get me,small & should have low rpm/freq. cutoff setting with around 3% voltage regulation. Self excitation based device is required,which will use residual magnetism of generator.

 

Do you already have the generator and need a way to control it?? Do you want to control voltage, frequency or both? What variations in RPM do you expect to see? OR do you just want to monitor the frequency and provide an indication that it is above or below a given value by more than 3%? Is it required that you use AVR or would other methods meet your needs?