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The rule of thumb is your fuse should be the weakest link in the system, so it fails before any other component fails. I don't believe delay fuses are appropriate for the systems with programmable motor controllers - the motor controller should limit the current before a current spike exceeding the fuse's rating would be produced. Fuse in that context really is to account for a short circuit either in the wiring, or in the motor controller itself. Likewise there should be no need to account for inrush currents to the motor, motor controller should ramp it up slowly enough that there is no spike exceeding the ratings.

Either way, that's just my opinion. To sum it up: fuse size = min(Imax(controller), Imax(wiring), Imax(battery), Imax(contactor)) - 5%.

 

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That sounds reasonable, but your motor controller vendor would be the ultimate authority on this. Is this what you got ? https://www.go-ev.com/PDFs/HyPer_9_IS_User_Manual_REV09.pdf

 

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"The rule of thumb is your fuse should be the weakest link in the system, so it fails before any other component fails. "

It's usually sized to the lowest current carrying capacity of the conductors (which includes connectors and switches) in a circuit, usually determined by insulation temperature limits. The primary reason for a fuse is fire prevention vs component protection.

A MOSFET will blow up a lot faster than any fuse can react (it becomes a fuse 😂). Don't ask how I know this....

Which is why I qualified the comment with "my opinion" - I knew you'd show up and disagree Now explain this - in a short circuit scenario in a circuit rated for 500A a fast acting fuse rated 1000A would blow what, 10 nanoseconds (arbitrary number pulled out of my butt) later than a 500A one ? Does that make a significant difference in terms of heat build up on the insulation to actually cause a fire ?

 

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It is a good point in the context of house wiring (dumb circuits), but it's not quite sufficient for circuits with secondary and tertiary protection systems. In an EV we have a motor controller that is supposed to limit the currents according to its programming, then we have a BMS that is supposed to protect the battery (unless of course you're a 15 year old who happened to find some Chevy Volt modules and a random charger, then you don't need a BMS), including the overcurrent conditions.

Just like you, I had numerous MOSFET failures that took out the fuses. I am still convinced everything else being equal, I'd give fuse a chance to blow before anything else.

 

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Yeah, so it's another great point. Sometimes the wiring (and connectors, contactor, etc) is actually beefier than say the rating of the motor controller. So if we were to spec the fuse ignoring the motor controller, we'd be in the position to fry the motor controller if it didn't take care of itself.

On that other question... What is your motor controller rated for ? I am skeptical that it can take 750A continuously. Typically the number that's being thrown out there is the peak rating for a small duration, something like 2 minutes, at least not without additional cooling measures. Same goes for the wiring - 2/0 may be rated for 325A continuously (actual rating will depend on insulation quite a bit, like remy pointed out), but it will take much higher currents for brief moments. Unfortunately I never see "peak" ratings for cabling, only continuous ones.

 

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I think this is the vendor's manual for the controller Netgain is pairing with that motor :

https://www.go-ev.com/PDFs/ACX1_TAU_SYNC_UserManual_En.pdf

There is a note:
Additional heatsink could be necessary to meet the desired continuous ratings. The heat sink material and system should be sized on the performance requirement of the machine. We recommend ambient temperature air to be directed over the heatsink fins to maintain heatsink temperature below 75 °C.

So basically the controller will put up to 750A, but will thermally saturate within moments unless you've added extra cooling. Based on similar ratings from other vendors for controllers in a similar form-factor, I'd say you got about 200A continuous. Anything above that will not be sustainable, and again referencing other vendors I'd say 2 minutes is your top at full current. Obviously if you just slightly exceed 200A, and perhaps on a cool day, you may get away with more, not a precise science.

Whether you want to go there that's the question. I still think you should not use a time delay fuse. If you program the controller to allow 600A, then use something like 650-700A fast acting fuse.