Two reasons to upgrade. One is the obvious noise on your existing AC line. Look at it on a scope. If you have a big SS amplifier on the same branch then the top and bottom of sinewave is likely lopped off. There will also be other kinks and distortions, perhaps a lot of hash. Things like laser printers, refrigerators, fluorescent lights all contribute different types of noise back onto the power line. And that is the same AC you are sending to your TT motor.
Fortunately, this is pretty easy to clean up with low pass filtering from a good AC line filter. That one trick alone will help to stabilize the motor so it is not trying to respond to any other signals other than the main 60Hz component.
The other reason is speed stability and variation. Ok, all the power plants are driven from master clocks, so long term drift is not an issue. Your clock on the wall will keep perfect time. However, there may be subsonic variations, although I can't imagine they are very big. But when it comes to constant pitch playback, every bit helps. We want the motor driven at a very steady pace. It is also desirable to change that pitch! Not every TT is machined perfectly or perhaps the belt drive is worn. A motor controller with frequency adjustability can fix this.
Oh yeah, and for 3-phase motors, again, they are not machined or wound perfectly. The magnetic fields may not be perfectly 120 degrees apart! Minimal vibration or cogging of the motor may require slight phase tuning of the three drive signals.
jh
