Following up on the ringing discussion on generatorlabs homebrew thread I decided to make a little experiment.
The question being: how much ringing is too much?
So this is the test setup:
Two film caps, 420µF each connected to the DC bus in a laminated fashion. The loop could be smaller, but still.
The Bus voltage is 33V. A rewound 190V ACIM is connected (not on this picture I know). Switching frequency is 17.6kHz.
So here is the first test: how much ringing can I see on the DC bus?
Scope is set to 5V/div
We see an spike to about +-/12V, so about 30% of the bus voltage.
Next the AC output between GND and L1. Scope is set to 10V/div
And we see a bit of ringing as some other phase switches.
Now I completely disconnected the bus caps, no snubbers, no nothing. This is the result:
100V/div so we see 400V spikes which is like 1200% of the bus voltage!
It means that with 100V bus voltage I could kill those 1200V IGBTs with hardly any current going through them.
On the AC side we see bad ringing but the amplitude is small compared to the DC side.