Power electronics for resistance spot welding equipment

Sammanfattning: Resistance spot welding is an inexpensive and efficient way ofjoining metals. It has extensive applications in household appliancesand in automotive industries. The traditional approach in relation tospot welding machines is to use 50 Hz welding transformers. Thedrawback associated with these transformers is that they are both heavyand bulky. Moreover, the fusing requirements become larger due toincreased welding power.With the development of high power semiconductor switches andDC-DC converter topologies, it is now possible to develop inverterdrive resistance spot welding equipment (RSE) which can be operatedat frequencies higher than the 50Hz frequency. The advantage of usinghigh frequencies is the reduction in the size of the transformer.Moreover, the fusing requirements are relaxed, as the power is sharedbetween three phases.In many industrial applications long welding arms are requiredbetween the transformer and the weld spot, which increases theinductance. The parasitic inductance in welding arms limits themaximum rate of change of the current. In order achieve a higher powerthe current has to be rectified. To rectify a current of the order of tenthof kA is challenging task and is one of the major sources of loss.The full bridge converter topology is used for the inverter drive RSE.The power switches used in the converter are IGBT. In RSE, the DClink capacitors are used to store high energy. In the case of circuitfailure, the stored energy can cause the IGBT device to rupture and inorder to avoid this, a protection scheme is discussed in this work.A controller circuit, using a DSPIC33FJ16GS502 controller, isdeveloped in order to drive a high frequency full bridge converter,which can also be used to drive the IGBTs in the RSE.The secondary side welding current is of the order of kilo amperes.A requirement for the welding control is that the current must be sensedprecisely and in order to fulfill this, a Hall sensor system has beendeveloped. This developed circuit is used in the feed-back control of theRSE. The presence of metallic objects and tools in the vicinity of theHall sensor system can affect its precision. We have estimated theivexclusion distance for the metal objects from the sensor by means of amodel developed in COMSOL Multiphysics software.