# When is a contactor recommended/required/necessary?



## Functional Artist (Aug 8, 2016)

I'm working with/testing a YALU 3000W 70A 48 60 72V BOMA Brushless BLDC Electric Motor Controller from Alfa Wheels (~$100.00)
Controller YALU 3000W 70A 48 60 72V f BOMA GoKart Brushless BLDC Electric Motor | eBay 

*Controller Features and Specifications*

*48 to 72 Volts, rated, 3000 Watt*
*70 amps Peak (almost 5000W peak!)*
*120 °*
*Connecting diagram: Please look at picture showing cables and functions*
*Features/ Connects to:*
*Battery*
*Motor (3 cables, power)*
*Motor (5 cables, hall effect)*
*Power lock switch (turn it on and off)*
*Brake switch (kills the power when braking)*
*Reverse switch (Reverse is half speed)*
*3 Speed selector (Hi-Mid-Low)*
*Speed Signal*
*Throttle (hall effect throttle) 3 cables*
*Cruise Control (connect to temporary switch for Cruise Control*
*Self Learn (To calibrate with new motor)*


To power/control a 60V 2,000W brushless motor (~$125.00)








2000W Watt 60V Volt BLDC electric motor w Base BOMA BM1024 T8F sprocket GoKart | eBay


Find many great new & used options and get the best deals for 2000W Watt 60V Volt BLDC electric motor w Base BOMA BM1024 T8F sprocket GoKart at the best online prices at eBay! Free shipping for many products!



www.ebay.com




*Specs:

60V DC
Rated 2000 Watt, 42 amps
Rated 5600 RPM
Chain drive (11 teeth sprocket) #8 chain 8mm pitch
Type of motor: DC, Brushless (BLDC)
Reversible

Dimensions:
- Diameter: 4 1/4"
- Length (no shaft): 5 1/4"
- Length (with shaft): 6 1/4"*

There is NO wiring diagram, available (that I could find) just a wiring definition 
...but, it does not mention a contactor.

So, is there a minimum voltage or amperage where a contactor is recommended, required or necessary?

Also, what about pre-charging? 

Is 60VDC of "inrush" current enough to damage the "cap's" in this controller?


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## remy_martian (Feb 4, 2019)

A lot of info. But nothing saying what you're using it in and how.


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## Rbertalotto (May 26, 2020)

subscribed


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## tks007 (Feb 16, 2020)

Ohm's law dictates that at high voltages with low resistance amperage goes sky high. It's that current which one tries to eliminate. Also the pre-charge lets you do a health check. If the power provided is dumped/lost/wasted you know that connecting the bigger relay won't be a good idea...

When a device is connected and caps are empty is when the biggest inrush current flows. It's a magnitude of 60amps.
Or your inverter has an system integrated.

I think you should use a contactor and a pre-charge resistor. They won't need to be from HV systems since you have 60vdc and 70amps.


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## Functional Artist (Aug 8, 2016)

remy_martian said:


> A lot of info. But nothing saying what you're using it in and how.


I'm testing it on Excalibur my racing style go kart (test vehicle) 








Excalibur Racing Style Go Kart


I designed & built an electric powered, racing style go kart. (Inspired by the movie Go Karts) It's propelled by (2) 48V 1,000W ZY-1020 brushed motors ...& powered by (4) 12V 15AH SLA batteries. It's pretty quick, does "donuts" ...& I got ~45 minutes of WOT/hard driving ...or (338wH) out of...




www.diyelectriccar.com








tks007 said:


> Ohm's law dictates that at high voltages with low resistance amperage goes sky high. It's that current which one tries to eliminate. Also the pre-charge lets you do a health check. If the power provided is dumped/lost/wasted you know that connecting the bigger relay won't be a good idea...
> 
> When a device is connected and caps are empty is when the biggest inrush current flows. It's a magnitude of 60amps.
> Or your inverter has an system integrated.
> ...


IIUC a controller that is capable of 70A (peak) will only produce that much current if/when connected to or powering a motor that can draw/use that much current
...but, this 60V 2,000W motor should only draw ~33A constant & maybe 60A for split second "spikes" or peaks

I build a lot of vehicles. I started off with 24V & 36V scooters & then, lots of 48V go karts & now I'm working with 60V 
...& probably 72V systems next

So, I'm looking for actual criteria for if/when a contactor &/or pre-charging was necessary (& why)

I'm using a 125VDC 65A circuit breaker to protect the system & as a "main cut-off switch" (for storage & maintenance)
...& I believe, it should be able, to be used as, a "manual cut-off switch" in the event of a "runaway" situation 
(as far as I understand, that's the main purpose/job of an electronic contactor)

* A pre-charge function could be easily be incorporated into the system (a momentary switch & a resistor)


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## kennybobby (Aug 10, 2012)

You might want to go ahead and remove the cover of your controller to examine the circuits and write down the value, rating, etc of the input capacitor, and go ahead to order a replacement--because if you don't use a pre-charge circuit and contactor then it is only a matter of time until it blows.

When a capacitor is given a large DC voltage step, such as 0 to 60V on startup, its initial response is like a direct short circuit to ground--there is a huge inrush of current to fill the empty capacitor. This inrush can cause internal damage and is the reason to use a precharge to limit the current to a safe rate. The contactor is then energized after the capacitor is full in order to by-pass the precharge circuit and provide a low-impedance path for the main operational current. The contactor has precious metal surface contacts with very low resistance, but it does not like to be switched while current is flowing, due to arcing across the contacts which damages the surface and increases the resistance.

Sure you can go ahead and try it and run it without the circuit for awhile, but it's a gamble until the controller caps blow, then it might be costly or impossible to repair. 

Pay now or pay later, either way an education is a costly expense.


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## tks007 (Feb 16, 2020)

Kennyboy says what I meant. Perhaps I should have stated the law which dictates the charging of a capacitor.

Anyway what you could do is place a fairly high resistor over that switch. Then you have your pre-charge fixed. The only thing is the controller will be always live so adding a second switch is needed then.

But remember switching on in the wrong order is still a weak spot then.


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## remy_martian (Feb 4, 2019)

Contactors have several purposes, but none of them is to actually switch heavy current on and off - they CARRY heavy current. If you want them to last. People love to cite ohms law, but it's a lot more complicated than that for caps and the circuits they hang on.

Also, a breaker should be used as a protection device, not as a switch. If it were my project, I'd have a bright red disconnect switch that's accessible no matter the kart's orientation.

Your speed controller may already have a precharge relay in it, since it has an on/off switch input. Several ways to find out.

I really like the packaging you've done there. What kart frame did you start with?


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## Functional Artist (Aug 8, 2016)

remy_martian said:


> Contactors have several purposes, but none of them is to actually switch heavy current on and off - they CARRY heavy current. If you want them to last. People love to cite ohms law, but it's a lot more complicated than that for caps and the circuits they hang on.
> 
> Also, a breaker should be used as a protection device, not as a switch. If it were my project, I'd have a bright red disconnect switch that's accessible no matter the kart's orientation.
> 
> ...


Well...I/we kinda went thru BRB's & manual vs. electronic contactors & even pre-charging, a few years back.
Here is the thread, if interested.








Big Red Button manual cut-off switch


I left this message awhile back, "Great thread, packed with good stuff ...but, most of this info on contactors & precharging is kinda old Are there any updates, new products, concepts or procedures? Now, that there is a lot more use & data available, Are electronic contactors...




www.diyelectriccar.com





From what I understand, a contactors job is to connect & carry the "heavy current" 
...or disconnect & sever the "main power supply" from the controller
...but also, to be able to the break the "flowing current", if/when necessary 
(like in an emergency runaway situation or like when the low voltage cut-off, limit has been reached)

I asked the original question, "is there a minimum voltage or amperage where a contactor is recommended, required or necessary?" 
...because this is a higher voltage system (60VDC) (compared to what I'm used to working with) 
...& this is a HUGE controller 
...with probably some "good sized cap's inside
...that are capable of drawing & releasing a lot of amps.

I mainly wanted to know (if anyone knew) if there is "actual" criteria for when a contractor is necessary/

Thanks!, my Excalibur kart is built from scratch. 
It's made outta regular old 1/2" schedule 40 waterpipe & based on a common "racing" design.
I just incorporated the battery "trays" into the chassis. 

I first tested it with (2) 48V 1,000W motors, powered by (4) 12V 15AH SLA's 
...it ran ~45 minutes, while draining ~345Wh out of the pack.

Next, I tried a 48V 35AH Lithium battery pack 
...but, the (2) motors seemed to be drawing too much from the pack & causing some heating issues
(I aborted the test @ only ~250Wh used)

So then, I removed (1) motor & tested it again 
...it ran like 3 hours, while draining ~936WH out of that pack.

* I've probably put ~10 miles on this thing, so far 

Now, I'm trying a 60V 2,000W motor
...& then, maybe even try/test (2) of them


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## kennybobby (Aug 10, 2012)

So if you can determine the size of the input capacitor in your controller, then you can calculatus the stored energy to charge it up,
E = .5 x C x V^2

The voltage squared term goes up quickly. When you went from 30V to 60V system, you doubled the voltage, but the energy went up x4 or more depending upon the caps.

With no resistance of an inrush current limiter device, the charge up happens nearly instantly. The current will be 60V divided by the resistance of the wire from the battery to the controller. If you used a 1 ft piece of 20AWG, 10mR per foot, then the inrush current would be 6,000 Amps. This would likely exceed any current ratings of the capacitor, the current would arc over and punch thru the electrode foil layers and short out the cap at least, and worst case explode. Any switch device used to turn ON the system will also be subject to this large current and sparking and arcing that may occur and damage the switch.

There is no one-size-fits-all rule of thumb. Each system and situation must be evaluated. But over 30VDC is considered getting into the lethal region and requires due diligence.


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## Functional Artist (Aug 8, 2016)

kennybobby said:


> So if you can determine the size of the input capacitor in your controller, then you can calculatus the stored energy to charge it up,
> E = .5 x C x V^2
> 
> The voltage squared term goes up quickly. When you went from 30V to 60V system, you doubled the voltage, but the energy went up x4 or more depending upon the caps.
> ...


 Yes, I agree & thank you that's the kind of info I was looking for.

I was under the understanding that once you get over 48V then, you were getting into the lethal range. 
(where the current can penetrate dry skin etc.)

I've noticed, many times, the spark/arch when connecting the power to my 48V controllers & initially energizing the caps.

Again, that's the reason for this thread, because this system is over 48V & there is no mention of or provision for a contactor or a pre-charge circuit. 

To help expand the conversation a bit more, I went ahead & opened this controller up.

I also, did a quick video, to help show/document the inside of this thing.




.


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## remy_martian (Feb 4, 2019)

8mA of AC through the heart can kill you, so it's not as simple as voltage. 

That inverter output can kill you even though it's _only_ on a 48V battery.


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## kennybobby (Aug 10, 2012)

That was a good job to open it up and get some pictures. On one of the pictures of the little black caps on the phases i could see the part number, CD228H, and used that to find the vendor is Nantung or Nantong. They have many varieties but i couldn't read the number on the big blue caps, but you read off the value so we know they are 470uF 100V.

They have 2 blue caps in parallel with an unpopulated spot for a 3rd. So the total is 2x 470uF = 940uF, and the stored energy is 1.69 Joules.

With nearly 1,000 microFarads uF, it would be to your benefit to use a pre-charge circuit or device. 

Maybe something simple like an NTC thermistor would work for you, but it's a little pricey at $33.





MM35 0R280 Ametherm | Circuit Protection | DigiKey


Order today, ships today. MM35 0R280 – Inrush Current Limiter 200 mOhms ±25% 80 A 2.677" (68.00mm) from Ametherm. Pricing and Availability on millions of electronic components from Digi-Key Electronics.




www.digikey.com





You need some switch for ON/OFF, maybe a big red panic button or E-Stop button switch could be used for that and you could just manually pre-charge your caps with a sufficiently sized resistor across the switch contacts. Then you don't need all the extra wiring and battery taps to operate the contactors. For a street car you would need full blown contactors, but a gokart could get by with a big resistor and switch.


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## Functional Artist (Aug 8, 2016)

kennybobby said:


> With no resistance of an inrush current limiter device, the charge up happens nearly instantly. The current will be 60V divided by the resistance of the wire from the battery to the controller. If you used a 1 ft piece of 20AWG, 10mR per foot, then the inrush current would be 6,000 Amps. This would likely exceed any current ratings of the capacitor, the current would arc over and punch thru the electrode foil layers and short out the cap at least, and worst case explode.
> 
> Any switch device used to turn ON the system will also be subject to this large current and sparking and arcing that may occur and damage the switch.
> 
> There is no one-size-fits-all rule of thumb. Each system and situation must be evaluated.


Thanks for the replies 
...it really helps me to learn more &/or clarify things that "I think I know"

Ever notice how more information almost always generates more questions? 

1.) How long does it take to charge up the caps, thru the precharge circuit?

Like for a DIY precharge circuit (resistor & push button/momentary switch)
...& also,
On my ElMoto, I shut down the entire system (right @ the circuit breaker) after every use (to eliminate parasitic draws)
...the next time I go to use it, I engage the circuit breaker (powering up the system) 
...then, turn the ignition key to "on" (to energize the 12V system)
...then, turn the Run/Stop switch to "run" (to turn on the SC) 
* kinda just like the stock ICE system was set up

But, I now realize that this scenario only allows for ~30 seconds of precharge, before the contactor get's engaged & the full voltage is connected to the controller.
* I haven't had any problems for over 500 miles  

2.) Does this "any switch" refer to a DIY precharge circuit? (resistor & push button/momentary switch)
...or the controllers On/Off circuit ? (power lock or KSI) 

3.) For a precharge circuit, what if the "switch" was connected in/on the controller side of the resistor, instead of the battery side?

Wouldn't it only have to deal with & switch the reduced current that's "now" being let thru the resistor?

4.) As for the On/Off (power lock), it connects a relay that just turns the SC's "logic" on/off 
(it's VCC/pack voltage but, draws & thus switches less than an amp)
...the caps draw their energy thru the big/main power wires

Would that "relay" be considered an internal contactor?


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## kennybobby (Aug 10, 2012)

to answer #1,
the time to fill the capacitors would be: t = 6 x R x C , where R is the resistor and C is the total capacitance, which in your case is 940uF. It will occur much quicker than 30 seconds. You can pick a time that you would like it to occur such as 1.5 seconds, then substitute this into the equation and solve for R. This will be the resistor value to achieve that pre-charge timing.

2) This only applies to the switching device in the main power line from the battery to the controller, such as a diy push button or E-stop, or a relay or contactor in this path also. If switching is done before precharge, then that device becomes the path for the hellacious current.

3 and 4) i'm not following your question. Probably a drawing of a circuit schematic is needed. The diy resistor/switch has these two in parallel and the resistor is manual touched across the terminals using your hand, held for the time calculated above, then the switch button is engaged to complete the path and the resistor is removed. If the resistor is attached to one side of the switch, then the open leg will have pack voltage on it--not a good idea to leave it floating and flopping about.


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## Functional Artist (Aug 8, 2016)

kennybobby said:


> to answer #1,
> the time to fill the capacitors would be: t = 6 x R x C , where R is the resistor and C is the total capacitance, which in your case is 940uF. It will occur much quicker than 30 seconds. You can pick a time that you would like it to occur such as 1.5 seconds, then substitute this into the equation and solve for R. This will be the resistor value to achieve that pre-charge timing.
> 
> 2) This only applies to the switching device in the main power line from the battery to the controller, such as a diy push button or E-stop, or a relay or contactor in this path also. If switching is done before precharge, then that device becomes the path for the hellacious current.
> ...


Kool! the start up procedure, I use, for my motorcycle should be fine.

3.) DIY precharge circuit
I drew up a diagram of the DIY precharge circuit that I have in mind.
...the red lines = pack voltage / constant current (VCC)
...blue lines = pack voltage / reduced current (VRC) 

4.) "Would that "relay" be considered an internal contactor?" This would be a NO
Because, we saw, how the big power wires (red & black) connect directly to the main "rails" inside of the controller.








So, NO there is NOT a relay or contactor in between the batt pack & the caps.
...& without an internal "switch" there can NOT be an internal precharge circuit.

FYI:
I took a pic of the power monitor. 
Notice, when the system is switched "on" (power lock plug shorted) it shows the pack voltage (64.36V)
...the wattage of the device (SC logic) is registering @ 1.2W
...the power draw (amps) is showing 0.02A 
...& energy use in Wh is showing 0Wh


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## kennybobby (Aug 10, 2012)

That looks like it will work with the correct sized resistor (value and power rating). What about adding a fuse in that line or in the Negative side path? The breaker is probably not a DC rated device is it?


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## Functional Artist (Aug 8, 2016)

kennybobby said:


> That looks like it will work with the correct sized resistor (value and power rating). What about adding a fuse in that line or in the Negative side path? The breaker is probably not a DC rated device is it?


A fuse in the precharge circuit?
...what would be the purpose, for this fuse?

Yes, this breaker is DC rated. 
It's a MidNite Solar - 50 Amp 150Vdc DC Circuit Breaker MNEPV50 
Their usually used in solar installations. (pics in post #5 show the specs)

I have a 10W 1KJ resistor, that came with a 36V 200A contactor, that I bought from Kelly Controls, that I can try/test.

So, to test this DIY precharge circuit, I assembled the (10W 1KJ) resistor with a small switch (rated 0.6A @ 125VDC &/or 0.3A @ 250VDC) 
...installed it onto the CB mount/housing 
...& connected it "across" the CB terminals

I didn't "do the math"
...but, it seems to function as anticipated  

Here is a video describing the concept & testing it out. 

Are these results, what's to be expected?


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## Duncan (Dec 8, 2008)

Pre-Charge Resistor

I use an old kettle element


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## kennybobby (Aug 10, 2012)

i was referring to a fuse in the red wire path from the battery pack positive terminal, or in the wire to the negative terminal. This was before i saw the actual breaker.

Did you get a datasheet with the breaker? Did you check the polarity of the breaker for installation? 

The microswitch with the resistor looks like a good method to do the precharge. You got lucky that it works ok without checking the math--i would call that semi-functional.


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## Functional Artist (Aug 8, 2016)

No, data sheet 
...but, most pertinent info seems to be labeled, right on the side (see pics in post #5)
...& it's a known brand so, probably easily found, if necessary.

FWIU the polarity is indicated by (++) on the "power in" side of the breaker
...so, that's where I connected the cable, from the batt pack.

Semi-functional?
...are you saying that it doesn't work as anticipated? (slowly filling the caps, upon command, with the main contacts "off" or open)
...or is that simply a "crack" for "not doing the math"?

TBH
I wasn't too worried because it was a resistor, sent with a contactor from Kelly controls (explicitly for the precharge function)
...so, I thought it would be appropriate for this test 
...& I already had it


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## kennybobby (Aug 10, 2012)

yes the breakers are shown as polarized in the datasheet, so good thing they marked the terminals.

yes that is a semi-functional and careless move to guess and not check the math; a resistor sized for a 36V system won't have the same margin in a 60V system. You got lucky. Would you still count on luck in a 96V system?


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## Functional Artist (Aug 8, 2016)

kennybobby said:


> yes the breakers are shown as polarized in the datasheet, so good thing they marked the terminals.
> 
> yes that is a semi-functional and careless move to guess and not check the math; a resistor sized for a 36V system won't have the same margin in a 60V system. You got lucky. Would you still count on luck in a 96V system?


I didn't do the math cause, I'm just a tow truck driver & this calculus stuff seems to be a bit "over my head" 
...but, I believe the necessary info is here:

"So if you can determine the size of the input capacitor in your controller, then you can calculatus the stored energy to charge it up,
E = .5 x C x V^2
&
They have 2 blue caps in parallel with an unpopulated spot for a 3rd. So the total is 2x 470uF = 940uF, and the stored energy is 1.69 Joules."

I just don't know, what goes where? 

A demo would be most appreciated


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## kennybobby (Aug 10, 2012)

> I didn't do the math cause, I'm just a tow truck driver & this calculus stuff seems to be a bit "over my head"
> ...
> A demo would be most appreciated


Okay i understand.
It's always okay to say it's over my head and ask for help; but it's not okay to skip the math check for resistor Power. If you try to put too much current thru a resistor it gets very hot, and can melt, throw arcs and sparks, start a fire, and burn stuff.

First, we got to figure out how much current will be flowing?
The basic equation is Volts = Amps x Resistance, V = I x R; In your case you know V and R, so you can calculate for current I = V/R= 60volts/1000 Ohms = 0.06 Amps. This will be the current flowing thru the resistor during inrush to fill the capacitors.

next is the Power: is the resistor power rating adequately sized to handle this much current?
The basic equation is Power = I² x R , the current squared times the resistance. So now that you know the current as calculated above, and knowing the resistance, in your case
Power = (0.06 Amps) x (0.06 Amps) x 1000 Ohms = 3.6 Watts.

Compare: Your resistor is rated for 10 Watts, so you have a margin of safety here because the Load Power is less than the Resistor Power rating, (that's why i said you were lucky).

Now it's your turn: what if your power supply was 120 Volts and you used the same 1k resistor. Can you run the numbers for this case and determine if the margin will be good, bad or ugly?
If you can do this you won't be semi-functional artist anymore.


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## Functional Artist (Aug 8, 2016)

Thanks, when explained like that, it's way more understandable.  

Challenge accepted 

OK, if the PS is 120V & the pre charge resistor is a 10W 1KJ, I believe the math would be:
120V / 1,000 Ohms = 0.12A
0.12A x 0.12A x 1,000 Ohms = 14.4W
So no, because in this situation, the Load Power exceeds the Resistor Power rating. 

Also, comparing other (common) voltages
(& practicing my new skills) 

36V / 1,000 Ohms = .036A
.036A x .036A x 1,000 Ohms = 1.29W

48V / 1,000 Ohms = .048A
.048A x .048A x 1,000 Ohms = 2.3W

60V / 1,000 Ohms = 0.06A
0.06A x 0.06A x 1,000 Ohms = 3.6W

72V / 1,000 Ohms = .072A
.072A x .072A x 1,000 Ohms = 5.18W

96V / 1,000Ohms = .096A
.096A x .096A x 1,000 Ohms = 9.21W

So, it looks like a 10W 1KJ resistor should be adequate for use, in a precharge circuit, with almost all of these systems
...but, @ 96V there doesn't seem to be much of a safety margin


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## kennybobby (Aug 10, 2012)

A+, i think you got it down. 

96V is definitely a close call and requires extra investigation. How long does the current flow? If it is for a short period, then it might be okay. But if it is a long time then the resistor will get hot. How to calculate this?
bonus points:
Knowing R and C, you can calculate the time; the equation was given in an earlier post in this thread, but repeating it here: Inrush time ~= 6 x R x C.


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## Functional Artist (Aug 8, 2016)

kennybobby said:


> A+, i think you got it down.
> 
> 96V is definitely a close call and requires extra investigation. How long does the current flow? If it is for a short period, then it might be okay. But if it is a long time then the resistor will get hot. How to calculate this?
> bonus points:
> Knowing R and C, you can calculate the time; the equation was given in an earlier post in this thread, but repeating it here: Inrush time ~= 6 x R x C.





kennybobby said:


> the time to fill the capacitors would be: t = 6 x R x C , where R is the resistor and C is the total capacitance, which in your case is 940uF.


Well, over the weekend, I tried it a couple of ways (C = 940uF & even, C = 1.69 Joules)

T = 6 x R (1,000) x C (940) 

So, 6 x 1,000 x 940 = 5,640,000 (then, I guess, convert from milliseconds to seconds)
...& that would be 5,640 seconds
...but, that can't be right  

Then I tried, T = 6 x R (1,000) x C (1.69)

So, 6 x 1,000 x 1.69 = 10,140 (milliseconds)
...& that would be ~10 seconds
...that seems more like it 

So (if I'm correct) it looks like, if a 10W 1KJ resistor is used to precharge a 96V system, it should take ~10 seconds to fill up the capacitators. 

* Would 96V @ 10 seconds be enough time to "overheat" a resistor?


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## kennybobby (Aug 10, 2012)

i should have explained about the units. Your first attempt was correct using C = 940uF, but i didn't explain that the unit of micro Farads, uF, requires you to divide by a million. 1uF = 0.000001 Farads. So the charge up time would be about 5.6 seconds. It would get too hot to touch in this time but wouldn't likely catch on fire.

The "J" in the resistor, as in "10W 1kJ", indicates that it has a tolerance in the value of +/- 5%, so its actual value could be anywhere between 950 and 1050 Ohms.


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## Functional Artist (Aug 8, 2016)

Ok, that makes a lot more sense.  

So, it should be/look like: 
T = 6 x R (1,000) x C (940) / 1,000,000
...& then doing the math, 6 x 1,000 x 940 / 1,000,000 = 5.64 seconds

...or as another example:

I took a look inside of a 48V 1,000W (brushed) speed controller & it has (4) 63V 220uF capacitators in it. (4 x 220 = 880uF)
...the math looks like:
T = 6 x R (1,000) x C (880) / 1,000,000 = 5.28 seconds

So, if a 1KJ resistor was used to precharge a 48V system (with 880uF total capacitance) 
...it looks like it would/should take ~5.28 seconds to precharge


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## Functional Artist (Aug 8, 2016)

Why is there such a tiny difference, in the time it takes to precharge this 48V system, than for a 96V system? 
(5.28 & 5.64)

Is it because the total capacitance (energy, required to fill the caps) in both systems is so close? 
(880uF & 940uF)


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## kennybobby (Aug 10, 2012)

Yes


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## Duncan (Dec 8, 2008)

My tuppence worth to this conversation
I use a kettle element for my pre-charge resistor
Nice low resistance and its designed to cope with 230v across it - admittedly when full of water
And effectively free

Oops just realized - kettles are not as ubiquitous in the USA as they are in most of the world


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## Functional Artist (Aug 8, 2016)

Duncan said:


> My tuppence worth to this conversation
> I use a kettle element for my pre-charge resistor
> Nice low resistance and its designed to cope with 230v across it - admittedly when full of water
> And effectively free
> ...


1st Congrats on your Devise being the "face" of the forum.  
2nd What is a "kettle element"?


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## Duncan (Dec 8, 2008)

Everywhere except for the USA we use electric kettles to boil the water for Tea (and even coffee)
They are rare in the USA as a 110v kettle is about 900 watts and takes forever to boil
AND coffee is best made at a lower temperature - not boiling

In the base of the kettle is a simple element - ours are between 2 and 3 kw - so the resistance is about 28 ohms and they flow about 8 amps
I just took one from an old kettle that had been exiled to the shed as it was tatty
There is something very similar in your hot water tank

Its a bloody shame that they have my car on the top when they have mortally wounded the forum -


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## Functional Artist (Aug 8, 2016)

That's a very unique & creative solution.  

They didn't get permission to use "your" car on their "forum"?
..."mortally wounded the forum"?

While doin' research on precharging, I came across this video:





In it, I noticed that they use (2) contactors (IIRC Duncan does too)
...but, they call the battery pack an "accumulator" & contactors AIR's (Accumulator Isolation Relay)
...& it also, mentions/discusses a discharge circuit (for discharging the caps down to a "safer" voltage level) 
(from ~400v down to ~60V)

I've never heard of a "discharge circuit" before. 
...not even on factory EV's

Is anyone in the DIY world using them?
...or are they mainly only used in racing?


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## kennybobby (Aug 10, 2012)

An electric range stove eye makes a good high load resistor too

51xFg--vFCL.jpg

All OEM EVs have discharge resistors on the HV capacitors for turn OFF, and pre-charge resistors for the turn ON. Some diy guys do also. Different languages can use different terms to describe the same device or function, the US and UK are two nations separated by a common language.


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## Duncan (Dec 8, 2008)

_They didn't get permission to use "your" car on their "forum"?_
They did get my permission - I submitted the picture to them

_..."mortally wounded the forum"? _
This forum is just a shadow of what it used to be


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