The dragging part before total failure is interesting.
Someone correct me if I'm wrong, but I think this is a 3 phase induction motor. I think there are three things I can think of that might cause dragging. 1) a bearing failure. 2) A winding failure. 3) An inverter partial failure. I've seen the first 2 occur in motors and the last one in a power inverter.
We've all probably seen a bearing failure. I'm thinking noise at first. Then as it starts to cease up motor drag and overheating, and finally possible gear failure as ball bearings go flying out.
As for a winding failure I had a table saw motor fail that way. It behaved very strangely until it finally failed for good. I first suspected the soft start motor controller, but after replacing that component it didn't fix the problem. Only replacing the motor's induction windings fixed the problem which surprised me as being a rare thing to fail. The defect in the end turned out to be improper and inadiquite physical support and wrapping of the windings to support the physical forces of starting up and running a heavy blade, i.e. the wires that produce the high magnetic fields were bending back and forth with the AC current. Normally motor windings are tightly wrapped to each other, or to some sort of frame to keep them motionless, i.e. from vibrating and/or thus bending with each cycle of the current.
In order to produce the huge amounts of torque in the Tesla, tremendous physical forces are put on the wires generating that force. This can cause tiny physical motion in these wires, and the motion in time can wear off parts of the wire-to-wire insulation, thus causing shorts between loops. This won't stall the motor completely, but will cause it to loose power, heat more and vibrate more. At first only one phase would be affected.
Normally in a three phase motor constant torque is produced so there is very little vibration in the rotor. But because you will have at least 3 separate windings, if one of them has a short between wire loops it will cause vibration in the torque output and excess heating, and roughness as the short at first can come and go with force. And this could cause additional issues like early bearing failure, and/or other windings to in time fail.
In the case of my table saw motor failure, Bosch had not properly wraped and glued the windings tightly to each other leaving them to vibrate somewhat with with the ac current. The damage didn't show up for a couple of years.
Finally if one of the three power supplies (choppers) partially fails, this would also cause torque vibration, as one of the phases would not be balanced with the other two. I don't know how their switching electronics is designed, but I'm guessing that for each phase there are paralleled power components (think paralleled transistor switches, though the silicon is much more advanced than that, probably using some sort of a FET (Field Effect Transistor)) in order to get the high power needed. (Someone please fill me in if you know how this is actually done.) From my experience repairing hundreds of heavy power supplies for main frames years ago, I imagine that if one of those switches fails, that the overall switch might be able to keep working, but not smoothly.
