Inductive Heating Coil Design For Tool Holder

Applications of Power Electronics

Jean Pollefliet , in Power Electronics, 2018

2 High Frequency Inductive Heating

Especially in the metal industry it is often necessary to heat objects, possibly for pouring, soldering, case hardening, melting or tempering of these parts.

Mostly the heat is generated outside the object and via radiation or convection the heat is transferred to the object or work piece. A much more elegant method is to generate the heat in the object (or part of the object) using eddy currents. These eddy currents are induced using high frequency magnetic fields produced in induction coils. This is called inductive H.F. heating. It is clear that only electrically conductive material can be heated in this way.

The induction coil that is placed around the object is called the work coil (fig. 15-9) This work coil is often a copper tube through which water flows to cool the coil.

The eddy currents in the work piece cause eddy current losses which result in heating. In ferromagnetic materials hysteresis losses also play a role. As we saw in chapter 5 depending on the frequency we have pronounced or non pronounced skin-effect. Induction ovens whose purpose is to melt metals operate with lower frequencies. In this case the skin effect plays virtually no role. Installations using higher frequencies (2 to 500 kHz) use the skin-effect to locally heat metals or to get them to glow. We limit our discussion to these installations. Due to the skin-effect the heating will be greatest on the outside of the work piece. This property is useful when we want to surface harden a work piece. The heat is concentrated in the outer layer (S_di ) of the work piece.

The skin depth S_dj follows from (5-32):

(5-32) $S_{d i} = \sqrt{\frac{ρ}{π \cdot μ_{0} \cdot μ_{r} f}}$

With μ ₀. = 4.π. 10⁻⁷ H/m this becomes:

(15-2) $S_{d i} \approx 503 \cdot \sqrt{\frac{ρ}{f \cdot μ_{r}}} mm$

Here f is in Hz and ρ in Ω mm²/m.

The magnetic field strength in the work coil is extremely high and with ferromagnetic work pieces magnetic saturation often occurs. This means amongst other things that the relative permeability μ_r is small. At the curie point (for steel at 760°C) the magnetic properties disappear (μ_r = 1) and the penetration depth will increase for a specific frequency.

It is obviously also the case that if the high frequency field is maintained long enough that the work piece will quickly heat up.

To work with a reasonable efficiency it can be shown that $\frac{S_{d i}}{d} \approx \frac{1}{8}$ . Here d is the diameter of the (round) work piece. The frequency is chosen such that the penetration depth is a maximum of $\frac{d}{8}$ . From (15-2) we find then:

(15-3) $f_{\min} = 16 . 10^{6} . \frac{ρ}{μ_{r} . d^{2}} (Hz)$

The efficiency decreases with increasing frequencies and in addition the efficiency of an R.F. generator also decreases with rising frequencies so that in most cases f_min is used.

Fig. 15-10 shows f_min as a function of the diameter of the work piece for different materials.

One of the most important applications of inductive high frequency heating is hardening steel. The heating duration is often a fraction of a second. Usually the power concentration is from 1 to 5 kW per cm². Steel types with a carbon content above 0.3% can be hardened in this manner. Shocking the work piece with a liquid is easy with this system since it is possible to spray for example on the workpiece with the openings between the windings of the work coil (photo p. 15.10).

For the R.F. generator a so called resonant converter is used. Fig. 15-11 shows the basic schematic of a DC-AC current source inverter with a parallel resonant load.

The induction coil and the load are replaced by an equivalent L_R and R_b . The capacitance C_R causes parallel resonance. With a sufficiently high Q-factor we obtain a practically sinusoidal voltage v_o across the parallel circuit. The current source inverter (three-phase bridge) and the square wave generator (single phase bridge) can for example be constructed using thyristors (fig. 15-12).

To prevent high di/dt values through the thyristors a small coil L₂ is placed in series with the load.

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HIGH TEMPERATURE PROPERTIES OF HPSN UNDER LOAD IN SIMULATED COAL GASIFICATION ENVIRONMENTS

M Hoffmann , ... R Danzer , in The Institute of Energy's Second International Conference on Ceramics in Energy Applications, 1994

3.3 Protection of the Induction Coil from the Sulphidizing Gas

The water-cooled induction coil consists of a high purity, oxygen-free copper which is attacked by the sulphidizing gas, resulting in the formation of CuS. Particularly for long term experiments corrosion of the induction coil has to be prevented. Electro-plating the surface of the coil with a thin gold layer, using a solution suggested in the literature [ 7] proved to be satisfactory. To achieve an adequate corrosion resistance, the porosity of the gold layer has to be minimised by preventing the formation of bubbles in the bath through limiting the current density to 30 A/m² [7].

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Magnetic field and inductive applicators and probes at high frequencies

Mehrdad Mehdizadeh , in Microwave/RF Applicators and Probes (Second Edition), 2015

7.5.3 Coupling circuits for H-field applicators and coils

The resonated circuit comprising the induction coil and the tank capacitor typically has a high impedance that needs to be matched to the power source's lower impedance. This is accomplished through a coupling circuit. One class of coupling methods involves the use of a capacitive or inductive component to provide the impedance needed. Three types of direct coupling methods are shown in Figure 7.28.

Capacitive coupling is usually preferred, particularly in low-power systems, because variability of coupling is often needed and variable capacitors are more readily available. The capacitor-coupled method shown in Figure 7.28A is very similar in operation and design to that showed in Figure 3.15 for E-field applicators.

The inductive coupled method shown in Figure 7.28B is used in situations where a fixed load provides the opportunity to use a fixed coupling inductor to save the expense and maintenance of issues related to capacitors. In higher-power systems, inductors are less expensive and more reliable than capacitors. Thus, they are preferred if such an option is available.

The dual variable capacitor method shown in Figure 7.28C is usually used for systems with a highly variable load such as ICPs, where the two capacitors are controlled by an automatic matching system. In the design of a dual capacitor system, it is often necessary to know the values for typical load situations. The following equation can be used for computing the capacitance values:

(7.53) $C_{1} = \frac{1}{ω^{2} L (1 - \frac{\sqrt{R_{s} Z_{0}}}{L ω})}$

where ω=2πf is the angular frequency of operation, L is the inductance of the coil, Z ₀ is the characteristic impedance of the power source (e.g., 50 Ω), and R _s is the equivalent total resistance of the induction coil, which includes the coil and load losses as described in Section 7.4.4. The capacitance, C ₂, can be calculated from:

(7.54) $C_{2} = \frac{X}{ω (X^{2} + R_{s}^{2})}$

where:

(7.55) $X = L ω - \frac{1}{C_{1} ω}$

The second method of coupling, which is transformer coupling, is shown in Figure 7.29. In this method, instead of a reactive component, flux linkage is utilized to provide the coupling impedance transformation.

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Through Coating Imaging of Early Marine Corrosion Using ECPPT

Yunze He , ... Ruizhen Yang , in Transient Electromagnetic-Thermal Nondestructive Testing, 2017

12.3.2 Through Coating Imaging

In this experiment, both the induction coil and camera were placed on the same side of the sample. The heating time was set as 200 ms, followed by a cooling time of 300 ms. The sampling frequency of the IR camera was 200 Hz and the sampling interval was 5 ms. Fig. 12.4A–C show thermograms for a coated sample with 6-month corrosion, at 50, 200, and 500 ms, respectively. The unit of temperature is DL. In Fig. 12.4A–B, the profile of corrosion can be seen. However, as time increases, the lateral blur effect severely affects the detectability of corrosion. Even in the best result (Fig. 12.4A), the characterization of corrosion is difficult. Fig. 12.5A shows the temperature responses for points A, B, and C. Their locations are marked in Fig. 12.4A. Point A is located in a defect-free area, while points B and C are located in the corrosion area. In Fig. 12.5A, the temperature of B is greater than A while the temperature of C is smaller than A. This is difficult to explain, as there are too many influential factors (as mentioned in Section 12.2). One potential reason is the huge difference in physical parameters of B and C. Another reason is the temperature gradient caused by nonuniform heating. In Fig. 12.4B, the detail of the corrosion area is polluted by the nonuniform heating effect caused by the coil shape.

Whole temperature responses, including heating and cooling phases, were processed by discrete Fourier transform (DFT). The total number (N) for the whole temperature response was 200 and the DFT was performed in MATLAB. The frequency resolution (f _r) was 2 Hz. Fig. 12.4D–F show phase images of a coated sample with 6-month corrosion at 4, 10, and 38 Hz, respectively. In Fig. 12.4D, the shape of corrosion can be easily sized and the detail of corrosion can be observed. The phase of the corrosion area is smaller than that of the defect-free area. Fig. 12.5B shows phase spectra for points A, B, and C. The phase of B and C, located in the corrosion area, is smaller than point A, located in the defect-free area.

It is also found that phase spectra are modulated or oscillated at a fixed frequency (Fig. 12.5B). This phenomenon has been explained in previous work [142]. The reason is that both heating and cooling phases of the temperature response were used as input data for the DFT. If only the cooling phase of the temperature response was used as input data, this phenomenon would not be observed. The modulated period is the reciprocal of the heating duration [228], and the phasegram whose frequency is at the peak of phase spectra can show the abnormal flaw. The frequency (4 Hz) of Fig. 12.4D, which shows corrosion, is located on the peak of phase spectra of Fig. 12.5B. The frequency (10 Hz) of Fig. 12.4E, without corrosion, is located on the valley of phase spectra. In Fig. 12.4F at 38 Hz, corrosion is hardly found due to energy damping.

Fig. 12.6A–B show thermograms of a coated sample with 3-month corrosion at 50 and 200 ms, respectively. The unit of temperature is DL. In Fig. 12.4A, the shape of corrosion can be seen. However, as time increases, the lateral blur effect and nonuniform heating effect severely affect the detectability of corrosion. Fig. 12.4C–D show the phase images of a coated sample with 3-month corrosion at 4 and 10 Hz, respectively. The phasegram at 4 Hz performs the best in terms of corrosion detection and sizing as shape, size, and detail can be easily characterized. The phase value of the corrosion area is also smaller than that of the defect-free area. Fig. 12.7A–B show thermograms of a coated sample with 1-month corrosion at 50 and 200 ms, respectively. Fig. 12.7C–D show phase images of a coated sample with 1-month corrosion at 4 and 10 Hz, respectively. The same conclusions can be drawn.

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Magnetic Properties of Dislocations

A. Sato , M. Nagao , in Encyclopedia of Materials: Science and Technology, 2004

1.2 Magnetic Measurements

(i): Electromagnetic induction . An induction coil is used to detect magnetization. For example, a vibrating sample magnetometer (VSM), which uses a secondary coil placed around a sample, is designed to detect an alternating voltage induced by a vibrating sample magnetized in an applied magnetic field.
(ii): Electron spin resonance (ESR). The momentum J or equivalently the magnetization M of an electron is detected by resonant absorption. The application of a magnetic field forces J to undergo Larmor precession, and this is amplified by superimposition of an electromagnetic wave with appropriate frequency. The spectrum and intensity of the absorption peaks provide useful information about M . ESR is alternatively called electron paramagnetic resonance (Bowers and Owen 1955, Ingram 1968, Wertz 1968).
(iii): Nuclear magnetic resonance (NMR). Resonance arising from the magnetic spin in the nucleus. This method picks up nuclear magneton as small as 1/1840 of the Bohr magneton. The shift of a resonance peak (Knight shift) in NMR provides detailed information about the behavior of electrons in metallic and nonmetallic crystals including Heusler alloys (Khoi et al. 1978). NMR is also applied to visualize a biologic organ by construction of a three-dimensional image (Gadian 1982) and to analyze the structures of proteins, nucleic acids, DNA fragments, etc. (Sarkar 1996).
(iv): Mössbauer effect. γ-rays with variable wavelength are generated by Doppler effect and utilized to detect by resonance small energy gaps induced by nuclear spins. The magnetic states of atoms can be analyzed by Mössbauer (1958) spectroscopy. Since the gaps are closely related to the motion of outer shell electrons, it can, for example, differentiate between Fe²⁺ and Fe³⁺. It has also permitted to identify a transition from ferromagnetism to superparamagnetism induced in Cu–Co by cold rolling (Nasu et al. 1968).
(v): Diffraction. Crystal structures and ordering are commonly examined by x-ray, electron, and neutron diffractions. Neutron diffraction provides direct information about electron spins interacting with nuclear magnetic moments. It has been used, for example, to analyze dislocation arrangements in deformed Fe (Göltz et al. 1986) and to discriminate between ferromagnetic and antiferromagnetic properties of Heusler alloys (Natera et al. 1970).

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NDE method for rebar corrosion degree of RC structures

H. Oshita , in Acoustic Emission and Related Non-Destructive Evaluation Techniques in the Fracture Mechanics of Concrete (Second Edition), 2021

9.2.2 Electromagnetic-induction heating

By charging high-frequency electric current onto an electromagnetic induction coil, an alternating field is generated around the coil and eddy current occurs on the surface of rebar. This current induces heat in rebar. In the case that the coil is circular as shown in Fig. 9.3, the alternating magnetic field is concentrically generated and the magnetic flux density becomes higher in the region of a dense arrangement of the coils. On the other hand, in the region of the center and the edge of the coil, the magnetic flux density becomes smaller. As a result, rebar set in the magnetic field is heated nonuniformly, as that temperature in the center and the edge regions of the coil becomes lower, while that in the intermediate region becomes higher, resulting in nonuniformly distributed temperature in the axial direction of rebar.

In the proposed system, it is very important to heat rebar uniformly in the axial direction. Consequently, the coil has been developed to satisfy such a condition. Various experiments for the coils were performed on the shape, the size of the steel tube, and the diameter of the steel tube. It is found that a rectangular coil is shown in Fig. 9.4 is of the most suitable shape to heat rebar uniformly because no heating gradients exist in the region of 60 mm × 300 mm. This electromagnetic-induction coil is equipped with a copper pipe of 10 mm diameter, inside which cooling is performed with water to reduce the heat of the coil due to radiofrequency current. The coil temperature becomes about 30°C in the case of the radiofrequency current charge, even if cooling is performed. Therefore, a styrene foam of about 10 mm thickness is set as an insulator at the concrete surface under heating.

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Advanced Machining Technologies

A.K.M. Nurul Amin , T.L. Ginta , in Comprehensive Materials Processing, 2014

11.13.6.2 Induction Heating Equipment

Induction heaters provide alternating electric current to an electric coil (the induction coil), as a result of which the induction coil becomes the electrical (heat) source that induces a high-frequency alternating electrical current into the workpiece to be heated. No contact is required between the workpiece and the induction coil acting as the heat source, and the heat is restricted to localized areas or surface zones immediately adjacent to the coil. This happens because the alternating current (ac) in an induction coil has an invisible force field (or magnetic flux) around it. When the coil is placed next to the tool approximately 5 mm above the workpiece surface, the lines of force concentrate in the air gap between the coil and the workpiece. The induction coil actually functions as a primary transformer, with the workpiece to be heated becoming the secondary transformer. The force field surrounding the induction coil induces an equal and opposing alternating electric current in the workpiece, with the workpiece then heating due to the resistance to the flow of this induced high-frequency alternating electric current. The rate of heating the workpiece is dependent on the frequency and intensity of the induced current, the specific heat of the material, the magnetic permeability of the material, and the resistance of the material to the flow of current. The induced currents are sometimes referred to as eddy currents, with the highest intensity current being produced within the area of the intense magnetic fields.

To heat the workpiece, a Portable Transistor Induction Heating Machine SP-25AB (25 kW capacity) and GP-35AB (25 kW capacity) were used for steel and titanium alloy Ti–6Al–4V, respectively. In the present study, a portable high-frequency inducting heating equipment was used for preheating the work material just prior to machining with the heating coil placed ahead of the cutting tool. This high-frequency current with alternative polarity generates eddy currents in the surface layer of the workpiece to heat up the layer.

This process has numerous advantages over other heating methods since the generated electric current is simpler to control than other processes. The heating system consists of three major components (Figure 18): high frequency transformer (Invertors), Matching Box (Transformer and Condenser), and cooling unit (designed for industrial use).

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Fluoropolymer Coating Processing Technology

Laurence W. McKeen , in Fluorinated Coatings and Finishes Handbook (Second Edition), 2016

12.3.3 Induction Baking

Heating by induction ⁶ is another approach to direct heating of the substrate. It is a common misconception that the substrate must be magnetic to be a candidate for induction heating. To be heated by induction, the substrate must conduct electricity. Technically, it must also resist the flow of electricity or have resistance, but that is true of all materials except superconductors.

The principle of induction heating depends on understanding that, when electricity flows, a magnetic field is generated, and the reverse is also true. Where there is a magnetic field and a conductor, electricity will flow.

Induction heaters make use of this principle. The heater uses alternating electricity in a coil to generate a magnetic field. When a piece of metal is placed close to (not touching) this coil, the magnetic field generated by the coil interacts with the metal, generating electric current. That current is called an eddy current, which is shown in Figure 12.12. The resistance to current flow in the metal leads to loss of electric power as described by the basic electrical formula in Eqn (12.5).

(12.5) $P = i^{2} R$

In this equation, i is the amount of current, R is the resistance of the metal, and P is the power loss or the heat gained. The equation also indicates that doubling the current quadruples the heat generated.

Because the coil uses alternating current, the magnetic field averages out to zero over time.

The strength of the magnetic field drops off with distance from the induction coil. Because the eddy currents are related to the strength of the magnetic field, the heating is strongest at the surface. The process seems simple, and in a way it is. It is complicated to control however, but it can be controlled. The heat-up rate of the metal underneath the coating being cured with induction heating depends on several properties of the substrate metal:

1.: Specific heat
2.: Magnetic permeability
3.: Resistivity.

All of these properties of the substrate vary with temperature. The weight and shape of the substrate metal will affect the heat-up rate. Since most of the heat is generated at the surface closest to the coil, the thermal conductivity of the substrate will also affect the peak temperatures at the surface as heat moves toward the cooler areas of the substrate.

Figure 12.13 shows a schematic of an induction heater on the left and a photograph of the coil heating a rod on the right. The control parameters of the induction coil include:

1.: Power
2.: Frequency.

There is a relationship between the frequency of the alternating current and the depth to which it penetrates the substrate. The induced current flow within the part is most intense on the surface. The current decays rapidly below the surface. The metal closest to the surface will heat more quickly than the inside. The "skin depth" of the part is described as the depth within which 80% of the heat in the part is produced. The skin depth decreases when resistivity decreases, permeability increases, or frequency increases. High frequencies of 100–400 kHz provide shallow penetration, which is usually ideal for curing surface coatings. Low frequencies of 5–30 kHz are effective for thicker materials requiring deep heat penetration such as those coated items with complex shapes.

Magnetic materials such as steel are easier to heat than nonmagnetic materials such as aluminum. This is due to a secondary heating mechanism called hysteresis. Magnetic materials naturally resist the rapidly changing magnetic fields within the induction coil. The resulting friction produces its own additional heat—hysteresis heating—in addition to eddy current heating. A visual explanation is given in Figure 12.14. A metal that offers high resistance is said to have high magnetic "permeability." Permeability can vary on a scale of 100–500 for magnetic materials; nonmagnetics have a permeability of 1.

The advantages of induction heating over conventional convection heating include:

1.: Fast cycle time. Heat can be developed directly and nearly instantly inside the substrate, allowing a much quicker start-up than conventional convection heating. Bake cycle times can be dramatically reduced
2.: Controlled directional heating. Very small areas of the substrate can be heated without affecting other surrounding areas or the fixturing that holds the part. With precise power input control, one can achieve the exact temperature required either slowly or quickly
3.: Repeatability. With modern induction heating systems, the heating pattern is always the same for a given set-up, cycle after cycle and day after day
4.: Noncontact heat. Nothing touches the coated part when it is placed in the induction coil, the process induces heat within the part without actually touching it
5.: Energy efficiency.

In summary, one can buy or formulate the finest quality fluorinated coating, but if it is not applied correctly and baked correctly, it may fail miserably in use.

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3D-printed miniature Savonious wind harvester

Dan Zhao , ... Arne Reinecke , in Wind Turbines and Aerodynamics Energy Harvesters, 2019

2.2.12 Electromagnetic convertor

In order to convert kinetic energy harvested from air motion into electrical power, induction coils are used to convert mechanical energy into electricity via an electromagnetic effect (also known as Faraday's law). A permanent magnet is attached onto the shaft at one end of the energy harvester, and placed in a wound of copper induction coil. When the torque exerted by the moving air onto the energy harvester exceeds the frictional torque, the harvester starts to rotate and so do the shaft and the magnet attached. The rotation of magnet causes the magnetic flux change in the induction coil, thus induces emf via mechanism as discussed in Chapter 4. According to the Faraday's law, the emf induced in the closed path (the coil) is equal to the negative of the time rate of change of magnetic flux enclosed by the path [70]. For a cooper coil having constant number of turns, the faster the energy harvester rotates, the larger the rate of magnetic flux change, and subsequently the greater the emf induced.

To build the electromagnetic convertor, an induction coil holder is designed as shown in Fig. 2.6B, the copper coil is wounded on both sides of the holder, and the magnet rotated in the middle of it. The air-driven energy harvester, together with the turbine supports and the electromagnetic generator is shown in Fig. 2.4.

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Mapping subsurface utilities with mobile electromagnetic geophysical sensor arrays

R. Birken , M. Oristaglio , in Sensor Technologies for Civil Infrastructures, 2014

12.4.3 Utility locating equipment

The most widely used utility or pipe locators are handheld EMI units using passive (or quasi-passive) detection and pushcart GPR units with a single transmitter–receiver antenna pair (Fig. 12.10). Induction locators, which can trace most conductive utility lines and estimate their depth, dominate One-Call services. During the last 5 years, GPR systems have penetrated the SUE market, with their ability to locate both conductive and non-conductive lines and provide more accurate depth estimates.

EM pipe locators

The basic components of a quasi-passive EM locating system are a current generator and induction coils. Commercial manufacturers offer a large variety of locators: from basic single-frequency systems with one current transmitter and a few tuned receiver coils to sophisticated multi-frequency systems with multiple current transmitters and broadband multi-coil receivers. High-end systems have integrated GPS and antennas for transmitting and receiving signals from RFID markers buried with utility lines ( North, 2010).

Single-channel GPR systems

Three manufacturers dominate the worldwide market for commercial single-channel GPR systems: GSSI, Sensors and Software, Inc., and Malå Geoscience. GSSI introduced the first commercial GPR system in 1974; advances over the last 40 years have included improvements in electronics and antenna efficiency, better on-screen displays and control software, and more advanced signal processing. One of the most important improvements was the incorporation of shielding on top of the antennas to ensure that most of the EM energy is radiated into the ground. Introduction of push-cart-mounted GPR systems – first done by the Canadian company Sensors and Software in 2000 – improved logistics dramatically for operation on city streets. Figure 12.10 shows a typical pushcart system.

GPRs are classified as ultra-wideband (UWB) radar systems. According to FCC guidelines (FCC, 2002), UWB refers to any radio technology whose frequency bandwidth exceeds either 500 MHz or a value equal to 20% of the system's center frequency. By comparison, air traffic control radar, which operates at 1030 MHz, has a bandwidth of only a few percent. The top right panel of Fig. 12.6 shows an idealized transmitted pulse and spectrum of an UWB GPR with a central frequency of 200 MHz. Modern systems are available at center frequencies ranging from about 100 MHz to 3 GHz. A single radar control unit can usually be used over the full range of center frequencies, but antennas of the right size and shape must be used in different frequency ranges. Some systems have integrated cross-polarized antennas, where each antenna package contains dipole antennas in two orthogonal orientations. Full system specifications, along with applications and case histories, can be found on the manufacturer websites.

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