The most common mistake in furnace transformer selection is starting from the kVA. Furnace duty is cyclic, heavily overloaded part of the time and lightly loaded the rest, so a nameplate rating chosen the way you would choose a distribution transformer will be wrong in one direction or the other. This guide sets out the order in which the decisions should actually be made.
Start with the melting cycle
What determines the rating is the process: charge weight, target tap to tap time, power on time per heat, how many heats per day, and how much overload the furnace is driven into during meltdown. Those numbers define the thermal duty, and the thermal duty defines the rating. A furnace that is pushed hard for a short melt and then idles needs a different design from one that runs steadily at moderate load, even if the average power looks similar.
Set the secondary voltage and the regulating range
Furnace transformers step down to a low secondary voltage with very high current. Published parameters for 35 kV furnace transformers show secondary voltages in the region of 130 V to 509 V depending on rating, with multi step regulation between the extremes. The range has to cover meltdown, refining and holding:
- Meltdown: maximum power, longest arc, highest secondary voltage.
- Refining: lower power, shorter arc, reduced voltage and current.
- Holding: minimum power to keep the bath at temperature.
On load tap changing is what makes this possible without stopping production. The number of steps matters: more steps give finer control over arc length and electrode immersion, which translates directly into electrode consumption and energy per tonne.
Choose the impedance deliberately
Furnace transformer impedance typically falls in the 4.5% to 21% band, well above distribution practice. The reason is arc stability. Higher impedance limits the short circuit current when the electrodes touch the charge, which happens constantly during meltdown, and it softens the current swings that would otherwise trip the furnace. The cost is reduced voltage regulation and a lower power factor at the furnace, which is why the value is chosen against the furnace and the grid, not maximised.
Verify short circuit withstand
Arc furnace service is a repeated short circuit duty. Every meltdown produces multiple electrode-to-charge shorts, and each one puts mechanical shock into the windings. The design response is mechanical, not electrical: braced and banded windings, short and balanced low voltage leads to limit electromagnetic forces, and verification against the short circuit requirements of the applicable standard. If your specification does not mention short circuit duty, add it.
Consider a series reactor
A series reactor is often fitted for the initial melting period. It lengthens the arc and lowers the current, which reduces electrode consumption and softens the impact on the supply during the noisiest part of the heat. It can be built integral with the transformer or supplied as a separate unit, and it is usually switched out for the refining stage.
Select the cooling for a cyclic load
- AN and AF: for smaller furnace units.
- OFAF and OFWF: for larger units, with cooling capacity sized against the cyclic load rather than a steady rating.
Redundancy matters more here than in most applications. With OFWF, two coolers each rated for the full losses allow the furnace to keep running through a cooler failure. Losing a heat to a pump fault costs far more than the second cooler did.
Harmonics and flicker
Arc furnaces are a notorious source of voltage flicker and harmonic distortion, and the transformer is part of the mitigation. Phase shifted arrangements and, where needed, an SVC or STATCOM on the supply side are used to keep the grid within limits. If your connection agreement has flicker limits, raise them at enquiry stage — the transformer design may change.
What to send us
Furnace type and capacity in tonnes, number of heats per day, tap to tap time, transformer primary voltage and system short circuit level, secondary voltage range and number of steps, impedance requirement or furnace manufacturer’s recommendation, duty cycle, cooling medium available, ambient conditions, applicable standard, and whether type test or witness testing is required.
How this is packaged on real projects
On most sites the unit described here is not installed on its own. It goes into an assembly that includes the incoming switching, protection and outgoing feeders, which you can see on the industrial power distribution, MV switchgear, transformer substation packages and furnace transformers pages.
Frequently Asked Questions
What should be confirmed before ordering a furnace transformer?
System voltage and frequency, fault level at the point of connection, the load with its duty, and the site conditions including ambient range and indoor or outdoor placement. With those settled the rest of the specification follows.
What makes a furnace transformer different from a distribution transformer?
The duty cycle and the load. Furnace duty brings repeated short-circuit and inrush events, very high secondary currents and frequent tapping under load. Windings, mechanical bracing and tap changer selection are all driven by that duty rather than by steady-state load alone.
How is the secondary current handled at these levels?
By designing the secondary for the current it actually carries, with the busbar or cable terminal arrangement settled at drawing stage. Conductor cross-section, support spacing and cooling are decided together, since the mechanical forces at high current are as significant as the thermal ones.
Which standard applies, and is it certified?
The applicable standard and clause set are agreed against your project specification before production starts. Where a project names IEC 60076 for the transformer, we work to those clauses and the routine test report ships with the unit.
What do you need from us to quote?
The single-line diagram or a description of the application, the voltages, the load with its duty, the site conditions, and any standard the project specifies. Send what you have – if something is missing, it can be worked out together rather than delaying the enquiry.