What is the main frequency of an induction furnace? A Guide to Optimizing Melting & Heating

The primary frequency for an induction furnace is not a single value, but rather a range chosen based on the furnace's specific application. Most modern induction furnaces used for melting metals like iron and steel operate in the medium frequency range, typically between 150 Hz and 10,000 Hz (10 kHz). However, other types, such as large holding furnaces, operate at standard line frequency (50 or 60 Hz).

The choice of operating frequency is the most critical factor defining an induction furnace's performance. It directly controls the depth of heat penetration and the intensity of the molten metal's stirring action, determining whether the furnace is best suited for rapid melting, holding large volumes, or specialized surface heating.

The Role of Frequency in Induction Heating

To understand why different frequencies are used, we must look at the core physics of induction. The alternating current in the furnace's coil creates a fluctuating magnetic field, which in turn induces electrical currents—called eddy currents—directly within the metal charge. The resistance of the metal to these currents generates immense heat.

The "Skin Effect": Controlling Heat Depth

The frequency of the alternating current dictates how deeply these eddy currents penetrate the metal. This phenomenon is known as the skin effect.

At high frequencies, the eddy currents concentrate in a very thin layer on the surface, or "skin," of the material. This is ideal for applications like surface hardening.

At low frequencies, the magnetic field penetrates much more deeply into the metal, inducing heat throughout a larger volume. This is better for melting large, solid pieces of scrap.

Stirring Action vs. Power Density

Frequency also determines the intensity of the electromagnetic stirring within the molten bath.

Lower frequencies produce stronger, deeper magnetic fields that result in a vigorous stirring action. This is excellent for homogenizing the chemical composition and temperature of a large melt.

Higher frequencies allow for a higher concentration of power in a smaller volume (power density), leading to much faster melting, but with a gentler, less intense stirring action.

Common Frequency Ranges and Their Applications

Induction furnaces are engineered around three main frequency bands, each tailored for a different metallurgical purpose.

Line Frequency (50/60 Hz): The Holding Furnace

These furnaces, often of the "channel" type, operate at the standard frequency supplied by the electrical grid. They function much like a transformer, where the furnace's induction unit is the primary coil and a loop of molten metal acts as the secondary.

This low frequency provides deep penetration and a very strong stirring action, making these furnaces ideal for holding, superheating, and maintaining the chemistry of very large volumes of already-molten metal. They are less effective for melting metal from a cold state.

Medium Frequency (150 Hz - 10 kHz): The Melting Workhorse

This is the most common range for modern "coreless" induction furnaces found in foundries. The power is supplied by a solid-state power supply that converts line frequency into the desired medium frequency.

This range offers the best of both worlds: it provides enough power density for rapid and efficient melting of cold scrap while still creating a sufficient stirring action to ensure a uniform final product. The ability to vary power and frequency gives operators precise control over the melting process.

High Frequency (>10 kHz): The Specialist

High-frequency furnaces are typically smaller and used for highly specialized tasks. This includes melting small batches of specialty alloys, precious metals, or for laboratory-scale applications.

The shallow heating depth associated with high frequencies is also leveraged for non-melting applications like brazing, soldering, and the surface hardening of steel components.

Understanding the Trade-offs

Choosing a furnace frequency involves balancing competing factors. There is no single "best" option, only the most appropriate one for the task.

Efficiency vs. Stirring

A medium-frequency furnace is highly efficient at melting a charge of small-to-medium-sized scrap metal. A line-frequency furnace would be less efficient at this task but would provide superior stirring once the metal is molten.

Capital Cost vs. Flexibility

Line-frequency systems are generally simpler and have a lower initial capital cost. However, medium-frequency systems, with their advanced solid-state power supplies, offer far greater operational flexibility, faster melt times, and better overall process control.

Refractory Wear

The intense stirring action of a very low-frequency furnace can accelerate the erosion of the furnace's refractory lining. This increases maintenance costs and downtime compared to a gentler medium-frequency stirring action.

Making the Right Choice for Your Goal

Your selection must be guided by your primary metallurgical objective.

• If your primary focus is holding and superheating large volumes of molten metal: A line-frequency (50/60 Hz) channel furnace is the most energy-efficient and effective choice.
• If your primary focus is rapidly melting batches of solid scrap iron or steel: A medium-frequency (150 Hz - 10 kHz) coreless furnace provides the optimal balance of speed, efficiency, and process control.
• If your primary focus is melting small, specialized batches or surface heat treating: A high-frequency (>10 kHz) system is required for its precision and concentrated power.

Ultimately, matching the furnace's operating frequency to the material and the process is fundamental to achieving an efficient, cost-effective, and high-quality result.

Summary Table:

Frequency Range	Primary Application	Key Characteristics
Line Frequency (50/60 Hz)	Holding & Superheating Large Volumes	Deep penetration, strong stirring, ideal for molten metal maintenance
Medium Frequency (150 Hz - 10 kHz)	Rapid Melting of Scrap (Iron, Steel)	High power density, efficient melting, good stirring control
High Frequency (>10 kHz)	Specialized Melting & Surface Heating	Shallow penetration, precise for small batches & heat treatment

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Contact us today to discuss how our induction furnaces can enhance your productivity and quality!