Electric car conversion: Project ForkenSwift

[MetroMPG]

Quote:
Originally Posted by SW
1. Are the the coils of the contactor solenoids intended to operate at 36 - 48V?

Yes - they're out of the forklift. It only had 1 pack (no 12v), so everything was 36/48 rated.

Quote:
2. Are you connecting the 12V battery ground and the 36V battery grounds together? Or is only one connected to the body (I assume atleast the 12V gnd is or you'd have to rewire the rest of the car).

12v is grounded to the chassis; 36/48 is isolated (not grounded to chassis - that's what you call electrically isolated, right?)

Quote:
3. The controller looks as if it regulates only the negative connection to the motor.

That's right.

Quote:
If the negative side of the battery is always connected to ground (once the neg contactor is closed) and the positive is always connected to the motor (once the positive contactor is closed), then there is a risk of any short (e.g. dropping a wrench) between ground and the negative motor connections throwing the motor into full power (serious lurch).

But for that scenario to happen, someone would have to be pressing the go pedal (to close the pack +ve contactor) while I was dropping the wrench. Not too likely to be wrenching around under those circumstances.

[MetroMPG]

professor SW continues...

Originally Posted by SW

Yes.

Well, yes isolating the ground should greatly reduce such a risk. BTW contactors sometimes do stick.

4th question:

4. Does the pot box micro switch have a normally-closed (NC) position?
If so, you may be able to further mitigate accidental lurches the way Jerry does with that latching relay, though in your case, you'd may need to use a DPDT relay. Let me think about that.

[MetroMPG]

If I understand Jerry's setup, his pot box latching relay prevents the pack +ve contactor from closing if the ignition is switched on while the go pedal is pressed.

His approach is sort of equivalent to forcing you to depress the clutch before starting a manual, or pressing the brake to shift out of park.

I omitted that precaution.

[MetroMPG]

SW continues...

Right again. It does make sense to omit that interlock if you're the only one who will drive it. Some people (like my mother) have a tendency to step on the gas pedal when turning on a car - that can be bad since there's no engine that must first be started. I believe the interlock is required (in the US) of EV manufacturers but I don't think it applies here.

OK, so back to the ground issue. Your shematic shows the negative connections to the contactor coils with a "chassis ground" symbol but there is no place where that ground actually connects to the negative side of the 36V. I assume those connections go to the point just after the shunt.

I would put a small fuse (1A or less)) on the + side of both the voltmeter and the ammeter. It's just a precaution and won't affect their accuracy.

Also you should fuse the controller +36V small wire, and the clock wire (it's better than having burned wires - see I do have a serious side ). The Ammeter fuse may not be needed but if the shunt ever comes loose, it'll save the meter from being fried.

Maybe like this?

[MetroMPG]

Awesome - thanks.

OK, so maybe I should have an interlock. I never thought it was a problem, but if there's a law...

Some (most?) controllers also have a built in "high pedal interlock", which uses internal logic to prevent the controller from working if the pot input is not "idle" when the Key Switch Input is switched on. But not this one.

So essentially everything gets fused? Should I put a fuse inline with the potbox microswitch too?

Here's another question: someone asked why I was using pack voltage to control the main contactors. Do you happen to know if they'll work on 12v? Or will they just pull more current and get hotter?

[MetroMPG]

PS - didn't realize I was mis-using the chassis ground symbol. Learned sumpin' new today! Just in time too - the day's almost over, here.

[MetroMPG]

SW...

I think it's more of a dummy-proofer. A boss of mine once told me "you can't make anything foolproof because fools are so ingenious."

I seriously think a kill switch would more worthy of your time and more likely to be appreciated by first responders. Another place for an interlock is on any opening compartment that exposes the traction wiring - but then again good insulating covers may be better overall. Those kind of interlocks too often get defeated while the thing is in "R&D."

Pretty much. Besides being a safety valve of sorts, it's also a way to disconnect parts of the circuit w/o putting switches everywhere. The potbox probably doesn't need a fuse because it's just a 'sense' circuit. The 5k pot varies from a dead short to 5,000 ohms and is just a voltage divider for the controller input. It shouldn't have much current flowing if all connections remain correct. Besides some controller internal malfunction, the only danger would come from some higher power wire coming in contact it. This is where the quality of your physical electrical connections will pay off in overall reliability.

It would be easier to wire if everything except 'power' was 12V but it depends on the parts themselves.

A 36 volt contactor coil energized with 12V won't pull more current because of the lower voltage (prob less) and thereby heat up, but there could be another problem with the lower V. If as you said, they came out of the forklilft as 36V devices, then 12V may not be enough to get them to close firmly. If they engage weakly, or worse chatter, then the power contacts will probably arc, self destruct, and otherwise ruin your day.

Also, if the correct V is 36V, then 48V into the contactor coils would entail more power draw and more heat, but I don't know if that's destructive. If it is and you still want to go to 48V or more, there are ways you could handle that with varying amounts of scope creep:

1. Just use a wire coming from the third battery (36V worth) for the contactor circuit. This one falls into the "if it's stupid and it works then it ain't stupid" category. It means assymetrical loading of the batteries but maybe only slightly.
2. Use an electronic regulator circuit to drop the 48V down to 36V for those coils (it's not as complicated as it sounds).
3. Change the contactors to 12V ones (prob the most expensive).

BTW, I'm pretty sure the small wire to the controller still has to be at full pack V.

[MetroMPG]

Battery news: just got off the phone with the company that sold us the forklift. Apparently we're welcome to any 6v floodies they swap out of customers' equipment - gratis.

They don't get them in too frequently, and the ones they have will be old and tired (around 4 yrs) and will probably have been neglected by the customer.

But I'm willing to try "free", to start. They'll have four for us to pick up on Monday.

[MetroMPG]

DAX:

Floodie = wet cell?

Yup. Flooded lead acid batteries, that need periodic watering to replace electrolyte lost during charging.

Golf carts, industrial floor sweepers, and most EV conversions use 6v floodies.

[MetroMPG]

Small world, this Internet...

Len joins the conversation, saying: That's my car...

I was just browsing along seeing how things were going with Project Forkenswift and I clicked on the link and... THAT'S MY CAR.

That's so funny.

I just purchased it in December and while it does run, there are problems with it--the Forward/Reverse contactor spits blue-green flame in reverse and when I try to go forward at low speed it jumps like a jerky start in a clutch--if I press down the pedal more it evens out but apparently I can't use low speed.

With the problems I haven't tried taking it further than the end of the block so far.