GB2213332A - Power control for an inductor heating system - Google Patents

Abstract

An induction heating coil 111 is connected in series with a first electronic switching means 224, 225, 226 between first and third lines R,T of a 3-phase A.C. supply RST, a capacitor 113 is connected in series with a second electronic switching means 115, 116 across first and second lines R,S of the supply, an inductor 114 is connected in series with a third electronic switching means 117, 118 across second and third lines S,T of the supply, and to reduce transients at each start up of the heater, the first switching means 224, 225, 226 is rendered conductive after the second and third switching means. Initially one of thyristors 117, 118 is fired at a crest of the voltage between the S and T phases to connect inductor 114. Thyristors 117, 118 are thereafter fired alternately at zero-crossings. After a few cycles of the supply, one of thyristors 115, 116 is fired at a zero crossing of the R and S phases to connect capacitor 113, and thereafter thyristors 115, 116 are fired alternately at zero crossings. After a further few cycles, thyristor 224 is fired at a predetermined non-zero phase angle of the R and T phases to connect the heating coil 111 via a current limiting resistor 219 and then thyristors 225, 226 are fired alternately at zero crossings to energise the coil 111 with a burst of A.C. cycles. The sequence is repeated prior to each such burst. Capacitor 113 may alternatively be connected before inductor 114. <IMAGE>

Description

Power Control for an Inductor Heating System The invention relates to controlling the supply of a three-phase alternating current supplied on three lines to an inductor heating system, for example, the type used in industrial induction heating of molten metal, wherein the inductor heating system includes a first inductor arranged to inductively heat a load connected between the first and third lines of the a.c. supply, together with a Steinmetz balancer system comprising a capacitor connected between the first and second lines of the a.c. supply, and a second inductor connected between the second and third lines of the a.c. supply.

Conventionally, in inductor heater systems including a Steinmetz balancer system the switching of the a.c. supply is controlled by contactors in each'line of the supply. To start such a heating system, the contactors are controlled to switch in the second line, followed by the first and third line. However, the start-up transients associated with this system are severe, even when using soft start contactors, and the average life of a contactor is short. Such transients may also damage other components of the system, and cause problems in maintaining a controlled heating system.

It is an object of the present invention to overcome these problems by reducing the transients associated with the starting up of a conventional inductor heating system including a Steinmetz balancer system.

According to the present invention there is provided an apparatus for controlling the supply of a three-phase alternating current supplied on three lines to an inductor heating system including a first inductor arranqed to inductively heat a load and connected between the first and third lines of the a.c. supply, a capacitor connected between the first and second lines of the a.c. supply, and a second inductor connected between the second and third lines of the a.c. supply, comprising:: a first electronic switching means connected in series with the first inductor between the first and third lines of the a.c. supply, and controllable to start to conduct current at a predetermined phase angle of the first and third lines of the a.c. supply, a second electronic switching means connected in series with the capacitor between the first and second lines of the a.c. supply, and controllable to start to conduct current at a predetermined phase angle of the first and second lines of the a.c. supply, a third electronic switching means connected in series with the second inductor between the second and third lines of the a.c. supply, and controllable to start to conduct current at a predetermined phase angle of the second and third lines of the a.c. supply, and a control means capable of independently controlling each switching means, and arranged to cause conduction of the second and third switching means before conduction of the first switching means.

Preferably, the control means is arranged to cause the second electronic switching means to conduct at a predetermined phase angle corresponding to a substantially zero-crossing point of the first and second lines of the a.c. supply.

Preferably, the control means is arranged to cause the third electronic switching means to conduct at a predetermined phase angle corresponding to a substantially crest voltage point of the second and third lines of the a.c. supply.

Advantageously, the control means is arranged to cause conduction of the third switching means before conduction of the second switching means.

An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings.

Fig. 1 shows a conventional thyristor arrangement for control of a transformer.

Fig. 2 is a timing diagram for the circuit of Fig. 1, showing the thyristor switching associated with a soft start control method.

Fig. 3 shows an alternative thyristor switching arrangement for a transformer.

Fig. 4 shows the thyristor switching timing associated with the arrangement of Fig. 3, together with the magnetising current.

Fig. 5 shows part of an embodiment of the present invention applied to an inductive heater system.

Fig. 6 shows the control circuitry associated with Fig. 5.

Fig. 7 shows a timing control system to provide a conditioning pulse to an inductive load.

Fig. 8 shows the control cycle associated with the thyristor phase angle controller of Fig. 7.

Fig. 9 shows the control pulses sent to the thyristor drive units by the signal conditioner of Fig. 7.

Fig. 10 shows the circuitry of the signal conditioner of Fig. 7.

Fig. 11 shows timing pulses of the thyristor phase angle controller 102 of Fig. 6.

A conventional thyristor arrangement for control of a single-phase transformer 1 comprising two inverse parallel thyristors 2 and 3 is shown in Fig. 1.

In order to reduce the surge current associated with switching on a transformer at the zero crossover point, the thyristors are controlled via signals to their respective gates to switch on initially for only portions of the power cycle of the a.c.

signal supplied to the transformer. This is shown in Fig. 2, where thyristor 3 is switched on at a point 4 during the power cycle and subseuently conducts for the shaded portion before switching off at a zero current point. Similarly, thyristor 2 is switched on at a point 5 of the power cycle and conducts for the following shaded portion before switching off at a zero current point. The phase angle at which the thyristors fire is advanced until full conduction throughout the cycle occurs. The rate of advance of the phase angle at which the thyristors fire can be altered so that, for instance, only thyristor 3 conducts for a portion of the cycle and both thyristors thereafter conduct alternately and fully throughout the cycle.However, this system has the disadvantage that if power factor correction capacitors are connected in the circuit, these capacitors will take a high inrush current if the thyristors are triggered at any point other than a zero crossover when starting.

The system illustrated in Fig. 3 overcomes this problem. This consists of first switching means comprising a pair of inverse parallel thyristors 7, 8 connected as in the conventional arrangement, and having also connected in parallel therewith the series combination of second switching means in the form of a third thyristor 6 and a current limiting element in the form of a resistor 9. Referring to Fig. 4, the thyristor 6 is controlled so as to switch on at the time t0 in relation to supply voltage 10 in order to provide a conditioning pulse.However, unlike the previous control arrangement, the thyristor 6 is connected in series with the resistor 9 and this arrangement limits the current taken by the transformer and any power factor correction capacitors and hence avoids both the problems of high capacitor inrush currents associated with starting at a non zero point and the problems of a transformer surge current associated with starting at a zero crossover.

The phase angle at which thyristor 6 fires and the value of the resistance 9 can be varied in order to achieve the optimum effect. Subseauently, the thyristors 7 and 8 fire whenever either of them is forward biassed to conduct full cycles of power, and continue firing alternately as long as desired.

A typical magnetising current produced by this control method is shown at 11 in Figure 4.

For a sizable transformer the amplitude of the initial current pulse will be quite large, and the resistor must therefore be able to withstand a high current. As, however, the pulse may only be required once, before a burst of full cycles, the resistor power rating and the thyristor heat sinking capacity is relatively modest.

The situation of a sizable transformer with power factor correction capacitors connected is commonly found in the field of induction heating (in particular induction heating of metal billets to be fed to an extrusion press) and the control system of the present invention is of particular use in this field. In these heating systems, a burst of a.c. cycles is applied via a transformer to an induction coil and billet to be heated, the length of the burst being determined in dependence on the amount of heating required, and it is clearly desirable that the transformer can be controllably switched on and off with the minimum of unwanted peak currents.

A related inductive heating system using power factor correction capacitors uses the metal to be heated as a short circuited secondary winding of a transformer, so as to induce heating directly by application of a burst of a.c. cycles to an inductor arranged in relation to the metal to form the primary winding of the arrangement. As the current taken by the corrected inductor in such a heating system is often very high (1OOOA, for example) it is usual to spread the current over the 3 phases of the a.c.

supply by use of a Steinmetz balancer system, comprising a capacitor bank and an iron cored reactor arranged as shown in Fig. 5.

A control system for such an inductive heating system is also shown in Fig. 5. Inductor 111 is powered by the R and T phases of a 3-phase a.c. supply to heat metal 110. The a.c. supply to the inductor is controlled by thyristors 224, 225 and 226 arranged together with a resistor 219 in the same manner as Fig. 3. Thyristors 224, 225 and 226 are controlled by thyristor drive units 227, 228 and 229 respectively.

A power factor correction capacitor 112 is provided parallel to the inductor 111. A capacitor or capacitor bank 113 is provided between the R and S lines of the 3-phase a.c. supply and an inductor 114 is provided between the S and T lines of the 3-phase a.c. supply.

Current to the capacitor 113 and inductor 114 is controlled by thyristor pairs 115, 116 and 117, 118 respectively, which are in turn controlled by thyristor drive units 119, 120 and 121, 122. A voltage sensor 108 is provided parallel to the capacitor 113. A resistor 123 is also provided parallel to the capacitor 113 to allow the capacitor to discharge over a period of time after the a.c. supply is switched off.

A control system for the thyristor switching units is shown in Fig. 6. Thyristor firing units 227, 228 and 229 (controlling the supply of current to the load) are controlled by signals from a 3-phase power controller 101 as modified by a signal conditioner 104 and a l-phase power controller 106. Thyristor firing units 119, 120 (controlling the supply of current to capacitor 113) are controlled by signals from a 3-phase power controller 102 via a diode arrangement 130. Thyristor firing units 121, 122 (controlling supply of current to inductor 114) are controlled by signals from a 3-phase power controller 103, as modified by a signal modifier 105 and a l-phase power controller 107. The operation of each 3-phase power controller is controlled by signals on lines 124, 125 and 126 from a signal delay unit 100.

In order to reduce the transients associated with the switching in of the inductor heater 111, capacitor 113 and inductor 114 sections, the signal delay unit switches in each section of the network consecutively.

Initially, the signal delay unit 100 sends a signal along line 126 to cause the 3-phase power controller 103 to produce a signal which is modified by the signal modifier 105 and l-phase controller 107 and then sent to one of the thyristor drive units 121, 122 to trigger the appropriate one of the thyristors 117, 118 upon the occurrence of a crest voltage between the S and T phases of the 3-phase supply. Thereafter, the thyristors 117, 118 are fired alternately as each is forward biassed to allow full conduction of the S and T phases. The inductor 114 is switched in at a crest voltage point, as this is the optimum switching point for reduction of transients.

After a period of 6 to 10 cycles of a.c. power, to allow the inductor section of the system to settle down, the signal delay unit sends a signal along lines 125 to 3-phase controller 102, which activates one of the thyristor firing units 119, 120 to cause conduction of one of the thyristors 115, 116 at a zero crossing of the R and S phases of the 3-phase a.c. supply. Thereafter, each of the thyristors 115, 116 is allowed to conduct alternately as each is forward biassed. In the case of a capacitor, the optimum point for switching in of the section is at a zero voltage crossing.

After a further period of 6 to 10 cycles of a.c. power, the signal delay unit 1OO sends a control signal along lines 124 to 3-phase power controller 10l to control the application of the R and T phases of the 3-phase a.c. power supply across the heater inductor 111. Tn a similar manner to the system of Fig. 3, thyristor 224 is activated to provide a conditioning pulse via resistance 219 to the inductor 111. The phase angle at which thyristor 224 fires can be similarly varied to achieve an optimum effect, for example thyristor 224 may be fired at a crest or peak voltage point. Subsequently, thyristor 224 is inhibited and thyristors 226 and 225 are fired to conduct full cycles of power for as long as desired.

Whilst the timing of the switching in of each section is important to reduce transients, the sequence of switching in of each section may be varied. For example, in some circumstances it may be preferable to switch in the capacitor section before the inductor section.

In order for the signal delay unit to initiate application of a.c. power to the system, a demand signal must be present for a few seconds before the signal delay unit will start the switching on of the system. This is to ensure that any transients remaining from the last burst of a.c. power have died away. The signal delay unit will also maintain the application of a.c. power for a few seconds after the demand signal is removed to allow any starting transients to disappear before switching off. Furthermore, the signal delay unit will be inhibited in the presence of a signal from the capacitor sensor 128 indicating the presence of a significant charge on the capacitor. This is to allow any charge which may develop on the capacitor to drain off before starting another burst. Such charges may be sustained for some while and therefore, voltages of effectively twice the normal peak voltage can occur across the thyristors.

An alternative arrangement is to sense the voltage across the thyristors and re-energise the system which fires thyristors 115 and 116 when this voltage passes through zero.

The signal delay unit 100 controls the sequential application of a burst of a.c. cycles to each section of the inductor heater and Steinmetz balancer system in dependence on a demand signal. This demand signal may be dependent on the desired temperature of the metal 110 to be maintained or, alternatively, may be manually set to apply, for example, 25% of a.c.

power to the system so that the system is, for example, switched on for 30 seconds and switched off for 120 seconds.

The 3- and l-phase power controllers are well known and commercially available devices used in the field of thyristor control to produce synchronised drive signals to thyristor drive units in dependence on the phase of the a.c. supplied. A description of a control system for use with a transformer and inductor heating device, including a thyristor switching mechanism using 3- and l-phase power controllers, will now be described with reference to Figs. 7 to 10 by way of illustration to aid comprehension of the present invention.

A transformer 20 powered by the T and R phases of a three-phase a.c. supply is connected to an inductor 21 to heat a metal billet (not shown). The a.c.

supply to transformer 20 is controlled by thyristors 24, 25 and 26 arranged together with a resistor 19 in the same manner as Fig. 3, which in turn are controlled via thyristor drive units 27, 28 and 29 by a signal conditioning circuit 40, a 3-phase power controller 30 and a l-phase power controller 31.

A resistor and capacitor arrangement 32 is arranged in parallel with the thyristors 25 and 26 in order to prevent any rapid rates of chanqe of voltage appearing in the circuit from trigqering the thyristors. Similarly a varistor 34, which will break down when the voltaqe across it is greater than a certain value, is also placed in parallel with the thyristors, in order to prevent any high but perhaps less rapidly changing voltages from triggering or damaging the thyristors inadvertently.

Varistors 35 and 36 are similarly provided to remove any high voltages from the circuit, such as may occur on turning the transformer off at low load, and a resistor and capacitor arrangement 33 is applied to filter out any large a.c. spikes from the supply, which could occur for example on contact bounce in the supply contactors. Values of the varistors and resistor and capacitor arrangements can be decided in dependence on the characteristics of the thyristors and the magnitudes of the other components of the circuit.

The length of the burst of a.c. cycles applied to the transformer is determined in dependence on a demand signal from a temperature controller, which is fed to the 3-phase power controller 30 through inputs 37, 38. At the start of a burst the 3-phase power controller 30 in conjunction with timing pulses R, S and T as shown in Fig. 8 related to the corresponding supply phase waveforms R, S and T supplied to inputs 54, 55 and 56 produces signals which are modified by the signal conditioner 40 to control the thyristor drive units 27, 28 and 29.

The l-phase power controller 31 continually produces bursts of firing pulses of width of the order of 15 microseconds and with a frequency of about 5 kHz and with each burst beginning at a predetermined phase relative to the T and R supply supplied at inputs 95, 96 (synchronised with the T and R phases supplied to the transformer). The phase of these pulses is determined by a manual control and this consequently determines the starting point of the conditioning pulse. Referring to Figure 10, the signal conditioner 40 is interfaced with the 3-phase controller at terminals 70 to 75. Terminals 70, 72, 74 are connected via 1 K resistors 90, 92, 94 to a 25 V positive line, and terminals 71, 73, 75 are switchable to ground via transistors 76, 78, 77. At the start of a burst an R current pulse is produced by switching terminal 71 within the controller to ground via transistor 76.This switches on transistor 66 via the resistor network 67, 68 and sends the next pulse produced by the l-phase controller 31 to the thyrister drive unit 27 to fire thyristor 24 and start the conditioning pulse. Drive unit 27 will fire thyristor 24 immediately on receipt of a signal from the l-phase power controller 31.

The possible range of the firing point of the thyristor 24 in relation to the supply voltage is shown by block D in Fig. 9.

The S pulse shown at ts in Fig. 8, produced by switching terminal 73 to ground, enables drive unit 29 v-ia inputs 44 and 45 which fires thyristor 26 as soon as it detects a zero crossing. The possible range of the firing point is shown by block E in Fig. 9.

The first T pulse shown at tt in Fig. 8 initiates a new logic output between terminals 52, 53 of the signal conditioner 40, enabling drive units 28 and 29, via terminals 48, 49 and 46, 47 respectively to fire thyristors 25 and 26 alternately upon detection of the appropriate zero crossing until the end of the burst. Thyristor 24 and resistor 19 do not conduct for the remainder of the burst as thyristor 25 offers a lower impedance path when forward biased. This is represented by block F in Fig. 9. Additionally a second opto isolator 80' prevents transmitter 66 from further conduction thus inhibiting demand signals to firing unit 27. Referring to rig. 10, transistor 77 switches on, grounding terminal 75, causing LD 80 to conduct and activate phototransistor 81.This in turn switches on thyristor 84, which then supplies current from terminal 70 of the 3-phase controller t0 via output 52 of the signal conditioner to the drive units 28 and 29 and back to terminal 53 of the signal conditioner. Terminal 53 is connected via diodes 97, 98, 99 to terminals 71, 75, 73 of the 3-phase controller to provide a persistent path to ground through transistors 76,77,78, depending on the signal from the 3-phase controller. As terminal 70 is always high thyristor 84 will conduct for the remainder of the burst.

Thus the control system shown in Figures 7 and 10 enables the transformer 20 to receive a conditioning pulse via thyristor 24 and resistance 19 prior to direct application of the a.c. power supply, the phase angle timing of the start of the conditioning pulse being set by the l-phase power controller 31.

In a similar manner the 3-phase power controller 101, l-phase power controller 106 and signal conditioner 104 control the firing of thyristors 224, 225, and 226 to supply a.c. power to the heater inductor 111.

The thyristors 117, 118 controlling supply of a.c. power to inductor 11 are fired by signals from the 3-phase power controller, modified by the signal conditioner 105. The timing of the first firing signal may be adjusted by the l-phase power controller to fire the appropriate thyristor at a crest or peak voltage of the phases S and T. However, if the 3-phase power controller 103 produces signals analagous to those illustrated in Fig. 8 there may be no need for signal modifier 105 or l-phase power controller 107 since the first control pulse R will be produced at a peak voltage of the S and T phase (shown as a dotted line in Fig. 8).

The 3-phase power controller 102 illustrated in Fig. 6 is slightly different from power controllers 101 and 103, in that it is timed using line to line R, S, T, cross-overs to produce current pulses timed to start with a particular line to line zero crossing as indicated in Fig. 11. By using a diode arrangement 130 to sum the current pulses R, S and T a control pulse indicated by block C in Fig. 11 may be provided to thyristor drive units 119, 120 to cause one of the thyristors 115, 116 to fire at a first zero crossing of the R and S phases and each consecutively thereafter, as desired.

It is to be understood that references herein to 'first', 'second' and 'third' lines are not to be construed as references to specific phase lines and various connection configurations to the supply can be employed.

Claims (5)

1. An apparatus for controlling the supply of a three-phase alternating current supplied on three lines to an inductor heating system including a first inductor arranged to inductively heat a load and connected between the first and third lines of the a.c. supply, a capacitor connected between the first and second lines of the a.c. supply, and a second inductor connected between the second and third lines of the a.c supply, comprising: a first electronic switching means connected in series with the first inductor between the first and third lines of the a.c. supply, and controllable to start to conduct current at a predetermined phase angle of the first and third lines of the a.c.

supply, a second electronic switching means connected in series with the capacitor between the first and second lines of the a.c. supply, and controllable to start to conduct current at a predetermined phase angle of the first and second lines of the a.c. supply, a third electronic switching means connected in series with the second inductor between the second and third lines of the a.c. supply, and controllable to start to conduct current at a predetermined phase angle of the second and third lines of the a.c.

supply, and a control means capable of independently controlling each switching means, and arranged to cause conduction of the second and third switching means before conduction of the first switching means.

2. An apparatus as claimed in claim 1, wherein the control means is arranged to cause the second electronic switching means to conduct at a predetermined phase angle corresponding to a substantially zero voltage-crossing point of the first and second lines of the a.c. supply.

3. An apparatus as claimed in claim 1 or 2, wherein the control means is arranqed to cause the third electronic switching means tQ conduct at a predetermined phase angle corresponding to a substantially crest voltage point of the second and third lines of the a.c. supply.

4. An apparatus as claimed in any preceding claim, wherein the first electronic switching means comprises: a first switching unit controllable to conduct current in either direction, and a second switching unit controllable to conduct current in one direction, connected in series with a current limiting element, both arranged in parallel with the first switching unit, and wherein the control means is arranged to hold the first switching unit in a non-conducting condition and to render the second switching unit conductive for a portion of a first half cycle of the first and third lines of the a.c. supply beginning at a predetermined phase angle corresponding to a nonzero-crossing point when power is first applied to the first inductor and subsequently to control the first switching unit to conduct, whereby a preliminary magnetizing current in the form of a conditioning pulse is supplied via the second switching unit and current limiting element before conduction of the first switching unit.

5. An apparatus as claimed in any preceding claim, wherein the control means is arranged to cause conduction of the third switching means before conduction of the third switching means.

6. An apparatus for controlling the supply of a three-phase alternating current supplied on three lines to an inductor heating system, substantially as hereinbefore described with reference to Figs.

5. An apparatus as claimed in any preceding claim, wherein the control means is arranged to cause conduction of the third switching means before conduction of the second switching means.

5 to 11 of the accompanying drawings.

Amendments to the claims have been filed as follows voltage-crossing point of the first and second lines of the a.c. supply.

3. An apparatus as claimed in claim 1 or 2, wherein the control means is arranged to cause the third electronic switching means to conduct at a predetermined phase angle corresponding to a substantially crest voltage point of the second and third lines of the a.c. supply.

4. An apparatus as claimed in any preceding claim, wherein the first electronic switching means comprises: a first switching unit controllable to conduct current in either direction, and a second switching unit controllable to conduct current in one direction, connected in series with a current limiting element, both arranged in parallel with the first switching unit, and wherein the control means is arranged to hold the first switching unit in a non-conducting condition and to render the second switching unit conductive for a portion of a first half cycle of the first and third lines of the a.c. supply beginning at a predetermined phase angle corresponding to a nonzero-crossing point when power is first applied to the first inductor and subsequently to control the first switching unit to conduct, whereby a preliminary magnetizing current in the form of a conditioning pulse is supplied via the second switching unit and current limiting element before conduction of the first switching unit.