A method of synchronizing a synchronous reluctance electric machine wherein said machine is speed and/or residual magnetization voltage sensorless and comprises power terminals and rotating masses whose mechanical rotation frequency (f M) is subjected to speed transients caused by power cut-off conditions, which method comprises at least the following steps: a) applying a control voltage (V C) with predetermined amplitude (v C) and duration (T C) to the terminals after the transients, for inducing an electric current (I i), said electric current having a harmonic spectrum that is a function of the frequency differential between the frequency (f C) of said control voltage (V C) and the mechanical rotation frequency (f M) of the rotating masses; b) detecting said induced electric current (I i) to restore power and synchronous rotation control of the machine, wherein said harmonic spectrum of said induced current (I i) has substantially null or negligible harmonic components in the audible range, in response to the application of said control voltage (V C) to the terminals; c) decomposing said induced electric current (I i) into a pair of vector currents (I d, I q) offset by substantially 90°; d) using a high-pass filter to filter out the direct current component and obtain respective filtered vector components (I' d, I' q) dependent on the instantaneous rotation speed of the rotating masses; e) setting an initial value for the instantaneous rotation angle of the rotating masses of the electric machine; f) adjusting said instantaneous rotation angle (θ ist) to determine a synchronized rotation angle (θ sinc), substantially in phase with the mechanical rotation angle (θ M) of the rotating masses; g) supplying the electric machine with a vector current (I al) having an electric rotation angle (θ al) which is a function of said synchronized angle (θ sinc) and a torque component (I al_torque) increasing from zero to the nominal value in a predetermined interval of time; wherein said step f) of adjusting said instantaneous rotation angle (θ ist) comprises a step of h) iteratively minimizing the angular error (ε θ) of the synchronised rotation angle relative to the mechanical rotation angle of the rotating masses from said initial rotation angle value (θ in), said iterative minimization step h) being obtained by means of a first phase-lock loop algorithm (PLL 1) for generating said synchronized rotation angle (θ sinc) as its output; wherein said power supply step g) comprises a step i) of conditioning said synchronized rotation angle (θ sinc) generated by the first phase-lock loop algorithm (PLL 1) as a function of the number of poles of the electric machine and a step k) of fine adjustment of said synchronized rotation angle (θ sinc) obtained using a second phase-lock loop algorithm (PLL 2), and has an initial transient in which said vector supply current (I al) has a substantially zero torque component (I al_torque) and a flux component (I al_flux) having a predetermined value, for generating a flux lower than the nominal flux in the electric machine. a) applying a control voltage (V C) with predetermined amplitude (v C) and duration (T C) to the terminals after the transients, for inducing an electric current (I i), said electric current having a harmonic spectrum that is a function of the frequency differential between the frequency (f C) of said control voltage (V C) and the mechanical rotation frequency (f M) of the rotating masses; b) detecting said induced electric current (I i) to restore power and synchronous rotation control of the machine, wherein said harmonic spectrum of said induced current (I i) has substantially null or negligible harmonic components in the audible range, in response to the application of said control voltage (V C) to the terminals; c) decomposing said induced electric current (I i) into a pair of vector currents (I d, I q) offset by substantially 90°; d) using a high-pass filter to filter out the direct current component and obtain respective filtered vector components (I' d, I' q) dependent on the instantaneous rotation speed of the rotating masses; e) setting an initial value for the instantaneous rotation angle of the rotating masses of the electric machine; f) adjusting said instantaneous rotation angle (θ ist) to determine a synchronized rotation angle (θ sinc), substantially in phase with the mechanical rotation angle (θ M) of the rotating masses; g) supplying the electric machine with a vector current (I al) having an electric rotation angle (θ al) which is a function of said synchronized angle (θ sinc) and a torque component (I al_torque) increasing from zero to the nominal value in a predetermined interval of time;