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.

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!

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.