Electrical Tech Inspection

OK, so the picture is actually design judging, but E-tech was really the thousand-mile-march of our first few days in Nebraska.

In the paddock space, reviewing what we have under control and what needs work with Jeff and Tyler, my co-conspirators, using the tech inspection checklist on the Note tablet.

Here’s the rundown of some of the things we had to come up with ways to fix:

• The judges noticed that our Tractive System Active Light shut off immediately on key-off, before the DC-link capacitance was discharged. This was because of the inclusion of independent contactors for each motor controller– the TSAL trigger circuit was looking at the battery side of those contactors and since they opened immediately on key-off, it wouldn’t see the voltage on the caps. Solution? Radio Shack hadn’t gone bankrupt yet, so we were able to get some simple diodes to place across the contactors, allowing the battery side TSAL trigger circuit sensing the DC link voltage.
• The firewall insulation we had applied in haste didn’t have an electrical insulation rating on file. Note to self: although generic spray on truck bed liner may be electrically insulative, the manufacturer doesn’t put a dielectric rating on the datasheet. Solution: Cover the firewall with Kapton tape.
• The charger we built at the last minute, an aluminum box with four Mean Well HLG supplies in it, didn’t have the conductive  case attached to line ground. Solution: a jumper between the inlet ground and the case.
• The first time we powered up the car the Insulation Monitoring Device tripped. Every time, with the same delay. Turned out that the motors were designed to be run on a vehicle which has chassis ground in common with battery ground– so the encoder had a high value (100k) resistor between chassis ground and signal ground. Which, in our isolated system created a ground fault. Solution: once I traced it back to the encoder, it was trivial to lift the ground.
• Finally, the big one. If you downloaded and read the ESF pages, you may have noted my treatment of a certain rule about fusible links between paralleled battery “strings”. This was not in line with the intent of the rules– the judges simply said, we view a single cell as a “string”, and demanded fusing between the paralleled cells. We had to construct an engineering argument on the spot and try to support it with manufacturer and test data to support that our pack was safe paralleled at the cell level. Fortunately we were able to come up with some salient points and as the rest of our tech inspection checklist went very smoothly the judges were willing to show slightly more leniency and let us pass without modification of the battery pack. In fact, with the comprehensive pictures I had taken for the judges (many of which I have posted here) the pack was not once opened at competition! Note that the 2015 and later rules have changed about this topic!

Finally passing E-tech was one huge milestone down!

And cause for celebration.