Page 66 - MetalForming October 2011

Resistance Welding
64 MetalForming/October 2011
www.metalformingmagazine.com
The single-gun machine resistance-
welds two nuts with ring projections to
a General Motors seat component. With
the previous AC welding system, elec-
trodes were only good for about 40,000 welds; the new MFDC system enables the machine to run 200,000 welds prior to an electrode change (although just to be conser- vative, operators change electrodes after 150,000 welds). Annual expenditures for con- sumables has dropped by more than $10,700.
amperage available from the machine. As a result, brazed-on Elkonite tips (tungsten-copper alloys from CMW Inc.) of the welding electrodes some- times fell off.
The problem finally came to the fore- front when compressors in the office air-conditioning system began to fail, and the company attributed the break- downs to the voltage drop occurring every time the welding machine fired.
In November of 2009, the plant’s maintenance team leader DeWayne Spann and general manager Terry Judy decided to convert the machine to MFDC technology, based on these per- ceived benefits:
• Energy savings;
• Balanced three-phase primary draw; • Improved power factor;
• Lower secondary-loop inductive
losses;
• More accurate weld-feedback
capability; and
• Reduced weld expulsion.
Emerging Technology
While relatively low-cost single
phase AC resistance welders remain more common than MFDC inverter technology in U.S. plants, and con- ventional single-phase transformers and controls can be more robust in some applications, the added efficien- cy of using three-phase mid-frequency DC current can be dramatic. In addi- tion, a metalformer applying MFDC technology to a new installation can realize significant savings over the cost of installing utilities to serve single- phase machines, since smaller three- phase electrical switches and wiring can be used.
At Metrican Stamping Corp., the existing RoMan single-phase 150-KVA 60-Hz welding transformer was replaced with a new RoMan 320-KVA, 50-percent-duty cycle, three-phase MFDC power supply operating at 1000 Hz. In addition, the existing welder control was quickly converted with an MFDC retrofit kit (from Medar, a division of WTC).
Although the MFDC transformer is rated at 320 KVA, it’s physically smaller and lighter than the 150-KVA