Frequency converter

In a frequency converter comprising an inverter with thyristors in a bridge circuit, the a-c outputs of which are connected to a resonant circuit and the d-c inputs of which are connected to an a-c voltage source, via an intermediate d-c link with a smoothing choke and a controlled rectifier and having a voltage sensor for determining the resonant circuit voltage and a current sensor for determining the inverter output current, the outputs of which are inputs to evaluating circuit, the output of which is connected to the control unit for the thyristors of the inverter, which evaluating circuit determines by means of a trigger pulse, prior to every zero crossing of the resonant circuit voltage, a firing instant for thyristors of diagonal bridge arms of the inverter, the position of which depends on the commutation time of the inverter and the protection time of the thyristors, the improvement comprising:

• a. the current sensor being one which measures and adds up to an ac current the currents in bridge arms with a common d-c terminal;
• and
• b. a monitoring circuit having as inputs the outputs of the current and the voltage sensor and developing at its output a pulse if, between a firing instant (t.sub.z) and the corresponding zero crossing (t.sub.o) of the resonant circuit voltage, the inverter output current and the resonant circuit voltage remain in phase or developing said pulse if said current and voltage are in phase opposition after the zero crossing.