# time constant for EV traction circuit



## sabahtom (Mar 1, 2011)

I asked Cooper for a matched set of fuse and box (the result of a previous thread). Their engineer asked this which looks like it needs it's own thread. I'm not sure how to answer but I guess it'll be similar for most LiFePO EV's?

I'd be driving for maximum 30 minutes at a time. Peak motor current of 250A for 10 seconds.

The loading profile is going to have to be more specific and also include off time, if there is any. I can do a continuous loop for being on, but I need specific time intervals and specific amperages. Also a maximum voltage and the time constant of the circuit will be necessary. I see below you mention 340Vdc, so I can assume this is the max DC voltage, but can you provide a time constant on an anticipated fault loop? 


I mentioned 340VDC since I won't be charging to 100%, I guess 340v is about right for a 99-cell pack of China Hipower as V under a light load? In a short circuit I'm not sure what V value to use.


Also what is the maximum available fault current that your battery can supply the circuit? A common problem here is having a large amperage requirement, 100-200A or so, and a low available fault current. The fuse sometimes sees this as an overload, which is a misapplication of a high speed fuse. They are designed for short circuit protection only.


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## dougingraham (Jul 26, 2011)

Your battery voltage is 340 v. The problem he needs to address is the fuse needs to be designed to handle the voltage. If the voltage is higher than the fuse is designed for when the fuse opens it can still conduct because an arc will jump the gap. So the maximum voltage the fuse can see is the important number. If you want to push it the maximum voltage during charge would be the one to give.

In your case the 250A is the maximum normal use current flow that the fuse will see. What he wants to know is what is the short circuit current of the battery. I dont know what your battery is so I cant guess at this. But if typical of say 100AH LiFeP04 cells 1000A is not a problem and probably a lot more. For you a fuse that will carry 250A for the duration of your longest run at max current, like perhaps up a steep hill at highway speeds but will blow quickly when subjected to that 1000+ amps the battery will provide when shorted.

Let us know what he recommends.


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## sabahtom (Mar 1, 2011)

Thanks Doug.

What should I say about the time constant on an anticipated fault loop? 

Is that how fast I need it to blow after the current passes 250A?


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## winzeracer (Apr 3, 2012)

ac circuit what time will voltage maxWhat should I say about the time constant on an anticipated fault loop? 

Is that how fast I need it to blow after the current passes 250A?

[/QUOTE]

The time constant in an RC circuit is Resistance X Capacitance, this is engineer speak really, shouldn't need it to size a fuse. This would be used for figuring out the precharge resistor value. As Doug said for sizing a fuse, the short circuit current it what he needs to size a fuse,which would typicaly be the Max Amps that the batteries are capable of so 100Ah at 10C would be 1000Amps.

Edit: Typically Ev's use "Slow Blow" fuses


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## dougingraham (Jul 26, 2011)

sabahtom said:


> Thanks Doug.
> 
> What should I say about the time constant on an anticipated fault loop?
> 
> Is that how fast I need it to blow after the current passes 250A?


If such a thing were possible you would like it to operate forever at 250A but blow instantly at 251 or more amps. In the real world you can get close to this only with active circuitry. You can't make a passive device do this and have a reasonable life expectancy.

The engineer should be able to spec a fuse that will hold at 250 amps but blow pretty fast at your battery short circuit current. That is about the best you can do.


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## sabahtom (Mar 1, 2011)

From Cooper - quick response, nice to see 

Without a specific loading profile, the best I could provide is based off what I have sized previously. Including your temperature derating we would be looking at approximately a 175A fuse, then a safety factor of 2 is the general factor for EV applications. That would put the fuse sizing at 350A. Now we are back to my previous comments, where a 1000A fault is not that significant to a 350A rated fuse. A time delay fuse would allow this current to flow even longer. Generally I would like to see a fault current of at least 8 times the rated current of the fuse. How locked into this design are you?

Since the fuse is the last traction circuit component to buy, I am locked in.


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## major (Apr 4, 2008)

sabahtom said:


> From Cooper - quick response, nice to see
> 
> Without a specific loading profile, the best I could provide is based off what I have sized previously. Including your temperature derating we would be looking at approximately a 175A fuse, then a safety factor of 2 is the general factor for EV applications. That would put the fuse sizing at 350A. Now we are back to my previous comments, where a 1000A fault is not that significant to a 350A rated fuse. A time delay fuse would allow this current to flow even longer. Generally I would like to see a fault current of at least 8 times the rated current of the fuse. How locked into this design are you?
> 
> Since the fuse is the last traction circuit component to buy, I am locked in.


Hey saba,

Your maximum fault current would be the short circuit battery current. I don't know what battery you're using, but a typical LiFePO4 100Ah cell might be 3 or 4000 Amps.

Also, I don't know what controller you use, but often the manual will recommend a fuse. 

We do talk a lot about fuses on this forum. Do a search for "Ferraz" or "Shawmut" and read some of those posts.

major


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## TEV (Nov 25, 2011)

Just send him this link : http://www.discountfuse.com/A50P250_4_Mersen_Ferraz_Shawmut_Amp_Trap_500_Volt_p/a50p250-4.htm

and ask for a fuse with the same specifications. It's what an OEM used in an 288V system with the same controller and motor.


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## TEV (Nov 25, 2011)

When will you post some pictures and/or videos with your build ? We love pictures on this forum


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## Ziggythewiz (May 16, 2010)

sabahtom said:


> Including your temperature derating we would be looking at approximately a 175A fuse, then a safety factor of 2 is the general factor for EV applications. That would put the fuse sizing at 350A.


Did he say anything else about the safety factor? Are these fuses really so terrrible that you have to double the spec to prevent an unintended blow?


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## sabahtom (Mar 1, 2011)

*$120 fuseholder - specced by engineer....*

FWA250A Type FWH 250A 500V $92.26 EA 
BH-1133 0-400 Amp 2500Vac High Speed 1 Pole Modular Fuse Block Connector Type 2 Hole $120.70 
*Item Total: * *$212.96* 
*International * *$40.00* 
*Total: * *$252.00
*
It's from onestopbuy.com

Is this reasonable?

The engineer also said I can connect a put the fuse on the fuse block stud and put a lug with my 20mm cable on top and bolt them down together. Sounds like a better (more normal?) arrangement.


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## major (Apr 4, 2008)

*Re: $120 fuseholder - specced by engineer....*



sabahtom said:


> FWA250A Type FWH 250A 500V $92.26 EA
> BH-1133 0-400 Amp 2500Vac High Speed 1 Pole Modular Fuse Block Connector Type 2 Hole for 6-250MCM CU 3/8-16 Stud $120.70 EA 1 $120.70
> *Item Total: * *$212.96*
> *International * *$40.00*
> ...


One of the EV parts advertisers on this forum has Ferraz Shawmut fuses for about $50 and Ferraz fuse holders for about $40. http://www.evolveelectrics.com/Fuses.html


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## sabahtom (Mar 1, 2011)

TEV said:


> When will you post some pictures and/or videos with your build ? We love pictures on this forum


Hi TEV

I've put some in an album


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## sabahtom (Mar 1, 2011)

*Re: $120 fuseholder - specced by engineer....*



major said:


> One of the EV parts advertisers on this forum has Ferraz Shawmut fuses for about $50 and Ferraz fuse holders for about $40. http://www.evolveelectrics.com/Fuses.html


Thanks Major. I had a look, the $50 ones are A30QS300, 300v. My pack is 309v nominal so I'll need the 500v which is $84, fuse holder is $64. Still $100 cheaper than the Bussmann setup.

BTW thanks for the link. Looks like I should start my shopping with the advertisers here.


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## sabahtom (Mar 1, 2011)

Ziggythewiz said:


> Did he say anything else about the safety factor? Are these fuses really so terrrible that you have to double the spec to prevent an unintended blow?


This was the complete email:

Without a specific loading profile, the best I could provide is based off what I have sized previously. Including your temperature derating we would be looking at approximately a 175A fuse, then a safety factor of 2 is the general factor for EV applications. That would put the fuse sizing at 350A. Now we are back to my previous comments, where a 1000A fault is not that significant to a 350A rated fuse. A time delay fuse would allow this current to flow even longer. Generally I would like to see a fault current of at least 8 times the rated current of the fuse. How locked into this design are you?


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