A Simple Energy-Absorbing Circuit for Current-Source Inverters

The design and performance are described of a simple circuit using one impulse commutated thyristor which absorbs excessive energy returned to the output clamp of a current-source inverter. It is well-known that the thyristor voltage stresses in a current-source inverter are roughly proportional to load current under full load conditions. This Is due to the return of energy trapped in the motor leakage inductance into the commutating capacitors. Most such inverters contain an output clamp circuit to provide reverse recovery suppression of the power circuit diodes, and to provide some limiting of peak capacitor voltage. It is difficult to obtain short-time overload capacity due to the tendency of the voltage stresses to climb dangerously high. The clamp discharge resistors cannot be made small enough to limit the voltage rise without a severe efficiency penalty under normal load. The proposed circuit functions to switch in resistance only when needed to limit the voltage. It is very simple in concept and requires few parts and no external control power supply. Experimental results on a 20-hp current source inverter will be described, showing the ability to attain 150 percent overload with minor increases in voltage stress over nominal load.