Frequency converter and method of operating same

Claim 1

A method, for operating a frequency converter comprising an inverter having thyristors in a bridge circuit, the output terminals of which are connected to a parallel resonant circuit and the input terminals of which are connected to an a-c voltage source through an intermediate d-c link containing a smoothing choke and a controlled rectifier with the input terminals of the inverter bridged by an auxiliary commutating arrangement comprising only a bridge circuit of thyristors the a-c outputs of which are coupled only across a capacitor including, during the start-up phase, firing thyristors of corresponding diagonal bridge arms of the inverter and of the auxiliary commutating device prior to every zero crossing of the resonant circuit voltage and, after the conclusion of the start-up phase, firing only thyristors of diagonal bridge arms of the inverter while the thyristors of the auxiliary commutating arrangement remain unfired, comprising the steps of: (a) firing the thyristors in two diagonal bridge arms of the auxiliary commutating arrangement at the start of the operation and maintaining said thyristors conducting until the voltage at the capacitor of the auxiliary commutating arrangement has reached a first predetermined magnitude; (b) firing the thyristors in two diagonal bridge arms of the inverter when said voltage magnitude is reached and maintaining conduction until a current of predetermined magnitude flows; (c) maintaining said current constant during the entire start-up phase by controlling the rectifier; (d) after said current magnitude is reached, firing the thyristors of bridge arms of the auxiliary commutating arrangement that conduct next; (e) after a constant time delay firing the thyristors of bridge arms of the inverter that conduct next, said time delay depending on the voltage and the current of predetermined magnitude; (f) subsequently, prior to every zero crossing of the resonant circuit voltage firing the thyristors of bridge arms of the auxiliary commutating arrangement and, delayed by said constant time delay, firing the corresponding thyristors of the inverter; and (g) continuing this switching cycle until a resonant circuit voltage of second predetermined magnitude which is sufficient for direct communication is reached.

Claim 2

A method, for operating a frequency converter comprising an inverter having thyristors in a bridge circuit, the output terminals of which are connected to a parallel resonant circuit and the input terminals of which are connected to an a-c voltage source through an intermediate d-c link containing a smoothing choke and a controlled rectifier with the input terminals of the inverter bridged by an auxiliary commutating arrangement comprising only a bridge circuit of thyristors the a-c outputs of which are coupled only across a capacitor including, during the start-up phase, firing thyristors of corresponding diagonal bridge arms of the inverter and of the auxiliary commutating device prior to every zero crossing of the resonant circuit voltage and, after the conclusion of the start-up phase, firing only thyristors of diagonal bridge arms of the inverter while the thyristors of the auxiliary commutating arrangement remain unfired, comprising the steps of: (a) firing the thyristors in two diagonal bridge arms of the auxiliary commutating arrangement at the start of the operation and maintaining said thyristors conducting until the voltage at the capacitor of the auxiliary commutating arrangement has reached a first predetermined magnitude; (b) firing the thyristors in two diagonal bridge arms of the inverter when said voltage magnitude is reached and maintaining conduction until a current of predetermined magnitude flows; (c) maintaining said current constant during the entire start-up phase by controlling the rectifier; (d) after said current magnitude is reached, firing the thyristors of bridge arms of the auxiliary commutating arrangement that conduct next; (e) after a constant time delay firing the thyristors of bridge arms of the inverter that conduct next, said time delay depending on the voltage and the current of predetermined magnitude; (f) subsequently, prior to every zero crossing of the resonant circuit voltage firing the thyristors of bridge arms of the auxiliary commutating arrangement and, delayed by said constant time delay, firing the corresponding thyristors of the inverter; and (g) continuing this switching cycle until a resonant circuit voltage of second predetermined magnitude which is sufficient for direct communication is reached.