Hi,
I'm trying to build a single phase VFD for controlling the speed of a 150W fan but somehow it seems I've made an IGBT toaster

.
I tested the attached schematic on a breadboard using a separation transformer on the input mainly so that I could use scope more freely, but it also limited the current to about 5A. I first tested everything with a lightbulb to represent a resistive load and all the waveforms were looking OK. Switched to the actual motor and it spinned as expected, I was able to control the speed. A thing to note here is that the input voltage at that point dropped to about 100V due to (undersized) separation transformer on the input. There were some spikes on the output from the inductive nature of the motor but the protection diodes (D3 and D4) took care of that.
The problems started when I switched to 230V directly. After the series current limiting resistor R13 turned glowing hot, I quickly disconnected everything only to discover the negative IGBT Q2 was toast. I changed it for a new one and repeated the whole process. Toasting included

After quite some head scratching I didn't get far.
My current working hypothesys is that the parasite inductance of the wound(!) series resistor prevents the line to absorb the spikes from the motor and in turn raises the voltage over the second IGBT, toasting in the process.
I am thinking of putting ~100n capacitors and 400V zenner (or better, tunnel) diodes across +IGBT and MotN (-IGBT and MotN on lower side). Also an 8uF cap in parallel with the output.
Any thoughts would be appreciated

The KiCad project is available on Github (along with all the code for the Arduino):
https://github.com/gztproject/VFD/tree/master/PCBThanks, Gapi