Electric Heating and Welding

Resistance heating dissipates I²R in a heating element or the workpiece itself; the energy H = I²Rt (joules) links directly to the temperature rise via the mass and specific heat. Induction heating induces eddy currents in a conductive workpiece for clean, localised heating.

Arc welding melts metal with heat from an electric arc; the heat input per unit length H = ηVI/(v) (with v the travel speed) determines penetration and the heat-affected zone. A drooping V–I source suits manual arc welding, a flat source suits MIG.

(Joule heating, resistance furnace)

(J/mm, v travel speed)

Relation 1 —

(Joule heating, resistance furnace)

Write this relation with symbols exactly as in Art & Science of Utilisation of Electrical Energy — H. Partab before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 2 —

(J/mm, v travel speed)

Write this relation with symbols exactly as in Art & Science of Utilisation of Electrical Energy — H. Partab before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 3 —

Write this relation with symbols exactly as in Art & Science of Utilisation of Electrical Energy — H. Partab before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

The temperature rise from a known heat input follows H = m c ΔT, so ΔT = H/(mc). Furnace design accounts for heat losses so the input exceeds this ideal value by the furnace efficiency.

Induction heating frequency is chosen for the required depth: higher frequency heats a thinner surface layer (smaller skin depth), used for surface hardening; lower frequency heats deeper for through-melting.

Resistance (spot/seam) welding passes a large current briefly through overlapping sheets; the contact resistance concentrates heat at the interface to form a nugget.

State assumptions explicitly before using any relation for electric heating and welding — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.

Wrong assumptions invalidate the entire solution even when the formula is correct. In Electrical Utilization viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

1. Read the question and list given data with SI units (common in Electrical Utilization papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to electric heating and welding.
4. Use equation 1: .
5. Use equation 2: .
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Electric Heating and Welding appears in industry and railways. In Indian electrical curricula this topic is tested because it connects theory to traction, illumination, and heating.

GATE and semester exams often combine electric heating and welding with earlier units — revise prerequisites before attempting mixed problems.

Industry interview panels sometimes ask: "Where did you use electric heating and welding?" — answer with a lab, mini-project, or plant visit example if possible.

• Forgetting furnace efficiency (real input exceeds the ideal mcΔT)
• Using average instead of rms current in I²Rt
• Ignoring the travel-speed term in welding heat input
• Mixing up the drooping vs flat source characteristic for the welding process

Before attempting electric heating and welding problems, confirm you can:

1. Induction heating: eddy currents in workpiece
2. Dielectric heating for insulating materials at HF
3. Resistance welding: spot, seam, projection types

Revise the solved examples in Art & Science of Utilisation of Electrical Energy — H. Partab and one previous-year GATE or university paper for this unit.