Recently started getting some readings from a large motor that I'm confused as to what the issue may be. firstly spec
Asset is a Kiln fan- Motor is a Brook Crompton 280kw 990rpm. runs on a VSD/VFD. hard to get completely consistent readings but usually take around 830rpm. readings have been low for years and within the last two months have started to creep up in mm/s ovr and g's ovr signal only on the NDE, the DE has continue to remain low at vert-1.878 mm/s horiz-1/747vmm/s and axial -1.215 mm/s. This may be an obvious sign to more experienced VA analyst but I'm new to the game and not totally sure what could be causing this increase. The dominate frequency is at 618hz with sidebands around at 13.8hz, the same as the running speed of the motor at the time it was taken. From what I was taught sidebands are never a good sign. this is why I'm slightly more concerned than normal. Also because this asset is critical to the plants production, I don't want to have to replace it until I know that there's a defect that's getting worse. I thought that an electrical fault would have sideband of either the pole pass frequency or 2x line frequency and that was it, so I've kind of ruled out the possibility of it being an electrical issue although I don't know for a certainty. the Temperature around the NDE and DE bearings are normal and haven't increased since this issue has arisen.
All general checks have been made including the Fan blades, the coupling, loose bolts. cracks in the concrete.
Any Ideas would be greatly appreciated