Electric car conversion: Project ForkenSwift

[MetroMPG]

http://www.youtube.com/watch?v=2Ins29dqbac

[MetroMPG]

DAX asks...

Nice! Is the 20 km/h in 1st gear? What do you estimate it will be with 48V?

[MetroMPG]

Dax, the current limit of the golf cart controller is 275 A. And that value drops as it heats up from max duty cycle - 275 A is its 2 minute rating; 200 A = 5 min; 125 A = 1 hr.

(EDIT: and it was in 2nd - but I didn't have it "wide open" I would say 1/3 to 1/2 on the pot).

The forklift controller limit is in the neighbourhood of 400A, but it also has a bypass, which will feed full battery current to the motor under certain (logic) conditions.

So with the golf cart controller at 36V, we're limited to the equivalent of 275 A * 36 V / 746 W/HP = 13.3 HP. At 48 V we'd get 17.7 hp. (Not counting losses.)

EDIT: aw, what the heck - let's throw in some loss estimates too. PWM controllers are typically described as "above 90% efficient", and the motor may be 75% efficient. So now we're down to:

HP (golf cart controller @ 36v) = 13.3 * .9 * .75 = 9.0 HP
HP (golf cart controller @ 48v) = 17.7 * .9 * .75 = 12.0 HP

Still, 9 HP is theoretically enough to go 55 mph in this car on level ground (assuming you don't run out of power before you eventually accelerate to that speed). What else am I missing?

[MetroMPG]

SW makes another good observation and then furthers my education some more...

Originally Posted by MetroMPG
The forklift controller limit is in the neighbourhood of 400A, but it also has a bypass, which will feed full battery current to the motor under certain (logic) conditions.

Originally Posted by SW

That must be analogous to a tourque-converter lockup clutch. It does make a lot of sense. At those high amperages, the semiconductors are just ohmic switches dissipating a lot of loss because of their even slight resistances. If the duty cycle of the PWM gets close enough to 100%, it might as well use a relay (metal) instead of silicon.

EDIT: that should mean that the system should be more efficient at the equivalent of WOT.

BTW, silicon resistance goes down with temperature. That's why there have to be preset thermal or current limits or they will fuse together (read self-destruct). When you do overload a power transistor, you will be able to see glowing silicon a split second after it pops the lid off the device's case and a split second before it clouds the view with smoke. Don't ask me how I know this.

[MetroMPG]

Some more nuggets from SW:

Quote:
Originally Posted by MetroMPG
Still, 9 HP is theoretically enough to go 55 mph in this car on level ground (assuming you don't run out of power before you eventually accelerate to that speed). What else am I missing?

Originally Posted by SW

That's a steady-state or straight and level cruise. I think your system's peak current limit will determine your acceleration rate and ability to climb hills. (edit: I mean that 9 hp may mean 0-50kmh in approximately 10 or 15 minutes ) - edit 2: when you hit the gas pedal, your laptop application can just show an hourglass, or better yet, one of those calendars with the individual date sheets blowing off in the wind.

[MetroMPG]

Originally Posted by SW
That must be analogous to a tourque-converter lockup clutch. EDIT: that should mean that the system should be more efficient at the equivalent of WOT.

Nice analogy!

That's one of the advantages of contactor controller setups, a la 3-stage setup in the CitiCar / ComutaCar. Notwithstanding the first speed stage (where a beefy resistor limits voltage to permit "creep" speeds), it's a 100% efficient controller approach, effectively always at WOT.

But the catch is that you're pulling high current more often with WOT, and Peukert says that'll reduce the total energy you can get from a PBA battery. Depending on driving needs/ability, you might get better range with a less efficient PWM (assuming you use the PWM to keep the amps low and accelerate gently). I suspect PWM is better for variable speed (ie: stop 'n' go) driving; and straight contactors would be more efficient in steady speed driving of longer duration.

Ideally, we want a PWM for stop 'n' go and a bypass circuit in with the Curtis for cruising efficiency (though it'll only work at one or possibly 2 speeds - 1st or 2nd gear).

I think your system's peak current limit will determine your acceleration rate and ability to climb hills.

Oh definitely. This will be a side street-using, hill-avoiding car for sure.

[MetroMPG]

Thinking about EV hypermiling techniques: I wonder if the car will coast further in neutral or in gear when off the go pedal...

[MetroMPG]

Someone tells me his landlord (electronics teacher) has a CitiCar, and wonders what kind of motor he should watch for to do his own EV conversion...

Sounds like you've got the ideal situation with your landlord being an electronics teacher.

I learned how to advance the brushes to change a motor from electrically retarded to electrically advanced, but I'm having trouble doing the same thing to myself.

Ask your landlord if he has his car listed in the EV Album (http://evalbum.com)

The type of motor you should watch for should be of the DC variety. I suspect the kind you threw out were AC motors. Some people do AC conversions, but the parts aren't as common and so tend to be a lot more money. Also the system voltages tend to be really hgh, which makes for some pretty spendy battery packs.

Just as a rough idea, the physical size of DC motor people most often use in small car conversions are around 6.5-8 inches diameter, about 12-16 inches long, and in the neighbourhood of 100 lbs.

Find yourself a forklift graveyard, or an existing conversion that someone started but didn't finish, or finished but eventually lost interest in (these will be cheap and have dead batteries).

One of the first things you need to do is ask what the minimum requirements of your conversion are: how far & how fast? The response to that is pretty much proportional to "how much"

[MetroMPG]

Schematic time...



(With thanks to http://www.evconvert.com/eve/schematic-update)

It's pretty simple, but I'm sure we've overlooked stuff. Fuses?

1. Amps and Volts are always on. (Is that bad? I thought it would be nice to be able to stick your head in the window and check the voltage without having to turn the key.)

2. Switching on the ignition activates a relay that sends power to both the pot microswitch, the controller Key Switch Input, and brings in the pack -ve contactor.

3. Pressing the go pedal a tiny amount activates the pot microswitch which pulls in the pack +ve contactor, activates the "hours" meter, and makes the controller "live".

4. Pressing the go pedal further moves the pot wiper which the controller interprets for controlling the motor.

EDIT: so I have 2 possible escapes from a full-on controller failure: releasing the go pedal, and/or switching off the key.

EDIT 2: the hour meter is just for fun. It came with the forklift, so why not use it

[MetroMPG]

About the schematic, SW inquires ....

OK, now you're talking. I have some questions.
1. Are the the coils of the contactor solenoids intended to operate at 36 - 48V?

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).

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