Blown digital VMs on rpc....

Existential Angst · 2009-10-30T02:32:28+00:00

Awl -- So as some of you know, I have a fairly kick-ass rpc system, with a switchable bank of 3ph motors AND a switchable bank of caps, that I switch in for starting, then switch for running, with increments of 20 uF, if nec. It's all manual/relayed, but eminently "tune-able" for load.Toward the end of tuning, I have 3 digital VMs at the rpc bank, and 3 digitals on the Fadal pendant, for my voltage viewing pleasure -- mostly to JMO, cuz I got the 3rd leg like within 2% of the line, BUT I have occasionally forgotten to switch out the starting caps and switch in the running, which the DVMs would immediately revealIt's the starting caps which make commercial rpc such crap, giving that 270 V wild leg with 240 line.God forbid they allow you to switch between a starting set of caps and a running set.Inyway, the DVMs, being $4 cheapies, run through 9V batts like crazy, so I wired them up to a little AC/DC charger, and voila, nice digits.So I hooked them up to the Fadal relay (an external 3 pole relay I have in series with the knife switch), turned the VM to 750 V, had nice 000 on the display, turned on the rpc, then the relay, and voila.... BLANK lcds, never (apparently) to turn on again.Same 750 V position as I had been using these for over a year -- until the batts died one last time.WTF happenned? The relays were not operating any load, so no spike was possible (fadal knife switch was off), nothing really changed from the previous hookup except for this DC 9V charger (little bitty thing like for cell phones or whatever).And no, no poss. of having set the dial to Amps, or ohms, or anything like that....Oh yeah, ALL THREE DVMs went blank simultaneously.I'd go with analog (indeed had analog), which are OK for big discrepancies like 270 V wild legs, but a pita for real tuning or general observation.Scratching my head over here....

Oct 30, 2009 41 Replies

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Winston 16 years ago

(...)

This is getting silly. :)

--Winston

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Michael A. Terrell 16 years ago

Yes, you do. The ability to measure the current in all three legs would be useful when first setting up a RPC.

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