Electric Shock Protection of A.C. Arc Welder with Inverse Parallel Thyristors and Influence on Drooping Characteristics

Abstract

The present paper describes an A. C. arc welder which is controlled by inverse parallel thyristors in the primary winding. These thyristors operate as a protector of electric shock of the welder. The experimental results show that this apparatus has several advantages of lower secondary voltage in no load condition, lower mechanical noise, etc. So, this apparatus is found to be better than the usual one with electromagnetic switches; considering noise and wearing problems of mechanical contacts.
Being drived by phase controlled thyristors, the stability of arc emission of the apparatus is increased in the small current region, but the sharpness of the drooping characteristics is reduced, and the abnormal current arises in the large current region. This abnormal current in the primary winding of the welder under some controlling condition of thyristors, is stational current and contains much D. C. component. Considering the cost of the circuit, it was found to be satisfactory for the purpose of suppressing the abnormal current, to add continuous pulses following a control signal pulse, suitable pulse interval being 5°el.
Next, a simplified equivalent circuit for the A. C. arc welder is introduced, and the influence of the phase control of thyristor upon the drooping characteristics of the welder is shown. The theoretical curves obtained from the equivalent circuit agree well with the experimental results for the efficiency, the power factor and the drooping characteristics. It was conLluded that a feed back compensation circuit or its equivalent is necessary for A.C. arc welder controlled by thyristors.
In the last section, the influence of magnetic saturation upon the conduction angle of thyristor is discussed.