Midland-ross Corporation

Medar, Inc.

Vanderhelst, Peter W.

Peter W Vanderhelst

Active snubber structure and method for use with an electronic power inverter to reduce turn off losses in electronic switches in the power inverter. The snubber structure includes a resistor and a capacitor connected in series with each other and in parallel with the load on the power inverter and parallel, inverse connected silicon controlled rectifiers connected in parallel with the resistor for selectively shorting the resistor. In the method of use of the active snubber structure, electrical energy is stored in the capacitor connected in parallel with the load during on time of the inverter and the resistor is shorted immediately prior to the off time of the inverter whereby the capacitor charges substantially to the value of the source of direct current electrical energy during the on time of the inverter and on shorting of the resistor during off time of the inverter, the voltage across the load changes slowly so that the switches of the inverter can transition from on through a resistor like turn off stage, to completely off before there is appreciable voltage across them.

Active snubber for an inverter

A computer-controlled welding apparatus includes a phase-controlled resistance welding circuit (A) for selectively conducting pulses of a welding current to a workpiece (B) and a control circuit (C) for controlling the conduction of the welding current. The control circuit includes a line clock generator (30) for determining the beginning of each voltage half line cycle and a conduction sensor (34) for determining whether or not a contactor (14) is conducting current to the workpiece. A microcomputer (40) is preprogrammed with a plurality of power factor correction curves, each of which represents weld current vs. firing angle for a fixed hangover angle. The computer is programmed to select one of the power factor correction curves. If the preceding half line cycle was conductive, the program selects the curve which corresponds to the hangover angle of the preceding half line cycle. If the present half cycle is the first conductive half cycle in a weld, the program selects the curve corresponding to a hangover angle of about 87.degree.. During pulsation welding, the curve is selected based upon the hangover angle of the most recent conductive half cycle and the number of nonconductive half cycles therebetween. During each half cycle, the present angle (duration since the beginning of the half cycle) periodically addresses the curve and a corresponding weld current is retrieved. The retrieved weld current represents the weld current which would be produced if the container were rendered conductive at the present time, i.e., the present weld current. A comparator (86) compares a preselected weld current for the present half cycle with the present weld current. When the present weld current becomes coincident with the preselected weld current, the contactor is rendered conductive.

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Birmingham, MI, United States of America

Peter W Vanderhelst