Electric heating and welding

In short

Electric heating converts electrical energy into heat inside the work itself or in a resistor near it, and electric welding uses that heat to melt and join metals. Both are standard chapters of "Utilization of Electrical Energy" in Indian B.E./B.Tech electrical syllabi and appear in GATE-EE utilization questions, in state-utility and PSU recruitment papers (SSC-JE, RRB-JE, DMRC, NTPC, SAIL, BHEL) and in diploma practicals. The subject matters industrially because electric arc and induction furnaces make most of India's secondary steel, and resistance and arc welding hold together every fabricated structure, pipeline and car body. Numerical work centres on heating-element design, furnace energy balances, duty cycle and welding-transformer ratings.

Explained

Why electric heating, and how it is classified

• Advantages over fuel firing: no flue gases or ash at the point of use, close temperature control (±1 °C possible), clean and safe, high efficiency (60–90% against 20–40% for coal firing), ability to heat the inside of the job (induction, dielectric), easy automation, and selective/local heating.
• Limitations: high cost of electricity per kWh of heat, dependence on supply reliability, high initial cost of furnaces and converters, poor power factor of arc and induction loads, and heavy harmonic injection by arc furnaces.
• Classification: (a) direct resistance heating — current passes through the charge itself; (b) indirect resistance heating — current passes through a separate element, heat reaches the charge by conduction/convection/radiation; (c) direct arc heating; (d) indirect arc heating; (e) induction heating (core type/ajax-wyatt and coreless); (f) dielectric (capacitive) heating; (g) infrared/radiant heating; (h) electron-beam and laser heating (special).
• Heat transfer basis: conduction (Fourier), convection (Newton's law of cooling) and radiation (Stefan–Boltzmann) all act; in high-temperature furnaces radiation dominates, which is why element and chamber temperatures are set by the fourth-power law.

Resistance heating and element design

• Direct resistance heating: electrodes dip into the charge (salt-bath furnace, billet heater, electrode boiler). AC is preferred to avoid electrolysis; heating is uniform through the section and efficiency is high, but the charge must be conducting and of regular shape, and temperature control needs a tap-changing transformer.
• Indirect resistance heating: nichrome/kanthal elements as wire, strip or rod in ovens, muffle furnaces, water heaters, cooking ranges, air heaters; heating of the chamber plus a charge-to-element radiation path. Ovens below about 400 °C use forced air circulation.
• Element material requirements: high resistivity (so a small length suffices), high melting point, low temperature coefficient of resistance, resistance to oxidation at temperature, adequate mechanical strength and ductility when hot.
• Common materials: nickel–chromium (Ni-Cr 80/20, up to ~1,150 °C, resistivity ~1.1 µΩ·m), nickel–chromium–iron (up to ~950 °C, cheaper), kanthal (Fe-Cr-Al, up to ~1,350 °C), silicon carbide rods (globar, ~1,450 °C), molybdenum disilicide, and graphite or molybdenum/tungsten in vacuum or inert atmosphere above 1,500 °C.
• Causes of element failure: oxidation and intermittency of operation, formation of local hot spots, embrittlement from grain growth, and contamination by furnace atmosphere.
• Temperature control methods: varying the applied voltage (auto-transformer/tap changer), series–parallel and star–delta regrouping of element groups (gives power in ratio 1:3 in star/delta and 1:4 in series/parallel), on–off thermostat control, and thyristor phase-angle or burst-firing control (modern practice).
• Goodenough's law (design rule): for an element radiating to the charge, the radiated power per unit surface area fixes the surface loading; too high a loading burns the element out even though the resistance calculation is correct. Design therefore involves two conditions — resistance for the required power at rated voltage, and surface area for permissible watt loading.

Arc furnaces

• Direct arc furnace (Héroult): arc strikes between graphite electrodes and the charge, so current flows through the metal — strong stirring, fast melting, temperatures of 3,000 °C+ near the arc. Three-phase, with three electrodes raised/lowered by automatic (hydraulic/electro-mechanical) regulators. Used for steel making, including scrap-based secondary steel.
• Indirect arc furnace: arc burns between two electrodes above the charge, which is heated by radiation only; no stirring (sometimes a rocking mechanism is used), so it suits non-ferrous melting (brass, bronze).
• Electrical characteristics: an arc has a negative resistance/voltage–current characteristic and is unstable, so a series reactance (choke or transformer leakage reactance) is essential on AC; DC arc furnaces use a rectifier and a series resistance/reactor.
• Supply-side problems: very low and fluctuating power factor (0.7–0.85), flicker, unbalance and harmonics — handled with SVC/STATCOM, harmonic filters and dedicated HV feeders. Indian steel plants take arc furnaces at 33 kV or 132 kV through a furnace transformer with on-load taps.
• Submerged-arc and resistance-arc furnaces (ferro-alloys, calcium carbide) operate with electrodes buried in the charge, so heating is part arc, part resistance.

Induction and dielectric heating

• Principle of induction heating: an AC coil induces eddy currents in a conducting charge; the charge heats by I²R of its own eddy currents, plus hysteresis loss if magnetic. No contact and no element, so very high temperatures and clean, selective heating are possible.
• Core-type (low-frequency) induction furnace: the charge forms a single-turn secondary loop; the Ajax-Wyatt vertical-channel type cures the pinch effect of the earlier horizontal design and runs at supply frequency; used for brass, zinc, aluminium melting.
• Pinch effect: the interaction of the heavy induced current with its own field squeezes the molten metal in the channel, which can interrupt the circuit — avoided by a vertical V-channel and by keeping the channel full.
• Coreless induction furnace: water-cooled copper coil around a refractory crucible; medium frequency (500 Hz–10 kHz) from a thyristor/IGBT converter; used for steel, alloy steel and precious-metal melting, and for vacuum induction melting.
• Depth of penetration (skin depth): heating concentrates in a surface layer whose depth decreases as frequency rises — hence high frequency (10–450 kHz) for surface hardening of gear teeth and crankshafts, lower frequency for through-heating of billets before forging.
• Frequency choice: roughly, low frequency for large sections and deep heating, high frequency for small sections and shallow case depth; small jobs at low frequency heat poorly because penetration exceeds the job size.
• Dielectric (high-frequency capacitive) heating: the non-conducting charge is placed between two plates as the dielectric of a capacitor; dielectric loss at 10–50 MHz heats it uniformly throughout the thickness. Used for plywood gluing, drying textiles and paper, baking foundry cores, curing rubber, food processing and plastic (RF) welding, as well as medical diathermy. Advantages: uniform internal heating of poor conductors, fast, no overheating of surface. Limitations: costly HF generators, low overall efficiency, risk of breakdown of the dielectric and radio interference (shielding is required).
• Microwave heating (2,450 MHz) is the same loss mechanism at higher frequency, with a magnetron source.

Electric welding: processes and equipment

• Welding classes: (1) fusion/arc welding — metal melts, with or without filler, no pressure; (2) resistance welding — heat from I²R at the interface plus pressure; (3) solid-state/pressure processes — friction, ultrasonic, explosive; plus gas welding and thermit welding (non-electric), soldering/brazing (filler melts, parent metal does not).
• Arc welding supply: needs a drooping (constant-current) characteristic for manual electrode welding so that arc length variation changes current little, and a constant-voltage characteristic for continuous-wire MIG/CO₂ welding, where the burn-off rate self-adjusts.
• AC welding set: step-down transformer (about 70–100 V open circuit, 15–30 V arc voltage, 50–600 A) with a series reactor or moving-coil/tapped winding for current setting. Cheap, efficient (about 80–85%), no polarity problem, but poor power factor (~0.3–0.6 uncorrected) and unsuitable for some coated/non-ferrous electrodes.
• DC welding set: motor-generator set, or more commonly a transformer–rectifier or inverter welding set (IGBT, light, high duty cycle, 85–90% efficient). DC gives a steadier arc and allows polarity choice.
• Polarity: straight polarity (DCEN, electrode negative) puts about two-thirds of the heat in the work — thick plate; reverse polarity (DCEP, electrode positive) puts more heat in the electrode — thin sheet, and is needed for the cleaning action in TIG welding of aluminium.
• Processes: SMAW/MMA (flux-coated stick electrode), TIG/GTAW (non-consumable tungsten, argon shield, high quality, thin and non-ferrous work, AC with HF for aluminium), MIG/MAG/CO₂ (GMAW) (continuous bare wire, argon/CO₂ shield, high deposition), flux-cored arc welding, submerged arc welding (SAW) (granular flux blanket, very high deposition, long straight seams such as pipe mills and pressure vessels), plasma arc, carbon-arc, stud and electro-slag welding.
• Electrode coating functions: forms shielding gas and slag, adds deoxidisers and alloying elements, stabilises the arc, slows cooling, and shapes the bead. Indian coated electrodes are classified by IS 814 (covered electrodes for manual metal arc welding of structural steel); low-hydrogen basic electrodes must be baked/dried before use.
• Weld defects: porosity, slag inclusion, lack of penetration, undercut, overlap, cracks, blowholes, spatter, distortion and residual stress; detected by visual, dye-penetrant, magnetic-particle, ultrasonic and radiographic testing.

Resistance welding and the Indian standards framework

• Principle: H = I²Rt at the joint, where the contact resistance is deliberately the highest resistance in the circuit; a very low voltage (1–30 V) and very high current (thousands of amperes) are used for a short, timed period, with mechanical pressure.
• Types: spot (overlapping sheets between two electrodes — car bodies, white goods), seam (roller electrodes, continuous leak-tight seam), projection (preformed projections localise the current), butt/upset (bars end to end), flash butt (arcing and flashing, then forging — rails, chains), and percussion welding.
• Welding cycle: squeeze time → weld time → hold (forge) time → off time; current, time and pressure are the three controlled variables, with timing by electronic/thyristor contactors.
• Electrode material: copper alloys (Cu-Cr, Cu-Cr-Zr, Cu-W), water-cooled, with high conductivity and hot hardness so they do not weld to the job.
• Duty cycle: welding sets are rated at a percentage duty cycle (commonly 60% for industrial sets over a 10-minute period, 20–35% for small inverter sets). Current must be derated if the set is used at a higher duty cycle than rated.
• Indian standards and statutory practice (quote current editions; several older IS numbers have been superseded): IS 814 for covered electrodes for MMAW of structural steel; IS 816 for arc welding of mild steel in general construction; IS 2825 for unfired pressure vessels; IS 800 (2007) for steel structures, which specifies permissible weld stresses and throat thickness; IS 4353 for submerged-arc welding; IS 7307/IS 7310/IS 7318 for approval and qualification of welding procedures and welders; IS 1181 for welder qualification. Equipment safety follows IS/IEC 60974 series for arc welding equipment. Electrical installation and safety are governed by the Central Electricity Authority (Measures relating to Safety and Electric Supply) Regulations, 2023 and the Electricity Act, 2003; workplace health and safety in welding shops now comes under the Occupational Safety, Health and Working Conditions Code, 2020, in force from 21 November 2025, which replaced the Factories Act, 1948.
• Energy-efficiency context: induction and arc furnaces are notified industries under BEE's PAT scheme; industrial furnaces and welding loads carry a specific energy consumption benchmark in energy-audit work.

Key points

• Direct resistance heating - current flows through the charge itself; uniform, efficient, needs a conducting charge of regular shape.
• Indirect resistance heating - separate element; heat reaches charge by radiation and convection; used in ovens and muffle furnaces.
• Element material - high resistivity, high melting point, low temperature coefficient, oxidation resistance: Ni-Cr, Ni-Cr-Fe, kanthal, SiC.
• Surface watt loading - the second design condition after resistance; exceeding it burns out an electrically correct element.
• Direct arc furnace - arc through the charge, heavy stirring, steel making; indirect arc - radiation only, non-ferrous melting.
• Arc has negative V–I characteristic - needs series reactance (AC) or resistance/reactor (DC) for stability.
• Pinch effect - electromagnetic squeezing of molten metal in a core-type induction furnace channel; cured by the Ajax-Wyatt vertical channel.
• Coreless induction furnace - medium frequency, water-cooled coil, crucible; preferred for steel and alloy melting.
• Skin/penetration depth falls with frequency - high frequency for surface hardening, low frequency for through-heating of billets.
• Dielectric heating - loss in an insulating charge between capacitor plates at 10–50 MHz; plywood, textiles, plastics, foundry cores.
• Drooping (constant-current) characteristic for manual arc welding; constant-voltage for MIG/CO₂ continuous wire.
• Straight polarity (DCEN) - two-thirds heat in work, thick plate; reverse polarity (DCEP) - more heat in electrode, thin sheet and TIG on aluminium.
• AC vs DC welding set - AC: cheaper, efficient, no arc blow, poor pf; DC: steadier arc, polarity choice, all electrodes.
• Resistance welding - H = I²Rt with pressure, very high current at 1–30 V; spot, seam, projection, butt, flash butt.
• Welding cycle - squeeze, weld, hold, off; current, time and pressure are the controlled variables.
• Duty cycle - percentage of a 10-minute period the set may carry rated current; current must be derated above it.
• TIG for quality, MIG for deposition rate, SAW for long heavy seams, SMAW for site versatility.
• Codes - IS 814 (electrodes), IS 816 (arc welding of mild steel), IS 800:2007 (weld design), IS 2825 (pressure vessels), IS/IEC 60974 (equipment safety).

Formulas and examples

Example 1 — Resistance oven element design. A resistance oven is to take 16 kW at 230 V single phase, three elements in parallel, using Ni-Cr wire of resistivity 1.1 × 10⁻⁶ Ω·m. Element temperature 1,000 °C, charge 500 °C, emissivity 0.9, radiation factor 0.6. Find wire diameter and length per element.

• Power per element = 16,000/3 = 5,333 W → R = V²/P = 230²/5,333 = 52,900/5,333 = 9.92 Ω.
• Current per element = 230/9.92 = 23.2 A.
• Radiated loading H = 5.67 × 0.9 × 0.6 × [(1273/100)⁴ − (773/100)⁴] = 3.062 × [26,268 − 3,572] → (12.73⁴ = 26,268; 7.73⁴ = 3,572) = 3.062 × 22,696 ≈ 69,500 W/m².
• Surface area needed per element = 5,333/69,500 = 0.0767 m² = πdL.
• Resistance condition: 9.92 = ρL/(πd²/4) → L = 9.92 × πd²/(4 × 1.1 × 10⁻⁶) = 7.084 × 10⁶ d² (with d, L in m).
• Substituting in πdL = 0.0767: π × d × 7.084 × 10⁶ d² = 0.0767 → d³ = 0.0767/(2.2256 × 10⁷) = 3.446 × 10⁻⁹ → d = 1.511 × 10⁻³ m ≈ 1.51 mm.
• L = 7.084 × 10⁶ × (1.511 × 10⁻³)² = 7.084 × 10⁶ × 2.283 × 10⁻⁶ ≈ 16.2 m per element.

Example 2 — Furnace energy and efficiency. An electric furnace melts 1,000 kg of brass from 25 °C. Specific heat 0.39 kJ/kg·°C, melting point 920 °C, latent heat 168 kJ/kg; furnace efficiency 70%. Energy at ₹8.00/kWh.

• Sensible heat = 1,000 × 0.39 × (920 − 25) = 1,000 × 0.39 × 895 = 349,050 kJ.
• Latent heat = 1,000 × 168 = 168,000 kJ. Total useful = 517,050 kJ = 517,050/3,600 = 143.6 kWh.
• Input = 143.6/0.70 = 205.2 kWh. Cost = 205.2 × 8 = ₹1,641.4.
• If the melt must be done in 1.5 hours, the furnace rating = 205.2/1.5 ≈ 136.8 kW.

Example 3 — Welding transformer and power factor. A single-phase AC welding set draws from a 415 V supply and delivers 300 A at an arc voltage of 25 V. Input power measured 9.0 kW.

• Arc (output) power = 25 × 300 = 7,500 W = 7.5 kW → set efficiency = 7.5/9.0 = 83.3%.
• Primary current = input power/(V × cosφ). At cosφ = 0.45, I = 9,000/(415 × 0.45) = 48.2 A; input kVA = 9.0/0.45 = 20 kVA.
• Correcting pf to 0.90 halves the kVA to 10 kVA and the primary current to 24.1 A — which is why capacitor banks are fitted to welding shops.
• Heat input at a travel speed of 150 mm/min = (25 × 300 × 60)/(150 × 1,000) = 450,000/150,000 = 3.0 kJ/mm.

Example 4 — Spot welding and duty cycle. Two 1.5 mm steel sheets are spot welded with 8,000 A for 10 cycles (50 Hz) with an interface resistance of 100 µΩ.

• Weld time = 10/50 = 0.2 s. Heat = I²Rt = 8,000² × 100 × 10⁻⁶ × 0.2 = 64 × 10⁶ × 10⁻⁴ × 0.2 = 1,280 J.
• Secondary voltage drop at the joint = 8,000 × 100 × 10⁻⁶ = 0.8 V; instantaneous power = 8,000 × 0.8 = 6.4 kW at the joint.
• A set rated 400 A at 60% duty cycle used at 100% duty cycle: I₂ = 400 × √(60/100) = 400 × 0.7746 = 310 A.

Example 5 — Star/delta temperature control. Three 1 kΩ elements supplied at 400 V three phase.

• In delta, each element sees 400 V: P = 3 × 400²/1,000 = 480 W. In star, each sees 400/√3 = 231 V: P = 3 × 231²/1,000 = 160 W — a ratio of 3:1, giving two heat settings from the same elements.

Where it is used

• Steel and metals: electric arc and induction furnaces in secondary steel (Jindal, SAIL, Tata, and the large induction-furnace sector in Punjab, Gujarat and Chhattisgarh); ferro-alloy submerged-arc furnaces; aluminium and brass melting.
• Automotive and white goods: robotic spot-welding lines at Maruti Suzuki, Tata Motors, Hyundai and Bajaj; seam-welded tanks; induction hardening of gears, crankshafts and camshafts.
• Fabrication, pipelines and power plants: SMAW/SAW/TIG welding of boilers, pressure vessels and penstocks at BHEL, L&T, Thermax; pipeline girth welding for GAIL and HPCL projects; welder qualification to IS/ASME procedures.
• Railways: flash-butt and thermit welding of long welded rails; induction heating for shrink-fitting wheels and bearings.
• Process industry heating: resistance ovens, salt baths and heat-treatment furnaces in forging and tool rooms; dielectric heating in plywood (Kerala, Haryana), textile drying and food processing.
• Job roles: utilization/plant electrical engineer, furnace engineer, welding engineer and QA/NDT inspector, maintenance engineer on furnace transformers, electrode regulators, thyristor converters and welding sets.

Interview questions

• Q: Why must a series reactance be used with an AC arc? A: The arc has a negative voltage–current characteristic, so without series reactance the current would run away and the arc would be unstable or extinguish each half cycle.
• Q: Why is AC preferred for direct resistance heating of a salt bath? A: DC would cause electrolysis and decomposition of the charge; AC avoids any net electrolytic action.
• Q: What is the pinch effect and how is it avoided? A: The heavy induced current in a core-type induction furnace channel interacts with its own magnetic field and squeezes the molten metal, risking circuit interruption; the Ajax-Wyatt vertical V-channel keeps the channel full and avoids it.
• Q: Why is high frequency used for surface hardening? A: Penetration depth falls as frequency rises, so the induced current and heat stay in a thin surface layer while the core remains cool and tough.
• Q: Which heating method would you choose for drying plywood, and why? A: Dielectric heating — plywood and glue are insulators, and dielectric loss heats the whole thickness uniformly, which surface heating cannot do.
• Q: Why does manual arc welding need a drooping characteristic? A: The welder cannot hold a perfectly constant arc length; a steep (constant-current) characteristic keeps the current nearly unchanged when the arc length varies, so the bead stays uniform.
• Q: What is the difference between straight and reverse polarity? A: In straight polarity (DCEN) the electrode is negative and about two-thirds of the heat goes into the work, suiting thick plate; in reverse polarity (DCEP) the electrode is positive, more heat goes to the electrode, suiting thin sheet and giving the cleaning action needed for TIG on aluminium.
• Q: Why is the power factor of a welding set so poor, and what is done about it? A: The series reactor and transformer leakage reactance make the load highly inductive; shunt capacitors across the supply correct the power factor and cut the kVA demand and cable current.
• Q: What does a 60% duty cycle rating mean? A: The set may deliver its rated current for 6 minutes of every 10-minute period; beyond that the current must be derated, approximately as I₂ = I₁√(D₁/D₂).
• Q: Why are electrodes in resistance welding made of copper alloy and water-cooled? A: Their resistance must be far lower than the interface resistance so heat is generated in the job, and cooling plus hot hardness stops them from softening or welding themselves to the work.
• Q: Why do arc furnaces need SVC or STATCOM compensation? A: Their current fluctuates violently, causing voltage flicker, unbalance and harmonics on the supply; dynamic compensation stabilises voltage and meets the utility's power-quality limits.
• Q: Soldering, brazing or welding — how do you distinguish them? A: In welding the parent metals melt and fuse; in brazing (above about 450 °C) and soldering (below it) only the filler melts and joins by wetting and capillary action.

Common mistakes

• Designing a heating element from the resistance condition alone and ignoring the permissible surface watt loading, so the element burns out.
• Using temperatures in °C inside the Stefan–Boltzmann fourth-power term instead of kelvin.
• Confusing direct and indirect resistance (or arc) heating, and so claiming stirring in an indirect arc furnace.
• Forgetting latent heat of fusion when the charge has to be melted, or omitting furnace efficiency from the input-energy figure.
• Saying induction heating works on any material — it needs a conductor; insulators need dielectric heating.
• Assuming higher frequency always heats better; low frequency is right for deep through-heating of large billets.
• Taking welding-set kVA as equal to kW, or ignoring duty-cycle derating when sizing cables and current.

Related topics

Illumination engineering and lighting design · Electric traction and drives · Energy audit and conservation · Power quality, harmonics and reactive compensation · Testing and commissioning of electrical equipment · Electrical safety and earthing