Often a component, device, or system is the darling of an industry, but rarely does any one item, no matter how snazzy, work by itself. Typically the dazzling item is a high-visibility unit supported by many critical components that work in the background, out of sight and out of mind, as the saying goes. A 1966 Dodge Charger outfitted with a 426 hemi wouldn’t move an inch under its own power without a set of spark plugs, a power steering pump doesn’t do any good without a hydraulic line, and a supercharger can’t do its job without a connection to the fuel tank.
On a tube or pipe mill, the induction welding unit is a marvel of technology, focusing a tremendous amount of electrical power at the coil. It sets up an electromagnetic field so strong that it turns steel red-hot almost instantaneously. Electrical current flowing through the tube or pipe’s wall, hindered by the metal’s resistance, generates a spectacular amount of heat in short order. This is followed by a squeezing force exerted by the roll tooling, thereby making this a forge welding process that joins the two edges to create a seam as the material moves through the mill.
The welding unit uses a common AC input, at 50 or 60 cycles per second (hertz), to develop waveforms at frequencies in the hundreds of thousands of hertz, providing an optimized output that corresponds to the tube or pipe to be made on the mill. Doing so means relying on sophisticated circuitry that multiplies the power and increases the frequency simultaneously. A key component—the unsung hero—is the transistor.
The original transistor, developed in 1947, can work as an amplifier, switch, or switchable resistor—and it ushered in the modern electronics era. Although vacuum tubes would continue to provide the basis for radios and televisions long after the transistor was invented, eventually the transistor came to dominate these markets. Using less power, generating less internal heat, needing less space, and costing less than a vacuum tube, the transistor was just one of several semiconducting devices that changed the electronics field. Circuits based on transistors, diodes, inverters, and gates would modernize radios and televisions and open up entirely new possibilities in circuit design and new products.
Because the transistor is a solid-state device—one that has no moving parts—it can switch on and off thousands of times per second. It’s this capability that allows it to contribute to sophisticated outputs that can be designed and tailored by the circuit designer.
High-powered Transistors for High-powered Applications
The original transistor, the field-effect transistor, led to numerous varieties, such as the junction gate field-effect transistor, the insulated gate bipolar transistor, and the metal oxide semiconductor field-effect transistor (MOSFET). While semiconductors that function in low-voltage circuits, up to 5 volts, are commonly used in consumer electronics gear, industrial systems use vast components that work on voltages in the hundreds or thousands of volts.
In induction welding power supplies, which typically have an input of 380 or 480 V at 50 to 60 Hz, the circuitry increases the frequency to a practical maximum around 500 kilohertz (for welded pipe to meet American Petroleum Institute manufacturing standards, a frequency between 100 and 500 kHz is typical). In such a power supply, the traditional MOSFET has a substrate made of silicon (Si). A 100-kilowatt induction welding unit made with this component type needs 64 MOSFETs, said Don Gibeaut, global tubular products manager for Ajax TOCCO Magnethermic.
However, this isn’t the end of the line. A power supply with the same input and output, based on another MOSFET type, one that uses silicon carbide (SiC) as the substrate, needs just four transistors. Comparing an SiC MOSFET to an Si MOSFET isn’t like comparing the transistor to the vacuum tube, but indeed the SiC version does have some remarkable improvements over the Si version, Gibeaut said.
Capabilities. Silicon carbide’s physical properties make it a more efficient semiconducting medium than silicon. Using several devices made from SiC leverages the material’s efficiency.
“SiC MOSFETs have three times the conduction rate of Si MOSFETs,” Gibeaut said. The conduction rate is influenced by current leakage. Both Si and SiC devices leak current when they are in the switched-off state; the SiC type runs more efficiently and has a lower rate of current leakage at a given temperature. In a side-by-side comparison of two MOSFETs developed by component manufacturer Semikron, when running at 1,200 V, the energy loss of the Si device is 104 megajoules (MJ) whereas the SiC unit’s energy loss is 15 MJ, an 85 percent improvement.
The SiC variety also has higher thermal conductivity, meaning it generates less heat, which is wasted power. The operating temperature is an essential parameter; removing excess heat is an essential function.
“In induction power supplies, transistors switch tens of thousands of times per second, which generates a quite a bit of heat,” Gibeaut said. The heat is such a liability that, on some conventional units, Ajax TOCCO used to remove the cooling fins and integrate a water-cooling circuit to wick heat away from the transistor. SiC units generate much less heat, alleviating the need for such workarounds.
The switching process also results in a power loss. Faster switching—running at higher frequencies—generates greater losses. Every transistor is prone to this phenomenon, but an SiC MOSFET experiences nearly 75 percent less switching loss at 30 kHz, Gibeaut said.
Another efficiency factor concerns the voltage rating, or breakdown strength. All else being equal, the SiC variety has 10 times the breakdown strength of the Si variety, Gibeaut said. A greater breakdown strength is used by the transistor manufacturers in one of two ways: a capacity increase or a size reduction. The new component either handles more voltage or takes up less space, providing more capability or more convenience.
These factors—lower current leakage, higher thermal conductivity, and higher breakdown strength—mean that SiC transistors run more efficiently than Si transistors. Using fewer transistors multiplies the efficiency gain.
“Our 500-kW MosWeld Si unit, 300-kHz output, has 620 semiconductors,” Gibeaut said. “Our MosWeld SiC unit has 20 semiconductors and generates 30 percent more power.”
Trials and Tests. In use for about 30 years in the power generation industry, the SiC MOSFET has been deployed in induction welding units built by Emmedi for about two years, Gibeaut said. The long delay from the component’s first use in the power generation industry to its use in induction welding applications is an advantage, Gibeaut said.
“The same thing happened with another component, the silicon-controlled rectifier,” he said. “It took about 15 years for induction industry to adopt the SCR. If anything, a delay like that is a benefit. The component gets tried out, refined, and updated in one industry long before it shows up in the other,” he said.
A Quick Look Back, a Quick Look Forward
“For some time, transistor manufacturers had been running out of efficiency improvements for the Si type of MOSFET,” Gibeaut said. Improvements in the electronics technology are just like improvements everywhere else, trickling upward as the component manufacturers explore new processes, purchase the latest equipment, and refine quality control practices; better components lead to better equipment. Improvements in the early years yield the greatest gains, and eventually the improvements hit a plateau until something comes along that disrupts the process and starts it over again. In induction welding, the new transistor stands to be just such a disruptor.
The capability of the SiC MOSFET brings an ancillary benefit: improved reliability.
“Reliability is the result of significant reduction in semiconductors and ancillary supportive components,” Gibeaut said. The difference isn’t a simple multiplier, but an exponential decrease in part count. Fewer parts and fewer connections mean fewer things that can go wrong. That translates into more uptime and fewer service calls. Although power supplies built with the new semiconductors haven’t been deployed in large numbers for long periods of time, the initial results are promising.
“We’ve had these power supplies in the field for two years, and we haven’t had a single failure associated with an SiC MOSFET,” Gibeaut said.
Does this mean that induction welding units with Si MOSFET technology are obsolete? Of course not. Sturdy, time-tested, and capable, they’re no more obsolete than a 1966 Charger with a 426 hemi. It doesn’t run as efficiently as a 2019 model Charger, and a 53-year-old car has more rigorous maintenance requirements than this year’s model, but if it’s still turning heads and burning rubber, there’s no sense in replacing it. The same goes for a trusty welding unit—if it ain’t broke, don’t fix it.
Now, if the costs are a little too high for comfort—either operating or maintenance costs—it might be time to think about retiring the welding unit. When the day comes, keep in mind that you’ll have a choice between two welding unit types, traditional and contemporary.
Ajax TOCCO Magnethermic, www.ajaxtocco.com