GB2146186A - Apparatus for electrically heating a metallic workpiece - Google Patents

Abstract

The apparatus is primarily for use in contact welding and induction heating applications and has a pair of bus bars 10, 12 for connecting a low impedance load (typically 0.01 ohms) to a high power high frequency alternating current generator. For impedance matching purposes, the generator has a step-down transformer for connection to an input end of the bus bars, and parallel capacitance 20A to 20F is connected across the bus bars 10, 12 adjacent the load to allow transformation of the resistive component of the load to a higher value of effective shunt resistance using the inherent inductance of the bus bars and the load. A variable inductance element 40 may be coupled in series in bus bar 12 between the capacitance and the load to achieve optimum matching with different loads. The inductance is varied by means of a fluid-cooled, member 46 slidable between plates 42, 44 projecting from bus bar 12. In a further embodiment, two or more loads can be connected to a single transformer coupled generator output via respective pairs of bus bars, each pair of bus bars having a shunt capacitance and a variable series inductance as mentioned above (Fig. 7). <IMAGE>

Description

SPECIFICATION Apparatus for electrically heating a metallic work material This invention relates to alternating current apparatus for electrically heating a metallic work material, and primarily to contact welding and induction heating apparatus.

Typically the resistive component of the load constituted by the work material in a contact welding system is in the order of 0.01 ohm. In order to generate sufficient heat in the work material, a large current must be generated not only because of the low resistance, but also as a result of the high Q (ratio of VA to power) caused by the inductance of the leads from the power supply to the work material.

High power high frequency generators used as power supplies in this field are inherently high impedance devices using valves in their output stages, and so conventionally a transformer, generally air-cored, is used to transform the power to the load impedance. An air-cored transformer has a very low coupling factor compared with a typical iron-cored transformer, which means that the VA transfer efficiency is low, typically between 20% and 40% when matched into an optimum load.

The transformer can be fitted either in the generator or at the work position. The electrical circuits of these two arrangements are shown in Figs. 1 and 2 respectively. Referring to Fig. 1, if a transformer T is fitted in a generator G, and it transforms directly to a very low load impedance Z,, there is a large loss of both VA and power in the feed bus bars to the load due to the high secondary current required. Taking the second alternative, if the transformer T is fitted at the work position, as shown in Fig. 2, there is still considerable VA loss in the primary bus bars, which are now more inductive since they have to be well separated to allow for the high primary voltage. With the transformer at the work position, these high voltages are also brought close to the operator.A further disadvantage is that the transformer at the work position has fixed couplings, so it is very inflexible and can only work into a small range of impedances. Both arrangements suffer from the drawback that even a single turn secondary coil has a large inductance compared with the load, due to the physical size of the transformer needed to carry the power, and this further reduces the VA transfer efficiency.

Calculations of the loaded Q and the required power input from the generator for typical arrangements according to the circuits of Figs. 1 and 2 are set out hereinafter in parts (i) and (ii) respectively of the Appendix. This yields typical loaded Q values for Figs. 1 and 2 of approximately 1 35 and 40, with required VA inputs to the transformer of 27 MVA and 8 MVA.

It is an object of this invention to provide an arrangement of higher efficiency which largely overcomes the disadvantages of the two arrangements described above.

According to this invention, the series inductance of the work is used to transform the resistive component of the load to a higher value of effective shunt resistance. By tuning out the effective shunt inductance with a shunt capacitor or capacitors, the effective load has a higher resistance which can be relatively easily matched into a typical generator output circuit. The required series inductance may be achieved by adding inductance to the load, so that together with the bus bar inductance an optimum load impedance for the generator can be provided.

Best results are obtained if a plurality of shunt tuning capacitors are connected across the bus bars at spaced locations adjacent the load. This arrangement acts as a low impedance distributed transmission line, reducing the risk of unwanted high frequency resonances. This technique can be applied to induction heating apparatus as well as to contact welding apparatus.

In addition to the advantage of improved matching and hence improved efficiency, the requirement for series inductance allows the distance between the work position and the generator to be increased compared with prior art arrangements, without a serious reduction in efficiency. As the bus bars are increased in length, the added series inductance can be decreased to the point where all of the required inductance is provided by the inherent inductance of the output bus bars.

The series inductance of the arrangement has the effect of improving isolation between the generator 'tank' circuit and the load circuit, largely preventing 'mode jumping' of the generator from the 'tank' resonant frequency to the load resonant frequency.

The invention will now be described by way of example with reference to Figs. 3 to 7 of the drawings in which: Figure 3 is a circuit diagram of an improved matching arrangement in accordance with the invention; Figure 4 is a perspective view of a pair of bus bars and impedance matching components at the work station end of contact welding apparatus in accordance with the invention; Figure 5 is a perspective view similar to that of Fig. 4, showing a pair of bus bars with an additional matching component in the form of a variable inductance device; Figure 6 is a perspective view similar to that of Fig. 5, showing the work station end of induction heating apparatus in accordance with the invention; and Figure 7 is a circuit diagram of a further arrangement in accordance with the invention in which two low impedance loads are connected in parallel to a single generator.

Referring to Fig. 3 of the drawings, in preferred embodiments of contact welding or induction heating apparatus in accordance with the invention, a generator G, having an output frequency of 40GkHz (only the output stage of generator G is shown in Fig. 3), has an air-cored output transformer T with variable coupling KT. Bus bars B, having an effective series inductance LB, connect the transformer secondary coil to a load circuit comprising a load having a resistive component RLs and an inductive component LLs, an added series inductance LTs (preferably variable), and a shunt tuning capacitance CT.

The approximate efficiency of the arrangement can be calculated as shown in part (iii) of the Appendix, yielding VA and Q values of 2.7MVA and 1 3.7 respectively with bus bars of ten times the length used for the earlier calculations. Compared with the loaded Q requirements of the prior art arrangement of Figs. 1 and 2, the loaded Q value for the arrangement of Fig. 3 is very low, resulting in relatively low losses in the tank circuit of the generator, and giving improved power efficiency to the system as a whole. With a typical tank circuit Q of 20 to 25, the required output power can be obtained with reduced coupling KT in the variable output transformer T.

Corresponding tank circuit Q figures for the prior art circuits of Figs. 1 and 2 are 1 35 and 39 respectively, even without the penalty of the tenfold increase in bus bar length.

In the preferred embodiment of the invention, both the effective bus bar inductance LB and the series inductance LTs are variable, these being set during installation of the apparatus to suit the length of the bus bars, so that the generator G is working into its optimum load for maximum VA transfer to the load. LB can be varied by the use of a tapped inductor, but since LTs has a relatively low inductance, a variable length shorted series stub is preferred, such as the screw adjustable water cooled device disclosed in our co-pending application filed with this application. The disclosure of the co-pending application is specifically incorporated in this application by reference.

The physical construction of the bus bars and matching components corresponding to the circuit discussed above will now be described with reference to Figs. 4 to 6. At the work station end of the apparatus, the bus bars are formed as a pair of spaced back-to-back copper channel sections 10 and 1 2 shaped and arranged for low inherent inductance and resistance combined with an adequate voltage breakdown capability. The sections 10 and 1 2 are typically 760mm in length (only portions thereof are shown in Figs. 4 to 6) and are connected to the transformer secondary by a pair of heavy cables and/or narrower bus bars, the need for low bus bar inductance being less as the distance from the load increases.

In the contact welding arrangement of Fig. 4, the bus bars terminate at the work station end in a pair of detachable tapered nose pieces 14 and 1 6 each having a spring loaded welding contact 18 for engaging respective parts of work material to be welded together. Referring to Fig. 4, the inductance of the end portions of the bus bars adjacent the load and the inductance of the load itself is tuned out by the parallel capacitance CT of six capacitors 20A, B, C, D, E, F mounted on the outwardly facing surfaces of the channel sections 10 and 1 2. Each capacitor is a low inductance, low loss, high voltage mica plate component comprising a stack of capacitor plates of which half are connected to a respective inner terminal 32 mounted directly on the outer surface of one channel section and half are coupled via a respective conductor 34 bridging the gap between the sections 10 and 12 and joined to the side of the other channel section. In Fig. 4 only four of the six capacitors are visible, the others being mounted on the channel section 10.By arranging the capacitors side by side in pairs and the pairs located alternately on one channel section and then the other at different distances from the load the bus bars in this region act as a very low impedance distributed line to reduce the risk of unwanted resonances.

The number of capacitors used can be varied very easily, making the whole system extremely flexible and capable of being set up to match a wide variety of loads and applications.

In the embodiment of Fig. 4, the inductance of the end portions of the channel sections 10 and 1 2 and the nose pieces 14 and 1 6 is fixed. To provide greater flexibility, a variable inductance device 40 may be fitted between the capacitors 20A-F and the nose pieces 14 and 16, as shown in Fig. 5. Effectively, one of the bus bars is broken between the capacitors and its nose piece and an extra length inserted across the break in the form of a shorted stub to add inductance. This can be achieved by making a right angle bend in one of the channel sections i2 to form a first outwardly projecting plate 42 and attaching a parallel channel section to the nose piece 1 6 to form a lower plate 44. A conductor bar 46 connects the two plates and is slidable towards or away from the bus bars 10, 1 2 to decrease or increase the series inductance. The bar 46 is hollow, having an interior space coupled to pipes 48 for cooling fluid, as described in more detail in the above mentioned co-pending application.

The inductance added by the stub corresponds to at least a part of the variable inductance LTs in Fig. 3.

A second important application of the invention is in induction heating apparatus. Referring to Fig. 6, the construction of the bus bars for this application is similar to that shown in Fig. 5 for contact welding with the exception that the spring loaded contacts 1 8 are replaced by a heavy gauge work coil 50 bolted or otherwise attached to the nose pieces 14 and 16. A typical induction heating operation for which the apparatus of Fig. 6 is suitable is the continuous welding of tubes produced from sheet material. A steel strip 52 preformed into a tubular configuration is continuously passed through the work coil 50 with its edges abutting each other so that the heat generated in the strip 52 causes the edges to be welded together.

The extent to which the power transfer efficiency can be improved using this invention and the consequent surplus power are such that it has become possible to drive a wide variety of loads from one generator, particularly with the facility of adjustable tuning of the load. For example, if, in a contact welding arrangement, the work material is changed, giving a different load resistance value, the transformer coupling KT (and hence the VA) can be re-adjusted to give the required power with a single knob servo driven control. If the welding contacts 14, 1 6 are moved to produce different heat patterns, or if a work coil is changed, the change in inductance is compensated for by re-adjusting the series inductance Lots, again with a single knob control.

Major changes in work VA can be catered for by fitting a different number of shunt capacitors to the distributed line shown in Figs. 4 to 6, and because of the isolation of the work from the generator output a single generator can be used to drive two different loads, as shown in Fig. 7, and the ratio of power delivered to each load adjusted by means of adjustable series inductors LB1 and LB2 in the bus bars, with fine adjustment carried out by adjusting LTS1 and LTS2.

Features not shown in the drawings include an optional HF voltage detection and trip circuit which may be connected across the bus bars, effectively across CT, to switch off the generator output if the detected voltage rises beyond a predetermined maximum permitted value, as may happen for example if the generator power is set too high or if the inductors are incorrectly set so as to result in severe mismatch. The detector can also be used to provide a meter indication to assist matching adjustments.

The variable coupling tranformer T allows the output VA and power to be varied without the use of chopping techniques normally used to vary the high tension supply to the oscillator valve V. Chopping requires considerable filtering to reduce the low frequency ripple on the output to an acceptable level-both for the welding or induction heating process and for operator safety.

Apparatus in accordance with this invention has particular application to contact welding as well as to single turn or other low impedance work coils used in induction heating and brazing applications, but it also has application in other high frequency low impedance power systems involving high currents.

APPENDIX (i) Analysis of circuit of Fig. 1 (Prior Art) Assuming typical circuit element values as follows:load resistance (series) RLs = 8 mE2 Load inductance (series) LLs = 0.03 yH Generator frequency f = 400 kHz Bus bar inductance (3 metres) LB = 0.1 yH Transformer KVA transfer efficiency KT = 30% (0.3) Output power P, = 200 kW The load current, l, is given by

Load inductive impedance, XLs = + j27fL,s = + j2m (400 X 10,) X (0.03 X 10-6' "1 = + j0.075Sl As this is high compared to resistance RLs, the load impedance is approximately equal to the inductive impedance of the load.Hence: Load voltage, V, = I, X XLS = 5000 x 0.75 V = 375 V Note: VA = 375 x 5000 VA = 1875 KVA (in load) VA 1875 and the load Q, Q, (= -----) =9.38 power 200 Bus bar inductive impedance, XLB = +j2#fLB = + j2# (400 x 10-3) (0.1 X 10-6) 2 = + j 0.25 # Generator transformer load impedance = + j 0.25 # + j 0.075 # = + j 0.325 # Generator transformer secondary current (= I,) = 5000 A Generator transformer secondary voltage, Vs = 5000 x 0.325 V = 1625 V Generator transformer secondary VA= 1625 X 5000 V = 8125 KVA VA 8125 Note:Output Q= = = 40.6 power 200 Since KT= 30%, the required tank circuit VA is given by: 8125 VA = KVA = 27 MVA 0.3 Thus, tank circuit loaded 0 is given by 27 x 106 Q= =135 200 x 103 (ii) Analysis of circuit of Fig. 2 (Prior Art) It is assumed that the values of RLS, LLS, f, KT, and P, are the same as for the circuit of Fig. 1.

Since the bus bar spacing is increased, assume bus bar inductance (3 metres), LB = 1 H As before, l,= 5000 A and V,= 375 V As before, load VA = 1875 KVA Thus, the VA input to the transformer primary is: 1875 KVA = 6.25 MVA 0.3 If the primary voltage, Vp = 8 kV rms, the primary current, 6250 Ip= A=781 A 8 Transformer primary impedance 8000 = +j #= + j 10.24 # 781 Transformer primary inductance 10.24 = H = 4.07 yH 2 #f Bus bar inductance = 1 H Voltage on tank capacitors in generator is given by:: (4.07 + 1) VT= x 8000 V 4.07 = 9966 V rms # Tank circuit VA = 9966 x 781 VA = 7.78 MVA Thus, the tank circuit loaded Q is given by: 7.78 x 106 Q= = 38.9 200 X 103 (iii) Analysis of circuit of Fig. 3 Assume the same values as before for RLS, LLS, f, KT and PL, but increase the length of the bus bars to 30 metres and add fixed inductance if necessary so that the effective bus bar inductance, LB = 2 H. Also, let the added series inductance, LTS = 0.01 yH and the added tuning capacitance, CT = 4 yF. Taking these values, the total series load inductance is given by: : L = 0.04 yH giving a series load reactance Xs= + j2srfL = + j 0.1 # Transformed effective shunt resistance, Xs2 0.12 Rp = = # = 1.26 # Rs 0.008 Transformed effective shunt impedance # + j 0.1 # (this is tuned out by the shunt capacitance, CT) Bus Bar current

Bus Bar inductive impedance, XB= + j27rfLB = + j2m (400 X 103) (2 X 10-6) = +j 5.03 u Generator output transformer load impedance

= 5.19 A Hence generator output voltage Vs = 5.1 9 X 398 V = 2063 V and generator output VA = 2063 x 398 VA =821 KVA VA 821 Generator output Q = ----- = -- Power 200 = 4.1 VA 821x103 Required tank circuit VA = - = VA== KT 0.3 = 2.7 MVA Tank VA 2700 and tank circuit loaded Q = Power 200 = 13.7

Claims (22)

1. Alternating current contact welding apparatus comprising: an alternating current generator including an oscillator and a step-down output transformer, a pair of elongate bus bars connected at one end across a secondary winding of the transformer for conducting power from the transformer to a workpiece, means for connecting the other end of the bus bars across the workpiece, at least one capacitor connected across the bus bars in the region of the said other end for tuning out the effective inductance of an end portion of the bus bar pair and the workpiece at the operating frequency of the generator.

2. Apparatus according to claim 1, including a plurality of capacitors connected across the bus bars at spaced apart locations in the region of the said other end.

3. Apparatus according to claim 1, further comprising at least one variable inductance device coupled in series in said end portion.

4. Apparatus according to claim 3, wherein the said at least one variable inductance device comprises a short circuited stub of variable length connected in series in one of the bus bars in said end portion.

5. Apparatus according to claim 1, wherein the pair of bus bars comprises, over at least part of its length in the region of the said other end, a pair of elongate metal strips mounted in a face-to-face spaced apart relationship, and wherein the or each capacitor is mounted on an outwardly facing surface of either of said strips with an inner terminal coupled directly to said surface and an outer terminal coupled to the other strip via a respective conductor bridging the gap between the strips.

6. Apparatus according to claim 5, including one or more pairs of capacitors connected across the strips, wherein said conductors of the or each pair of capacitors are attached to respective opposite sides of said other strip.

7. Apparatus according to claim 2, wherein the pair of bus bars comprises over at least part of the length in the region of the said other end, a pair of elongate metal strips mounted in a face to face spaced apart relationship, and wherein said capacitors are mounted alternately on an outwardly facing surface of one strip or the other at each consecutive location.

8. Alternating current apparatus for electrically heating a metallic work material comprising: an alternating current generator including an oscillator with a transformer-coupled output, a pair of bus bars having an input end connected to the output of the generator and an output end having means for connection to a load of less than 0.5 ohm, capacitance means coupled between the bus bars in the region of said output end.

9. Apparatus according to claim 8, wherein said means for connection comprises a pair of contacts mounted on the output end of the pair of bus bars for engaging work material components to be welded together on application of alternating current power to said bus bars.

10. Apparatus according to claim 8 including an electrical load mounted on the output end of the pair of bus bars, wherein said load comprises an induction heating coil.

11. Apparatus according to claim 8, wherein the pair of bus bars, over at least a part of its length, comprises a pair of channel sections mounted back to back and terminating at said output end, the capacitance means being mounted on said channel sections.

1 2. Apparatus according to claim 11, further comprising a series variable inductance in the bus bar pair between the capacitance means and the load connection means.

1 3. Apparatus according to claim 11, wherein at least a part of said capacitance means comprises: at least one capacitor stack having an inner terminal mounted on an outwardly facing surface of one of said strips, and an outer terminal extending beyond a side edge of said one strip and bridging the gap between the strips to engage the other of said strips.

14. Apparatus according to claim 13, wherein said capacitance means comprises a plurality of capacitor stacks mounted on the outwardly facing surface of both strips.

1 5. Apparatus according to claim 14, wherein said capacitor stacks are mounted at a plurality of locations at different distances from the outer end of the bus bar pair to form a distributed capacitance between the bus bars.

1 6. Alternating current apparatus for electrically treating metallic work material, the apparatus comprising: an alternating current generator including an oscillator and a step-down transformer, at least two output circuits coupled to the transformer, each circuit comprising: a pair of bus bars having an input end coupled to the transformer and an output end for connection to a respective low impedance load, and capacitance means coupled between the bus bars in the region of the output end.

17. Apparatus according to claim 16, wherein each output circuit includes a series connected adjustable inductance device in said bus bar pair between the capacitance means and the output end.

1 8. A bus bar assembly for connection to a low impedance load in apparatus for the electrical treatment of a metallic work material, the assembly comprising a pair of elongate metal strips mounted in a face to face spaced apart relationship, a plurality of capacitors connected in parallel between the strips, each capacitor being mounted on an outwardly facing surface of one of said strips and having a conductor bridging the gap between the strip and coupled to the other of said strips, and load connection means at adjacent ends of the strips.

1 9. A bus bar assembly according to claim 18, further including a variable inductance device coupled in series in one of the bus bars between the capacitors and the load connection means, said device comprising a pair of plates extending outwardly in a parallel spaced apart relationship from a break in one of the metal strips, and an adjustable position shorting element located between the plates.

20. Contact welding apparatus constructed and arranged substantially as herein described and shown in the drawings.

21. Alternating current apparatus for electrically treating a metallic work material, the apparatus being constructed and arranged substantially as herein described and shown in the drawings.

22. A bus bar assembly constructed and arranged substantially as herein described and shown in the drawings.