There is something to be said for using current as a cue for switching. Marlec did this with their old FM1800 control box for switching between transformer taps. They used a current trasnsformer and some op amps with feed back to make it stable (ish).
As for a simple method to determine speed, the best idea I came up with was to build a pilot coil into the stator using very thin wire, and then power the star/delta relay off that. It might be better to use 3 coils and a rectifier to get a cleaner switching action with DC.
I confess I did not persevere with the thing and get it working well. The problem with star/delta for me is that you are using 2 stator configurations, neither of which is really suitable for the mid-range winds where most of your energy is generated.