Saturday, June 13, 2015

The Journey Never Ends

Well, that didn't take long.  It seems I have validated my doubts about the longevity of the ballast resistors I used in my motor starter project, though that is not to say that the resistors were all that had failed.

Ahh, memories

A while after being successfully installed, the motor start ramp became noticeably shorter.  At the time, I wasn't in the mood to even guess what had gone wrong, knowing that it would eventually get worse.  Indeed, within a few days, the resistors open-circuited and left the motor to start hard after the timed delay.  Upon inspection, two of the resistor legs were opened, and the third pair was intact ... but the corresponding SSR channel was shorted.

I had mentioned building three of these SSR's to use in an undervoltage start configuration and that I had subsequently watched them get violently destroyed when a relay collision dumped the branch SCC through the nine triacs.  One of those nine triacs actually survived and was re-used in the resistor starter configuration where it eventually failed.  Although I still question my snubber design and whether these triacs are genuine, the resistor-start configuration of the relay involves much lower voltages and switch stresses.  With no real evidence in support, I'll just assume that most recent triac failure was a product of the prior severe overcurrent.

That brings us to the resistors.  I clearly had little confidence in these things from the start, but they should have only been operating at about 20-25% of their allowable short-time overload rating. Did the SSR failure overstress them?  My rough calculations say no; shunting one leg during start would only increase average power in the remaining resistors by about 30%.  That's based on the false (but convenient) assumption that doing so did not also decrease the time required to reach steady state.  The increase should be negligible in comparison to the margin.

Let's take a closer look at the resistors.  When I decided to open these up, I honestly expected to find a 3W four band metal film resistor in a silicone blob.  I was surprised to find what looked like a wirewound element of decent construction.  The wire is 26 AWG, approximately 3.9 ohms/m.  My guess at an alloy would be alloy 294, maybe.  The ceramic former, end caps and terminations are all very sturdy.  The plastic end plugs are of a hard composition and are very tight in the extrusion.  Much like my other experiences with some of these cheap components from china, it seems there isn't an overarching theme of inadequacy.  Every bit of the part seems quite appropriate and well made -- all except one bad idea or decision made with complete disregard for part functionality.  In this case, the encapsulating cement ruined what was otherwise a useful part.

Blurry attempt to capture the porosity and coarseness of the cement sand

One might expect to find a wirewound resistance element like this encapsulated in a very fine-grained alumina cement made for the purpose.  The properties of interest might be its electrical conductivity (low), its thermal conductivity (high), and its maximum operating temperature.  Contrary to the goal of increased thermal conductivity, the resistor is packed with a porous and friable cake of coarse sand.  I honestly can't tell if there was originally any binder used or if the sand had just been weakly sintered from the heat of what must have been a yellow-hot resistance wire. The remaining material serves better as an insulator than anything else.  Fetching a questionably applicable thermal conductivity for packed sand of 0.4-0.5 W/m*K, rough calculations place the element temperature near the melting point after the first second's delivered energy.  All of the resistors showed this pattern of overheating (scaling, sagging, shorting wires), suggesting that the failures of the resistors and the triac were unrelated after all. 

At this point, I'm questioning my sanity and double-checking prices on soft-start modules.  No, they're still unaffordable.  It's interesting how much time and failure is still cheaper than a motor starter.  I guess that only works when your time is worth nothing.  Working with the assumption that the part failures were merely coincidental, I figured I'd rebuild it again with parts that weren't shit.  I rebuilt the relay using actual ST BTA26-800CW triacs from Mouser, and I replaced the chassis mount ballasts with three massive 6 ohm corrugated element resistors.  These resistors are not only 300W, but they have a 1000% 5 second overload capacity.  Compared to the prior configuration, these are so ridiculously oversized, they barely get warm.  Surprisingly, I only got these because they were the cheapest option.  I did have to machine some bushings and make some brackets though.

The resistor equivalent of a bigger hammer

With a few tweaks to the relay (I added a contact state indicator and adjusted trigger currents), everything seems to be working fine again.  I have no reason to think that the new ballast will be an issue.  Hopefully everything stays working. 

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