2013
04.12

When machining the scope mount spacer rings in a previous post, I had been planning to finish them when Max had stopped by one evening.  Simple enough – fire up the RPC and turn on the lathe.  Only thing was, the red ’start’ button on the RPC was stuck and I couldn’t press it in.  What the heck?  Being in an impatient, Neanderthal-esque mood (I was hoping Max could take home all the parts for the project that night), I powered down the system and removed the button so that I could hammer it into a variable on/off mechanism rather than being restricted to the single, not-terribly-useful state of ‘off’.  With my ‘fix’ complete, I re-installed the switch in the semi-dangling sheet metal front panel (I plan to fully enclose the whole thing nicely one of these days, honest).

After throwing the mighty disconnect to ‘on’ once more, I pressed the just-tweaked red start button again, and the system fired up.  However, things were not sounding right, so I pressed the black stop button right away.  I surmised that the growling must be due to loose items sitting atop the plywood sheet that adorns the RPC cart (and acts as more horizontal storage space than I care to admit).  So I pressed the red start button again, and let the system growl angrily while I prepared to turn some perfectly good aluminum tubing into swarf.  After perhaps 10 seconds, there were loud popping sounds coming from the RPC, followed by me hopping around trying to turn off every electron flow nearby.  After I finally hit the main disconnect, I had a look at the carnage underneath the plywood sheet, where we both saw some rather impressive green flames emanating from what had been the bank of starter capacitors.

“Um, what temperature do electrons burn at?” he quizzed, a highly amused grin on his face.  After I had finished blowing out the flames (note to self – buy fire extinguisher for garage) and giggling over the absurdity of the situation, I determined that finishing the scope rings would simply have to wait for another night.  We pulled out the smoking remains of the capacitor bank and had a look.

So, that’s what’s inside an electrolytic capacitor – foil, paper, and magic smoke.  As everyone knows, once you let out the magic smoke, nothing works correctly, and you can’t get the smoke back into the component it escaped from.  After I checked my notes (that is, my previous blog posts on actually building the RPC), I re-familiarized myself with the circuit.  I had connected the starter capacitors directly through another pushbutton module, as I didn’t have a suitable relay at the time.  I recalled consulting with our EE at work about the use of just the pushbutton for the starter capacitor array, and he said that while it wasn’t ideal given the amperages in play, it would probably hold up just fine for several dozen to a hundred startups, which sounded just fine to me at the time (and then I promptly forgot about that limited lifespan).  I don’t know exactly how many RPC startups I’ve had since building it, but it’s certainly within that range – he gets major bonus points for accurate estimation of the failure mode and timeframe.

Really, I should have noted the signs of excessive current when I first pulled the pushbutton out of the panel.

The melted plastic housing right next to the screw terminal at the very rear of the module is a dead giveaway for “something ain’t right over here”.  So I replaced that module with a spare that I had, and decided to actually do things correctly this time.  Well, more correct at any rate – if it blows up again, I may have to reconsider my approach.

I used an Allen-Bradley contactor (the gray device on the left) with the 120v coil wired in to the start switch by way of the step-down auto-transformer (black box in the upper right).  The previous bank of ten 64mfd start capacitors (bits of their innards still sprinkle various components) has been replaced with three 208mfd caps (which I would have used initially if Surplus Center had them in stock at the time).  The big rectangular run capacitors remain unused, the poor things (well, it’s a better life than erupting into a cloud of electrolyte and flame).

Starting the newly repaired RPC was not quite as nail-biting an experience as the very first time, but still caused me to grit my teeth when pressing the red button.  Fortunately, all my work had been properly done, and the RPC started up in a flash without issue.  In fact, it seems to be even more reliable at this point – I’ve started it up perhaps 5 times or so, and each time was perfect.  I didn’t have to release the start button after 1.2 seconds because it didn’t sound quite right – every start has been confidently powered, with no hesitation.  The scope rings were turned with ease, and the system is better than ever!

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