# Switching main contactor OFF when throttle is off?



## madderscience (Jun 28, 2008)

You want the main contactor to open when you are off the throttle and when the ignition is turned off for safety reasons. Basically, if the controller shorts out you would have a runaway motor. But opening the main contactor means that you will still be able to cut power as you instinctively go for the brakes or turn off the ignition.

Assuming you are working with a curtis or similar controller with a separate input voltage for the logic circuits, you want that input also switched by a separate "kill switch relay" for the same reasons. This relay would be wired in parallel (opens and closes in unison) with the main contactor. The reason for wiring it in paralel is this relay should still open even if the main contactor fails to open. When the controller logic circuit loses power it will shut down.

Most people seem to want to install a precharge circuit as well which is basically a big 1000-ohm or so, 20W or so resistor across the main contactor terminals to reduce arcing in the main contactor and reduce voltage fluctuations on the capacitors in the controller.

I just finished retrofiting a precharge circuit to my car; I had not originally put one in (and everything is still working fine; I am approaching 4000 miles since getting it running) but I decided that at best it is a good idea, and at worst it will hurt nothing.

Don't forget the circuit breaker or mechanical disconnect (a big anderson connector that you can yank via a cable or lever is a common setup) that are accessible from the cabin, and fuses for the traction battery too.

Good Luck.


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## Tesseract (Sep 27, 2008)

VoltC said:


> ...Question: Is it a good idea to disconnect the main contactor when the throttle is off? (by wiring coil of the contactor in series with the dead-man switch of the throttle potentiomitter?). Or can it cause arching of the contactor on rapid throttle apply/release?
> thanks!


This is done in golf carts for safety reasons because people are constantly getting in and out of the cart with the "ignition" on. One rarely does this in a car, though - a driver is always present to kill the power should the controller suddenly pick that moment to short out internally. Also, the contacts in a contactor undergo a wiping motion when they meet to help keep them clean. Obviously, there are only so many "wipes" they can undergo before the plating wears through, which is another reason to not turn off the contactor every time the throttle returns to zero. Finally, if your setup does not have a precharge resistor then every time the contactor turns on it will be looking into dead short (charging up the input capacitors inside the controller). Needless to say, this is rough on the contactor, but also on the capacitors and wiring inside the controller.

Lots of reasons not to do this, not really a single good one to do it, IMO.


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## tj4fa (May 25, 2008)

In the Electric Vehicles of America (EVA) design, there are two contactors. One is closed when you turn on the ignition switch and stays closed until you turn the ignition switch off.

The second one is closed when you step on the accelerator and opens back up each time when you release the accelerator (take your foot off the gas).

In their design, they do not use any precharge resistors. 

The PB-6 potbox in their setup has a microswitch that opens the contactor when you take your foot off the accelerator.

This would seem to place a lot of wear on the second contactor and it's an annual maintenance item to swap it with the first contactor so it would even out any wear. I haven't noticed any wear but have only been using the setup for about a month and a half.

I'm assuming this same type potbox and microswitch does this in single contactor designs as well but probably depends on how your system is designed and what potbox/controller you use.

Here is a photo and you can see the microswitch behind the potbox speed control lever in the square opening.


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## ClintK (Apr 27, 2008)

I've been wondering a similar question.

I'm thinking of using the approach of 2 contactors. One triggered by key switch. When you turn the key to "on" the first contactor closes.

The second contactor has a precharge resistor across the leads. So after I turn the key to "on" the controller can start its precharge. When I give it gas the contactor closes, bypassing the precharge resistor, and power goes freely to the controller.

Since the controller has an onboard DC-DC converter to handle its internals, it is best to have constant power. When I release my foot from the gas the second contactor opens, allowing power only through the precharge resistor. That keeps the controller's internals working, but it is also a safety measure.

If the controller decides to go full throttle on me (be it either at a stop or on the road), I can just lift my foot off the gas and the 2nd contactor opens. When full amps try to go through my little precharge resistor it will blow and effectively cut power. (I should probably use a small 10A fuse or such along with the precharge resistor.)

That way I don't have to think about pulling the emergency cut-off or turning my key to off - just the ingrained-in-my-brain action of lifting my foot off the gas will stop the acceleration.

My concerns are:
Does the controller's internal 12V power overload the small resistor?
If I were to quickly release my foot from the gas after a hard acceleration, the second contactor opens allowing current to only flow through the precharge resistor. Will the controller try to pull excessive power through the small resistor at that instance?

Think it will work? Thanks!


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## ragee (May 25, 2008)

tj4fa said:


> The PB-6 potbox in their setup has a microswitch that opens the contactor when you take your foot off the accelerator.
> 
> This would seem to place a lot of wear on the second contactor and it's an annual maintenance item to swap it with the first contactor so it would even out any wear. I haven't noticed any wear but have only been using the setup for about a month and a half.


This is exactly the way mine is set up also. I dont have any noticeable wear on contacter either. But makes sense to me. I will keep an eye on this. 

Thanks,
R Agee


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## VoltC (Jan 5, 2009)

Thanks a lot of the excellent comments - looks like the choice is not obvious - Tesseract objects the switching OFF strongly and others seem to be using it. I currently use a single contactor (Just like "Convert it" book), but I like the 2 contactors as well - another decision to make. Switching OFF on throttle OFF appears safer - I will stay with it for now. Is there anyone who has run that setup long enough to observe contactor actually wearing off? (probably most visible in dual contactor setup) and how long does it take to see the wear? 

I also use separate relays to turn off the contactor and the voltage to the controller, on turning off the ignition switch.


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## Frank (Dec 6, 2008)

I use the same setup as ClintK describes including the precharge resistor. These contactors are designed for zillions of cycles and should last forever. If you use Albright contactors you can replace the bits that wear should that be a problem. Opening the throttle at speed does nothing. I've only got about 2500 miles on my conversion so far but everything seems fine.


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## tj4fa (May 25, 2008)

Frank said:


> I use the same setup as ClintK describes including the precharge resistor. These contactors are designed for zillions of cycles and should last forever. If you use Albright contactors you can replace the bits that wear should that be a problem. Opening the throttle at speed does nothing. I've only got about 2500 miles on my conversion so far but everything seems fine.


Frank/ClintK, 
Do you have a pre-charge resistor on the secondary contactor, the contactor that closes/opens when you depress/release the accelerator?

If so, what value PC resistor are you running?

I'm considering it not so much as wear on the (Albright) contactor, but so that the controller's internals don't get hammered everytime the accelerator pedal is depressed/released.

Thanks.


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## ClintK (Apr 27, 2008)

tj4fa said:


> Frank/ClintK,
> Do you have a pre-charge resistor on the secondary contactor, the contactor that closes/opens when you depress/release the accelerator?
> 
> If so, what value PC resistor are you running?
> ...


Yes, my current design has the pre-charge resistor on the secondary contactor that is triggered opened and closed by the accelerator pedal. I picked up a 1k ohm 50W resistor for the pre-charge. There's a small discussion on my build thread out pre-charge that explains the sizing a bit http://www.diyelectriccar.com/forum...ead-23830p2.html?highlight=precharge+resistor

Disclaimer though! I have not tried out this arrangement yet (still waiting on my controller to arrive). I'm going to try it out, and if I see sluggish response by the controller or start blowing resistors I'll probably modify it.


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## Frank (Dec 6, 2008)

My precharge resistor is across the set of contacts that are operated via the microswitch on the potbox. My controller is a Curtis 1231C and I used what they recommended, I think it's a 750 ohm 25-watt but can't swear to it. Some folks use a heavy-duty incandescent light bulb.


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## bblocher (Jul 30, 2008)

ClintK said:


> Yes, my current design has the pre-charge resistor on the secondary contactor that is triggered opened and closed by the accelerator pedal. I picked up a 1k ohm 50W resistor for the pre-charge. There's a small discussion on my build thread out pre-charge that explains the sizing a bit http://www.diyelectriccar.com/forum...ead-23830p2.html?highlight=precharge+resistor
> 
> Disclaimer though! I have not tried out this arrangement yet (still waiting on my controller to arrive). I'm going to try it out, and if I see sluggish response by the controller or start blowing resistors I'll probably modify it.


If you are running 144v then a 1k resistor would require just over 20 watt rating. You'll be fine there. The only thing is knowning how long you need to wait to charge the capacitors fully.


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## tj4fa (May 25, 2008)

Thanks for the replies guys! 

I'm still sorta dumb on this topic and have read RFEngineer's wiki stuff but still will have some questions about it later (I have to go to work right now ).


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