Circuit for generating a sinusoidal synchronization voltage for firing thyristors

Claim 1

A circuit for generating a sinusoidal synchronisation voltage for the firing of thyristors in a current regulation circuit of a drive for an electric motor powered by a power supply network, comprising, in combination: a phase-locked loop delivering an output voltage; means for forming the synchronisation voltage as the product of the output voltage of the phase-locked loop and the mean value of the magnitude of the network voltage of the power supply network such that the output voltage of the phase-locked loop essentially equals the synchronisation voltage when the network voltage is sinusoidal and its peak value possesses a predetermined reference value; means for comparing the synchronisation voltage with the network voltage and generating a first control signal when the network voltage essentially is equal to the synchronisation voltage; said phase-locked loop having a reference frequency-input; reversing switch means for conducting, during a first switching state thereof, the network voltage to the reference frequency-input of the phase-locked loop and, during a second switching state thereof, the synchronisation voltage to the reference frequency-input of the phase-locked loop; means for comparing the phase of the voltage applied to the reference frequency-input of the phase-locked loop with the phase of the output voltage of the phase-locked loop and for generating a second control signal when the corresponding phase difference essentially equals null; and said reversing switch means being controlled by both control signals such that said reversing switch means, upon absence of the first control signal and simultaneous presence of the second control signal, assumes its second switching state, otherwise assumes its first switching state.