# Need Pre-Charge Resistor Help



## Earthwindwater (May 2, 2011)

Hi,

I need some help understanding precharge resistors. I have researched a bit, but I am still a bit confused. 

1) What exactly do they do? Buffer the incoming electricity?
2) Where are they used?
3) How are they used?
4) Are they necessary?

Thank you!


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## major (Apr 4, 2008)

Earthwindwater said:


> Hi,
> 
> I need some help understanding precharge resistors. I have researched a bit, but I am still a bit confused.
> 
> ...


Hi E, W & W,

Read over this http://www.diyelectriccar.com/forums/showthread.php?t=25419 Any questions after that, post up 

Regards,

major


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## Earthwindwater (May 2, 2011)

major said:


> Hi E, W & W,
> 
> Read over this http://www.diyelectriccar.com/forums/showthread.php?t=25419 Any questions after that, post up
> 
> ...


Thank you Major. I will be sure to read some more!


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## Earthwindwater (May 2, 2011)

major said:


> Hi E, W & W,
> 
> Read over this http://www.diyelectriccar.com/forums/showthread.php?t=25419 Any questions after that, post up
> 
> ...


OK, I have questions. I read one person using a 60 watt bulb as a resistor. Does it stay on once the system is on? So the bulb acts as a buffer?

Also, I am a visual person. Do you or some else have some images that you could post of a precharge configuration? 

Thank you for helping.


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## major (Apr 4, 2008)

Earthwindwater said:


> OK, I have questions. I read one person using a 60 watt bulb as a resistor. Does it stay on once the system is on? So the bulb acts as a buffer?


Hi again EWW,

I don't understand what you mean by buffer, but I think the answer is no. The main contactor is a switch in the high current battery path (cable) to the controller. When it closes (turns on) it connects the battery to the controller. The controller has capacitors in it. A capacitor looks like a short circuit when in a discharged state and a voltage is suddenly placed across it, such as happens when the main contactor closes. So at the instant that contact closes, full short circuit current from the battery pack attempts to flow. But the characteristic of the capacitor is such that the voltage cannot change instantaneously, so what results is an arc across the contacts of the main contactor as they close. The high current and arc combine to damage the contacts and may actually weld them shut.

The solution to avoid this is to use a resistor in series with the battery and controller to limit the current and charge the capacitor to or near battery voltage. If the capacitor and battery are at the same or close to the same voltage, then the main contactor can close with no arc. The resistor needs to be placed across the contacts of the main contactor to conduct current around the open contacts.

Once the contacts of the main contactor are closed (on state), this resistor is short circuited by the contactor itself and therefore has essentially zero voltage and no current. So if a light bulb is used as a resistor, when the contactor is on, the light bulb is dark. 

If the resistor is permanently connected across the contactor, the controller remains charged all the time.* So this is why some precharge circuits use a small contactor (relay or switch) in series with the resistor (light bulb) so it isn't on all the time when the controller is off. Then you must close the small relay to turn on the resistor (or bulb) in advance of closing the main contactor, hence called precharge. Depending on the size of the capacitor in the controller, the battery voltage and the Ohmic value of the resistor (or bulb), it will take several seconds to a minute or so to charge that capacitor before it is safe to close the main contactor. If you use a light bulb, just wait until it goes dark 



> Also, I am a visual person. Do you or some else have some images that you could post of a precharge configuration?


No pictures here. Maybe another member will post one for you. The schematics in the wiki should suffice. If you can't read a schematic, I suggest you learn. It will come in handy building an EV 

Regards,

major

*edit: Changed reason.


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## Guest (Jul 3, 2011)

In a nut shell the resistor is used to bypass the main contactor and is not allowing much through but enough to pre-charge the capacitors. You want your capacitors charged before you apply full power through the contactors. Once the capacitors are full no more power is being drawn through. The resistor only allows a minor amount of power in and you should fuse it with like 5 amps or less. Just in case. You don't need the amps but you need the voltage to precharge. It is simple and it can save your contactors and components in your controller. 

Thats about it. 
Yes you need to do this. I have on all my systems but my current one is done within the controller. No need for me to add one. Most setups will require you to put one one.


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## gojo (Feb 1, 2011)

The light bulb is nice because you can actually see it working, and it is fun to watch. 

I have a light bulb (40 watt) for my DC DC capacitors. I use a 750 ohm, 25 watt resistor for my controller. and I just leave it on all the time, but if it isn't working, I would have no way of knowing until damage was done. I may do another light bulb for my controller. The little C7 sockets work nice.


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## Earthwindwater (May 2, 2011)

Thanks yall. By buffer I ment that it slows down the full force of the pack coming in. Same as you described. I didn't have the terminology correct. 

Unfortunately for this forum, I will have many more questions! I am researching for my first ev build. 

Thanks again!


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## major (Apr 4, 2008)

major said:


> ....so what results is an arc across the contacts of the main contactor as they close. The high current and arc combine to damage the contacts and may actually weld them shut.


Just last week I had a contactor weld shut on me. I was working on tuning a different controller and motor for the first time on the bench. This particular controller is supposed to handle the precharge and close the contactor when conditions were right. I don't think it was doing this properly.

It was an eBay find, so we didn't have all the details. It was just at 48V with a completely unloaded motor. A new contactor. Properly wired. It welded up before we ever got the motor to turn over. We broke it loose, but it continued to stick. I don't think it ever saw any load current, just the inrush.










Hopefully you can see the weld marks on the button contacts. This was sufficient to prevent the spring from separating the contacts when the coil voltage was removed. *This is why you pre-charge*


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## Guest (Jul 7, 2011)

Looks kinda like an el-cheap-o contactor to me. One more suited for 12 volts than 48. But that looks like it had to have seen some current. Could be that the coil is weak too. For 48 volts I'd still use like the kilovac 200 contactor. What controller did you use? I know of no cheap brand controller that offers any sort of precharge built in. The only other two are not cheap-o brand controllers either.


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## major (Apr 4, 2008)

gottdi said:


> Looks kinda like an el-cheap-o contactor to me. One more suited for 12 volts than 48. But that looks like it had to have seen some current. Could be that the coil is weak too. For 48 volts I'd still use like the kilovac 200 contactor. What controller did you use? I know of no cheap brand controller that offers any sort of precharge built in. The only other two are not cheap-o brand controllers either.


Hey got,

I didn't say it was a "cheap brand controller". I think the contactor was rated for the voltage. 100 A I think. But never had been used with a loaded motor. The motor never rotated at all. The contactor had a 24V coil and the controller was set to deliver 24V to it and did.

*The point of the post was to show the effects of closing the contactor into a capacitive load.*

I think we have the issues worked out with the controller parameter settings and it works now and motor rotates. We tried the Kilovac, but the controller didn't like it. Too low coil resistance, I think. Now have a big honkin' Albright in there. Works fine. Open contacts and we can see no arcing. 

Regards,

major


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## Guest (Jul 7, 2011)

I know your point you are getting across. When I see things with vague information I always try to troubleshoot. Kinda one of those mechanic things you know. What kind of controller are you using? Never heard of a controller that did not like the Kilovac either. Odd. Another vague thing that I want to troubleshoot so good information can be passed along to others in the same situation. If all you did was put 24 volts to that burnt contactor I'd almost have to believe that the contactor was toast before you even used it. You know, parts do get repackaged and resold when they are returned to a store for what ever reason. On ebay I'd suspect it even more. 

Anyway it is good to see what happens and what points look like when burnt beyond use. Nice to hear your using that honkin Albright. I have a couple of those but the Kilovac is doing just fine. I'll stay using that. 

Still, again, what controller? Can't be that big of a secret.


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## major (Apr 4, 2008)

gottdi said:


> I know your point you are getting across. When I see things with vague information I always try to troubleshoot. Kinda one of those mechanic things you know. What kind of controller are you using? Never heard of a controller that did not like the Kilovac either. Odd. Another vague thing that I want to troubleshoot so good information can be passed along to others in the same situation. If all you did was put 24 volts to that burnt contactor I'd almost have to believe that the contactor was toast before you even used it. You know, parts do get repackaged and resold when they are returned to a store for what ever reason. On ebay I'd suspect it even more.
> 
> Anyway it is good to see what happens and what points look like when burnt beyond use. Nice to hear your using that honkin Albright. I have a couple of those but the Kilovac is doing just fine. I'll stay using that.
> 
> Still, again, what controller? Can't be that big of a secret.


The controller type is off topic. The contactor was a White-Rodgers 124-903. New old stock, not from eBay, Grainger years ago I think. It shows no evidence of ever being installed or used previous to my test. It is rated 100A continuous, 400A inrush, 24V contacts on inductive load rated. It should have handled a no-load bench test without problem. Never intended to go onto vehicle. We just needed to tune the controller.

To my knowledge, the contactor was bought maybe as long ago as 10 years. Been sitting in the cabinet in the lab. I have not watched it continuously for that time, so it is possible something unknown to me happened. But there were no obvious tool marks on it and terminal threads looked virgin.

I agree, the arc damage on the points looks severe for having never seen true load current. But its behavior, working at first, then welding shut, indicated to me that our controller settings were off or something along those lines and the point damage was caused by arcing upon closing into an uncharged capacitor. 

Again, it is presented as an example of why precharge is needed. Certainly a higher voltage system (like 144V) into a larger controller would have the capacity for even greater arcing without precharge.

major

ps. The Kilovac coil is just 3.3Ω. The W-R was 53Ω. Albright about 60Ω. I think the Kilovac coil current was too much for the controller coil driver circuit.


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