"do you know why such a great speed overrun is needed to hit 60Hz
and why the motors seems to try to output only it's rated Hz and input
voltage? "

The field actually rotates in the rotor. It does the same thing when it is a motor, when you load it and it pulls more current, you are fighting the magnetic field's desire to not rotate in the rotor. At 60 Hz it should be 1,800 RPM but it isn't. It's 1,750. That is actually nominal but it is accepted as the normal loss for motors that run machines rather then say, clocks.

This is the same **** only opposite. It's also a hell of alot less eficient when not made for the purpose. In a way it's like running an internal combustion engine backwards, just not as bad.

J