A method of determining instability of an ultrasonic blade includes monitoring a phase angle ϕ between voltage Vg(t) and current Ig(t) signals applied to an ultrasonic transducer, coupled to an ultrasonic blade via an ultrasonic waveguide, inferring the blade temperature based on the phase angle ϕ, comparing the inferred temperature to an ultrasonic blade instability trigger point threshold, and adjusting a power level applied to the ultrasonic transducer to modulate the temperature of the blade. The method may also include determining a frequency/temperature relationship of an ultrasonic blade that exhibits a displacement or modal instability and compensating for a thermal induced instability of the ultrasonic blade. The method may be implemented in an ultrasonic surgical instrument or by a control circuit in a power generator for the ultrasonic surgical instrument.
