Loss of rotational power?

Make sure that the carbon brushes are not worn out or if you have already had them out make sure that they have been put back in properly. There is one corner ground off each brush that needs to line up with a blocking corner in the brush holder. Then get your multi meter out and check the unplugged main cable for continuity to the speed control panel. Then check each half of the field assembly for continuity from the speed control plate to a brush holder. Check you have no broken wires to the speed control plate. Check that the switch on the speed control plate is functioning. If all that tests OK I'd replace the phase board, it's worth not much and worth a shot.

The two plastic screws are the carbon brush holders, not something you should be fiddling about with unless you are changing the brushes. It is quite common for the brushes to wear out and need replacing so taking the caps off and checking the brush length would be a good idea. The brushes only fit in one way, one corner is ground off to allow it to go past a blocking key in a corner of the holder. Use a torch to see which way to put the brushes back in. If they are too short get new ones.

Undo the black plastic caps, one on each side. You probably need new carbon brushes. Remove them carefully and note that the brushes will only fit one way there is a blocking stop on the inner edge of the brush holder, you can see it with a torch when brush is out. Line the chamfered off corner of the new carbon brush up with the blocking stop. On one side it is at the back at the top and the other at the back at the bottom ( I think )

Check the carbon brushes, look inside the hole, it is keyed, the brushes only go in properly one way. Match the missing corner on the brush to the blocker inside the brush holder.

Hi, ANNEQUESTRIA, I hate it when that happens. Those ‘two big black plastic spring loaded screws’ are your mixer’s electric motor brushes. The slotted black threaded plastic parts are just the Brush Holder Caps and are protective covers to the brush holder assemblies. They allow access to the brushes when they need to be replaced without have to take the whole mixer head apart. What prompted you to remove them?
From what’s been done already, still I suspect that one or both of the brushes is (are) not seated properly.
The brushes need to be installed so that they both seat correctly on the armature's commutator. See the illustration below:

I'd suggest removing them again and making sure the concave contact face rests completely on the armature’s commutator. If you've turned it 90 degrees too much, it won’t seat properly and you get no power to the motor.
It would also be helpful to have the Parts List manual on hand. If you're lacking one, you can download a PDF version from the KA website at: http://www.kitchenaid.com/flash.cmd?/#/page/home.
Click on Customer Care>Locate Manuals & Guides> Enter the 6 character model number, in your case use KSM50P (Your KSM90P mixer falls within this manual), and select the last file KSM90PSWW0.pdf (354.08 KB) listed under Parts List (16) and then check to verify this matches your mixer model.
Turn to pages 6 & 7, titled “Motor And Control Parts”, which provides an exploded view for the disassembly and reassembly of your mixer’s electrical components, along with all the parts. Locate Illus No 32, Brush holder assembly that includes Illus. Nos. #18, 21, 22, 23 & 31, and Illus. No. 19, Brush & Spring assembly respectively.
The Parts List illustration along with the picture above regarding the orientation of the brushes should get your mixer powered back up and operational. Good luck! I hope this helps. Howard, Burke, VA
If this solution has been helpful to you, please rate it, thanks! -hslincoln

Hi COVINIO. Yes, the "large black spring loaded magnetic screws on the sides of the mixer" are your mixer's brushes, which supplies electricity to the electric motor. (The black cap is the brush cover). I suspect that one or both of the brushes is (are) not seated properly. Did you remove the brushes to replace them or were you just inspecting them, when you replaced your plastic worm gear?
The brushes need to be installed so that they both seat correctly on the armature's commutator. See the illustration below:

I'd suggest removing them again and making sure the concave contact face rests completely on the armature’s commutator. If you've turned it 90 degrees too much, it won’t seat properly and you get no power to the motor.
It would also be helpful to have the Parts List manual on hand. If you're lacking one, you can download a PDF version from the KA website at: http://www.kitchenaid.com/flash.cmd?/#/page/home.
Click on Customer Care>Locate Manuals & Guides> Enter the 6 character model number, in your case use KSM50P (Your K5SS mixer falls within this manual), and select the first file KSM50P.pdf (438.31 KB) listed under Parts List (4) and then check to verify this matches your mixer model.
Turn to pages 6 & 7, titled “Motor And Control Parts”, which provides an exploded view for the disassembly and reassembly of your mixer’s electrical components, along with all the parts. Locate Illus No 47, Brush holder assembly that includes #28, 32, 33, 34 & 46 which is the brush holder insert, and Illus. No. 29, brush & Spring assembly respectively. The illustration along with the picture above regarding the orientation of the brushes should get your mixer powered back up and operational. Good luck! I hope this helps. Howard, Burke, VA
If this solution has been helpful to you, please rate it, thanks! -hslincoln

do not know the device specifically, but with brushes in general it should be fairly obvious by inspecting the alignment of the brush housing to the rotor to determine the proper orientation. perhaps use a flashlight to look into the brush housing and try to see how it meets the rotor? chances are the "point" of the brush would be in the same direction as rotation. it may also be helpful to try one orientation and turn the shaft slowly by hand to confirm the brush is smoothly contacting the rotor..

Hi, The KitchenAid mixers are usually quite easy to work on if you are competent at DIY. It sounds like a gearbox fault. Gearbox parts are easy obtained for most of the models and are easy to repair.

This is a service manual for a lot of the KitchenAid mixers. Might give you an idea on how difficult it is to work on before taking the plunge.

Hope this helps

I just ran into this problem when the PLASTIC worm gear broke and then the mixer would not power on after I replaced the gear. The problem was due to the was I tried to dissasemble the mixer. I removed the two plastic screws near the top center of the mixer when attempting to disassemble. These do not have anything to do with the assembly it turns out the attached square pieces (attached to these screws by springs) must be in the proper orientation to run. I put them in the way it showed in a diagram but it did not work until I tried all four different combinations that they could be rotated. Two of the rotations on the square could be eliminated by closely looking at the end where it contacts the shaft. The round shaft marking will be evident on the square inserts. I'mnotsure how to better describe them, but the are attached by springs to the plastic screws on both sides of the top of the mixer.

I had removed carbon brushes and when tried to start nothing happened kept removing brushes it eventually started