I work for Toyota Ky. I teach resistance welding and have seen many problem welds in the past five years. I would like to share an unusal problem with everyone and see if anyone has had the same problem or can give a solid answer. Inductive reactance caused by soft iron outside the secondary loop. This problem only shows up when welding thin-coated metals. The welder must be very close to a clamp or a datum support and be perpendicular to the shanks. I do know as the soft iron is turned more parallel to straight parallel with the shanks, weld quality increases. Lower resistance of the work is not the problem but is high lighted by either an increased flux density and or warping the magnetic fields of the welder. Test with a impedance meter did not show the results I wanted to see however the impedance meter can not generate the flux density needed for this test. G.W. Riddell

-- Gary Wayne Riddell (gwriddell@hotmail.com), January 30, 2000

Answers

The inductive reactance and magnetic flux seen in the secondary of a resistance welder is present always when weld current is flowing. It is in basic terms the same as your primary voltage is inducing into the transformer core of the welding transformer. The secondary is also producing a magnetic flux, and with the higher amperages lower volts, the magnitude of the field is much larger than seen on primary windings. With any iron content being introduced into or around the secondary loop, the magnetic flux will be induced into it. The size of the field determines how close or far iron content materials will have an influence. More times than not, the larger iron mass and the location of the mass to the lines of flux will be seen as a greater loss in actual welding current. You stated that the problem only shows up when welding the thin coated metals. It would be my belief that it becomes more apparent when welding these coated materials because of the higher required currents. It should be present at the same weld gun location with just plain mild steel. At lower currents the field would be smaller, resulting in less of an affect yet still present in comparison. A possible solution would be to change the material of the clamp and datums to a stainless steel or a material of non-magnetic properties and review your results after the change. I hope I answered a little of what you had asked.

-- Spot (Spotweld@juno.com), February 09, 2000.