Induction Heater

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Induction Heater


(or inductor), an electromagnetic device used for induction heating. An induction heater consists of two basic parts: a primary coil, which generates an alternating magnetic field, and current feeders, which connect the primary coil to the power supply. An electrically conductive object placed in an alternating magnetic field heats up as a result of the thermal action of eddy currents induced in the parts of the object that are directly enveloped by the primary coil.

Basically, all types of induction heaters may be grouped in two categories, those with simultaneous heating and those with continuous sequential heating. In the first case the area of the primary coil is approximately equal to the area of the surface being heated, so that all regions of the surface can be heated simultaneously. In the second case the article being heated is moved relative to the primary coil, so that the surface regions of the article are heated sequentially.

Induction heaters for surface heating and hardening of various articles, through-heating of forgings, heating of sheet material, and melting of metals differ in design, the frequency of the current being supplied, and the material of the magnetic circuit of the induction system. An induction heater used for heat-hardening simple cylindrical parts by the simultaneous method is shown in Figure 1. The primary coil is made very large to avoid overheating and melting. Frequency of the current supplied to such an induction heater is 10 kilohertz (kHz). There are openings in the surface of the primary coil for supplying quenching water to the heated workpiece after the current has been shut off. Thus, the induction heater itself is also cooled simultaneously.

Figure 1. Induction heater for hardening cylindrical parts by the simultaneous method: (1) funnels for equalizing pressure of quench water in chamber (2), (3) primary coil with openings for quench water, (4) piping for water cooling system

A solenoid, which is used to harden internal surfaces of parts, is the simplest multiturn induction heater. Solenoid induction heaters are capable of heating internal cylindrical surfaces with a diameter of 50 mm or more. Loop induction heaters with a magnetic circuit (Figure 2) are used for openings with diameters of less than 30 mm, and rod heaters are used to heat internal cylindrical surfaces with diameters of less than 15 mm. Such heaters have a tube whose diameter is somewhat smaller than the diameter of the opening being treated. The tube is located coaxially relative to the opening. Induction heaters used in through-heating of forgings are made from a tube; very long tubes are divided into sections, each of which is cooled separately.

Figure 2. Loop induction heater for hardening internal cylindrical surfaces by the simultaneous method, with rotation of the workpiece: (a) design with separate chambers for cooling the induction heater and supply of quench water, (b) design without continuous cooling; (1) magnetic circuit, (2) primary coil, (3) piping for water cooling system

Flat surfaces of articles are heat-hardened using induction heaters with a primary coil in the shape of a flat spiral or zigzag (for small areas) or using the continuous sequential heating method, in which the part being heated is moved over the primary coil (Figure 3). Sectional induction heaters with separate electric current feed for each section are also used. Surfaces of varying width and the required shape may be hardened (heated) by switching the sections on and off in a predetermined sequence. End surfaces are heated using zigzag-shaped induction heaters; uniform surface heating is achieved by rotating the workpiece.

Figure 3. Induction heater for hardening flat surfaces by the continuous-sequential method: (1) primary coil, (2) magnetic circuit, (3) spray device for supply of quench water, (4) piping for water cooling system

Figure 4. Diagram of induction heaters for heating sheet material in a transverse magnetic field, with the following arrangements of the primary coil: (a) on one side of the sheet to be heated, (b) on both sides of the sheet; (1) primary coil, (2) magnetic circuit, (3) yoke of magnetic circuit

Sheet and strip material is heated most efficiently in a transverse magnetic field (Figure 4). In this case the thickness of the sheet must be smaller than the penetration depth of current (usually at frequencies of 10–70 kHz). Gear teeth are hardened using loop induction heaters that enclose the tooth from both sides. To harden the tooth space, the primary coil is located along the circumference of the gear. In this case parts of magnetic circuit are placed opposite the tooth spaces and enter them in the operating position.

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