An Introduction to Induction Heating

This article serves as a brief guide to the fundamentals of induction heating. When current flows through an inductor, it creates a magnetic field around the wire. In an alternating current (AC) circuit, this magnetic field will collapse and switch polarity as the current does. If a second conductor is positioned in this alternating magnetic field, it will generate an alternating current. The current in this second conductor is directly proportional to that of the first and inversely related to the square of the distance between them.

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Induction heating leverages this principle by utilizing a coil and a workpiece. The alternating current flowing through the coil generates a magnetic field that alternates in polarity at the frequency of the power supply, inducing an alternating current in the workpiece.

Heating Methods in Induction Heating

Induction heating can be achieved through two primary methods:

• Eddy Current Heating: This occurs from the losses within the workpiece itself.

• Hysteretic Heating: This method generates energy inside the workpiece by an alternating magnetic field, which changes the magnetic polarity. Hysteretic heating is most effective until the Curie temperature is reached (around 770 °C / 1420 °F), beyond which it substantially decreases. It’s worth noting that hysteretic heating occurs primarily in magnetic steels; non-magnetic materials like copper and aluminum do not experience this effect.

Understanding Operating Frequency

The operating frequency of an induction heating system plays a crucial role in determining the depth of heat penetration. To grasp this, one must understand the 'skin effect.' When the magnetic field induces current in the part, it tends to flow mostly at the surface. A higher operating frequency leads to a shallower skin depth, while a lower operating frequency will result in a deeper skin depth, affecting the heating penetration.

Induction Heaters Explained

It is important not to confuse induction heaters with induction cookers.

An induction heater is essential for all types of induction heating. These devices typically function at medium frequency (MF) or radio frequency (RF) ranges.

Modern induction heaters are composed of four major systems:

• The control system, which may include a control panel or ON/OFF switch.
• The power unit (often referred to as the power inverter).
• The work head (or transformer).
• The heating coil (inductor).

How Induction Heating Works

Induction heating is a non-contact method that heats conductive materials using a strong magnetic field. Induction heaters may operate at supply frequencies of either 50 Hz or 60 Hz, with some specialized units functioning at 400 Hz (for aerospace applications).

It's essential to differentiate between induction heating and induction cooking. The systems operate based on different principles, with induction heating creating oscillating magnetic fields to generate currents in ferrous materials, ultimately exciting and heating the material.

Main Equipment Components

Typically, an induction heater comprises three key elements:

Power Unit

This component, also known as the inverter or generator, converts mains frequency to a range between 10 Hz and 400 kHz. The output power typically ranges from 2 kW to 500 kW.

Work Head

This section contains a combination of capacitors and transformers, enabling the connection between the power unit and the work coil.

Work Coil

The inductor is responsible for transferring energy from both the power unit and work head to the workpiece. These coils can range from simple solenoids to complex machined components crafted from solid copper, and the design of the coil plays a critical role in the overall effectiveness of the heating process.

Understanding Induction Heating Frequencies

Induction generators operate at various frequency ranges, significantly impacting their applications:

• Radio frequency (RF) generators function between 100 kHz and 10 MHz, suitable for smaller components.
• Medium frequency (MF) generators operate from 1 kHz to 10 kHz and are often used for medium to larger components.
• Mains frequency induction coils directly use the standard AC supply, typically designed for localized or low-temperature heating.

History of Induction Heating

The principles of induction heating were first discovered by Michael Faraday in the 19th century. He observed the relationship between magnetic fields and electrical current, although practical applications came much later, initially focusing on melting steel in the early 20th century.

For more insights into induction technology, you may want to check out the article on induction tankless water heater.

As technology progressed, particularly in the 1940s, significant advancements were made in the utilization of induction heating techniques, especially in manufacturing and metal treatment processes.

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Further Reading

• Induction Forging
• Induction Hardening
• Induction Heating
• Drum Heater