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Old 11-16-2015, 08:02 PM
galderdi galderdi is offline
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Default Autocross EV special

Hi from Australia,

I am towards the end of a project to build a special EV for motorkhana / khanacross. Most of you would know these events as similar to Autocross.

While there are other electric race cars around this will be the first specifically for motorkhana and khanacross.

There are a lot of rules governing these specially built cars. I won't get into too much detail on the rules other than to say they have forced me to do some things I wouldn't have otherwise done. Such as the height of the sides.

It is running 12 x 12v high cranking AGM batteries (I don't need much range). The motor is an Advanced DC 9 inch and controller is a Curtis.

I anticipate the vehicle will weigh around 600kg or 1300 pounds (including batteries but excluding the driver). I have gone with front wheel drive because in motorkhana there is a lot of handbrake turns which chew up rear tyres(tires) and a front wheel drive allows me to put track tyres on the front and old rubbish tyres on the back to save expense. Most of the special vehicles I compete against a quite a bit smaller than the one I have made. This does give them an advantage as they can slalom their way through the flags without dropping as much speed. I have stuck with a two seat arrangement becuse it allows me to retain all the running gear (driveshafts etc) from the donor car. Plus it allows me to take passengers and scare the pants off them.

I am after some advice regarding safety. I am really after some way to detect ANY voltage on the chassis and trigger a shut down of the contactors to remove the source of the voltage. But I want to do this without introducing any connection from the Battery (Traction) -ve to the chassis. Any ideas?

I started the project back in March and I am on track to finish before Christmas and that gives me a few weeks for refinement before the first event on Feb 7th.

I currently use a Nissan Pulsar N13 for these events
Here is an example of a motorkhana:
https://www.youtube.com/watch?v=bLq14ZV8xGE
Here is an example of a khanacross:
https://www.youtube.com/watch?v=PodS9wEFQNg


I look forward to any thoughts you might have
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  #2  
Old 11-16-2015, 08:05 PM
galderdi galderdi is offline
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Default Re: Autocross EV special

Here are a couple more, earlier photos
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File Type: jpg 20151005_154410_resized b.jpg (82.9 KB, 61 views)
File Type: jpg 20151005_155335_resized_1b.jpg (166.0 KB, 50 views)
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Old 11-16-2015, 08:23 PM
bigmouse bigmouse is offline
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Default Re: Autocross EV special

In order to measure voltage (or current) between the chassis and the HV battery, you have to have some sort of connection between them. High-value resistor chains approximate isolation for safety purposes. Something in the range of 5 mega-ohm would be acceptable to most regulatory bodies. Measuring the voltage across one of those resistors will tell you how much current is flowing. You need to have two of these setups in case the loss of isolation happens on the same node as the resistor (no current will flow).

There are products you can buy that do this.

If you REALLY need to have complete isolation, then you'll have to get fancy with a capacitor and AC excitation to detect losses of isolation.

As for opening the contactors, you'll have to integrate something with the isolation detector to drive the contactors either directly or through a relay.
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Old 11-17-2015, 03:03 PM
galderdi galderdi is offline
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Default Re: Autocross EV special

Someone outside the forums made a suggestion to split the battery packs to reduce the voltage. I am thinking about taking that principle one step further. I think if I run the -ve from the -ve of the last battery in the chain to the -ve of my dual pole circuit breaker. Then run the other side of the circuit breaker to the chassis. The most it would ever see is 12v if the insulation failed but it would still be enough to trip the circuit breaker well before anyone ever comes in contact with it. I will do a test tonight to prove my theory. The two outcomes I will be looking for are (1) does the circuit breaker trip when connected to the -ve of the last battery? (I think this is fairly certain). (2) With 2 batteries as the test do I see 12v or 24v across the circuit breaker (again I am fairly certain it will be 12v).

I will do a circuit diagram to demonstrate the concept.
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Old 11-17-2015, 03:45 PM
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Default Re: Autocross EV special

Isolation detection is tricky to do right. Your most likely source of loss of isolation is an instrument that you thought was isolated followed by something like a buildup of brush dust in the motor. This can cause a bridge from the brush holders to the motor case. And in DC motor controllers usually the positive rail is connected through to one brush all the time.

As soon as you are hooked on the electric version of this switch to Lithiums. 1/4 the weight for the same stated capacity although in practice it can be as much at twice this.

Good luck on your racing. It looks like it will be a lot of fun.
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Old 11-18-2015, 05:58 AM
arklan arklan is offline
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Default Re: Autocross EV special

could u use a comparator for this?

and also as doug said,
my car has a 4Mohm connection from the negative to the chassis through the motor, if u get a bit of wire from the positive and scrape it along the car, it will get little sparks as if it were from a 9v battery, also makes me feel warm and fuzzy if i bump anything while playing around in the battery section.
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Old 11-19-2015, 08:51 PM
galderdi galderdi is offline
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Default Re: Autocross EV special

Thanks for the ideas. The resisitor idea is a good one.

I did a test this morning.
The circuit breaker will trip if it sees any positive voltage on the chassis but the maximum it will see is 12v.
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Old 11-26-2015, 04:18 PM
galderdi galderdi is offline
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Default Re: Autocross EV special

Hmmm actually rethinking my idea....it isn't any good. Yes it will trip the circuit breaker and yes it will only see the 12v under normal circumstances. But I didn't consider that connecting the -ve from the last cell is also connecting the +ve from the remaining 11 cells. This would mean if for some reason I came into contact with the chassis and the Traction pack earth I would have 132V running through me (exactly what I am trying to avoid).

However I am thinking I could solve that complication using a big diode to prevent positive voltage passing back through the circuit breaker. I will perform another test this weekend.

What advantage is gained by earthing the Chassis?
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Old 11-26-2015, 05:50 PM
arklan arklan is offline
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Default Re: Autocross EV special

semiconductor fuses are big fat diodes so u can do 2 things in one

earthing the chassis means that u dont need to run so many wires for your 12v system, thats it.

if u r talking about earthing your traction pack, yeah, dont do that
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Old 11-26-2015, 05:58 PM
galderdi galderdi is offline
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Default Re: Autocross EV special

Quote:
Originally Posted by arklan View Post
semiconductor fuses are big fat diodes so u can do 2 things in one

earthing the chassis means that u dont need to run so many wires for your 12v system, thats it.

if u r talking about earthing your traction pack, yeah, dont do that
Agreed, Earthing the chassis is what I am trying to avoid.

But I want some protection incase the insulation ever fails.
So my current thinking is to run the -ve from the last cell through a diode (or multiple to handle about 15amps) through to a 10amp circuit breaker then to the chassis. Then if the insulation ever fails and puts +ve voltage to the chassis (anything more than 10amp) it will trip the circuit breaker and disconnect all the power circuits.

Effectively I will be earthing the chassis from the traction pack but only from the last cell. Not the full 12 cells. I will attach a diagram shortly.
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