A line-frequency (LF) induction furnace is a mains-frequency (50/60 Hz) coreless or channel-type melting unit that delivers high melt mass per kVA at the cost of poor cold-start behaviour and large inrush current [S1].
Selection hinges on three engineering decisions — melt mass per shift, scrap/charge mix, and the lining/refractory duty — with 250 kg, 500 kg, 1 t, 2 t, 3 t and 5 t iron-melt capacity classes appearing repeatedly on Chinese OEM datasheets [S2][S4]. Typical China export pricing for 1 t intermediate-frequency iron furnaces sits between US$10,000 and US$1,000,000 per set, with mid-tier packages around US$78,000 to US$85,000 per set [S5].
Mains-frequency vs medium-frequency: where LF still wins
Line-frequency furnaces operate directly on 50 Hz or 60 Hz mains without static frequency-conversion stages, giving a 100% electrical efficiency path from transformer busbar to melt and avoiding IGBT/SCR inverter losses [S2]. For high-tonnage iron foundries running 3 t to 5 t per heat on continuous shifts, the absence of a frequency-conversion stage is the reason mains-frequency units remain the dominant installed base in ductile-iron and grey-iron job shops [S2].
Medium-frequency (MF) coreless units — typically 1 kHz class — are the competing technology: smaller, faster to melt, gentler stirring, and better at cold-start on a solid heel.
Capacity sizing: tonnage, kW and t/kWh
Iron foundries using LF coreless induction furnaces commonly run at 600 to 800 kWh per liquid-tonne for steel and 550 to 650 kWh/t for grey iron, which is the engineering basis for sizing the transformer against shift output [S2]. A 3 t LF iron-melt unit is typically paired with a 1,800 to 2,500 kW furnace transformer, while a 1 t unit sits in the 700 to 1,100 kW range [S2][S4].
Buyers should size the furnace to roughly 1.3× the per-shift melt requirement to keep the lining below its thermal-cycle fatigue limit; charging above 1.5 t/heat in a 1 t-class unit shortens crucible life sharply because the refractory sees deeper thermal gradient swings [S3]. Published 1 t class iron-melt units quote 250 kg, 500 kg, 1 t, 2 t, 3 t and 5 t nominal capacities, so the catalogue map already covers the typical small-job-shop through mid-size foundry bracket [S2][S4].
Lining, coil and refractory duty
The two dominant failure modes for HF/MF induction furnaces — and they apply to LF units as well — are disruptive discharge through the coil insulation and fracture of the refractory lining [S3]. On LF units the lining is typically a dry-vibrated silica or alumina-magnesia spinel mix rammed around a mandrel, then sintered in place over 2 to 3 iron melts before production heats begin.
Operators report lining campaigns of 200 to 400 heats for a well-sintered silica lining on grey iron; the figure drops to 80 to 150 heats if the bath is held at peak temperature for long holding periods or if the charge mix swings between light borings and heavy scrap [S3]. For ductile-iron shops running higher tap temperatures (1,500 to 1,550 °C), high-alumina or magnesia-based ramming mixes are specified in place of silica because silica softening drops sharply above 1,600 °C [S2].
Power supply, compensation and harmonics
A line-frequency coreless furnace draws a low power factor (typically 0.15 to 0.25 at start of campaign, 0.30 to 0.40 at running temperature) because the air-gap coupling between the primary coil and the charge is weak at 50/60 Hz; the supply is therefore paired with a capacitor bank sized at 1.0 to 1.2× the active kW to bring the network PF to 0.95 or better. [S1]
Inrush current at scrap drop-in is the second mains-frequency headache: a cold 1 t LF unit can pull 3 to 5× its running current for the first 2 to 4 minutes as borings and light scrap couple into the field, so the upstream transformer and busbar must be rated with a 1.5 to 2× sustained-current margin or the foundry accepts nuisance breaker trips during cold start [S2].
Selection criteria, side by side
Three specifications differentiate a workable line-frequency furnace from a mis-sized one. (1) Tonnage class — 0.25/0.5/1/2/3/5 t iron-melt ratings are catalogue standards, and the 1 t class dominates the China export datasheets at US$10,000 to US$1,000,000 per set [S5]. (2) Furnace transformer rating in kW, which should be cross-checked against the 550 to 800 kWh/t specific-consumption figure for iron/steel. (3) Refractory specification — silica for grey iron below 1,450 °C, high-alumina or magnesia-spinel for ductile iron and steel.
For higher-temperature superheat, faster melt rate, or melt mass below 1 t, a medium-frequency coreless unit is the correct choice, while a 3 t-plus continuous-shift iron foundry is still better served by mains-frequency coreless or a channel-type holding furnace. For detailed frequency and capacity selection criteria, see the spec-driven comparison in How to Choose an Induction Furnace: Frequency, Capacity and Sourcing Levers, and for the upstream mould-line equipment that follows the melt deck, see automatic molding line and molding line reference specs.
Who it is for — and who should not specify LF
LF coreless induction furnaces fit grey-iron and ductile-iron job shops running 1 to 5 t per heat on continuous shifts, copper and brass mills melting 0.5 to 3 t of red metal, and holding/duplicating stations downstream of larger cupolas or channel furnaces [S2]. They are a poor fit for small prototype foundries running 50 to 200 kg batches (the cold-start penalty dominates the energy bill), for steelmakers needing tight composition control (LF stirring is gentle, so alloy homogenisation is slow), and for any site with a weak mains supply unable to absorb the 3 to 5× cold-start inrush [S2].
Sourcing, standards and what to ask the supplier
Buyers specifying a line-frequency furnace in 2026 should request four pieces of evidence from the OEM: (a) the furnace-transformer kVA rating and the matching capacitor-bank kVAr, (b) the published specific-consumption figure (kWh/t) at the rated tap temperature, (c) the lining chemistry, grain size and recommended sinter schedule, and (d) a documented commissioning log showing PF correction and cold-start inrush measured on the buyer's incoming supply. Lead time on standard 1 t-class units from Chinese suppliers is typically 20 days after payment for export packaging in plywood case or nude wooden box [S4].
Watch the induction furnace category reference for the related medium-frequency and high-frequency sub-types, the resin-sand line for the mould-prep equipment that consumes the melt, and the conveyor sorting line for downstream casting handling. China HS-code and import-tariff classifications for "frequency induction furnace" should be checked against the latest MFN column on the customs database, as some sub-codes carry anti-dumping or CCC-certification overlays [S6]. The next trackable signal is the Q3 2026 release of revised specific-consumption benchmarks in the China Foundry Association equipment register, which will tighten the 550 to 800 kWh/t spread cited above.