Thursday, February 7, 2008

Parts on the way, and motor notes

I got notification from Azure Dynamics that my controller, motor, DC-DC converter, and interface harness have shipped. They should arrive in Colorado tomorrow. I'll put pictures up when they get here; in the meantime, here are some notes on why I selected the motor I did.

Basically, a sports car (like the volt914) is designed with a gearing ratio that encourages higher RPM. A SUV, like the ElectroJeep, is heavier, and the tranny typically has a lower gearing ratio. So, I was looking for low-RPM torque, and not too concerned with high-RPM perf. Here is a chart I made that combined two charts from Azure (along with an estimated "AC24L" motor that TimK mentions in this post on his 914ev blog). This plots torque vs. RPM. Note the *much* higher torque at low RPM, but the lower torque at higher RPM. The crossover happens roughly at 3000-3500 RPM. Edit - I added data for the Advanced DC 9" motor for comparison purposes:

Most people are accustomed to thinking of motor performance in horsepower, so here is the same data recalucated for HP:

I think this chart illustrates well why the AC55 is suitable for truck / SUV applications, whereas the AC24 is much better for smaller, sporty cars. This higher torque / HP is going to be especially important for the Jeep, since its curb weight is 50% higher than the Porsche, and its battery weight will similarly be 50% higher.

Time to look into 4-wheel disc brakes...


Dave Hale said...

What do you think about an AC55 for a 914 conversion?

I had a brief conversation with ElectroAuto, and before we lost connection they suggested I use the AC55 instead of the AC24 for the porsche 914 (opposite of what you are saying).

I just purchased a '72 914 and am getting things together to perform the conversion in the fall.

Ross Cunniff said...

I don't know why they would say that. There are several strikes against the AC55 in a 914:

1. It requires many more batteries - 26 instead of 18 - you will have to fill your rear trunk with batteries to make it work

2. The motor is much bigger and heavier - bigger means less room for batteries in the engine compartment, heavier means, well, heavier :-)

3. The increased weight of the motor plus the increased battery weight means around 600 extra pounds - a 20% increase in overall finished gross vehicle weight.

If you are looking for higher performance, you are probably better off going the DC route (compare the curves of the Advanced DC 9" - higher peak, more low-RPM torque, works at 144V, and so on).

laptop battery said...
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Dave Hale said...

I agree with your assessment, Ross. In any case, I ordered the AC24LS with my kit. "6-8 weeks" now, as they say...