Furnace transformers are rated against a duty cycle rather than a steady load, and getting that cycle wrong is the most common reason a unit turns out to be either oversized or uncomfortably hot.
What duty cycle means here
Duty cycle is the pattern of load over time: how long the unit runs at each level, how far above rating the peaks go, how long the pauses are, and how often the whole sequence repeats. On a furnace it is not a smooth curve.
For thermal purposes what matters is the heating the cycle produces, which depends on both the magnitude of the current and the time spent at each level. A short hard overload and a long moderate one can produce the same temperature rise by very different routes.
Duty cycles by furnace type
- Electric arc furnace. Distinct phases: meltdown at maximum power with frequent electrode-to-charge short circuits, refining at reduced power with a more stable arc, then tapping and charging with the transformer essentially unloaded. The cycle repeats every tap to tap time, often under an hour.
- Ladle furnace. A more stable arc and lower power, but still cyclic, and running in parallel with the EAF cycle rather than in sequence with it.
- Induction furnace. More continuous operation with varying power levels, generally a gentler duty than an arc furnace.
- Submerged arc furnace. Heavy and comparatively continuous, with the load set by the electrode position in the charge rather than by an open arc.
The thermal problem with cyclic loading
During high-load periods the windings heat and expand. During low-load and idle periods they cool and contract. Each cycle moves the insulation and the structural components, and the cumulative effect is ageing that a steady load of the same average power would not produce.
The insulation system and the mechanical clamping both have to tolerate that movement. This is why insulation class and temperature rise are specified against the cyclic duty rather than against a steady rating, and why the top-oil and winding temperature limits matter as much as the kVA figure.
What follows for selection
- Start from the melting cycle: charge weight, tap to tap time, power on time per heat, heats per day, and the overload the furnace is driven into during meltdown.
- Set the rating from the thermal duty those numbers produce, not from average power.
- Check cooling capacity against the peak of the cycle, not the average.
- Confirm the permitted temperature rise and insulation class against the cyclic duty.
- State the expected overload magnitude and duration explicitly in the specification.
A furnace that is pushed hard for a short melt and then idles needs a different design from one running steadily at moderate load, even when the average power looks similar.
Send the melting cycle and the load profile, and we can come back on the rating and the cooling arrangement. See the furnace transformer range, or contact us.
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Related Applications
- Transformers for Steel Industry
- Transformers for Electric Furnaces
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Related equipment
If you are specifying this as part of a larger distribution scheme, the surrounding scope is covered on the industrial power distribution, MV switchgear and transformer substation packages pages.
Frequently Asked Questions
What is the first thing to establish with furnace transformer duty cycle?
Start from the load and the environment, then let the equipment follow. Working from a catalogue rating backwards is how projects end up with either a marginal unit or one that costs money sitting under-loaded.
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.