Kermit comes home

Kermit left his home of the past 15 months at TechShop, and made the 10 mile trip back to my house in Mountain View this evening. It was a little bit nervewracking setting out on the open road for the first time, but we made it without assistance from AAA. I set the maximum current to the motor at 30 amps, and limited the maximum RPM to about 3600, so I definitely didn't set any speed records. It was a trick accelerating from a stop through certain traffic lights before they turned yellow again, and I'm sure I irritated more than one other driver. Accelerating from 0-5 mph was very difficult, with torque not picking up until the car got going a bit. Once rolling, though, it felt peppy, at least compared with the stodgy original acceleration.

I need to fiddle with the voltage, current and frequency parameters a bit until I get acceptable performance. Mostly, I wanted to be assured of getting home without overheating my motor, which was no problem. I could hold my hand on the motor once I got home.

It feels really good to have come this far. Even though the car is by no means done, it is a usable means of transport at this point, and I will be driving it to work and dragging groceries home.

I'll keep updating this space with further news as Kermit rolls on.

We don't need no stinkin' capacitors

One solution for the inrush shenanigans described in the last post is to construct an inrush limiting circuit with a large resistor and a contactor- the capacitor is charged through the resistor until the operating voltage is reached, when the contactor is closed, bypassing the resistor. The other way is to yank out the capacitor. The purpose of the capacitor is to filter the rectified AC mains supply so that it is a smooth DC source for the inverter. A battery is already a nice smooth DC source, so the capacitor isn't really needed. I already ditched the 20-plus pound inductor that was the other part of the line filter, so I had no qualms about getting rid of the capacitor. Luckily, the capacitor was its own easily removable module:
The other plus is now the inverter can be put in a much smaller box, and will have considerably less need for interior ventilation, making weather sealing a possibility.

Of course I tested the capacitorless inverter/motor setup the way my pappy taught me -- by revvin' er up real good a few times.

Kermit on a roll

Well, the big day finally arrived. Kermit moved under his own power for the first time! It was very exciting doing a few laps around TechShop's parking lot. Matt Heck took some video, which I'll post here soon.

There are a few issues before I can drive it home, however. The first is that when I connected the pack to the inverter for the first time, the inrush current to charge the capacitors caused a spark that was, well, cinematic. Of course I had all my safety gear on (gloves, face shield, etc.) and I was expecting a spark, so no injuries or soiled britches, but jeez... the blinding flash and deafening KAPOW were pretty exciting. Obviously, we can't have that kind of drama each time we fire up Kermit to go get some milk.

The second issue is that the 12V system isn't working yet, both because the DC-DC converter isn't converting, and because interfacing with the MGB wiring is a real head scratcher.

The third thing is related to the control issues mentioned in the last post. As I was driving around the parking lot, it was very difficult to accelerate and decelerate smoothly. The good news is, there's plenty of torque.

Control issues

The way the motor controller is currently hooked up, I have the throttle position sensor supplying a voltage to the speed control input of the inverter. The problem with this is that in all other cars, pressing on the accelerator gives more torque, not more speed. Some inverters will allow a torque input, but with my motor/inverter combination, that isn't possible. What my inverter does have, however, is an analog output that represents the current being given to the motor at any instant. If I connect an opamp with the positive input to the throttle position sensor and the negative input to the current output from the inverter, and feed the output of the opamp in to the speed control input, the throttle position sensor should control the current supplied to the motor. Current is related to torque, so this hack should make the car a lot more drivable.

I'll wait until it gets around the block under its own power to try this, though.

Kermit gets some batteries

I took a couple of days off of work to make a final push on getting the car rolling, and it paid off. I finished all the sheet metal work around the battery box, and gave it a shot of primer gray. I also finished up all the wiring. I used 3/4" flexible waterproof conduit run in the tunnel above the driveshaft and transmission from the battery box to the front of the car where the inverter lives. I got 70 cells (the remaining 28 are still being conditioned) and installed them:I was hoping to take it for a little spin with this partial pack, but 244 volts from these 70 cells was not enough to start up the DC-DC converters for the 12 volt system, and without 12 volts, the main contactor would not close, so no joyriding yet. I should get the remaining cells put in sometime this week.