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Discussion Starter · #1 ·
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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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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Discussion Starter · #4 ·
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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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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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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Discussion Starter · #7 ·
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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Discussion Starter · #8 ·
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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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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Discussion Starter · #10 ·
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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Discussion Starter · #14 ·
On Friday I weighed the car. It is currently 475KG. I still have a few things to add so I expect that to grow to about 530kg but thats still a lot less than the 600kg I was predicting.
 

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Is there a body and windshield?

Also how long are your runs and how many amps will you draw?

Looks like a weapon

Cheers Kiwi
 

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Discussion Starter · #18 ·
Is there a body and windshield?

Also how long are your runs and how many amps will you draw?

Looks like a weapon

Cheers Kiwi

The body will be a continuation of the right side you can see in that latest photo. It will continue around the back and sides at that same height. It is 0.4mm polycarbonate and weighs next to nothing.

There will not be any windscreen. We run on bitumen and are required to wear a helmet with visor.

The runs are anything from 20 seconds to 90 seconds. Theoretically it could draw 400amps but in reality I don't think it will need anything like that. I really won't know until the first drive. My best guess is peaks of 300amps for a couple of seconds here and there.

In between each run there are gaps of 5-10 minutes where the car will remain idle, giving everything a chance to cool (including the driver :) ) The hardest part will be looking for shade between runs to assist the cooling.

I don't think it will be the most competitive in outright speed as it is a bit bigger and heavier than some of the petrol driven competitors. Most of those are single seaters. But I am aiming to compensate using the torque and reliability. I'll let you know if it worked at the end of 2016.
 

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Discussion Starter · #19 ·
Looks like a fun car

Shame you are on the wrong side of the ditch - I would love to try "The Device" against you and EVSonic

http://www.diyelectriccar.com/forum...-dubious-device-44370p8.html?highlight=duncan

I suspect I am older than the two of you put together so you would win - but it would be fun
Swing on over with or without the car. I'd be happy to let you drive the BATTmobile in a Motorkhana. The dates for suitable events are Sept 4th, or Nov 27th.
That is one of the reasons I made it a two seater I get almost as much enjoyment letting other people thrash it.

I just hope the performance is up to scratch.
 

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Discussion Starter · #20 · (Edited)
Here is a challenge for you all.

I have plugged all the figures into a performance simulator found on this forum. So I have a hypothetical idea how it will perform. But I was wondering what other people might predict. So please post your 0-35MPH prediction here by Feb 6th. I will be competing on Feb 7th so I will have the results and video evidence. I have chosen 35mph as the target because the event is a motorkhana and 60mph may not be practical at this course.

The vehicles specifics are:


  • total weight without driver 1168 pounds
  • total weight with driver 1433 pounds
  • it is frount wheel drive with a viscous LSD
  • tyres (tires) are treaded track tyres 205/15/16
  • I generally run the tyres at 25 PSI cold
  • The event is in Feb and this test is likely to occur around 9am so I expect the ambiant temp to be about 84°F (29°C)
  • The voltage is 149V before I start
  • The current is unknown at this time but the controller and motor can handle 400 amps
  • The batteries can discharge at 410amps for up to 20 seconds (Assuming 77°F (25°C))
  • The gearing is as follows - 1=3.063 2=1.826 3=1.286 4=0.975 5=0.83 Final drive=4.167
 
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