A high current furnace transformer is defined by its secondary: very large current at very low voltage, delivered through conductors that would be impractical in any other application.
What high current means in practice
Melting furnaces are supplied at low voltage because the arc or the melt cannot use anything else, and the power required is large. The two together produce secondary currents in the thousands of amperes, rising into the tens of thousands on the largest arc furnaces.
At those levels, the secondary winding stops looking like a winding. It becomes a set of heavy conductors and busbars, and its design is dominated by effects that are negligible at distribution current.
Conductor and busbar design
- Large cross-section copper conductors, sized for current density rather than for mechanical convenience.
- Multiple parallel conductors, because at high frequency the current crowds toward the conductor surface. Splitting the conductor reduces both skin effect and proximity effect losses, and the strands have to be transposed correctly to share current evenly.
- Water-cooled busbars, common where the current is high enough that air cooling of the leads would need an impractical amount of copper.
- Short, balanced low voltage leads, which limit both losses and the electromagnetic forces between conductors.
Electromagnetic forces
Current flowing in adjacent conductors produces force between them, and that force scales with the square of the current. At short circuit the current multiplies, and so does the force. On a high current furnace transformer this is the governing mechanical load, not vibration.
The response is structural: bracing and clamping designed against the calculated forces, supported by finite element analysis on the larger units, and verification against the short circuit requirements of the applicable standard.
Losses that only appear at high current
Two loss mechanisms become significant at these currents and are easy to underestimate.
- Stray losses in structural steel near the secondary leads, which can cause local overheating in parts not designed to carry current.
- Eddy and circulating currents induced in the busbar enclosure and in any parallel conductor arrangements that are not correctly transposed.
Both are design-stage problems. Once the unit is built, the fix is expensive or impossible.
Cooling
Losses in the secondary and its leads are a substantial part of the total heat load, so cooling is sized against the whole unit rather than the core and winding alone. Forced oil circulation with forced air over the coolers covers most large units; forced oil with water cooling is used where the heat load is too high for air.
What to send us
- Secondary current and voltage range required.
- Primary voltage and system short circuit level.
- Duty cycle and expected overload during meltdown.
- Cooling medium available and ambient conditions.
- Furnace type, so the short circuit duty can be assessed.
See the furnace transformer range, or contact us with the requirements.
Related Products
Related Applications
- Transformers for Steel Industry
- Transformers for Electric Furnaces
- Transformers for Industrial Equipment
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