OPERATION OF INVERTERS SUPPLYING MUTUALLY COUPLED'INDUCTION HEATING LOADS
Induction heating of thin strip The heating of thin strip metals using induction heating techniques presents a number of problems. The poor coupling between solenoidal type coils and the very narrow cross section of strip has been much improved by using transverse flux coil systems [ l],as shown in Fig. 1.
One problem which remained unresolved until recently was the difficulty of obtaining an even temperature profile across the width of the strip. Strip is usuadly fed through a heating coil on a roller system, entering the acl.ive section of the coil, cold, and exiting hot a few seconds later. Unlike billet heating there is no soaking period associated with strip heating therefore a uniform temperature profile can only be achieved with uniform heating. A novel method of induction heating was recently proposed which allows closer control of the heating profile within the workpiece . The ,system,known as Complementary Profile Heating (CPH) use a number of separate inverter-fed heating coils to supply the metallic strip. Each coil establishes a heating profile (using the principle of transverse flux) which is complementary to the others. In order to alleviate the problems of electromagnetic forces acting on the thin strip the frequencies of the currents supplied to each heating coil are significantly different from those of the other coils. One disadvantage of the system is that a parasitic mutual inductance exists between each of the induction coils and this affects the operation of the inverters.
I Coil conductors Figure 1 Transverse flux arrangement for heating thin strip metals Medium frequency power supplies for induction heating The immediate application for CPH is the heating of strip metals and it is envisaged that pow-r 1 vels ould __ be several MW, with an operating frequency of around 3kHz. One of the most common types of inverter used for such applications is the current-fed load-commutated inverter shown in Fig. 2. A.M. Green is with UMIST, Manchester
0 1996 The Institution of Electrical Engineers. Printed and published by the IEE, Savoy Place, London WCPR OBL, UK.
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Figure 2 Current-fed load-commutated inverter The most simple example of CPH is shown schematically in Fig. 3. Two electrically isolated heating coils, fed by separate inverters operating at different frequencies, are us.ed to set up two complementary heating profiles within the strip. The effect of the parasitic mutual inductance between the two coils is to cause voltages to be induced into each heating coil, the frequencies of which differ from those of the inverter which supplies the coil. This results in a modulation of the coil voltage, at the beat frequency of the two systems. ,
It was thought that the load-commutated type of inverter would be particularly sensitive to the mutually induced voltages because it relies on the load voltage waveform to achieve commutation of the thyristors. It would, however, be desirable to be able to use any type of inverter in a CPH system in order that metal processing companies could use existing power supplies.
Inverter 1 &
Inverter 2 & /compensation
Figure 3 Equivalent circuit of simple CPH system
Simulation and experimental results In order to investigate the operation of inverters connected in this manner models were created of two commercial inverters, using the computer package, PSPICE. The inverters were significantly different in their design, one being a 450kW load-commutated current-fed inverter and the other a 300kW selfcommutated hybrid arrangement. Both units were capable of operating at around 3kHz. The simulations predicted that the load commutated inverter would fail to commutate with quite small values of parasitic coupling. The information was used to design the experimental circuit, illustrated in Fig. 4.
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TO SELFCOMMUTATED INVERTER R
The two inverters were used to supply separate induction heating loads;. The effect of the parasitic mutual coupling was provided by two additional coils. The amount of coupling was varied by changing the reluctance of the flux path between the cloils.
The results of the simulation and the experiment are shown in Figs. 5 and 6 reslpectively.
Figure 4 Equivalent circuit of experimental rig
Figure 5 PSPICE simulation of loadcommutated inverter failure.
Figure 6 Memured waveforms of loadcommutated inverter failure.
The predictions of the PSPICE simulation show close agreement with the experimental results. The loadcommutated inverter failed due to short turn off time for the thyristors. Although the self-commutated inverter continued to operate the induced voltages caused increased stress on the semiconductor switches and capacitors. It was concluded that it is not possible to simply connect standard inverters to the coils of a CPH system unless the parasitic mutual coupling is very small. The value of this coupling is dependent on the application. An anti-phase injection scheme has been proposed as a solution to this problem and is currently under development .
References 1. Ireson, R.C.J., Induction heating with transverse flux in strip-metal process lines, Power Engineering Journal, March 1989. 2. EA Technology LTD,1993, Int.Pub.No WO 93/11650, (Patent). 3. Green A.M. and Williamson A.C., The elimination of interference voltages from mutually coupled induction heating coils, Proceedings of UPEC 96, 18 - 20 September 1995, Vol. 3, pgs 1073 - 1076.
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Medium frequency power supplies for induction heating The immediate application for CPH is the heating of strip metals and it is envisaged t...