Outboard Motor HV Interlock Circuit
The picture above shows an FSAE EV rear left corner assembly. A thick orange HV cable plugs into a motor inside of a wheel. A thin black wire with an inline autosport connector is attached to the wheel by a loop of safety wire. The black cable is routed out of view of the camera back to the chassis of the vehicle. The failure mode will be a detached wheel unplugs or breaks the communication cable.
Tips from a Tech Inspector
At the FSAE MI June competition, I got a lot of questions about "how to do it next year" when inspecting the High Voltage Interlock Loop (HVIL) for outboard wheel motors. I will lay out some things I have seen and what I am looking for when making sure your interlock is safely implemented.
There are two main ways that teams have gone about solving this issue. Both are totally legal and have their own pros/cons. The first relies on the pull force of a crimp or connector. The second relies on the interlock wire physically yielding. I will use 22 AWG wire for some comparison, but note that no team passed or failed due to interlock wire size.
Method The First: Crimp/Connector Pull Out
If you rely on pull forces of a crimp or a connector, these are quite predictable (if you use a correct crimp die, otherwise the pullout force is likely reduced by some margin). To make sure you open the HVIL like this, there has to be less slack in the interlock wires than on the HV three phase going to the motor.
Crimps:
Pull-out forces of crimps are almost always on the data sheet of the crimp, or in a spec like SAE AS7928, UL – 486 A, or UL – 486 C. Sometimes the data are on the crimp tool data sheets rather than the crimps themselves. A 22 AWG crimp is not that hard to pull out by hand. The aforementioned SAE and UL specs list 22AWG crimp pull out forces at around 8-15 lbs. of pull force. I have seen claims of much higher pull forces on Amazon products, but I would not take those as good data. The easier this is to pull out, the more vulnerable you may be to an unnecessary failure of the system.
The picture above shows UWashington’s 2023 FSAE EV car facing the camera. Attention is drawn to the front left wheel assembly. A large service loop of thick orange HV cable can be seen hugging the side of the chassis. A thin, taught black wire comes out of a splice in the orange cable and attaches to the top of a wheel motor inside the wheel assembly out of view of the camera. The failure mode will be a detached wheel unplugs the short communication cable.
Connectors:
Like with crimps, connectors have a pull force requirement. Connectors have two pull forces to look out for:
the pull force required to separate the two halves of the connector
the pull force required to de-pin the connector.
Some connectors like spade terminals are meant to have quite low connector separation forces and are intended to separate before they depin. Other connectors, especially motorsport spec connectors will de-pin before separating the two halves.
The picture above shows Carnegie Mellon’s 2023 FSAE EV car facing away and to the right. Attention is drawn to the front right wheel assembly. A thick, orange HV cable can be seen between the chassis and wheel assembly. A silver sheathed HVIL cable can be seen wrapped tightly around both legs of the upper wishbone. The silver cable is terminated in a spade connector on both ends close to the chassis. The failure mode will be a detached wheel unplugs a spade connector of the communication cable.
The picture shows the rear of an FSAE EV car. Attention is drawn to the rear left wheel assembly where a thin, white HVIL cable is routed from an outboard motor in the wheel assembly to the chassis, terminating in a large autosport connector. The expected failure mode will be a detached wheel pulls the pins from the communication cable connector.
Method the Second: Shearing the Wire
This one is often simpler in concept: wheel falls off, wire breaks. No pull forces are needed to prove to tech that it’s safe or will fail as intended. It's pretty clear that a small wire will fail before a big wire. You need to demonstrate that the interlock wire has less slack than the HV cable running to the outboard motor. I will pull on it (gently) to check.
The picture shows the rear of an FSAE EV car. The vehicle is on jacks and the wheels have been removed. Attention is drawn to the rear right wheel assembly where a thin white HVIL cable is routed from the chassis, along the upper wishbone, through a bracket on the hub motor and back to the chassis. The wire has some slack, but it is more direct than the large orange cables seen slightly behind it. The expected failure mode will be a detached wheel breaks the communication cable.
Wire retention:
This method relies heavily on securely retaining the wire. I do not believe that a zip tie around an a-arm will stay in the installed position or predictably shear through a wire, especially after sitting in a hot wheel well or the summer sun for hours. The cable should be retained such that the retention device cannot move and allow for more slack in the cable.
If you desperately want to use a zip-tie, please do your homework. I will absolutely buy off on a zip tie if you show me testing data or a convincing tensile strength comparison between the wire and the tie-strap. Most zip ties have a yield strength of maybe 50lbs under the right conditions. 22 gauge wire has maybe 10-20lbs. Did you use heat shrink, what kind, what does that change? It's totally feasible, but pull testing by hand is not sufficient to get a zip tie to pass tech. It's also such an easy change that it’s less work to use something else.
A nice to see alternative to a standard zip-tie would be a metal zip-tie, safety wire (properly installed so it will not untwist under load), or even a purpose-built bracket.
Remote Actuation:
Some teams opt to use a non-conducting link to disconnect/open the HVIL. This is totally legal. Don't use fishing line, it's not going to pass. Fishing line cannot withstand heat and degrades in the sun. We have no way to know how hot your wheel assembly gets, and how long your fishing line has been left outside.
As an alternative, safety wire, steel braided wire, Kevlar, etc. are common and easy to implement. Regardless of the cable material, I would heavily advise against routing the cable indirectly, such as bending around a hole in the bodywork. This will make it harder to reduce the slack in the line.
The picture shows an FSAE EV car from behind. Attention is drawn to the rear left wheel assembly where a steel cable connects a bridged connector in the HVIL to the outboard motor. The expected failure mode will be a detached wheel pulls mechanical cable, unplugging a connector and opening the HVIL.
P.S. If we envision a catastrophic event that would remove the wheel assembly from your car, we could safely assume the loads are not applied at a smooth, slow, constant rate like the pull force tests of a crimp or the yield strength of a cable. This lowers the margin of error for a lot of this analysis during tech, so we have to be really thorough when looking at it. This should also make you do very thorough work on your vehicle's safety systems!
Good luck!
Original photos by Noah Dropkin.