LDU Overcurrent and broken inverter

[bexander]

I'm helping a friend getting his LDU to run.
Followed Damien:s video and everything was fine spinning the motor smooth and controlled on 22V via heater element.
Reconfigure with full pack, 72S (12 Tesla model S modules). First spin on HV battery in forward works fine (very smooth and at low speed), when selecting reverse and trying to spin, no response, and OVERCURRENT. No jerks, no sounds, contactors still on, fuse still ok.
Now the inverter power stage is broken and I can't for my life understand what's gone wrong?

Parameters:

2025_02_18_Setup.json

(1.5 KiB) Downloaded 317 times

It was a bit of a hassle to get the Wifi-CAN-board to connect to the Inverter board, giving "No connection between ESP and STM" very often. Needing to do a lot of refresh to get it to connect but once connected it never failed or dropped out. Could there have been a bad connection on the inverter logic board causing the inverter to fail?

[johu]

The inverter proves blown with the diode test mode and you get a red light on one of the gate drivers?

Parameters look like the bin file I usually flash, including the hidden flags.

With a good CAN connection the wifi module should be very stable by now, not sure what happened there

[P.S.Mangelsdorf]

So it worked fine forward, didn't work in reverse, and now doesn't work in either?

[bexander]

Diode tested it before my first post. All three low (if I remember correctly) transistors show 0,0V when doing diode test. Was 0,3V before, so indeed broken. I have not checked gate drivers for red light but I think diode test is enough.

Thank you johu for checking the parameters. Now that can be ticked of as not the cause.
Regarding the CAN-wifi it's a bit concerning as it might point to glitching cables, maybe? But once it hade connected it was very stable. If wiring is faulty then it probably should drop out sometimes?

Yes, when on full battery voltage for the first time I spun it up very gently in forward, up to maybe 60rpm on the output shafts. Then releasing the pedal very gently so the motor stoped. Did notice a 30A current draw according to BMS while in standstill but didn't bother about it at the time as it stoped when I put the inverter in reverse. When depressing pedal this time, no reaction on the motor. Checked faults and OVERCURRENT was present. Tested to restart the inverter a couple of times but got OVERCURRENT fault as soon as PWM-output should start, both in forward and reverse direction. After this the diode test concluded that the inverter was broken.

My conclusion is it's down to bad luck in this case, meaning you never know what a drive unit bought from a wrecked car have experienced prior. So will most likely try to source a replacement inverter and hope everything works fine next time. I can't find any other explanation then that the drive unit was damaged prior as everything worked fine and then just died at a very strang time.
If anyone disagrees with my conclusion and have any ideas what might have caused this, please speak up. I would be very sad to repeat this once more with another inverter...

[P.S.Mangelsdorf]

I've got nothing - that's a super weird failure. I'd love to know if after stopping the motor in forward, if you had pressed the pedal again what would've happened (because that could say if it died on regen or on the attempted reverse), but obviously impossible to know. Otherwise it sounds like your set up was fine and your conclusion is correct. Sorry about that, I know it sucks damaging an inverter.

[bexander]

Are there any checks that can be done to the OI logic board to rule it out as the cause, just to verify the same thing doesn't happen again?
Could it even have been the cause in this case? Seem very unlikely to me.

[johu]

Did notice a 30A current draw according to BMS while in standstill

Was that a fluke or an actual standby current? If so that would be worrysome.
I helped a client here in Germany and their inverter would gradually heat up when left in run state with no motor movement and no PWM going. So there seems to be many shades of "broken". They were also able to run the car on it but only very slowly and a quick overheat. An inverter swap solved it.

An intermittent gate drive signal could be unhealthy, perhaps... I like to check the connector pins with tweezers to really make sure they are soldered tightly to the narrow pads

[bexander]

Was that a fluke or an actual standby current? If so that would be worrysome.

I can't really say regarding the current. Will pay more attention to it once the inverter is replaced.

An intermittent gate drive signal could be unhealthy, perhaps... I like to check the connector pins with tweezers to really make sure they are soldered tightly to the narrow pads

Good tip, thanks! Will check all connector pins before runing it next time.

[P.S.Mangelsdorf]

The only thing I could add would be to give the whole board a good look-over visually and tactilely. Look for stray solder and ensure everything is tight and well attached the board. Last year I ignored a loose part, and it dropped 12V while driving, killing a nice LDU sport inverter.

[bexander]

I have a theory to what might have caused the failure.
While doing low voltage testing, using a 6S battery to power the inverter power stage via a heater element in parallel with a 30A fuse, the motor was jerky and the fuse blow because the encoder pins needed swaping. Swaped the encoder pins and replaced the fuse. Everything worked fine.
However, the heater element that was used have a resistance of 77ohm when cold. When the fuse broke there needed to be a current of at least 30A and suddenly that current need to pass throught the heater element resulting in 77x30=2310V... for a very short period of time but probably still enough to damage the transistors... The capacitors in the power stage will absorbe some of the energy so the actual voltage would not have been that high, but still high enough.
I think the heater element or resistor used while testing should be of lower resistance (less than 20ohm) and perhaps a lower value fuse (10A) as well to keep everything safely below specifications?