I also use Delta's PWM at my rigs.
As the other said, the possible thing is those fans have refurbished (adding old motor into the new case), even the case was broken. As far as I know, Delta fans case was a solid and strong enough. Next, maybe you also need to add PWM controlling board (with slight modifications to the wiring scheme) to manage all of your running fans (for adjusting fan speed needed at several conditions).
As the other said, the possible thing is those fans have refurbished (adding old motor into the new case), even the case was broken. As far as I know, Delta fans case was a solid and strong enough. Next, maybe you also need to add PWM controlling board (with slight modifications to the wiring scheme) to manage all of your running fans (for adjusting fan speed needed at several conditions).
On some rigs I have used to PWM of the motherboard in order to regulate the speed based on the graphic cards temperature (with fancontrol on linux). I thought that it will prevent them from burning, but there was no change. Some died even if they were running at 70% max speed.
In my setup, tree fans are used for each server case, and they are powered by the same atx cable that is able to sustain 20A (same as the graphic cards) so there should be no issue there. The three fans take about 8A so there's a room for it.