 PM
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How in the world were you able to make a 20 amp Fusetron function in a 15amp socket? or are Fusetrons different than Fuse-Stats, which none of the current ratings are interchangable? And I also wonder about why the line voltage would be that high.

Cabe Atwell
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Re: Good catch
Cabe Atwell   4/5/2013 2:46:18 AM
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When it comes to machines, "beefing up" the power would have been my first response to any power issue. I have my own machineshop and fabricate countless items, I have little time to make it so the gear scrimps by with the weakest options. Depending on the complexity of the machines, different loads will have a huge influence on the power demands.

I have the same ethos with motors and circuit protection. I've fused together too many motor coils to not protect it all...

C

DJP
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Silver
Re: Good catch
DJP   4/5/2013 10:50:48 AM
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William, the Fusetron is a variant on the regular screw in fuses that were in common use before the circuit breakers. The regular fuse is one time use, the Fusetron is a time delay variant that should tolerate minor over-current events a few times; the older version were more robust than available now. There are also push-button reset C/B's that can be used, and all are physically the same size, so one has to use intelligence to choose the correct fuse for the circuit. Mine were in a separate switch box for the AC unit. I suspected the high inrush motor currents for the compressor & fan due to the very high voltage exceeded the capabilities of the fuses, so I increased the fuse size by one step.

As to why the line voltage is so high I am unable to fathom. I do know that many years a go a linear power supply for thermoelectric coolers (120 VAC to 12 VDC) often went up in smoke & flames early in the morning in rural motels. It was noted by some people washing up first thing in the morning. It was suspected the local power authority switched from an overnight level to a higher morning level to cope with increasing demands made as people awoke. These were approved units, and yet failed in a spectacular way that was not anticipated. That design was dropped.

William K.
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Platinum
Re: Good catch
William K.   4/5/2013 11:13:49 AM
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DJP, thanks for the clarification. I was thinking of "FuseStats", which are more expensive than fuses and which are not interchangeable. Each size is a different length so that putting a hgher rated uit in a lower current position will not work. 

The methods used to compensate for voltage drop in distribution systems can be quite different in different areas, and it seems that some methods are not adequate for the real worlod, as your example shows.

Rob Spiegel
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Re: Good catch
Rob Spiegel   4/5/2013 10:08:24 AM
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Hey DJP,

Your story might make a good Sherlock Ohmes. Please send it in to rob.spioegel@ubm.com

Think in terms of a 300-word minimum.

William K.
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Platinum
Re: Good catch
William K.   4/5/2013 10:25:49 AM
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Rob,I can relate a true horror story about a multi-speed starter for a very large (150HP) motor and quite weak 380 volt three-phase power lines. The supply voltage drooped so badley that the starter "chattered" during the first on-site start, and as a result large portions of the started were destroyed. The local supplier for that brand claimed that the starter was obsolete and that parts were not available, nor was a replacement. So our startup team was trapped there for several weeks under a "somewhat hostile" condition while a replacement starter was obtained, with modifications that would allow it to function correctly with such poor incoming power supply. 

Based on that experience of a predecesor designer, I have avoided using multispeed startersever since. An electronic soft-starter or variable speed drive is a far safer choice. It is valuable to learn from the mistakes of others, and much less damaging to one's career.

TJ McDermott
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Intolerant Drive
TJ McDermott   4/4/2013 12:27:31 AM
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+/- 5% on AC voltage input is NOT a very friendly drive.

The specifications for a popular drive in the USA shows that at 480V AC input, the drive has an overvoltage trip of 576V, and an undervoltage trip of 300V. (+/-20%)

A modern drive in this scenario could have been purchased in the USA for running at 480VAC and still work perfectly well in China at 380V, or even the sag at 350V.

+/- 5% is just too small, especially for a system destined for export.

+/- 10% is generally considered the standard for voltage input.

 

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