Hi There,
A very basic test you can do is to measure whether there is a short between the DC bus and the outputs. If the drive has got external connections marked DC+ and DC-, you can even do it without opening it up. There should NOT be any short or low impedance path between
DC- and L1, L2(or N for single phase), L3, R, S, T
DC+ and L1, L2(or N for single phase), L3, R, S, T
If it does, you may have a busted rectifier or IGBT. In case of the smaller inverters, they are often integrated in a single power module.
Good hunting.