Reactor (Choke)
The reactor, choke, or DC link serves as the critical component connecting rectifier unit and the inverter in an induction furnace system. Its primary function in induction furnaces is to create a DC current source at the inverter’s input. This helps prevent the inrush currents that can occur during the furnace start-up and reduces current fluctuations during operation. Additionally, the use of DC links plays an important role in filtering high-frequency harmonics from the rectifier’s output current. This filtering improves the quality of power drawn by the coil and helps in preventing electrical damage to the furnace’s electrical infrastructure.
Three Essential Parameters in the Design and Construction of DC Links
The three primary parameters in the design and construction of DC links are inductance, current (power), and the voltage across the terminals. Inductance is the most crucial electrical characteristic of a DC link, representing its inductive power. During start-up, it is at its highest level but gradually decreases over time, reaching a stable, lower value as the system stabilizes.
DC links can be classified into two categories based on the variability of their inductance: simple (without taps) and tapped links. In a simple DC link, the inductance is at its maximum during start-up and then follows a completely decreasing trend, eventually approaching zero. In contrast, a tapped DC link initially experiences a decrease in inductance after start-up, but this decline eventually halts, stabilizing at a specific operating range. Once the current exceeds this range, the inductance starts to decrease again.
Structurally, the primary difference between these two types of DC links lies in their core designs.
With stepped air gap (two steps) DC link
with uniform DC link air gap
Apart from the classification of DC links into simple and tapped types, they can be further divided into several groups based on their structure and assembly method. Below, we will discuss the advantages and disadvantages of each group.
Inner Core & Helical Type Coil DC Link
In DC links with a helical core, the core structure consists of two columns and two yokes, with air gaps designed along the paths of the columns. The core columns pass through the interior of the helical coils. In this design, the coils are wound in one or two layers in a helical manner, with water inlet and outlet connections branching from specific points on them.
The main advantage of this type of link is the low cost of raw materials and manufacturing. However, a significant drawback is the lack of ability to disassemble and perform maintenance in the event of failure of any part of the core or coil. This limitation can lead to operational downtime in case of unexpected failures.
Most DC links manufactured by India;s Electotherm company, fall into this category.
Outer Core & Crossover Type Coil DC Link
As the name suggests, this type of DC Links consist of crossed, multi-layered coils and a separately arranged core, with no physical connection or integration between the core and the coils.
The primary advantage of this type of DC link is the ability to replace a faulty coil with a spare coil. This capability significantly reduces the time required for repairs and reactivation of the link in the event of a failure. Considering that the operation of a complete induction furnace system often depends on the functionality of a DC link, having a spare coil is of great importance.
The main drawbacks include the high volume of materials used, such as copper, core, and insulation, which leads to higher manufacturing costs. Additionally, the overall size and dimensions of the link are substantial.
This type of DC link is more commonly used by German furnace manufacturers, where it is favored for its advantages despite the associated costs and size.
Inner Core & Crossover Type Coil DC link
Inner-core DC links with crossed coils consist of crossed, multi-layered coils and an internal core. Essentially, this type of DC link is a combination of the two previously mentioned designs.
The primary advantage of this design is that it allows for the replacement of a damaged coil unit with a spare unit, avoiding the need to replace the entire defective coil and thus saving on high costs. This is facilitated by the modular structure of the coil, which means that if a fault or failure occurs in any coil unit, only the damaged unit needs to be replaced. Additionally, the compact coil structure reduces the length of the core compared to other types of links, leading to lower core manufacturing costs and a reduction in the overall size and dimensions of the link.
A notable disadvantage is that due to the complex structure, any fault or wear in the insulation of the coil is more probable. The physical integration between the coil and the core means that repair or replacement of a damaged unit requires disassembling the link, which can be time-consuming.
This type of DC link is commonly used in Chinese furnace constructions due to these advantages, despite the associated complexity and potential repair challenges.
“I” Core & Crossover Type Coil DC link
In this type of DC link, the link’s coils are arranged in a crossed, multi-layered configuration on an I-shaped core. The core, referred to as a “complete core,” consists of magnetic steel arranged in an I-shape with two series-connected magnetic paths and air gaps. The presence of a magnetic core in part of the magnetic flux path results in an increased inductance of the link compared to coreless DC links. Additionally, the air gap in the core’s magnetic flux path enhances the core’s saturation level compared to DC links with a complete magnetic core. This design achieves a balance between a high level of inductance and maximum magnetic saturation current compared to other types of DC links.
The primary advantage of this design is its balanced performance between high inductance and maximum magnetic saturation. The I-shaped magnetic core enhances the efficiency of the link while maintaining a manageable size and cost.
Similar to the internal-core design with crossed coils, the occurrence of faults and short circuits in the coil sections of this type of link is relatively common.
This type of DC link is predominantly used in the construction of induction furnaces by Megatherm, India.
Air Core & Crossover Type Coil Reactor
Air-core reactors with crossed coils, often referred to as Current Limiting Reactors (CLR), consist of multi-layered crossed coils without a magnetic core. These reactors are typically installed in the negative bus path between the rectifier and inverter in induction furnaces. The terminology “reactor” is used here to distinguish them from DC links used in parallel induction furnaces, reflecting their different application and purpose.
These reactors generally have much lower inductance (usually less than 1 milli-Henry) compared to DC links. This lower inductance is sufficient for their specific applications without the need for a magnetic core. They help in reducing the ripple in the voltage output from the rectifier. Also, they prevent high inrush currents from damaging sensitive electrical and electronic components by limiting current surges during short circuits. They also assist in discharging energy stored in power circuit capacitors, improving the system’s reliability.
To enhance reliability, these reactors are completely insulated through a resin impregnation process after the coil is constructed. This ensures effective electrical isolation. Sometimes, magnetic spheres enriched with resin are used within these reactors to increase the effective inductance without using a magnetic core.
Air-core reactors with crossed coils are primarily used in the power circuits of induction furnaces manufactured by Inductotherm and similar companies.
Commutation Coil
The commutation coil, sometimes referred to as the di/dt coil, consists of a generally single-layer, short helical coil made of copper with an electrolytic section. It is used to protect power circuit equipment in furnaces from transient overcurrents, depending on the current passing through the equipment. Commutation coils, which have a very simple structure, are installed at specific points in the power circuit of induction furnaces, depending on the type of protection considered in the design.
High-Frequency (HF) Transformer for Induction Furnaces
HF transformers used in induction furnaces serve as step-up transformers at the output of the inverter and between the resonance tank of the induction furnace—primarily in low-power and low-voltage furnaces—to increase the voltage across the coil and consequently reduce the current drawn from the power circuit of the furnace.
In addition to reducing the current drawn by the resonance tank, HF transformers provide electrical isolation between the induction furnace’s hardware on the load side and the power supply side, preventing faults and failures on the load side from electrically propagating to the power circuit, thereby offering greater protection to the electrical equipment used in the induction furnace’s power circuit. This function is somewhat similar to the role of the DC link in protecting against inrush currents.
Although the application of HF transformers is not entirely similar to that of DC links, their structure and components are very similar to them. Generally, HF transformers consist of two air-core coils for the primary and secondary sides, wound around a toroidal core with complex winding patterns. For structural stabilization and additional insulation, similar to air-core reactors with cross-coils, they are insulated with resin after construction.
HF transformers are predominantly used in the low-tonnage induction furnace designs of Inductotherm.
