Design and Implementation of a Low Losses Current-Fed Portable Induction Furnace

: This research is concentrated on the practical design of a portable Induction Heating Furnace (IHF) composed of a Current-Fed Parallel Resonance Inverter (CFPRI) as a power supply of an Induction Coil (IC) and its specimen. The design is intended to reduce the losses in both the switches of the inverter and those due to non-sinusoidal currents in the conductors connecting the power supply components. The IC was connected in parallel to a suitable capacitor to form a tank load in this furnace. Detailed design steps for each implemented part are represented in this research. The implemented


1.INTRODUCTION
Induction heating is a popular heat treatment technology in industrial and domestic products due to its higher efficiency, precise control, and lower emission characteristics than other heat treatment methods [1].The most common power supply for induction heating is the CFPRI because the VFPRI switches suffer from high stresses [2-4], while low stress affects the CFPRI switches [5], reliable short-circuit, and overcurrent protection make it more suitable for high power applications.The purpose of this work is to design a reduced harmonic and switching losses power supply for a portable induction heating furnace (IHF).In this type of furnace, a long connector must exist between the IF power supply and the tank circuit load to afford flexibility in field applications.Then this connector must be specially designed to form a low inductance, low resistance, and low power loss since it carries a square wave current characterized by a high harmonic content.The main components of the IHF are shown in Fig. 1.It is composed of a DC power source part as a three-phase FCFWR with its triggering circuit followed by an ( ) smoothing filter.The current fed to the H-bridge inverter forms the DC voltage source, passing through a choke coil to form a current source.This combination represents a current-fed parallel resonant inverter (CFPRI) [6].The load is a parallel tank circuit, including the induction coil with its specimen and the suitable parallel tank capacitor.Due to the dependence of the induction coil equivalent circuit on the temperature variation during the heating process, changing the tank circuit resonance frequency, a self-tuned inverter was designed for this purpose using a feedback arrangement with a phase-locked loop (PLL) [7] tuning circuit.This work designs the triggering circuit of the FCFWR and the triggering and the PLL tuning circuits of the self-tuned H-bridge inverter.The implemented power supply showed approximately very close results to the simulation.The design seeks the reduction of the switching losses by obtaining the zerophase shift between (  ) and (  ), which can be achieved when the inverter switching frequency corresponds to the resonance condition.
Fig. 1 The CFPRI for IHF Type LC Load.

2.CURRENT-FED PARALLEL-RESONANT INVERTER (CFPRI)
The idea of feeding the H-bridge inverter with a direct current source to form the CFPRI is to replace that known as a voltage-fed parallel resonant inverter (VFPRI).The output voltage of the CFPRI was of a sinusoidal waveform, while its current was a square wave.This feature leads to the following (Refer to Fig. 1): (1) Where  represents the quality factor of the tank circuit.Since    is an imaginary current, and   is the real current, the inverter will only feed the real power, which means that the CFPRI is compatible with a high voltage low current DC source.On the contrary, the VFPRI is suitable for low voltage, high current DC sources.Fig. 1 depicts a diode in series with a switch because, if not used, the IGBT parallel diodes would cause the load-reactive current components to reach the DC source [8].
Current-source topologies provide low switching   ⁄ and reliable short-circuit and overcurrent protection [9].The following guidelines are necessary for the current-source inverter to function correctly: The top or bottom switches of the various legs cannot be turned off simultaneously.Since a short circuit across the output voltage   might result, each switch must have a diode connected in series unless the circuit will be damaged.In order to apply the theory practically, an overlapping time must be considered in the control signals of the top or bottom switches of the various legs.Due to the tank circuit load shown in Fig. 2  The relationship between the rms value of the fundamental component of the square wave output current of inverter (  ) at resonance condition as a function of the inputdirect current fed to the inverter (  ) is [11]: Since the real power dissipated in the equivalent resistance of the induction coil (  ) due to tank circuit imaginary current passing in the induction coil �   � is: The following features apply to current-fed inverters (CFIs) [12]: 1-For the commutation process, overlap time is necessary; then the switching stages of the inverter are as follows [13]: a) SW1 and SW4 were ON during the positive half cycle.
b) SW1, SW2, SW3, and SW4 were ON, during the overlap time.c) SW2 and SW3 were ON during the negative half cycle.d) SW1, SW2, SW3, and SW4 were ON, during the overlap time.2-In the event of a short circuit, all semiconductors were required to conduct.3-ZVS occurred at the resonance frequency.4-ZCS occurred above the resonance frequency.5-Transistors must be capable of handling high voltage.Parasitic inductance (  ), due to the parasitic inductances of the switches, diodes, and connectors, is a source of high voltage stress across the switches which might cause the degradation of semiconductors.The parasitic inductances also influence the switching process when the inverter's commutation is capacitive.Still, when   is considered, the voltage at the inductance's terminals opposes the voltage on the tank.Hence, commutating at capacitive switching offers a more dependable inverter operation when considering   .Capacitive switching is favored in CFPRI, even if the reverse recovery currents on diodes are still an issue.A detailed analysis is in references [8,14] about the quality factor () effect on the suitable switching frequency for perfect ZCS and ZVS conditions.This analysis reveals that for a high () tank circuit, the resonance frequency and the zero-phase shift frequency will be very close.It is preferable to choose the switching frequency in the capacitive region to avoid the severe effect of   on the switches' safety and reduce the switching losses during the heating process.

THE PLECS SIMULATION AND RESULTS
In this study, the power supply of the IHF feeding linear specimen load is of a high-quality factor ().Fig. 3 depicts the simulation circuit diagram for the IHF.The system was powered by a mains transformer of 1 MVA, i.e., 3-phase, 380 V, and 50 Hz, with secondary equivalent resistance and inductance of 612.5 μΩ and 12.5 μH, respectively.By passing its output voltage via an L-C smoothing low-pass filter with   = 2.9 mH and   = 13200 μF, the FCFWR rectifies this AC voltage.In this design, a chock coil  ℎ  = 50 mH acted as a current source, simulating a low-ripple variable DC voltage power source.According to Eq. ( 12), the tank circuit's induction coil   =3 μH, tank capacitor   = 10 μF, and the resonance frequency (28.57kHz) are required.
To account for the loads, the equivalent load resistance (  ) was modified to (0.1Ω).For each case, three different applied terminal voltages were assessed by changing the FCFWR's triggering angle to (42.74 °, 60 °, and 75 °).Show the system currents and their harmonics for the input current of phase A, the direct current of the filter, the capacitor filter current, the input current fed to the inverter, the output current from the inverter feeding the tank circuit, and the induction coil current.These figures include all of the harmonics in each current according to the descriptions of load and triggering angles given below.Fig. 19 The Capacitor Current at the Resonance Frequency (  ).

Fig. 20
The CFIHF Power Supply's Harmonics in Each Branch.
Due to the harmonic results abbreviated in Fig. 20, the most severe contents of harmonics were that at the connector carrying the square wave current from the inverter feeding the  tank circuit load.Hence, this connector must be designed to reduce the heat generation within it during the process and reduce the voltage drop across it due to its length in such furnaces.Also, its self-inductance must be as small as possible.Practically, it is noticed that when using a traditional copper connector between the inverter terminals and the tank load, this connector was subjected to a continuous increase in its temperature during the operation time, and it reached an unacceptable range of temperature.To reduce the drop in the applied voltage across its terminals and to avoid the continuous increase in its temperature, the same procedure adopted by the reference results to choose the most suitable Litz's wire gauge from the AWG table, considering that the frequency closest less than or equal to that of the fundamental frequency.(b) From the data given in the AWG table of that chosen wire gauge, the rated power per meter length of that wire (  ) can be calculated by applying the commonly used power equation (  =   2   ), where (  ) in (A) is the rated current of that gauge, and   is its resistance per unit length of that chosen wire.(c) The chosen wire resistance with respect to each harmonic can be found by referring to Fig. 21, which describes the harmonic effective area �   �, and applying the following equations consequently:

Fig. 21
The Harmonic Effective Area.Due to the obtained spectrum of (  ), shown in Fig. 17, represents the highest load current value leading to dissipating 52 kW power in the specimen and requiring input DC power of 57 kW, with a system theoretical efficiency of about 91%.Since the fundamental frequency   = 28.570kHz, the nearest compatible wire gauge was gauge number (20) due to the AWG with the following data, as shown in Table 1.To calculate the ability of the rated power dissipated along the unit length of Litz's wire of gauge ( 20) �  20 �, the following formula can be used: ∴   20 = (11) 2 × 0.033292 = 4.028332 W Then using Eqs.( 16)-( 19) to calculate the    for each harmonic in Ohm per unit length of the connector using Litz's wire of gauge (20).To calculate the total drop in voltage per unit connector length for each harmonic current �   �, the following formula can be used: Also, the power dissipated due to the harmonic current in the unit length of the connector can be calculated from These data are represented in Table 2.The total drop in the voltage along the unit length of the conductor due to the total harmonics (VTnrdrop) is: ∴    = 16.663V • m −1 Similarly, the total dissipated power due to harmonic currents per unit length of the conductor �   � will be: ∴    = 893.692W • m −1 Hence, the total number of Litz's wires of gauge (20) required for this connector ( 20 ) is: To reduce the parasitic inductance of this group of 222 Litz's wires, they must be divided into two groups of 111 wires, and every two wires of each group twisted together, such that the twisting of the first group must be in CW sense, while that of the other group must be in CCW sense.This arrangement will reduce the stray inductance of this connector drastically.Hence, the connector will be quite suitable when connected to the CFPRI induction heating furnace.To get a smooth DC voltage, the rectified voltage was applied to an  filter, as shown in Fig. 24.The filter was composed of a 2 mH coil designed by [18], but by reducing its airgap, its self-inductance reached a value 2.9 mH.The capacitor of 800 V and 13200 μF was used to obtain an acceptable smoothing of the output DC voltage in a high triggering angle.
Fig. 3 displays the simulated circuit diagram created by the Plecs computer package.When �  = 0.1 Ω� and ( = 42.74°) was the triggering angle, the system voltages were expressed as follows: Accordingly, Figs.4-7.show the input voltage of phase A, the voltage across the chock coil, the DC voltage supplied to the inverter terminals, and the output voltage of the inverter.The cause of this disturbance in the input voltage occurred due to the nonsinusoidal input current feeding the controlled rectifier shown in Fig. 8.The four sharping edges (up and down) in the current waveform caused four disturbances in the voltage waveform since { = (  ⁄ )}.Figs.8-19.

Fig. 3
Fig. 3 Simulation Circuit of the IHF Using Plecs Software.

Fig. 14
Fig. 14 The Harmonic Current of the Fundamental Frequency of (300 Hz) of the Current that Flows to the Capacitor Filter at   = 0.1 Ω and  = 42.74°, 60 °, and 75 °.

Fig. 15
Fig. 15 The Waveform of the Inverter's Input Current.

Fig. 18
Fig. 18 The Current Flowing Through an InductionCoil at the Resonance Frequency (  ).
[15]  was applied, as shown below: (a) Determine the severest amplitude of the fundamental harmonic shown in the above   () Wire length in (m).  : Electrical resistivity of copper = (17.2359244× 10 −9 )in (Ω • m).  : The radius of the chosen copper conductor (  =   ) for the fundamental frequency.It was considered an external radius with respect to the other highest harmonics.   ∶The internal radius of the harmonic effective area (m).  : The frequency of the harmonic of order () (Hz).   : The copper relative permeability =(0.9793025) .

=
. ≅  Wires The designed cable will dissipate   20 W per unit length and cause a drop in voltage for each meter of length   20 equals:    =      (26) ∴    = .  •  −

Fig. 22
Fig. 22 Circuit Diagram of the Firing Unit for the 3-∅ FCFWR.4.TRIGGERING CIRCUIT FOR THE 3-∅ FCFWRThe most important part of designing the FCFWR is the triggering circuit.The circuit diagram is shown in Fig.22using the integrated circuit (TCA785)[17].Its data sheet is addressed in Appendix (A).The trigger angle can be changed from 0 ° to 120 ° by variable resistance to feed a variable DC voltage from 513 V to 0 V.The power circuit consists of six thyristor type SEMICRON (SKKT 132), see Appendix (B), and FWD type SEMICRON (SKKD 212), see Appendix (C), as shown in Fig.23representing the implemented circuit.

Fig. 24
Fig. 24 Smoothing LC filter.The LC filter represents a Low pass filter.The cutoff frequency due to Eq. (27) is 25.72 kHz. =  �

Fig
Fig. 25 Circuit Diagram PLL IC 4046.6.PRACTICAL AND SIMULATION RESULTSIn this section, the practical results were compared with the corresponding simulation results.These findings were: The selfinductance of the induction coil   = 2.08 μH, parallel tank capacitor 40 μF, and the switching frequency   =   = 17450 Hz.Fig. 26.

Fig. 31
Fig. 31 The Induction Coil During the Practical Test of the Designed CFPRI.