Sunday, April 26, 2009

Todo list

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

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.

Sunday, April 5, 2009

Inverter overhaul

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.

Saturday, March 14, 2009

Precharge

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!

Thursday, January 22, 2009

Snubbed!

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.

Saturday, November 1, 2008

Still learning

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.

Monday, September 22, 2008

Winding update

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.