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Discussion Starter #1
The reason I ask is because I am collecting parts to do a super cheap conversion of a Mazda MX-3. I basically have everything I need already. It will be a low performance city runabout/commuter and to keep costs way down I am using an Alltrax 7245 that I picked up for $300. I am going clutchless, using a 7.5"X18" prestolite motor. I am using an Allbright SW200 contacter with blowouts, whatever that means!

Questions:
I hear Alltrax controllers are bulletproof, but has anyone ever seen one fail to cause a runaway (other than incorrect installation or lack of precharge)?
Is my contactor a good enough safety measure or do I need more redundancy? Golf carts only ever have one contactor and I have never heard of one scaring any golfers.

Thanks!
 

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Questions:
I hear Alltrax controllers are bulletproof, but has anyone ever seen one fail to cause a runaway (other than incorrect installation or lack of precharge)?
Is my contactor a good enough safety measure or do I need more redundancy?
Hi mech,

I have never used an Alltrax. I have never had a DC motor controller fail "on" and cause a runaway. But I have had a number of other motor controllers (AC and DC) fail which resulted in the main switch (mosfet or IGBT) being shorted. And I was behind the wheel of a very expensive EV which was "stuck" full throttle reverse inside a building. I have also known other throttle stuck incidents resulting in wrecks. A full throttle condition can result in wheel torque well in excess of that which the brakes can hold and even when properly fused, will give you the ride of your life. In my case, I was lucky, or good enough to hit the off button in time to save the car from damage but did leave about 10 feet of rubber on the floor.

In the controllers which I have had fail with shorted IGBTs, all have quickly blown the fuse. It is possible that a shorted type failure could take longer to blow a fuse. It depends on a lot of factors.

It is my recommendation that you use a redundant safety system on the propulsion system which includes the second contactor or manual disconnect within reach of the operator. At the race track, or golf course, is one thing. But taking your converted EVcar on public streets is quite another. We don't want some innocent bystander run over due to a failure in something you built.

I like to use 2 contactors in the main battery feed circuit plus the appropriate fuse. A contactor in the negative battery line turns on with the keyswitch. It can always be turned off with the keyswitch. A contactor in the positive battery line handles the precharge and is controlled by either the motor controller or by a switch for system enable located on the instrument panel. For the race EVs (cars, bikes and carts), a kill switch is also required on the steering wheel (handlebar) and also clearly marked on the outside rear panel.

It is not just a single case failure you protect against. Maybe the Alltrax controller failure runaway odds are so great you can justify not having that redundancy. But the thing which will kill you is that which you haven't foreseen. Make the thing as safe as you can.

major

edit:
I recall a runaway electric cart shown on TV this past football season. It mowed down several people on the field before it could be stopped. It wasn’t due to a runaway controller, but an item fallen against the throttle pedal, I think. Fortunately nobody was injured. But it does show that sh!t happens.
 

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Discussion Starter #3
My plan is to install some sort of manual disconnect tied to the ex-clutch pedal, but unfortunately this car has a hydraulic clutch so it might be a bit of work to make it happen.

Just a side note. I have been talking this project over with my electrical engineering buddy who has worked on electric vehicles. He says that the number one rule in electrical engineering is never disconnect a circuit on the negative side, so I should only put my redundant safety devices on the positive side. That seems counter to what is often done in DIY projects, where contactors are put on both the positive and negative side. I realize that he is an engineer, often working in the theoretical world, but maybe he's right.

To me, opening a circuit is opening a circuit, whether on the positive side, negative side or even between any two batteries in a string.
 

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My plan is to install some sort of manual disconnect tied to the ex-clutch pedal, but unfortunately this car has a hydraulic clutch so it might be a bit of work to make it happen.

Just a side note. I have been talking this project over with my electrical engineering buddy who has worked on electric vehicles. He says that the number one rule in electrical engineering is never disconnect a circuit on the negative side, so I should only put my redundant safety devices on the positive side. That seems counter to what is often done in DIY projects, where contactors are put on both the positive and negative side. I realize that he is an engineer, often working in the theoretical world, but maybe he's right.

To me, opening a circuit is opening a circuit, whether on the positive side, negative side or even between any two batteries in a string.
Your buddy's advice would make sense only if the chassis was tied to the negative side. In an EV where the HV circuit is essentially floating relative to the chassis, you have to regard both sides as equally capable of causing injury or arc damage, and it's not really 'safe' unless both sides are disconnected from the controller.

On a regular car, it's wise practice when disconnecting the battery, to disconnect the negative terminal first. The reason for that is, if you swing a wrench that is touching a battery terminal and the other end touches the chassis, you don't want to be completing a 12V-to-ground circuit through the wrench. Having personally done something like that where the circuit also involved rings on my fingers, I can attest that it's Very Bad.
 

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.......my electrical engineering buddy who has worked on electric vehicles. He says that the number one rule in electrical engineering is never disconnect a circuit on the negative side,....
That is BS. There is no #1 rule and there is no universal reason not to switch the negative side of a circuit.
 

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While it may not matter which side of the pack you switch, it does matter how you connect it. Make sure that you follow the polarity marked on the Contactor since it has Blowouts. They won't extinguish the arc if connected backwards.
 

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Discussion Starter #7
That is BS. There is no #1 rule and there is universal reason not to switch the negative side of a circuit.
I would agree.

Maybe I should get my engineer buddy to explain why Ford cars do the switching of most circuits on the negative side (but maybe that's an argument against it, being Ford and all!). Or why most electronic switching of fuel injectors is done on the negative side.

But back to the subject at hand. I guess one contactor is not enough for safety sake. I will plan on using a redundant disconnect of some sort.
 

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I heard of a Gizmo with an Altrax that had a fail full on which ended up burning up the Gizmo so it can happen. The Gizmo has flammable fiberglass body parts. After that the owner added a BRB to all the Gizmos he had. One place you can get them is at http://www.tecknowledgey.com/catalog/product_info.php?cPath=135&products_id=1514 If you put you could put a copper bar across both posts to share the current. Just place the BRB where the driver can hit it in an emergency.
 

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I would agree.

Maybe I should get my engineer buddy to explain why Ford cars do the switching of most circuits on the negative side (but maybe that's an argument against it, being Ford and all!). Or why most electronic switching of fuel injectors is done on the negative side.
Mostly it is because someone noticed way back in the Model T days that a switch on the "return" side didn't arc or burn as much as the hot side and therefore could seemingly handle more current without destruction.. Down side is un commanded operation if the circuit gets chassis grounded, but that is why the fuses are on the HOT side.
 

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Mostly it is because someone noticed way back in the Model T days that a switch on the "return" side didn't arc or burn as much as the hot side and therefore could seemingly handle more current without destruction.. Down side is un commanded operation if the circuit gets chassis grounded, but that is why the fuses are on the HOT side.
Can someone explain this to a simple metal-basher? A basic mechanical switch consists of two contacts, so whichever way the current flows one will be positive and the other will be negative. What difference does it make which 'side' of the circuit it's on?
 

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MalcolmB,

It's because his has magnetic blowouts. If you don't get the polarity right, the magnets don't push the arc out of the way when you open under load, they actually pull the arc closer to the contacts, prolonging the arc. This decreases the ability to break current. You could go from being able to break 200A to only being able to break 100A.

That can be important when you're breaking a few hundred amps.
 

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Yes, I understand that magnetic blowouts make a switch polarity sensitive Travis, but piotrsko gave an example of switches used in Model T days, which I doubt had magnetic blowouts...
 

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Oh, I gotcha.... wasn't sure, so I explained, more for the original poster.

I'm not sure why, if there was a reason, that it should be one way as opposed to another.
 

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Discussion Starter #14
I'm not sure why, if there was a reason, that it should be one way as opposed to another.
I do know that switches tend to last longer when switching on the negative side because of less arcing. This is why newer ECUs usually switch higher amp circuits like fuel injectors on the negative side.

Flashbacks from trade school. It was originally thought that electricity flows from positive to negative (conventional theory), which is why the majority of circuits are switched in the positive side. Now it is understood that electricity actually flows from negative to positive (electron theory), which confuses me more about what my engineering buddy was taught. I will ask him about it.
 

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Discussion Starter #15
Ok...major sidetrack from original question, but it was my questions so I can do this!
I just talked to engineer buddy and he says switching on the neg side isn't the question. He said that you are never supposed to put a fuse/breaker on the neg side. This would mean a contactor for an emergency runaway. All emergency breakers must be installed on the pos side. This is what is practiced electrical engineering.

So while Ford switches most vehicle electric circuits on the neg side, the fuse is obviously on the pos side.
 

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He said that you are never supposed to put a fuse/breaker on the neg side. This would mean a contactor for an emergency runaway. All emergency breakers must be installed on the pos side. This is what is practiced electrical engineering.
Mechman,

This advice sounds logical for grounded systems, like the automotive 12V auxiliary where 12V negative is common to the chassis. But with the EV propulsion battery pack (high voltage), there is no "ground". It is isolated from the vehicle chassis both positive and negative side. Therefore, the redundant switch (contactor or disconnect) serves the interest of safety better to be on the opposite polarity lead from the battery, or in some cases, mid-pack in the battery.

Regards,

major
 

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Discussion Starter #17
I agree, Major. But said engineering buddy has designed EV traction systems and is adamant that he is right.
Obviously nobody here has had any ill effects of putting circuit breakers in "non-engineering accepted" places or your advice would be much different.

Now I just need to invent a super low cost disconnect that will be activated by my ex-hydraulic clutch. Eep.

Thanks!
 

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IObviously nobody here has had any ill effects of putting circuit breakers in "non-engineering accepted" places or your advice would be much different.
"non-engineering accepted" :confused: All you have is second hand hearsay. Give me a real reference please. Something like from SAE or IEEE or UL or NEMA or NEC.
 

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I do know that switches tend to last longer when switching on the negative side because of less arcing. This is why newer ECUs usually switch higher amp circuits like fuel injectors on the negative side.....
Actually... the reason why the negative end of stuff is switched more often is a purely economical one - N-channel mosfets and NPN bipolar transistors are much less expensive than their opposite polarity counterparts.

EDIT - your "EE" friend might be adamant, but he is still wrong - there is absolutely no difference between switching the positive or negative leg of a circuit. Kirchhoff proved that in the 1800's.
 
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