Why does a permanent magnet AC motor need a VFD?

Why can't we operate a permanent magnet (PM) AC Motor directly connected to the grid? The grid produces the rotating stator magnetic field at the grid frequency. I suppose the rotor should rotate because its magnetic field is either attracted or repelled by the rotating stator magnetic field continuously at the grid frequency, but why does the rotor just vibrate?

A: Assuming it's either an IPM or SPM motor, they are synchronous motors and upon start up, the magnets locate themselves. Depending on brand, they will have internal sensors or Hall ICs.

Using an IPM or SPM, you should be aware;
1. Use soft start, PM motor reach full speed/full torque nearly instantly. The shock load often damages connected mechanical components.
2. Voltage must be controlled.
3. You should not run more than 1 motor on 1 variable frequency drive (VFD).
4. VFD capacity must match motor (do not mix a 1hp VFD with a 1/2 hp motor).
Note; a spike can demagnetize the magnets leaving you with an expensive boat anchor.

Finally, not just any VFD will work.
You must use a variable frequency drive with a PM mode.
A VFD with dual CPUs greatly increases the accuracy so the PM motor can be used for positioning.
One of the best I am aware of are the Yaskawa V1000 or A1000 (Yaskawa actually has a very informative white paper on PM motors).
Hitachi and Toshiba offer some lower cost options.
Gozuk Drives recently developed one as well.