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Discussion Starter · #1 ·
Hi all,
I am new to the forum and have a general question. I've searched this forum and found multiple discussions on specific projects regarding EV system Fuse sizing, but nothing that addresses a general rule of thumb. Is there a basic formula for determining adequate fuse protection for my wiring and battery pack? I have a system that was packaged for me through an EV parts distributer, and I want to determine how the fuse size was chosen.
I have a hyper9 LV motor, 108V battery pack, Orion2 BMS, Elcon 3.3KWh charger. My system has a gigavac maintenance switch, dual contactors (motor and BMS), and has a 600amp class T fuse (amongst other items of course).
Given that the low voltage hyper 9 can draw 750amps, and my batteries can deliver 3C at 900amps, i guess the fuse can pass higher than rated amperage for short durations. as you can see, i'm no electrical engineer, nor a particularly smart guy, so how did they figure out the fuse size would allow transient current above rating but still provide adequate protection for my wiring etc?馃く
cheers
 

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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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Discussion Starter · #3 ·
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%.
Perfect. That leads me to another question I had about precharge circuits. I dont have one, but there is a delay following my key switch activation before my main contactor closes. I assumed SME controller had some form or internal precharge circuit, as the KSI input wire delivers HV pack voltage through the harness. My kit didn't specify a precharge circuit. thoughts?
 

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Discussion Starter · #5 ·

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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 馃槀). Don't ask how I know this....
 

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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 馃槀). 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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Boils down to a race between the fuse and the wiring. To your argument, sticking a 30A fuse in a fuse location rated for 15A won't burn your house down. But it has because not all circuit faults are 1e-25 ohm dead shorts.

There are curves for blow time....too lazy to look, but it's not nanoseconds or even microseconds, iirc
 

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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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Discussion Starter · #12 ·
so, if Im following you guys, the fuse is for wire and connector protection. I have 2/0 cable throughout my truck. when I use online wire calculators, I keep getting warnings that my cable isn't thick enough. If I my motor can pull 750A, and my cells are 300Ah 1C and capable of 900Amps for short durations, im utterly confused and concerned that the 600 amp fuse and my 2/0 cables are inadequate.
 

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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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Discussion Starter · #14 ·
That makes sense. The 750amps is definitely peak. I鈥檓 not sure what continuous rating is, as Netgain only publishes the 750amp rating. I think my confusion arises from the idea of peak vs. continuous current. Do I size my fuse based on the continuous current with a time delay fuse that can sustain short burst of increased amps during acceleration or the occasional hill? If that鈥檚 the case, then am i adequately protected with a 600amp time delay fuse?
 

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A time delay fuse should protect everything but the controller - what I said about the MOSFETs becoming fuses. The controller should limit its current, so unlikely to need fuse protection.

The right way to do this is measure the temperature with an isolated thermocouple while you do stress tests and limit the controller current. Kinda complicated for DIY...
 

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

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 掳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.
 

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

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 掳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.

The sample drawing from EV West uses a 600 amp fuse on the Hyper 9. (For what it's worth)
 
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