The invention relates to a method and a system for controlling the frequency of the supply voltage for the ultrasonic piezoelectric power transducers. According to the invention, the method is based on estimating the motion current derivative and using the estimation in a control loop depending on the error, so that there is provided the maximum mechanical power developed by the transducer, based on the measurement of the transducer voltage and current, by controlling the frequency of the supply voltage, in order to maintain the motion current derivative to zero. As claimed by the invention, the system comprises a power amplifier (AP) operating in commutation at high frequency, which supplies a piezoelectric transducer (E) with periodical electric voltage directly or by means of a voltage step-up (T), the voltage and the current of the transducer (E) are measured by means of a voltage transducer (Tu), and a current transducer (Ti), respectively; an estimating current (CC_DCM) uses the measured voltage and current values for estimating the motion current derivative, the output thereof is connected to the inverting input of a frequency control circuit (RG-f) of PI type which operates depending on the error between the motion current estimation and zero, the output of the frequency controller (RG-f) is connected to the voltage control input of a voltage-controlled oscillating current (GF_CT) which generates an impulse sequence with a frequency proportional to the control voltage provided by the controller (RG-f), and the output of the voltage-controlled frequency generator (GF_CT) is connected to a control circuit (CC) of the power amplifier (AP).
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