Sub-synchronous static converter cascade

Claim 1

In a circuit arrangement for operating a slip-ring induction motor in the sub-synchronous range wherein the slip energy is fed back into the mains by means including an intermediate direct current circuit comprising a diode rectifier and a mainscontrolled static inverter connected in cascade, the combination comprising a first resistor and a silicon controlled rectifier switch arranged in series therewith and which is connected in parallel with said inverter, said rectifier switch and said inverter being correlated such that said rectifier switch is rendered non-conducting when said inverter is operating, a first detecting and control means monitoring the voltage of said intermediate circuit and which applies its output to effect a firing of said rectifier switch when the voltage of said intermediate circuit exceeds a predetermined value for an operating mode when said inverter is cut off, and a circuit breaker having a second resistor arranged in parallel therewith and which is connected between said diode rectifier and the junction point at which said rectifier switch connects with said inverter, said circuit breaker being normally closed when said inverter is operating, and a second detecting and control means monitoring the direct current in said intermediate circuit and which applies its output to effect an opening of said circuit breaker when the current in said intermediate circuit exceeds a predetermined value thereby to introduce the resistance of said second resistor into said intermediate circuit.