The goal of this document is to take you through the process of designing a system from start to finish. It goes into excruciating detail, and shows you how to take a bad idea and slowly improve it. A lot of the steps include: you try this on the car, and you break the car and destroy multiple of hundreds of dollars. Note that the ideal thing is to avoid that and to think carefully about all the possibilities before that happens. But even I have things that I overlook. Writing this document made me really drill in and realize things about the rules that I’d never looked at before because I was like “oh the precharge system? that’s really basic I know all the rules for that” and then when you have to write it all out it becomes really clear what all the problems are. This document took forever to make, but I hope you find it useful as an example of the design process.
Even if you think you know them, its always good to read them again.
Here is the relevant section (in fsae, we’ll get to fhe in a second):
Seems simple enough. But lets thing about the interpretation. From this I have two questions:
If it’s not fused, what’s the current limiting device?
What the hell is the precharge relay. Don’t we already have two AIRs
#1 is easy, we go to fsaeonline and read the FAQ: https://www.fsaeonline.com/cdsweb/rqa/ViewFAQ.aspx?faqnum=358. There we learn that the leads have to be less than 150mm long. That will be important when we build later but useless to this specfic discussion
#2 Threw me for a loop. (To be clear, I expected this document to be very straightforward to write) I searched around reddit and found that the FSAE interpretation of no HV outside the accumulator is very different from the one in FH+E:
They require a separate precharge relay to close, rather than having the resistor constantly connected in parallel with the AIR. This is safer, since it guarantees that the precharge resistor, which is not sized for continuous current, does not get burnt up in the case that AIR1 (the one powered by the shutdown circuit) gets stuck closed when AIR2 opens. One possible scenario this happens is the precharge control malfuncitons and both AIRs close at once.
Sometimes, reading the other set of rules helps explain the first one:
A precharge circuit is a circuit which charges the large intermediate capacitor of the motor controllers. It does this by first closing accross a big resistor, which charges the capacitor, and then closing the second AIR when the voltage is nearly at the top. We can see why this helps using this graph from the GV240 (one of our old AIRs):https://www.littelfuse.com/media?resourcetype=datasheets&itemid=114c3cd0-280c-436d-8fce-82b894fe0315&filename=littelfuse-dcnevt-