A machine at work hadn't been used for a week, switched it on and BANG!

Turns out the inverter in it (Moeller DF4-120) is knackered, there is
blackening on the circuit board.

To those in the know, is it likelier that the inverter is at fault or
that something external has caused it to go pop?

I'm reluctant to simply buy another one and plug it in in case the same
happens.

I can't see anything untoward on the control side and everything is
clean and dry so wondering if the motor it's controlling is at fault,
how would I test a 3 phase motor to be sure this doesn't happen again?

Ta.