Showing posts with label Demand Pulse. Show all posts
Showing posts with label Demand Pulse. Show all posts

Demand Pulse Welding: A Brief History

Demand Pulse Welder
Demand Pulse Welder
One of the oldest welding technologies still in use today is resistor controlled arc welding. The technology is old, but when combined with more modern controls, provides exceptionally high quality performance. Resistor controlled arc welding incorporates edge-wound resistors using a special current-controlling alloy.

From the 1930’s until 1990, nearly all Multiple Operator Systems were the same general layout – a big bulk power supply, with “grids” connected by means of cable to form a system of distributed power. Originally designed for use in shipyards, this system minimized the use of high voltage primary power, distributing 75-80 volts of secondary voltage instead.

Historically, the most common application of resistor controls has been in Multiple Operator Welding Systems, widely used in shipyards and heavy construction. Nearly all the Nuclear Electric Power Plants were built using Multiple Operator Systems.

Big Four Manufacturing Company of Saint Louis, Missouri, pioneered the concept of connecting Multiple Operator systems in a complete loop configuration. This LOOP concept greatly improved the voltage stability of  Multiple Operator systems, because with all the power connected to a single circuit, voltage drops were reduced or eliminated due to the nature of a DC circuit. In 1977, Big 4 Manufacturing introduced their innovative and very successful Series 77 Control.

After the Three Mile Island incident and the subsequent slowing of the Nuclear Power Industry, Big 4 decided it was in their best interest to expand into the general fabricating, refinery and petro-chemical markets. Their popular soft arc was great for most welding, but was inadequate on pipe root passes with 5P, then the most common method use for oil related pipe. To provide an arc better suited for pipeline, Big 4 devised a circuit to make their resistor grid “think” it was a Lincoln generator. They called this new system “Arc Ram”. Arc Ram worked beautifully with 5P and similar electrodes and led to the design of a MIG system which they called  Demand Pulse.

Not to be confused with ...

"Short-circuiting Metal Transfer" (Short Arc) welding was introduced by Linde in the late fifties for welding sheet metal. During the 1960s, short-arc became widely used, sometimes for the wrong applications. When using short-arc the wire can contact the molten weld pool resulting many times with the weld not fusing to the base metal. This is called a "cold lap". Cold lap associated with Short Arc became a reason why many welding shops won’t use “wire” welding processes on pressure welds. Unfortunately, mis-used Short Arc welding gave ALL GMAW welding on pressure welds a bad name.

Demand Pulse to the Rescue

Demand Pulse rarely extinguishes the arc. Metal transfers through the arc, and the arc’s are much shorter than those from Pulse Spray. The transfer occurs above the molten pool, so spatter doesn't explode from the puddle. It is a "constant current" process. The operator selects a base current, similar to some GMAW Pulse Spray applications, adjusts the wire feed speed to give the correct voltage, and begins to weld. Lower voltage will cause the puddle to freeze faster, higher voltage will cause the puddle to be more fluid.

In short arc, a current pulse is triggered by the short-circuit condition caused by the wire driving into the work piece. Demand Pulse forces the transfer to occur before short-circuit, at a voltage selected by the operator, above the weld pool. This has two important effects: the arc does not extinguish and the spatter level is greatly reduced because the molten tip of the wire does not contact the weld pool. Because the arc is not extinguished, cold laps are virtually eliminated.

Many reference books describe short arc as a "random pulse" method of welding. Random in the sense that the pulse is triggered as a function of wire feed speed. The faster the wire feed speed, the faster the short circuits (pulses). Demand Pulse is exactly the same, except it does not short!

This blog post was abstracted from a 2004 press release from Aftek Welders, the only remaining company providing resistor controlled welding machines, and parts supplier for all Multiple Operating Systems. You can read the full version of  "AFTEK – the Best Welders you never heard of…” here.

Welding Application Note: Demand Pulse Technology Saves Three Weeks Production Time

Demand Pulse technology
welder (courtesy of Aftek EHS)
Demand Pulse is a unique, patented welding process. Because of the low peak current, open butt pipe welds are easier in all positions.

Short arc and pulsed spray both have peak currents in the 375+ amp range, and are at peak many times longer than the Demand Pulse process.

With Demand Pulse, welding current and volts are similar to those seen when “short-arc” welding, but there are NO short-circuits - the arc never goes out, and cold laps are almost impossible.

Demand Pulse allows for the use of very light torches, and still experience extremely long tip life. A 140 amp hand torch will not overheat running 200 amps on overlay work. Its even been reported that some customers only use one tip per week with Demand Pulse,  as opposed to several per day using conventional welders.

The first two pictures below are of an initial job done by a new customer using Demand Pulse MIG. The large skids pictured have (580) open butt, 316 SS pipe welds. The smaller manifold shown has (24) welds, for a grand total of (604) welds.

Only two qualified welders worked on this demanding job. They tested other types of equipment, all with unsatisfactory results. Then, after testing a Demand Pulse system, a good coupon was produced on just the second try, so the customer purchased a system and production started immediately upon delivery.

Astoundingly, the job was completed three weeks ahead of schedule, saving the fabricator 1600 hours of production. Most importantly, there wasn't a single repair required, and all welds passed 100% X-ray to ASME Code.

Large skids with 580 welds, all open butt 316 pipe.

Small manifold with 24 welds.

After experiencing Demand Pulse, this customer commented he'll never use anything else and that the welder paid for itself on the first job.

Below are close-ups of two of the welds on the large skid. The customer insisted on TIG wash on the cap. The TIG cap had 7 repairs - welds using Demand Pulse needed NO repairs.

TIG welds need 7 repairs.

Demand Pulse required 0 repairs.

For more information, contact:

AFTEK-EHS Welders
2960 East State Street Ext.
Hamilton, NJ 08619
Phone # 609.588.0900
Fax # 609.588.8333
www.hotfoilehs.com
Email: dap@hotfoilehs.com

Demand Pulse MIG Welding

Demand Pulse MIG Welder
Demand Pulse MIG Welder
(courtesy of Aftek-EHS)

Demand Pulse is possible because of the MOSFET, an extremely fast electronic switch. Using constant current DC power, a wire is fed into the arc zone. As the wire approaches the work, the voltage drops as a function of the arc length. At a pre-selected arc voltage (usually 15 volts), the MOSFET fire a pulse of current to expel the tip of the wire across the arc gap. This current is supplied by means of a parallel resistor in the main current control circuit and is generally 100 amps. Switching time is measured in nanoseconds, and the total cycle time is very short, perhaps 15 nanoseconds. An inductor is not required, nor desired, since fast switching is essential. Perfectly tuned, the pulse cycles 200 times per second or slightly less.

The arc looks and sounds like “short-arc”, but a close look will show that there is no short-circuit. The arc length is from .020” to .090”, very short compared to GMAW-P done with conventional machines. This short arc length, combined with the short duration of the pulse, and the low current required to effect transfer gives a total arc “heat” far below that of either short-circuit transfer, or conventional GMAW-P. Fusion is excellent, because the arc never goes “out”.

Below is a demo video using this technology on 18 gauge stainless steel, without any burn through.

For more information contact:

2960 East State Street Ext.
Hamilton, NJ 08619
Phone # 609.588.0900
Fax # 609.588.8333
Email: dap@hotfoilehs.com