MF welding is not necessarily specialised

A Bosch Rexroth product story
Edited by the Manufacturingtalk editorial team Jul 9, 2003

Many automotive manufacturers use modern Medium Frequency Resistance Welding techniques that make less demand on plant power systems, shorten weld times and delivers greatly improved quality.

Many of the most quality-conscious automotive manufacturers in the UK and Europe employ modern Medium Frequency Resistance Welding techniques.

These companies, renowned for their stringent management control systems, feel that MF welding is less demanding on plant power systems, requires shorter weld times and considerably less energy, yields savings on consumables and delivers greatly improved quality.

Moreover, the Electric Drives and Controls division of Bosch Rexroth, an acknowledged world leader in MF technology, suggests that many processes using spot, projection or seam resistance welding will benefit equally from this system, emphasising that payback times are becoming shorter as energy costs rise and reject rates fall.

Adding to the benefits, Rexroth backs its own PSI 6000 range of MF welding controls with an online ultrasonic quality assurance system, which is the only method that allows welds to be monitored in process.

Coupled with MF welding techniques, the PSQ 6000 system leads to further savings on destructive testing and rejects, eventually resulting in reduced plant and manpower requirements.

Resistance welding takes advantage of a workpiece's inherent resistance to the flow of electrical current, thus generating heat; unlike other methods, it does not utilise flux and filler rods, the weld nugget being formed directly from the base material itself.

Medium frequency (1000Hz) resistance welding works by rectifying a three-phase 50Hz AC current and supplying it to an inverter, which converts the current to a frequency of 1000Hz and passes this through a transformer/rectifier, typically integrated into the welding gun, so that the welding current is always DC.

Compared with AC welding at 50Hz, the use of direct current MF welding offers a number of technical and operational advantages.

Since the MF inverter draws balanced line current from all three phases, MF welding is less demanding on the plant's power system, resulting in reduced electricity bills.

It is also more tolerant of mains disturbances, as energy is supplied via the inverter rather than directly, while cabling economies are also permitted.

Rexroth estimates that for a given welding application, power savings of around 25% are achievable against a single-phase AC system.

Medium frequency DC means that weld times are 15% shorter too, because there are no current fluctuations.

Furthermore, due to the 1000Hz output from the inverter and its more compact waveform, weld times can be controlled to the millisecond, giving twenty times more accuracy than is possible with a 50Hz AC supply.

Also, unlike AC systems where there is inherent resistance in the welding arm, there is no resistance to overcome, allowing different arm lengths and tip characteristics to be specified, without affecting weld times.

Tip life is also extended, resulting in considerable savings on tip dressing and consumables.

With MF systems, peak voltage is virtually the same as RMS voltage, which helps to obtain high quality welds, with none of the 'splash' or sparks that are characteristics of AC welding.

Since welding is a function of current, time and pressure, consistent weld strengths can be obtained at lower pressure.

Thanks to the higher operating efficiency, even the MF transformers are lighter and more compact than AC types, especially where larger currents are required, making them particularly suited to robotic applications.

The controllability of MF also makes it ideal for difficult-to-weld materials like high strength steel, aluminium and laminated steel/polymer composites.

Rexroth provides for a whole range of MF resistance welding control applications, with its advanced PSI 6000 series, which combines an inverter, weld-time controller and selectable I/O board in a single, compact functional unit.

The PSI 6000 concept provides parameterised control functions for spot, projection and seam welding processes, in order to realise specific, complex sequences.

All the common I/O fieldbus systems can be used to connect the unit to the allocated machine or robot controller; while a choice of communications interfaces enables programming and data retrieval via Ethernet or other local area networks.

Rexroth's MF control options include one model in the 3-6kV current range for precision microwelding; water and air cooled versions for body-in-white manufacture, with a working range up to 36kA; one designed for high current aluminium and special welding applications (9-60kA); and another for seam and projection welding, where the payback can be exceptionally fast.

A BT6 operating and diagnostic unit can also be connected to the PSI 6000 series, providing direct access to the welding control parameters.

Find out more about this article. Request a brochure, download technical specifications and request samples here.
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