How to calculate energy efficiency and savings for a welding power source.
A welding operator at Ag-Chem uses a Miller system for TIG welding. With Miller inverters, operators can call up pre-set programs to quickly change jobs and reset welding parameters.
Ag-Chem pulse MIG welds using Miller inverter/60M packages. A 60M micro-processor rapidly switches between high, peak current to low, background current in precisely regulated intervals. Peak currents pinch off spray-transfer droplets, while low, background currents cool the weld puddle with little or no metal transfer.
Pulsed MIG welding with correctly set parameters creates little, if any spatter. The pulsed spray transfer projects and sprays tiny molten droplets of metal across the arc, which is established between the electrode and the base metal.
Buying a welder should be like buying a major household appliance, where an "energy guide" sticker on the side influences the purchasing decision. Unfortunately, energy efficiency traditionally takes a back seat to expenses such as gas, wire, rods, labor, and overhead when calculating total welding costs. This is because most welding power sources do a poor job of converting incoming line power to welding output power; efficiencies of 60% to 70% are the norm. Until recently, most manufacturers accepted these losses as part of doing business. But, by taking a practical formula into consideration, companies can save up to $1,700 per year, per welder, over the life of the machine. Essentially, the welder will pay for itself in less than two years.
Welding department superintendents and purchasing agents can use this formula for calculating the energy efficiency of a welding power source:
Since the annual cost to power a welder usually exceeds its purchase price, manufacturers with multiple power sources and/or high duty-cycle applications will want to calculate the energy efficiency of a welding machine and the subsequent cost to operate it.
Companies should ask welding distributors or manufacturers for information on power efficiency across a range of welding outputs, as well as power used while idling. Local power utilities can then provide information on the cost of energy in cents/kW-hr.
This data, along with some simple math and the following formulas, permits calculation of energy efficiency and energy costs to run the machine.
For example, if an application calls for GMAW welding at 409 amp and 34.3 V, the welder should generate 14.0 kW of output power (409 3 34.3 = 14,028 w). Miller's Dimension 652 power source is 82.66% electrically efficient at these welding parameters, so it requires 16.97 kW of input power to generate 400 amp of welding power (14.0 kW output 3 0.8266 efficiency = 16.97 kW primary). Compared to another welder, one with 63.46% electrical efficiency in similar conditions (requiring 21.43 kW of input power), the Miller power source draws 4.46 kW less input power. Manufacturers running Dimensions three shifts daily, with 25% arc on time and 75% idle time, can save $710 annually when power costs are, for example, $0.08/kW-hr.
To calculate the cost for operating a welder, use the following formulas:
[Input kW 3 Arc on time] +
[Idle kW (power source on but not
running) 3 Idle time = kW-hr
One example uses the Dimension 652 in an eight-hour shift with 75% arc on time and 25% idle time:
Compare this to a competitor's power source in the same conditions:
These calculations show that, not including any utility rebates, a Dimension 652 welder could pay for itself in less than two years based on power savings alone.
Equipment maker expects big payback
Ag-Chem pairs a Miller inverter-based power source with a 60M series programmable wire feeder.
At Ag-Chem, welding plays an important part in manufacturing a line of self-propelled, off-road chassis for row-crop machines and high-floatation vehicles. This Terra-Gator fertilizer applicator shows one of the company's finished products.
Ag-Chem Equipment Co. Inc. has purchased 33 Miller inverter-based welders and paired each one with a 60M Series programmable wire feeder. By using this package for pulsed MIG welding, Ag-Chem can expect to save $30,000 per year in welding wire (by standardizing with 0.045-in.-diameter solid wire), save $37,000 per year by eliminating the use of anti-spatter spray, and reduce electricity consumption by about one-third.
The company manufactures and markets a line of self-propelled, off-road chassis for products that include row-crop machines and high-flotation vehicles. Frequently, chassis include a product application system used to apply liquid and dry fertilizer, crop protection products, and biosolids. Some of these systems are computer-controlled and require data management support — Ag-Chem provides both the high-tech systems and related support.
Because Ag-Chem's 103 welding operators work with mild and stainless steel from 20 gage sheet metal to 1 /2-in. plate, its welding equipment must be accurate and provide flexibility.
"The Miller inverters were the best equipment to purchase because, as Welding Engineer Clark Thomas reports, operators could easily read the digital volt/amp meters and fine-tune parameters."
He says, "Operators control voltage to tenths of a volt and wire-feed speed within inches. This accuracy improves the weld."
For process flexibility, 60M feeders can store up to eight different programs, each containing custom-tailored parameters for different jobs. An optional data card, which Thomas strongly recommends, stores up to 32 additional programs and facilitates downloading programs to different machines, which eliminates the need to individually program each machine.
The feeders, with an E-Prom, are programmable off-line, using a custom-manufactured 60M trainer feeder.
"Previously, operators changed jobs and had to re-set the parameters," Thomas adds. "With the 60M, they click the trigger on the welding gun to call up pre-set programs — where optimum parameters have already been determined."
He points out another important benefit: "Pre-established programs also set welding parameters for our new employ-ees with no fabrication or welding experience."
In addition, he says, "The pre-set pulsed MIG programs helped train new operators in welding thick and thin material without having to teach the difference between MIG weld metal transfer modes."
"Purchasing the inverter-based welding packages cost us $130,000 more than if we purchased conventional MIG equipment," says Stan Nelson, welding engineer technician. "To justify the expenditure, we spent six months collecting data. From it, we deduced that the Miller package will save us $130,000 in two years, and completely pay for itself within five years." To see exactly how, it is necessary to understand pulsed MIG.
Pulsed MIG is a modified spray-transfer process. 60M microprocessors enable rapid switching between high, peak currents and low, background currents in precisely regulated intervals.
Peak currents pinch off spray transfer droplets, while low, background currents "cool" the puddle with little or no metal transfer. This gives the weld puddle a chance to freeze slightly and lowers the total energy input in a weldment, thus helping to control distortion and shrinkage.
Benefits of pulsed-spray transfer include enhanced bead wetting, good bead cosmetics, excellent penetration, high deposition rates, an ability to use larger diameter wires than with conventional MIG, and almost no spatter.
Nelson can, with the 60Ms, program optimum peak and background current, pulse duration, and the number of pulses per second for each application. He recommends that anyone working with pulsed MIG clearly understands how these variables affect the arc.
With Ag-Chem's conventional MIG equipment, the company ran 0.052-in.-diameter metal-core wire for heavy plate work (this wire has a high deposition rate, but low penetration) and 0.035-in.-diameter solid wire for thinner materials. Since switching to pulsed MIG, the company has largely standardized on 0.045-in.-diameter E70 S6 wire for dirty materials and 0.045-in.-diameter E70 S3 wire for clean material.
Nelson says, "we expect to save $30,000 annually because the 0.045-in. solid wire costs $0.70/lb, as compared to $1.32/lb for 0.052-in. metal-core wire."
Correctly setting parameters results in little, if any, spatter from pulsed MIG welding. The process projects and sprays tiny, molten droplets of metal across the arc, which is established between the electrode (wire) and the base metal. This fine spray creates a visually smooth weld bead without displacing metal droplets from the weld pool.
Both short-circuit transfer — where the wire shorts against the weld pool up to 250 times/sec — and globular transfer — where large globs of weld metal transfer across the arc — create spatter.
Pulsed MIG requires the operator to maintain an arc gap, which helps decrease the amount of spatter produced. Since switching to the pulsed MIG process, Ag-Chem has eliminated an annual $37,000 spatter-spray bill and dramatically reduced weld clean-up time.
Low energy costs
Welding in CV MIG mode, Ag-Chem would spend about $4.05/eight-hour shift, or an annual cost of about $69,500 for all 33 Miller inverters (the company welds on two shifts, five days a week, and pays $0.08 per kW-hr for electricity).
Pulsed MIG welding draws substantially less energy than conventional MIG welding because the unit operates at low background currents for a portion of the arc-on time. As a result, Ag-Chem will spend about $24,000 annually to power its inverters. Pulsed MIG welding will yield a 290% cost reduction compared to the same Miller inverter power supply for conventional MIG welding.
"We expect to save one-third of what we were spending on the CV machines we replaced," says Nelson. "The old machines were drawing an average of 26 amp, and the Miller inverters average 6 to 8 amp."