What is a furnace transformer?
A furnace transformer is an electrical device that regulates alternating current (AC) voltage, either stepping it up or down, to match the operational requirements of furnace – based systems. In industrial metal – smelting applications (e.g., for steel, special steel, white corundum, electrolytic aluminum), it functions as a specialized power transformer capable of withstanding large – current loads and repeated mechanical and thermal stresses from electric arcs, typically featuring a primary voltage not exceeding 72.5 kV, large capacity, and utilizing carbon rods as electrodes. In household and HVAC contexts, it converts high – voltage electricity to a lower, safe voltage for furnace components (such as thermostats and ignition systems), enabling efficient operation through electromagnetic induction for energy transfer, with its installation location ranging from inside the furnace to outdoor AC units depending on the specific system design.
Commonly Used Types of Furnace Transformers
The furnace transformer acts as the “heart” of an industrial furnace. Its primary function is to convert the grid’s high voltage and low current into the low voltage and high current required by the furnace, delivering precise and robust power for processes like melting, heating, and refining. Depending on the specific furnace type, operating principle, and process requirements, furnace transformers vary significantly in their design and technology. The following are some of the most widely used types in the metallurgical, chemical, and materials industries.
1. Arc Furnace Transformer
(1) Primary Application: Primarily used in steelmaking arc furnaces, serving as the core equipment in modern electric arc furnace steelmaking.
(2) Working Principle & Characteristics: Generates a high-temperature arc between graphite electrodes and metal scrap to melt the charge. It operates under extremely harsh conditions, frequently facing short-circuit impacts, drastic load changes, and operational overvoltages.
- High Overload Capacity: Designed to withstand frequent short-circuit impacts during the melting period, featuring high mechanical strength and electrical stability.
- Multi-Stage Voltage Regulation: Often equipped with on-load tap changers to provide appropriate voltage and current for different stages (melting and refining), optimizing energy efficiency and process control.
- High Impedance: Features increased impedance to limit short-circuit current, protecting the electrodes and the power supply system.
2. Submerged Arc Furnace Transformer
(1) Primary Application: Used in submerged arc furnaces for producing ferrous alloys, calcium carbide, industrial silicon, yellow phosphorus, etc.
(2) Working Principle & Characteristics: Utilizes resistive arc heat generated by electrodes buried in the charge to reduce ores.
- Very Low Secondary Voltage, Extremely High Current: Compared to arc furnaces, its secondary voltage is lower, but the current can reach tens or even hundreds of thousands of amperes, demanding extremely high short-circuit withstand capability and cooling design.
- Continuous Operation: The production process is nearly continuous, requiring the transformer to have exceptionally high reliability and continuous operation capability.
- Multi-Winding Structure: Large SAF transformers often employ multiple secondary windings to supply power to several electrodes separately, ensuring balanced power distribution within the furnace.
3. Power Frequency Induction Furnace Transformer
(1) Primary Application: Supplies power to line-frequency (50/60 Hz) coreless induction melting furnaces.
(2) Working Principle & Characteristics: Heats and melts metal charge by inducing eddy currents based on the electromagnetic induction principle. The transformer itself does not directly participate in melting but provides suitable power to the inductor coil.
Special Load Characteristic: The inductor acts as a large inductance coil with a very low power factor. Consequently, the transformer must work in conjunction with a capacitor bank for reactive power compensation to raise the power factor close to 1.
Stable Power Supply: The key requirement is to provide a stable and adjustable voltage to meet the needs of different metals and melting stages.
4. Ladle Refining Furnace Transformer
(1) Primary Application: Used in Ladle Refining Furnaces for the secondary refining of molten steel from a primary furnace (e.g., an arc furnace).
(2) Working Principle & Characteristics: Its working principle is similar to that of an arc furnace, but the process objective differs, focusing on heating, composition adjustment, and molten steel purification.
- Higher Voltage Regulation Precision and Stability: The refining process demands more precise temperature control and a stable arc, requiring finer and more stable voltage regulation.
- Relatively Smaller Capacity: Typically has a smaller capacity than the primary arc furnace transformer, as its main role is to maintain temperature and heat up rather than perform full melting.
5. Electroslag Remelting Furnace Transformer
(1) Primary Application: Powers Electroslag Remelting furnaces for producing high-quality special steels, superalloys, and pure metals.
(2) Working Principle & Characteristics: Current passes through a molten slag pool with high electrical resistivity, generating resistive heat to gradually remelt a consumable electrode, which then solidifies into an ingot in a water-cooled mold.
- Requires Extremely Stable Current: The entire remelting process must maintain highly stable current and melting rate to ensure ingot uniformity and purity. The transformer typically offers a very stable volt-ampere characteristic.
- Single-Phase or Three-Phase: Can be single-phase or three-phase, depending on the furnace size.
6. Resistance Furnace Transformer
(1) Primary Application: Supplies power to direct resistance heating furnaces or furnaces using heating elements like silicon carbide or molybdenum disilicide.
(2) Working Principle & Characteristics: Heats the charge using Joule heat generated when current passes through a resistive body (either the workpiece itself or specialized heating elements).
- Diverse Voltage Requirements: Provides different low voltages based on the heating element material and connection method (star/delta).
- Relatively Stable Load: Compared to arc furnaces, the load variation is gentler, imposing less stress on the transformer, with greater emphasis on operational stability and energy efficiency.
7. Salt Bath Furnace Transformer
(1) Primary Application: Powers electrode-type salt bath furnaces, primarily used for metal heat treatment such as quenching, tempering, and thermochemical treatment.
(2) Working Principle & Characteristics: Current passes through a molten salt bath, utilizing its electrical resistance to heat the workpieces.
- High Current, Low Voltage: Similar to submerged arc furnaces, it requires converting high voltage to a low voltage and high current suitable for conduction through the salt.
- Special Start-Up Characteristics: Solid salt is non-conductive, requiring an auxiliary start-up system to melt the salt between electrodes and form a conductive path before the main transformer can operate. The transformer must accommodate this start-up process.
8. Single-Phase Graphitization Furnace Transformer
(1) Primary Application: Supplies power to graphitization furnaces like Acheson furnaces, which convert carbon products (e.g., electrode blanks) into graphite crystalline structures at ultra-high temperatures.
(2) Working Principle & Characteristics: The furnace charge acts as both the resistive heating element and the material being processed.
- Very Large Capacity and Massive Current: This type has the largest capacity and output current among all furnace transformers, with secondary currents often exceeding 100 kA.
- Unique Voltage Regulation: Due to the enormous power required, a combination of on-load tap changers and series-parallel reconnection is typically used to adjust voltage and current over a wide range, meeting the long heating and soaking profile of the graphitization process.
9. Single-Phase Arc Furnace Transformer
(1) Primary Application: Used mainly in small steelmaking arc furnaces, foundry melting furnaces, or laboratory-scale research arc furnaces.
(2) Working Principle & Characteristics: The basic principle is the same as for three-phase arc furnaces, but the use of a single-phase supply leads to different application considerations.
- Relatively Small Capacity: Limited by single-phase power supply capacity, it is typically used for small to medium-scale production.
- Significant Grid Impact: A large single-phase load can easily cause a three-phase grid imbalance, limiting its application and often necessitating compensating devices.
In summary, different types of furnace transformers are products of their specific industrial processes. They are specialized in their design parameters (voltage, current, impedance), voltage regulation methods, overload capacity, and operational regimes. Correctly understanding and selecting the appropriate furnace transformer is crucial for ensuring production safety, improving product quality, and reducing energy consumption.
Voltage Regulation Methods of Furnace Transformers
Due to the low secondary voltage, large current, and wide voltage regulation range of furnace transformers (requiring adjustment from the maximum value to 25%~50% of the maximum value), their voltage regulation methods are substantially different from those of power transformers. The main methods are as follows:
1. Variable Flux Voltage Regulation
Its principle is to set voltage regulation taps or separate voltage regulation windings on the primary winding. When the primary voltage is constant, the voltage per turn is changed by altering the number of turns of the primary winding connected to the circuit, thereby changing the secondary winding voltage. The core flux changes during voltage regulation.
- Features: Simple structure, and the windings can be arranged concentrically or in a staggered manner. When the number of turns of the primary winding taps is equal, the secondary voltage step difference is unequal; if equal step difference is required, a stacked winding must be used. When the secondary voltage is at its maximum, the idle part of the primary winding is prone to voltage oscillation, which is unfavorable to insulation. When the secondary voltage is at its minimum, the core magnetic density is low, which is uneconomical for products with a wide voltage regulation range. The situation can be improved by using D-Y transformation.
- Application Scope: Small and medium-sized furnace transformers with a primary voltage of 35kV or below and a voltage regulation range of less than 20%.
2. Series Transformer Voltage Regulation
A series transformer is added in the tank of the main transformer. The low-voltage windings of the main transformer and the series transformer are connected in series. The voltage of the high-voltage winding of the series transformer is changed through the separate voltage regulation winding of the main transformer, thereby changing the composite voltage of the low-voltage windings of the main transformer and the series transformer.
- Advantages: Wide voltage regulation range, and the secondary voltage step difference is equal when the number of tap turns is equal. The primary voltage is not limited by the insulation level of the tap-changer and can be directly stepped down from 63~220kV, eliminating the need for an intermediate substation. The capacity of the voltage regulation winding is much smaller than the output capacity of the furnace transformer, making it easy to select a tap-changer with a high safety factor. The voltage regulation winding can be connected to a compensation capacitor to improve the power factor. The main transformer operates at constant flux, making the design of products with large capacity and wide voltage regulation range more economical.
- Disadvantages: Complex structure, time-consuming to manufacture, and inconvenient to maintain. When at negative taps, the load loss is large, and the load loss at the minimum secondary voltage is slightly different from that at the maximum secondary voltage, and the impedance voltage increment is large.
- Application Scope: Furnace transformers with a capacity of 10000kVA or more, or furnace transformers with a high primary voltage (63kV or more).
3. Autotransformer Voltage Regulation
It consists of an autotransformer and a furnace transformer with a fixed transformation ratio, which can be installed in the same tank or separately, with a maximum insulation level of 35kV.
- Features: The common neutral point of the autotransformer must be reliably grounded (when the voltage levels of the primary and secondary sides are different); it can be ungrounded when the voltage levels are the same. When the number of tap turns is equal, the low-voltage voltage step difference of the furnace transformer is equal. The impedance change is small during the voltage regulation process. For single-phase products, a three-limb core with unequal cross-sections can be used, with one limb for the autotransformer voltage regulation winding, one limb for the furnace transformer winding, and one limb as a common yoke, resulting in a more economical design. Large-capacity products should adopt a separate structure, and the connecting bus between the autotransformer and the furnace transformer should be shortened and enclosed to prevent damage to the autotransformer due to extremely large short-circuit current in case of a short circuit.
Key Design Considerations for Furnace Transformers
1. Winding regulation principle
Due to the low number of turns and high current intensity in the low-voltage winding, tap changers are usually set on the high-voltage winding side. By changing the number of turns in the high-voltage winding, the secondary voltage can be regulated. Different tap positions will cause changes in the core flux density. The upper limit of the secondary voltage determines the core size, and the voltage regulation range directly affects the total number of turns in the winding. These two parameters jointly determine the material usage and manufacturing cost of the transformer.
2. Impedance Characteristics and Structural Optimization
The regulation range of secondary voltage is inversely proportional to the square of the winding impedance. In standard designs, the impedance should be controlled within 15%. A larger tap range increases the complexity of the winding structure, while a smaller tap range is conducive to achieving a compact design. When grouping the windings, it is necessary to ensure that the impedance characteristics of each low-voltage coil remain consistent. A common approach is to divide the high-voltage winding into multiple parallel coils and use a center tap to achieve rapid voltage switching.
3. Key points of structural design
The box body and clamping structure are all made of steel. For high current conditions, special attention should be paid to:
- The busbar is arranged in groups corresponding to the windings.
- The adjacent busbars are arranged with reverse currents.
- Maintain a safe distance to avoid magnetic leakage and heating.
The three-phase connection adopts a low-voltage side delta connection. In modern designs, the connection points are moved outward to the outside of the box, effectively reducing internal losses. When the transformer is far from the electric furnace, a busbar collector should be set up on the side close to the electric furnace to reduce line losses.
4. Cooling and Protection
Large electric furnaces prefer water-cooling designs, which are more economical when there is a stable water supply. If the water quality does not meet the standards, natural cooling schemes should be adopted. To adapt to dusty environments, transformers need to have a fully sealed structure, and special sealing devices should be set at the wall-penetrating points of busbars to effectively prevent dust from entering.
V. Winding Arrangement of Furnace Transformers
Furnace transformers consist of high-voltage, voltage-regulating, and low-voltage windings, with their arrangement following these key principles:
1. Core Principles
Magnetic Potential Balance: For staggered windings, each leakage magnetic group’s high and low-voltage windings must have equal magnetic potential and symmetric structure. For concentric windings, the magnetic potential unbalance rate should not exceed 6%.
Impedance Growth Control: Adjust winding configuration to limit impedance increase at low output voltages, ensuring a hard external characteristic for stable furnace operation.
Material Efficiency: Maximize winding filling factor and keep the voltage of core-adjacent windings low for economical design.
2. Staggered Winding Arrangement
Windings are axially divided into sections, with high and low-voltage segments arranged alternately. Each leakage magnetic group’s high and low-voltage segments have balanced magnetic potential and radial dimensions (difference ≤5%). High-voltage segments are often placed at both ends. For deep voltage regulation, configurations like “coarse-fine tuning” adjust leakage magnetic groups to control impedance.
3. Concentric Winding Arrangement
(1) Low-voltage windings are external for easy lead-out. Voltage-regulating windings are usually internal, with high-voltage windings in the middle; for ultra-wide voltage regulation, voltage-regulating windings are placed centrally.
(2) In series transformer voltage regulation:
- Main transformer: Low-voltage (external) → High-voltage (middle) → Voltage-regulating (internal).
- Series transformer: Low-voltage (external) → High-voltage (internal). Basic windings (if added) lie between voltage-regulating and high-voltage windings.
In summary, staggered and concentric arrangements are selected based on magnetic potential balance, impedance control, and design economy, ensuring stable and efficient transformer operation.
Core Structural Components of Furnace Transformers: Differences from Power Transformers
Due to the need to adapt to the working conditions of electric furnaces such as high temperature, large current, and frequent voltage regulation, furnace transformers have targeted differences in core component structures from ordinary power transformers. The following is a detailed description of the key distinguishing component.
1. Conjugate Core
- Structural feature: The reactor core and transformer core share a common yoke, forming an integrated “conjugate” design.
- Difference point: The transformer core and reactor core of ordinary power transformers are mostly independent separate structures without a common yoke.
- Design purpose: Compress the overall volume through the shared yoke, reduce leakage magnetic loss, and improve the coupling efficiency of the core magnetic circuit to adapt to the dynamic regulation needs of electric furnace loads.
2. Staggered Windings
The windings of furnace transformers adopt a special staggered design, which is different from the conventional winding method of power transformers. They are mainly divided into two types:
- Direct-wound staggered windings: Conductors are arranged alternately at specific pitches during direct winding, not following the uniform sequential winding logic of power transformers.
- Assembled staggered windings: Windings are prefabricated in sections and then assembled in sets, with the winding direction and position of each section of windings distributed alternately.
- Core advantage: Enhance the short-circuit resistance of the windings, improve the distribution of electric and magnetic fields inside the windings, reduce local loss, and adapt to the inrush current condition during electric furnace startup.
3. Concentric Windings (Special Design for Voltage Regulating and Low-Voltage Windings)
The concentric windings of furnace transformers have significant differences from power transformers in the structural forms of voltage regulating windings and low-voltage windings:
Voltage Regulating Windings
- Winding types: Include four special designs: continuous type, interleaved type, spiral type, and cylindrical type, with more targeted application scenarios.
- Difference point: The voltage regulating windings of power transformers are mainly continuous type and on-load voltage regulation dedicated structures. The interleaved and spiral voltage regulating windings of furnace transformers focus more on withstanding impulse voltage, while the cylindrical type optimizes the magnetic field stability during voltage regulation.
- Adaptation requirement: Meet the needs of wide-range and frequent voltage regulation during electric furnace smelting, and improve the insulation reliability of the windings.
Low-Voltage Windings
(1) Special structural types:
- Double-disc low-voltage windings: Wound in double-disc segments, with a larger heat dissipation area compared to the conventional low-voltage windings of power transformers.
- “Figure-8” shaped low-voltage windings: Windings are distributed in a “figure-8” shape, effectively reducing leakage magnetic flux and adapting to large current transmission.
- Plate-type low-voltage windings: Adopt a flat-plate structural design, reduce the skin effect under large current, and improve current-carrying capacity.
(2) Core difference: The low-voltage windings of power transformers are mainly cylindrical and spiral types. The low-voltage windings of furnace transformers are all designed to adapt to large current working conditions, focusing more on low loss and high heat dissipation performance.
4. Low-Voltage Lead Wires
- Structural feature: Low-voltage lead wires use large cross-section conductors, and the layout path is optimized to a short-distance, low-impedance design. A shielding structure is used in some scenarios.
- Difference point: The low-voltage lead wires of power transformers focus more on insulation protection and neat wiring. The low-voltage lead wires of furnace transformers are mainly optimized around “large current transmission” with a larger conductor cross-section and lower impedance to reduce lead wire loss.
5. Low-Voltage Outlet Terminals
(1) Special types:
- Copper plate terminals: Processed from a single piece of thick copper plate, with a large contact area, suitable for medium and large capacity current transmission.
- Water-cooled copper tube terminals: Use hollow copper tubes as the carrier, combined with a water-cooling system, which can quickly dissipate the heat generated by large currents.
(2) Difference point: The low-voltage outlet terminals of power transformers are mostly conventional copper bars or bolt-type terminals without a dedicated water-cooling design. The terminal structure of furnace transformers is specially designed to withstand large current and high-temperature working conditions, ensuring current transmission stability and service life.
