Why Are VFDs for Induction Motors More Complex Than for Synchronous Motors?

To be clear, I am in by no means an experienced electrical engineer or designer.
I have been studying the fundamentals of induction motors and synchronous motors, and I've been struggling with understanding the differences in variable speed control of the two machines. I believe variable frequency drives (VFD) for induction motors are significantly larger and more complex than VFDs for equivalent synchronous machines, but I don't understand why exactly this is. Does this difference stem from the fact that induction motors have to deal with slip between the rotor and stator magnetic fields, and the feedback necessary to compensate for this is more complex than for synchronous systems? Better understanding the general requirements for variable speed control of these two systems will help me answer why designs for electric vehicles have opted to use synchronous motors instead of induction motors.

Really?  The motor drive electronics are larger in volume for induction motors than those for synchronous of the same speed and power rating?  What leads you to say that?  For the same bus voltage, the main thing that drives size is current.  I would expect the current draw to be a little higher in an induction motor because of the magnetizing current that is needed, and possibly because it is less efficient (winding and/or rotating losses) but I wouldn't expect those differences to be significant.  Complexity could be higher because it is harder to estimate where the rotor field is in the induction motor but that is in logic and software, not hardware.  -JD

Oh, and re-reading your question, I think the preference of using synchronous motors is likely due to them being permanent magnet motors (I assume) which can be significantly smaller  and lighter (and more expensive) than induction motors of the same performance.  All my experience with motors and drives has been limited to small sine-modulated permanent-magnet motors for personal transportation and robotics.  Someone with broader experience with larger electric vehicles may be able to help better here.

The power factor of a synchronous motor can be adjusted to lagging, unity or leading by varying the excitation, whereas, an induction motor always runs at lagging power factor.Other differencesSynchronous motors require an additional DC power source for energizing rotor winding. Induction motors do not require any additional power source.Slip rings and brushes are required in synchronous motors, but not in Induction motors (except wound type induction motor in which slip ring motors are used to add external resistance to the rotor winding).Synchronous motors require additional starting mechanism to initially rotate the rotor near to the synchronous speed. No starting mechanism is required in induction motors.The power factor of a synchronous motor can be adjusted to lagging, unity or leading by varying the excitation, whereas, an induction motor always runs at lagging power factor.Synchronous motors are generally more efficient than induction motors.Synchronous motors are costlier.