KermitTheCar

Todo list

2009-04-26T15:38:00.001-07:00

Here's a rundown of what needs to be done before the car is drivable.

1. Mount inverter on soft mounts
2. Find an automotive fan and build a mount and shroud so it cools the motor and inverter.
3. Upgrade motor leads. I have 3x10 gauge wires running to the motor right now, but will upgrade to 4 gauge welding cable soon.
4. Sanitize the engine compartment. It's a mess.
5. Build precharge circuit. Brad pointed out that an analog precharge circuit is overcomplicated. He suggested to use a microcontroller to measure the bus voltage and switch the main contactor on. That got me thinking- the inverter has some functions that will do just that. So, here's the new plan: the "start" position of the key switch will activate the precharge relay, which will charge the bus cap through a 1k resistor. Once the bus voltage reaches some minimum value, say 290 volts, the inverter will close one of its built in relays, which will then energize the main contactor. The current to do this will flow through the "on" position of the key swich, so that when the key is turned off, the contactor will open.
6. Get BMS working. Hopefully not much to do here, just wire it up.
7. Find a good location for the charger.
8. Wash. The car has been sitting for so long, it's embarrassingly dirty.

That's not too bad- I should be driving in a year or so---

Inverter fixed

2009-04-26T15:31:00.000-07:00

Today I put the finishing touches on the inverter overhaul, and tested it on the bench. Everything looks good, so I put it in the car for a test fit.

I'll add some mounts and a fan + shroud for the motor and inverter, and the engine compartment will be done. I'm happy with the repackaging- the inverter takes up much less space under the hood, and now can be made weather tight.

Inverter overhaul

2009-04-05T20:05:00.001-07:00

Today I did most of the overhaul of my motor drive. I got a surplus enclosure and am repackaging all of the guts of the drive to be more compact and more weather resistant. Here is the power stage in the new enclosure, with a pen for scale:

I made a crimp tool from a pair of bolt cutters for crimping the lugs onto the 4 gauge wire. It cost $20 rather than the $200 that crimp tools this size normally cost. Obviously, less engineering went into this one, but it seems to work fine.

Here's the drive with the gate drive circuitry in place:
and here it is with the control board in:

and here I've added the main DC link capacitor. I still need to wire it up.

and, finally, here is the buttoned-up box. The heat sink fins stick out the bottom. I will set the whole works on rubberized standoffs on top of the motor subframe. I also need to mount the cooling fans.

Precharge

2009-03-14T10:30:00.000-07:00

After learning that we do in fact need capacitors, I bought a nice big 230 uF capacitor made especially for this purpose:

The problem of limiting inrush current still remains- the main contactor wouldn't last very long under the 1kA or so that cap would draw as it's connected across the battery. I ruled out having the cap upstream of the contactor, because that would mean the contactor would need to be in the fron of the car, and the main high voltage cables running underneath the car would have no means of disconnection, other than manually pulling the pack splitting connections in the back. So, I set off to design a precharge circuit. The principle of this is that when the key is turned on, it applies 12 volts to a small relay that charges the main cap through a 1k or so resistor. This takes about 1 second. Meanwhile, another RC timing circuit charges up and turns on the main contactor. I used the free circuit modeling tool LTSpice to model the timing, and I think it will work nicely. Here's a screen grab of the circuit tool:

Note that for modeling purposes, the 12v ground (V1) and the negative terminal of the battery (V2) are connected, while in the car, they are isolated. The main contactor is modeled by the winding resistance, R1, combined with a voltage-controlled switch, S1. As C1 charges through R2, Q1 turns on and draws current through R1 and turns the main contactor on. The main capacitor C2 has been charging through R4 and the small relay S2. The Schottky diode D1 is to make contactor turnoff instantaneous, and D2 is to subtract D1's bias voltage from the gate of Q1. R3 is to bleed off C1's voltage while the system is off.

Feel free to copy this design at your own risk!

Snubbed!

2009-01-22T21:03:00.000-08:00

Well, it turns out that my assumption that the car failed from overheating was wrong. I dug into the inverter, and discovered that two of the IGBTs had died. The only thing that could really cause this, I reckon, is voltage spikes caused by switching. The problem was that when I removed all the DC bus capacitance (except for some small snubbers near the IGBTs) I didn't count on all the inductance I had added to the system in the form of cabling and the battery pack itself (essentially a big loop of wire). When the IGBTs try to turn off, this inductance causes the bus voltage to rise until something happens, in this case catastrophic failure of the IGBT itself. Pictures of the carnage soon!
Luckily, I was able to source off-the-shelf replacements, which will be arriving along with some bigger snubber caps later this week.

Still learning

2008-11-01T15:14:00.000-07:00

I have learned quite a bit about motors since setting out to rewind mine. First, I was wrong about my old motor being a 2 pole machine. It turns out that most 4 pole motors use something called consequent, or phantom poles, where there are 6 coil groups, 2 for each phase, so identical windings to a 2 pole motor. The difference is, the coil groups opposite each other (in the same phase) are wired to produce the same magnetic polarity, say North, in the air gap. This produces two consequent South poles at 90 degrees to the two North poles, making four total poles for each phase.

As far as the rewinding, I have put that off. I have bought a nicer motor-- aluminum frame, inverter duty, and will probably rewind eventually. For now, though, I will convert it from star to delta topology, which is as simple as bringing the central star point out in three leads. This has the effect of converting a 208 volt motor to 120 volts, meaning I can run at 3033 rpm instead of 1750 at full (240) volts. This means I can run the motor at 10 Hp continuous instead of the 5.5 rated. Since peak torque is roughly 2.5 times rated torque for a motor of this type, I should be able to get 25 Hp peak. How long I can keep that up will depend on my cooling scheme, which I haven't really settled.

Winding update

2008-09-22T18:50:00.001-07:00

It turns out that I made an error in assuming that there must be an even number of slots per pole. There is a method of winding called lap winding (as opposed to concentric winding) that allows almost any number of slots per pole by winding coils with constant pitch. I will post more as soon as I learn how to do that in this case.

Nameplate mystery

2008-09-21T22:38:00.000-07:00

I decided to tackle removing the motor and beginning the process of rewinding by having a look at the old windings. It turned out that the nameplate was wrong--- It's not a 4-pole motor at all. It is a 2-pole motor, meaning the full-load speed was around 3500 rpm, not 1750 at the nameplate advertised. There are 36 slots in the stator, meaning that 6 slots are used for each pole, times 2 poles, times 3 phases. I would like to increase the number of poles to increase the torque. Rewinding for 4 poles is impossible, as 36 slots divided by 3 phases gives 12. 12 divided by 4 poles gives 3 slots per pole. The number of slots per pole have to be even, though. The number of poles also has to be even, and there are only two even numbers that multiply together to get 12 -- 6 and 2, so this motor has to have to have 6 poles with 2 slots per pole or 2 poles with 6 slots per pole. Two slots per pole isn't enough because it results in a poor approximation of a sinusoidally varying magnetic field around the stator, so I guess I'm stuck with 2 poles. This means that at 180 Hz, the motor is spinning about 10000 rpm, which isn't too good for the transmission, I'm sure.

Heat death

2008-09-19T20:14:00.000-07:00

Since the last post, I drove the car back and forth to work (2.5 mi each way) a few times, and enjoyed getting the control settings to be a little more driveable. The car accelerates well, but has only a 35mph top speed. The problem seems to be that the torque falls off fairly rapidly as the motor is driven above 60Hz because of field weakening. The way induction motors work, they need a voltage proportional to the frequency to maintain constant torque. As my drive system peaks out at 240 volts or so, the torque falls off, because I am not able to force enough current though the windings. The solution to this is to rewind the motor to run at a lower voltage, say 80 volts. Then, we don't run out of voltage in a 240 volt drive system until we hit 180 Hz. The result is that the motor will have usable torque over a much wider speed range.

Luckily, I was able to speed up the process of rewinding the motor by overheating it to the point the windings shorted. I thought it would be a good idea to drive down to Stanford and back (10 miles) on a hot day, and as I was about to pull into my place, the inverter shut down because it had detected a fault. I knew perfectly well what had happened. I had been meaning to install some sort of temperature instrumentation, as well as a blower for cooling air, but had been putting it off. I figured I would test my luck with my $150 ebay motor...

I've never rewound a motor before, so I am practicing with a smaller motor from my blower, which I also got on ebay, and which I will be installing in the car when I have the motor off. I bought the blower with a 575 volt motor, which I will rewind as a 220 volt motor and run off a smaller inverter. Here's the stator of the small motor:

It's a 2-pole, 1/2 hp motor, with 4 slots per pole, star wound. The windings are 25 AWG, which I am replacing with 20 AWG to do the voltage conversion. There's about 2.5 lbs of copper in this, which works out to about 1000 feet of 20 gauge.

Here's the rotor, along with the end bells and the impeller from the blower. Such elegant machinery.

Kermit comes home

2008-08-18T23:40:00.000-07:00

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

2008-08-18T00:11:00.000-07:00

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

2008-08-17T23:45:00.000-07:00

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

2008-08-10T20:27:00.000-07:00

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

2008-08-10T20:17:00.000-07:00

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.

Contact!

2008-07-07T23:29:00.001-07:00

I bought what will (hopefully) be one of the last components for the car:

This is a sealed contactor, which will connect the battery pack to the motor controller when I turn the key.

Apologies to Pininfarina

2008-07-07T23:18:00.001-07:00

I have finished the battery box and tacked it in its proper location in the car. Unfortunately, to avoid striking the rear differential, the box had to be moved back to the point where the corners protrude from the rear skirt of the car.

This spoils the lines of the car somewhat, but the protrutions will at least be hidden behind the bumper. I don't feel too bad about cutting up the car anymore, though. I wirebrushed off some paint to do the welds, and discovered quite a bit of bondo, so apparently Kermit has had more than one surgery.

Throttle test

2008-06-14T23:14:00.001-07:00

I did the first test of the throttle control of the motor and drive. Everything went well; the drive was flexible enough to account for the idiosyncracies of the ebay special throttle position sensor I am using. More good news, the motor drive will fit in the engine compartment, freeing up more space in the back, and simplifying the wiring. I am running off mains power here, but it was a good feeling sitting in my car and revving the electric motor.

Battery box

2008-06-14T22:56:00.000-07:00

I have started a sheet metal box to hold the battery. This will be welded in place of the spare tire and gas tank. I need 100 cells, give or take, and the squarest pack I could make with about that number turned out to be 7 cells wide and 14 cells long, giving 98 cells. I started with a pristine sheet of 16 gauge steel:
Which got a little less pristine as I taught myself sheet metal fabrication:

The finger brake at TechShop wasn't big enough to bend all four sides of the box, so I had to do some old fashioned whacking it with a hammer and a dolly to get a nice fold on the last two sides. I won't show any closeups of the hammer marks.

Next, remove the gas tank. I'm going to try to find a new home for this, as it is in good shape.

Next, I had to grit my teeth and cut a bloody great hole in the bottom of my car. Not an easy thing to do with a classic car with decent sheet metal.

The hole will be bigger in the end, but I don't want to get too excited about cutting and end up getting rid of some metal that would help hold the box in later.

Test fit!

Astute readers will notice the complete lack of welds. Unfortunately, Saturday is welding class day at TechShop. so I wasn't able to get a welder all day. I need to weld some separators between the rows of cells as well.

Mmmm, toast!

2008-04-11T23:01:00.000-07:00

I needed a way to discharge the battery pack during my BMS tests, so I headed down to my local K-mapart to pick up a space heater. It's spring here in Califoria, though so there weren't any. I figured a nice Toast-R-Oven uses lots of electricity too, and I can make toast with it later. It's pretty funny to go to the store to find something that uses the most electricity.

One Logger per Child

2008-04-11T18:10:00.000-07:00

I got one of the xo laptops from the One Laptop per Child project while they were having their give one get one promotion. It's really a neat device, but I haven't had much time to play with it. It really makes a good datalogger, though. I can hook it up to the battery pack and collect data on charge/discharge cycles while using my grownup laptop for grownup things.

The BMS finally works

2008-03-27T22:21:00.001-07:00

It's been three months, but I finally have a working battery management system. Here I'm testing it with 24 cells in series and the Manzanita charger. I deliberately discharged one of the cells, and the system is bringing it back in balance as it charges the whole pack. It's been a lot of work, but this system should give the best life possible to my pack.

I'm sure there will be some fiddling with the control algorithms, but at least the hardware is done.

What a thousand boards looks like

2008-02-19T21:38:00.001-08:00

The production run of BMS boards arrived today. Richard Hatfield of Lightning Motors went in with me on an order of 1000 to bring costs down.

Beginnings of a battery pack

2008-01-31T23:01:00.001-08:00

I've finally entered the home stretch with the BMS system, and have 3 final prototypes installed on cells for testing. Cat for scale.

Stickers!

2007-12-26T16:25:00.000-08:00

I am spending Christmas in Wyoming with family, so no work on the car lately. I have been debugging the BMS code, and designing a sticker for the car:
The equation is the differential form of Faraday's law, which is one of Maxwell's equations, which describes induction. If this seems like something that belongs on your electric car, I can send the full size image.

Battery charger and other stuff

2007-12-17T22:46:00.000-08:00

It has been over a month since I last posted to this journal. I have been very busy at work, where we just launched our first product- a miniature XRD/XRF instrument for identifying minerals in the field. Even so, I have made some progress on the car, mostly buying parts and working on the next version of the BMS. I am now the proud owner of a PFC-20 battery charger from Manzanita Micro. This charger is nice because it can output the 408 volts that my pack will require. One drawback is that it is not isolated, which means that the negative side of the battery pack is connected to one leg of the mains. This should not be a safety issue, as all the other equipment I am using is fully isolated.

Oh yeah, I said next version of the BMS. Although I'm sure the voltage regulator style BMS that I already developed would perform just fine, I have gone ahead with the Eierlegende Wollmilchsau (literally, an egg-laying woolly milk pig-- a German expression that can better be translated as "Swiss army knife") version. This version has a small microcontroller (an Atmel ATtiny25) on each cell that monitors the cell voltage and communicates with a tiny linux computer over an optoisolated i2c bus. Each microcontroller can bypass current, either on its own or on command from the computer.

The advantage of this system is that it gives individual information on the state of each cell. Also, if a cell controller fails, the computer would immediately notice, and alert the driver that the pack needs service.

The other positive aspect is that each cell controller will cost less. Microcontrollers are truly amazing. For under a dollar, you can get a chip that is much more powerful than, say, the Tandy Color Computer 2 that was my first computer.

Mr Fusion

2007-11-09T23:53:00.001-08:00

I've started buying the safety equipment needed to deal with a 300 Volt battery pack with something like 15 kilowatthours of energy (roughly enough to run our house for three days.) In addition to some fancy insulated screw drivers to keep from shorting things out while I'm working, I bought this monster fuse. It will go between the battery pack and everything else. Lens cap for scale.

British racing green

2007-11-09T21:23:00.000-08:00

Well, I've finally fallen into the biggest trap for people who have electric car websites-- I've been working on my website instead of my car. At least reading my journal (I'm allergic to the word "blog") won't make your eyes hurt so much anymore. Plus, it's now authentic British racing green.

BMS, round two

2007-11-05T22:23:00.000-08:00

I received and assembled the first three samples of the BMS boards. Everything looks good. I did some thermal tests, and each unit should be able to bypass one amp with a heatsink installed on the main transistor, and about twice that with a fan in addition. If I use the BMS to cut back on charging current when any cell goes over voltage, the bypass current should only be a few tenths of an amp, and I'll be able to dispense with the heatsink. That would be nice, as the heatsinks are a fair amount of work to install. Here's the top of the board, with heat sink installed:And here's the back. The cool thing about making your own layout is that you can write whatever you want on the board:

Only 93 to go...

Ol' sparky

2007-11-05T22:09:00.000-08:00

I have finished figuring out where the batteries will sit. The spare tire well is almost perfectly sized for the pack. I will probably relocate the spare underneath the car in place of the gas tank.

While I was looking things over, I learned why British cars are infamous for their dodgy wiring.

Nothing like unsupported wires rubbing against the gas spout... and is that a twisted-together "connector"? I hope that was an aftermarket accessory. The factory connectors look pretty smashing as well:

I'm hoping these practices won't rub off on me when it's time to do the high-voltage wiring

BMS round one

2007-10-21T21:48:00.001-07:00

I received my first prototype board from the fab house this weekend, and soldered it together.
The circuit worked the way it was supposed to, so I have ordered 3 more "final prototypes" that will have a few small changes, as well as a more polished look, with solder mask and silk screen and a slick oval shape. Once those arrive, I will probably borrow a few batteries and test the whole system.

TPS Report

2007-10-21T21:35:00.001-07:00

Throttle Position Sensor, that is. I need a way to communicate to the motor drive that I want the car to go. Traditionally, electric car builders have used something called a "Curtis potbox", which is a variable resistor in a box, controllable by a cable hooked to the accelerator pedal. Trouble is, this device is made for controlling forklifts, and so isn't really suited to the abuse of thousands of stop-and-go cycles in a car.

Luckily, (newer) cars already have something very similar built in. The throttle position sensor tells the engine management computer to adjust spark timing and fuel mixture according to how much go the driver wants. It should be easy enough to adapt it to my purposes. The best part is, I got this one on ebay for $5. I think it came from a Ford. I 'm gonna keep all the useless-but-cool gasoline-related accessories on the module.

Batteries...

2007-10-07T12:36:00.000-07:00

With the bulk of the mechanical work done, it's time to start thinking about the electrics. I've chosen the Lithium Iron Phosphate batteries from Thunder Sky, which don't catch fire like some lithium batteries do when overcharged. Nevertheless, the batteries can be damaged if they are overcharged or overdischarged. In a long string of cells wired in series (I'm using 96) some cells are inevitably stronger than others, and will end up being overcharged. The string voltage remains constant, so this means that some cells get severely discharged (even negatively charged). Both of these destroy the affected cell, so I need a way to keep the voltage of each cell between 2.5 volts and 4.25 volts. There are several ways of doing this. Charging each cell individually, then monitoring all the cells for undervoltage during discharge is the best way, but constructing a charging system to individually charge 96 cells, then connect them all in series to the drive circuitry would be a difficult design challenge, as well as a wiring nightmare.

Instead, I'm taking a simpler approach. Each cell will have a shunt regulator across its terminals. A shunt regulator is a device that conducts no current until the voltage across it reaches a set value. When the voltage rises above the set value, the device begins to conduct just enough current to maintain the voltage at the set point. This takes care of the overvoltage protection. Undervoltage is a bit trickier. I think I've come up with a clever way to signal the controller to shut off when the voltage of any cell drops below the setpoint using a single circuit (instead of 94), but I need to develop it a bit first.

The idea of building 96 circuit boards is a bit daunting though. Each board will have ~10-15 components and cost about $5 for parts. It only adds about 5% to the cost of the battery system, so is well worth it, but spending my evenings hunched over a soldering table doesn't sound like fun. Maybe I will send it out for assembly... with 100 pieces, the prices should be pretty good. I just need to prototype one or two to get everything right.

Back on track

2007-10-07T12:29:00.000-07:00

I finally finished machining the new hub, tearing apart the clutch to get the spline, welding the two together, and putting the motor back in. I fired up the motor, and everything worked fine. I enjoyed sitting in the car listening to the motor & transmission turning for a while...

Setback

2007-09-10T22:59:00.000-07:00

I finished the subframe and got the motor properly aligned with the transmission (not so easy) and bolted everything down for a live test (without the driveshaft installed-- no driving through the shop wall). I rigged up a 300-volt DC power supply to run the motor drive, just to make sure it wouldn't complain about running off of batteries instead of its normal diet of 3-phase 240 Volt AC. It worked great. On to the powered test of the motor/transmission assembly. I ramped up the speed of the motor slowly, and everything was fine. I took things up a notch by quickly accelerating the motor, to simulate an actual load. Suddenly, the adaptor hub that I had spent a few evenings turning on the lathe twisted right in two. Oops. I had followed the design guidelines given by Lovejoy, the shaft locker supplier in machining the hub. The lock holding the transmission to the adaptor hub works by compressing a thin tubular section of the hub onto the shaft. I had my doubts that such a thin tube would be strong enough to carry the 150-220 ft-lbs (200-300 Nm) of torque Lovejoy advertised. It wasn't. Although, strictly, the shaft lock device was still securely in place, so I guess the book value was right. That's a little bit like building a ship in a bathtub, though...

Nevertheless, I shall continue! The next plan is to snag a splined hub from an old clutch disc, and attach that to the hub that is still locked on to the motor shaft, using bolts or perhaps my new ~~bowhunting~~ TIG welding skills.

It's a motor!

2007-09-05T21:45:00.001-07:00

After quite a bit of sweating and head scratching, I got the motor into place.
Don't worry-- the C-clamps are temporary. The engine bay looks cavernous, but soon enough it will be crammed with batteries.

This style of design and fabrication is quite a change from my job, where everything gets designed using computer tools and precisely manufactured. This is more the "Orange County Choppers" school of metal work, only with a lot less swearing and throwing things. I sometimes wonder about those chopper shows on TV, whether they have a whole sweatshop full of 3D CAD guys sitting in a poorly lit room somewhere, swilling mountain dew and churning out chopper designs.

Subframe

2007-09-02T23:21:00.000-07:00

To make the motor, transmission, and car more or less one unit, I need to build a subframe. This will also serve as a platform for the front battery boxes. It was difficult to measure everything outside the car, so I decided to install the transmission, block it up to where it should be, and build the frame to it. Here's the transmission in place with the first cross member:
This is my first stab at TIG welding, so the welds aren't going to win any beauty contests. I'm happy with the overall result, though. Next, the top half of the subframe:

Mr Motor, meet Mr Transmission

2007-09-02T23:07:00.000-07:00

I have been out of town for the last couple of weeks, so I was excited to work on the car all weekend. The shaft locks arrived while I was gone, so I set to work attaching the motor to the transmission. I had one small setback. The motor is a metric 112M size, which means that it is 112 mm from the base to the center of the shaft. It also should mean that it has a 28mm diameter shaft. For some reason, though, this one had an 1 1/8" shaft. This was a problem because the shaft lock I bought is 28mm. No worries, though. 28mm is smaller than 1 1/8", so I just turned the shaft down on the lathe.
This was good, as it gave me an excuse to take the motor apart.

Next, I test fit everything, and ran the motor for the first time attached to the transmission

Everything ran smoothly, so time to start putting it all in the car!

Shaft locking

2007-08-07T16:23:00.000-07:00

Since I finished pulling the engine, I have been working to source parts to couple the electric motor output shaft to the transmission input shaft. Instead of using a splined or keyed shaft, I will be using keyless shaft locks from Lovejoy, combined with a hub I will machine myself. The keyless shaft locks work by drawing two concentric wedge profile rings together with jack screws. As the rings come closer, they wedge against the outside of the shaft and the inside of the hub, to form a very tight fit. I will post some pictures of this once the parts arrive.

Engine pull day two

2007-07-19T21:53:00.000-07:00

Here is the sequel to last week's movie, where the engine finally comes out. It took a bit of work to get the tail of the transmission over and out, but there were no mishaps.

Engine pull day one

2007-07-09T00:01:00.000-07:00

Today I started pulling the engine. I nearly finished, but the hoist I scrounged up was not strong enough to lift the engine and transmission. I am doing the work at TechShop, down the street from my house. It's nice to have a place with enough space to work and all the tools I need. The big engine hoist they have there needed some repairs, so I will finish the job later this week. Meanwhile, check out the time-lapse video from today:

Thoughts about energy

2007-07-07T22:53:00.000-07:00

Since I've been thinking about building an electric car, I've gotten a lot of questions about them. I'm far from being an expert on the subject, but I have picked up a few answers as well. I think the most frequent question is something like "Can you put some solar panels on the roof and make it run that way?" The answer is yes, but the car would be slow even by classic British standards. For comparison, an average-sized solar panel makes about as much power on a bright day as a healthy adult in a hurry on a bicycle. You could maybe fit three of these panels on a car without looking ridiculous or getting in trouble with the law. That doesn't mean it is a stupid idea. Solar car races (I guess "rayces" if you're into that sort of thing) are pretty cool. And then there's this guy if you prefer the Mad Max look. The best thing is to have a solar powered carport to park your car under while you're at work, or just a roof full of panels at home. Even if you get all your electricity from the power company, though, electric cars are an improvement over the gas variety from an environmental view. The movie "Who killed the electric car?" presents arguments from this side quite well.

Engine sold

2007-07-07T22:34:00.000-07:00

I listed the gas engine on Ebay last week, and there was a fair amount of interest. Some in the motor itself, but also quite a bit in the idea of an electric conversion. I'm glad I started this journal, so I could point to it as a place to begin learning about electric cars. The auction ended last night, at a little over two hundred dollars. I'm mostly happy to have the engine out of the way so I can begin the conversion, but I don't hate two hundred dollars. That's almost two batteries!

Bench test

2007-06-29T13:26:00.000-07:00

I tested the motor + drive system last night, and overall it was a success. I ran the motor up to 7500 rpm with no problems. The cooling fan sounds like a jet taking off at that speed, though. It's nice to have a lot of cooling air, though. Unloaded, the motor consumes about 750 watts at top speed, which seems reasonable. I can run the motor down to about 30 rpm, where it uses about 50 watts. It would have been nice to get an idea of torque output, but I don't have an easy way of loading the motor.

The motor controller has two modes: V/f and sensorless vector. At its simplest, V/f mode makes the voltage to the motor linear with frequency, so for example, to run the motor at 30 rpm, the drive would supply 10 Volts at 1 Hz, and at 7500 rpm it would supply 230 Volts at 250 Hz. This is fine for constant load applications, but if the load increases too much, the motor can stall. In sensorless vector mode, the drive uses a mathematical model of the motor to calculate the torque applied at any time. It can then change the voltage and frequency to match a requested torque or speed. This is much better for an electric car, because motorists are used to an accelerator pedal that requests more torque from the motor. Because the motor's rated capacity is much smaller than the drive's (5 HP vs 30 HP) the drive was unable to model the motor, so sensorless vector mode is not working. I will see if I can input parameters specific to the motor and get it working. Otherwise, I will probably need a bigger motor eventually. In the meantime, I can set the acceleration and deceleration times for the drive to be long enough so that everything works. This just means that the drive will take the car's inertia into account a little better, and won't ask the motor to turn at 7000 rpm when the car's speed is telling the motor it must turn at 500 rpm.

Now to get rid of that pesky gas engine so I can start putting things in the car...

The motor drive is here!

2007-06-26T19:42:00.000-07:00

I ordered a motor drive last week from the internet. In the past, these were the kind of things you would have the purchasing representative from you company call the regional sales rep about, a price would be quoted, and a lot of paperwork generated. Now, there are a couple of places to get industrial equipment on line. I bought this one from factorymation.com, as they had brands that I had heard of before. Whatever that's worth.

The drive weighs about 30 kg.

The business end: This is where the cables from the batteries and to the motor will run. The fans dissipate waste heat. The unit is about 90% efficient, but that's still nearly 2kW of waste heat at full power.

DC or AC?

2007-06-22T21:37:00.000-07:00

Most electric cars built by hobbyists (see http://www.austinev.org/evalbum/ for a nice collection of home-built and other electric vehicles) run on DC - direct current. It's pretty simple - get a bunch (4-15) of batteries, hook them together, and run a motor with it. Speed/torque control is done by limiting the current through the motor by an electronic chopper (Pulse Width Modulation). AC systems, on the other hand, need higher voltage (300 Volts DC for a 240 Volt motor, or around 600 Volts DC for a 480 Volt motor) This means a string of 26 batteries to run a 240 Volt motor. Also, instead of a single switch to control current, as in the DC case, AC motor drives need to have 6 switching transistors and a bunch of digital signal processing logic to produce the three sine waves needed to run a 3 phase motor. AC Propulsion and Siemens and a few others make automotive-type AC drives. See http://acpropulsion.com/ and http://metricmind.com/for examples. These go for about $20k, so they're quite out of most hobbyists' reach. So why bother with an AC conversion?

Pure snobbery
Simpler, more efficient motors
Regenerative braking
Higher voltage means lower current, which means lighter wires and other components

An interesting industrial-component AC system from Australia is here:
http://www.austinev.org/evalbum/1149

2007-06-22T21:24:00.000-07:00

Here's the new motor. It's a 5 horsepower (3.75 kW) 3 phase, 240 volt induction motor (framesize 112M -- ebay $150).
My approach with this conversion is to use off-the-shelf industrial components as much as possible. Industrial AC motor drives are coming down in price significantly these days, and AC induction motors have always been cheap compared to their DC counterparts. This motor was manufactured in 1980 and apparently sat in a warehouse for the last 27 years, as it was brand new in the original crate.

The old motor

2007-06-22T21:22:00.000-07:00

Here's the old gas motor, which is for sale, by the way. Abingdon's finest.

Begin at the beginning

2007-06-22T21:09:00.000-07:00

Here's the car. Not exactly Kermit-green, but kinda looks like a frog. I'm a big fan of cars with names. As far as I know, this car doesn't have a name yet. So. Little green MG, I dub thee Kermit The Car.

Why?

2007-06-22T19:15:00.000-07:00

I've started this journal as a way to keep track of my progress converting a 1970 MGB GT to run on electricity. Why a pushing-40 British car? Why rip out a perfectly good gas engine and replace it with something that belongs on an air compressor? I don't know. I probably won't know when I'm all done.