What causes the stirrer fan to arc
When the microwave is started there are sparks in the top. So took out the stirrer fan and found some carbon on the fan blade and small area on the plastic shroud that had over heated.
The carbon on the blade was probably absorbing the microwaves causing it to short out and heat up and melt the plastic. I'd replace the fan. A model number will help to get to the correct part.
"Posted by: J. Kobari 20 Sep. 2020
I had the same problem, as burning smells moment after MW oven is turned on. My MW oven model is “Sharp Insight Pro. 24 inch” I opened the oven enclosure and found burned black carbon powder around the shaft of stirrer fan (serious attention should be paid to avoid turning the MW on, while its enclosure is opened) . I removed insulation sheet from inside of MW oven compartment at its top, which covers the stirrer blade, and I found more black powder and burning residue at the joint of plate and shaft of the stirrer blade (Model # FPLT-B002MRY0A). I also noticed that the stirrer blade is not turning while the fan’s motor is running, I was sure fan’s motor is working properly as I had removed and test the fan separately (Model; RMOTDA283WRZZ ).
Here is the root cause of the problem; the shaft and plate of stirrer blade are metal parts, firmly connected as one piece with zero resistance in between. For some reasons the joint between shaft and plate became loose, (fan’s motor is turning without turning the blade). Voltage induced by MW on these 2 metal pieces (Plate and Shaft) which are electrically disconnected or having a resistance more than zero, causes Arc at this joint, produces excess heat and burn the plastic bracket at its top.
The Fix is either to change the Stirrer Blade, or thoroughly clean the loose joint using a used tooth brush by removing all dirt and carbon powders from the joint. Then try to fix the plate and shaft joint tight and strong together, using a special tool such as chisel and hammer. Test the ohmic resistance of the 2 parts, which is be zero, before reinstalling the stirrer blade."