Basics of induction heating, Part 2: Principles

Induction heating is widely used in industry and even consumer appliances as a contact-free heating technique with many distinct advantages.

The understanding of induction heating begins with basic electromagnetic principles and that very ancient electrical component still in very wide use: the transformer. However, an induction-heating transformer does not resemble a standard AC-line power transformer, signal transformer, or RF transformer, even it has the same schematic and underlying principle (Figure 1).

Fig 1: The schematic diagram and functional relationships of the induction-heating transformer are similar to that of the standard AC transformer, but is very different in implementation. (Image: Circuit Globe)

Induction-based heating begins with a coil of a conductive material such as copper. The induction heating arrangement is a form of “open” air-core transformer design without the conventional core used to contain the magnetic flux and maximize efficiency. As current flows through the coil, it produces a magnetic field in and around the coil.

This primary-side current must be a non-steady, non-DC, alternating current (AC) for the transformer induction to work, again following Faraday’s discovery (we’ll look at the frequency of this AC later). The ability of the of the magnetic field to do work depends on the coil design as well as the amount of current flowing through the coil.

The driven inductor serves as the transformer primary side while the part to be heated – often called the workpiece – becomes the secondary side (even though it is a low-resistance or even short circuit. When a magnetic (ferrous) or non-ferrous metal part is placed within the inductor and enters the AC magnetic field, circulating currents called eddy currents are induced within the part (Figure 2).

Fig 2: In induction heating, the alternating magnetic field of the primary side induces eddy currents in the workpiece or load which is functioning as the secondary side of the transformer. (Image: AZO Materials)

The current transmitted through the secondary side is proportional to the inverse of the square of the distance between them. Since there is no magnetic core to direct and constrain the flux, the secondary side or workpiece is usually surrounded by the primary coil, to maximize the transfer of magnetic energy transfer (Figure 3). (This is not the situation in consumer-appliance designs, discussed in Part 4).

Fig 3: A induction-heater primary coil surround a metal bar can bring that bar up to glowing red temperature in a few seconds. (Image: Digilent)

As the eddy currents flow through a medium, there will be some resistance to the movement of the electrons. The eddy currents flow against the electrical resistivity of the metal, and they generate localized heat without any direct contact between the part and the inductor.

Thus, induction heating is a form of I²R heating, except that the “connection” between power source and heating element is not a direct wire connection with electron flow. Instead, it is an alternating magnetic field which induces electrons to move – that’s current flow – and this movement of electrons encounters resistance in the form of the resistivity of the metal, which then causes heating.

This heating occurs with both magnetic and non-magnetic electrically conductive parts. From a physics standpoint, this is often referred to as the “Joule effect”, referring to Joule’s first law which defines the relationship between heat produced by electrical current as it is passed through a conductor.

There is another heating mechanism at work in some cases. Additional heat is produced within magnetic parts through hysteresis and hysteric heating, – internal friction that is created when magnetic materials pass through the varying magnetic fields which modify the component’s magnetic polarity. This resistance also produces internal friction which in turn produces heat.

However, this secondary effect of hysteretic heating only occurs in a component up to the Curie temperature, the temperature at which the material’s magnetic permeability decreases to unity (typically, 500-600°C/1000-1150°F). In contrast, the heating due to eddy current continues.

One of the important attributes of the standard electrical transformer is that it provides galvanic (ohmic) electrical isolation between primary and secondary sides using a changing magnetic field to transfer energy across a barrier. This is also the case with induction heating. The material to be heated can be located in a setting isolated from the power supply’s primary side; it can be grounded, submerged in a liquid, covered by isolated substances, in gaseous atmospheres, or even in a vacuum.

The next part of this article looks at the overall induction-heating system and some important overall characteristics.

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