high frequency eddy current heating pdf

high frequency eddy current heating pdf
November 1, 2020

Both of these factors contribute to a reduction in the real power throughput. However, it is observed that higher the frequency employed in induction heating, the greater will be the tendency of the induced heating currents to remain at the surface of the material being heated. The vertical purple line marks the other end of the capacitive load region, where the inverter load current transitions again to lagging "Inductive" load current. (vi) Unskilled labour can also operate the equipment. This is undesirable for many solid-state inverters as the leading load current causes loss of ZVS and leads to forced reverse-recovery of free-wheeling diodes incurring raised switching losses and voltage overshoots. Introduction The conductive strip grating is a periodic planar arrangement of conductive strips with a long length and small thickness. Therefore it is useful to combine all of these losses into a single "loss resistance."

But the capacitive part is in parallel with the work coil's own tank capacitor, and in practice these are usually one and the same part. Other applications are welding, drying of paints, melting of precious metals, sterilization of surgical instruments and forging of bolt-heads and rivet heads. Finally the transformer design should be optimised for minimum inter-winding capacitance and good insulation at the expense of increased leakage inductance. Combing these two capacitances into one leads us to arrive at the LCLR model for the work coil arrangement, which is commonly used in industry for induction heating. The GREEN traces represent the current passing through the matching inductor, and therefore the load current seen by the inverter. In addition to this, the transformer must be cooled to remove excess heat generated by the high currents flowing in its conductors. 1 Recent advances in eddy current testing that are responsible for its enhanced use by the industry. Coating Characterization Using Eddy The lack of high frequency harmonics reduces heating in the transformer due to skin effect and proximity effect within the conductors. Other applications include melting, welding and brazing or metals.
Such a situation is ideal for supporting ZVS soft-switching in the inverter and preventing free-wheel diode reverse-recovery problems. In high power designs it is common to use a full-bridge (H-bridge) of 4 or more switching devices. When a lossy workpiece is inserted into the work coil, this damps the parallel resonant circuit by introducing a further loss into the system. Altering the inductance of the matching inductor adjusts the value to which the load impedance is translated. However, it should be realised that proper control of the inverter operating frequency should ensure that it tracks the resonant frequency of the tank circuit. At this frequency current through the work coil is also maximum and the largest heating effect is developed at this frequency. (iii) It can easily take place in vacuum or other special atmosphere, where as other conventional types of heating are not possible in such places. The squarewave voltage generated by most half-bridge and full-bridge circuits is rich in high frequency harmonics as well as the wanted fundamental frequency. Another advantage of the LCLR work coil arrangement is that it does not require a high-frequency transformer to provide the impedance matching function. Finally some control electronics is usually employed to control the intensity of the heating action, and time the heating cycle to ensure consistent results. We can think of the tank circuit incorporating the work coil (Lw) and its capacitor (Cw) as a parallel resonant circuit. It is fed from a smoothed DC supply with decoupling capacitor across the rails to support the AC current demands of the inverter. See point 6 in the section below about "Power control methods" for further details.). Keywords: high frequency eddy current testing, evanescent waves, metamaterials, flexible printed circuit boards, carbon fiber reinforced plastics 1. This property of the parallel resonant circuit can make a tenfold reduction in the current that must be supported by the inverter and the wires connecting it to the work coil. Therefore the current drawn by the parallel resonant tank circuit increases when a workpiece is entered into the coil. It is sized sufficiently large that it does not take part in the impedance matching, and does not adversely effect the operation of the LCLR work coil arrangement. In practice the resistance of the work coil, the resistance of the tank capacitor, and the reflected resistance of the workpiece all introduce a loss into the tank circuit and damp the resonance. However, in this case it only has to carry the part of the load current that actually does real work. This method of controlling power level by detuning is very simple since most induction heaters already have control over the operating frequency of the inverter in order to cater for different workpieces and work coils.

Firstly there is more resonant rise in the LCLR network when there is no workpiece present. Calculation of Power & Frequency Requiremen, Work-coil acts like primary of transformer and generates, Workpiece acts like single turn shorted secondary an, Induction heating has two mechanisms of energ, Sole mechanism of heat generation in Non-magnetic ma, Primary mechanism in ferro-magnetic materials b, Induction Coil design as per the heating req, Capacitor to correct Coil Inductive Reactance and get uni, Inverter for frequency control – typically square wave voltage, sine, Rectifier for power / voltage level control, When a lossy work-piece Rs is inserted in, Inverters for parallel tanks are Current Fe, Current in the semiconductors Q times low, Stray Lead inductance becomes part of Ls e, In case of operation in the Capacitive Current re, Heavy commutation losses with high commutating currents due to, De-tuned to operate in the inductive region, Current lags in phase and diminishes in amplitude, Lagging power factor ensures that devices turn on with zero voltage, However de-tuning on the inductive side means operating at higher, Increases reactive power drawn from the inverter, Inverter commutating close to resonance frequency so commutation current and losses, Care to be taken to avoid saturation of matching transformer, Induction Cook-top needs ferromagnetic vessels to, For lower losses in the coil and optimum use of Copper – the coil is made of Litz wire, Considering the voltage across resistance is the first harmonic, Thus power delivered to the workpiece can be controlled by, Matching of Coil+Load with Power Supply, So heating depth d is low below Cuire and high above Curie T, For Ferro-magnetic materials, the Control Circuit needs to sense the operating, Efficient as heat is generated inside the workpiec, Speed, lack of interference, upto 3000 deg C, Invisible part of the spectrum contains more energy, only active power transferred from primary, Decrease current through the semiconductors, Used for low impedance inductors – capacitance and capacitor, Coil inductance is matched by Capacitor at resonant frequency to, Current drawn from the Power Source = 10000/11 =, Addition of Capacitor in load circuit to achieve unity power factor, Placing the Capacitor nearest to the work-coil reduces the, Placing the Capacitor in the Primary of the transformer - reduces capac, Capacitors in the tank circuit must carry 100s ~ 1000s of Am, Operation at high frequencies causes losses due to di-electric heati, Journal of the Japan Society of Powder and Powder Metallurgy. This allows us to get a feel for how the network behaves with a large lossy workpiece to having no workpiece present at all, and all loads in between. In addition to this, the high frequency used in induction heating applications gives rise to a phenomenon called skin effect. The power factor seen by the inverter always remains good because the inverter is not detuned from the resonant frequency of the work coil, therefore reactive current flow through free-wheeling diodes is minimised. This significantly reduces turn-on switching losses due to device output capacitance in MOSFETs operated at high voltages. Inside a transformer the passage of eddy currents is highly undesirable because it causes heating of the magnetic core and represents power that is wasted.). Yet another common application for induction heating is a process called Zone purification used in the semiconductor manufacturing industry. The matching network is used to make the tank circuit appear as a more suitable load to the inverter, and its derivation is discussed in the section above. Therefore the leading power factor condition should ideally not arise, and should certainly not persist for any length of time. The load current is then left to freewheel through the devices body diodes during the deadtime when both devices are turned off. Correct choice of dielectrics and extended foil construction techniques are used to minimise the amount of heat generated and keep effective-series-inductance to a minimum. Induced eddy current is of greatest magnitude at the surface of the material to be heated and its value decreases with the distance normal to the surface as we go inside the material. It also increases with increasing graphite, Induction heating is widely used in various industrial processes. This makes heating steel above 700°C more of a challenge for the induction heating systems. The comparatively low currents make the inverter less sensitive to layout issues and stray inductance. The requirements for capacitors used in high power induction heating are perhaps the most demanding of any type of capacitor. Firstly, locating the transformer in this position means that the impedances at both windings are relatively high.

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