GB1569473A

GB1569473A - Regulating device for direct current

Info

Publication number: GB1569473A
Application number: GB1186277A
Authority: GB
Original assignee: Societe de Traction CEM Oerlikon
Current assignee: Societe de Traction CEM Oerlikon
Priority date: 1976-03-26
Filing date: 1977-03-21
Publication date: 1980-06-18
Also published as: DE2710980C3; FR2345760A1; IT1084660B; FR2345760B1; DE2710980A1; DE2710980B2; CH598655A5; BE852888A

Description

(54) REGULATING DEVICE FOR DIRECT CURRENT (71) We, SOCIETE DE TRACTION CEM OERLIKON, a French Company, of 37 Rue du Rocher, 75383 Paris Cedex 08, France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an electric current regulator adapted to provide a direct current for application to a load.

Current regulators for a direct current are known which apply to a load a direct current of for example a constant value notwithstanding changes in the voltage of a d.c. input to the regulator. A known regulator of this kind is described in more detail hereinafter in which the current to a load is controlled by a thyristor which is fired repetitively in dependence upon the current to be applied to the load. The regulator includes an extinction circuit which is arranged to extinguish the direct current flowing through the thyristor from the input to the load, between firings of the thyristor, so as to permit the thyristor to return to a nonconducting or blocked condition between firings. This known current regulator suffers from the disadvantage that the extinction circuit cannot be made to operate over a wide range of input voltages without incurring substantial energy losses within the regulator.

With a view to mitigating this disadvantage, the present invention provides a current regulator for providing a direct current for application to a load, comprising an input for receiving an input voltage, an output for applying the current to a load, a first thyristor arranged so that when it is fired it passes current from the input to the output, an extinction circuit including charge storage means for storing an electrical charge and means for producing a discharge of the charge stored by said storage means after the thyristor has been fired such that the discharge reduces the current flowing through the thyristor and permits the thyristor to assume a blocked condition, and means for limiting the charge stored by said storage means to a maximum value independent of said input voltage.

In order that the invention may be more fully understood and readily carried into effect, embodiments thereof will now be described by way of example and by way of contrast with a prior art regulator reference being had to the accompanying drawings in which: Figure 1 is a schematic circuit diagram of a known current regulator, Figure 2 is a schematic circuit diagram of a first example of a current regulator according to the invention, Figure 3 is a graph illustrating currents and the voltages developed at different points of the circuit of Figure 2, as a function of time, Figure 4 is a schematic circuit diagram of a second example of a current regulator according to the invention Figure 5 is a schematic circuit diagram of a modified embodiment of the circuit that includes the third thyristor of the regulator of Figure 2, Figure 6 is a schematic circuit diagram of another modified embodiment of the circuit that includes the third thyristor, Figure 7 is a circuit diagram in block form of the trigger circuit for the third thyristor of the current regulator shown in Figure 2.

The prior art regulator of Figure 1 will first be described and an embodiment of the regulator of the present invention will then be described by way of contrast. Like components of the two regulators are marked with the same reference numerals in Figures 1 and 2.

Referring firstly to Figure 1, the regulator includes input terminals which receive an input d.c. voltage UE, and an output which delivers a direct current to a load 1. The regulator has a main thyristor Thl which controls the supply of current to the load. The thyristor Thl is fired repeatedly by a control circuit, not shown, and the relative durations of the periods that the thyristor is fired (i.e. conducts) to the periods that the thyristor is blocked (i.e. does not conduct) controls the magnitude of the mean d.c. current applied to the load 1.

Once fired, the thyristor Thl will not return to a blocked condition until the direct current flowing through it is reduced to zero, and an extinction circuit is provided to automatically inject a current into the thyristor Thl after it has been fired, to return the thyristor to a blocked condition. The extinction circuit consists of a resonant circuit formed by a series connected capacitor C1 and inductance coil L1, and the resonant circuit can be completed either through a thyristor Th2 or through diodes D1 and D2.

The output of the regulator includes a rectifying diode D3, a smoothing capacitor C2, and a smoothing inductance L3. In practice, the coil L3 may be omitted from the circuit of the regulator if the load 1 is constituted by an inductance such as for example the coils of an electric motor.

In operation of the known current regulator, the thyristors Thl, Th2 are fired repetitively at a rate dependent upon the required magnitude of current to be applied to the load 1, the thyristor Thl providing output current pulses which are smoothed by the inductance L3 and the capacitor C2 to provide a d.c. current for application to the load 1.

When the thyristor Thy is blocked, the capacitor C1 of the extinction circuit is charged to a voltage V1 closely approximating to UE. When both of the thyristors Thl, Tb2 have been fired, the resonant circuit oscillates; during the first half cycle of its oscillation, the charged capacitor C1 causes a current to flow through the thyristor Th2 and as will be apparent from the description hereinafter, the charge on the capacitor is inverted. During the next half cycle of oscillation of the resonant circuit, the thyristor Th2 is blocked, and an extinction current is discharged through the diodes D1 and D2, causing the thyristor Thl to become blocked and permitting the thyristors to be fired again to control the load current.

Thus, by varying the frequency at which the thyristors are fired, the magnitude of the d.c.

output to the load can be controlled and for example the regulator can supply to the load 1 a constant direct current notwithstanding variations in the voltage UE. In such a known direct current regulator, the "switching power", that is to say the maximum current that the extinction circuit is capable of supplying during a time to which is sufficiently long for the thyristor Thl to recover its blocked state is, in simplified form, proportional to the amount of charge Q = ClVl stored in the capacitor Cl. Now the voltage to which the capacitor C1 is charged, depends upon the magnitude of the input voltage UE. It will therefore be understood that, in the known regulator, the maximum current Imax which it is possible to supply through thyristor Thl to the load 1 is proportional to the input voltage UE of the regulator. Now, generally, the load 1 is required to be fed with a current which is totally independent of the input voltage UE of the regulator and it is therefore necessary to dimension the elements of the extinction circuit C1, L1 Th2, D1, D2 so that they are capable of ensuring the commutation of the thyristor Thl when it passes the required maximum current Imax for the minimum voltage UE likely to occur in use. This requires using a capacitor C1 of large value.

Apart from disadvantages of the increased volume, mass and price of such a large value capacitor, a major disadvantage of the known regulator is that the energy i C1 V12 stored in the capacitor C1 which is required to ensure the commutation of the maximum current Imax when the regulator is fed with a minimum input voltage UE, increases rapidly as the input voltage UE increases. The result is that for the highest occurring values of the input voltage UE, very large magnitude currents circulate in the extinction circuit, causing considerable energy losses, and requiring over-rated circuit components to withstand the currents, which of course adds to the cost and the size of the regulator.

These problems are overcome by the embodiment of current regulator in accordance with the invention, which is shown in Figure 2. The circuit of Figure 2 is similar to that of Figure 1 except that the circuit of Figure 2 includes an inductance coil L2 connected to receive the input current, the inductance L2 being arranged to limit the growth of a current 12 through the thyristor Thl, and also the growth of a current 15. The circuit of Figure 2 also includes a thyristor Th3 connected between the capacitor C1 and the inductance L1 to control the charge developed on the capacitor C1.

The operation of the circuit arrangement is as follows: let it be assumed that at the moment tl, the thyristors Thl, TH2 and Th3 are in the blocked state, that the current Il is flowing through the load 1 and the diode D3. Let it likewise be assumed that the capacitor C1 has been charged through the elements L2, D1, L1, L3 to the d.c. input voltage UE.

At the moment tl, the thyristor Thl is fired. A current 12 then becomes established through the thyristor Thl and becomes equal to the current I1 at the moment t2. From the moment t2 on, the diode D3 ceases to be conducting. Between the moments t2 and ts, the feed voltage UE is therefore applied, through the thyristor Thl to the terminals of the diode D3.

When it is desired to interrupt the current 12 passing through the thyristor Thl, the thyristor Th2 is fired and rendered conducting at the moment t3. The resonant circuit consisting of the capacitor C1 and the inductance coil L1 is closed on itself through the thyristor Th2, and the current 14 which becomes established in this circuit effects a half-period of oscillation between the moments t3 and 4. At the moment t4, the charge of the capacitor C1 has therefore changed polarity. The current 14 then becomes inverted, which has the effect of blocking the thyristor Th2.

From the moment t4 on, the current I4, which has changed direction, flows through the diode Dl, the capacitor C1, the inductance coil L1 and the inductance coil L3 and progressively replaces the current 12 entering the thyristorTh1. From the moment t5 on, the current L becomes greater than the current I1 and the thyristor Thl ceases to conduct. The difference (L - I1) then flows through the diode D2 and the diode D1 towards the other end of the resonant circuit consisting of the capacitor C1 and the inductance coil Ll.

The oscillation thus excited continues and the capacitor C1 progressively recovers a charge, the polarity of which is the same as that which existed at the moment tl.

From the moment t6 on, the magnitude of the current 14 begins to decrease. At the moment t7, the current 14 becomes equal to the current I1 so that the difference (I4 - I1) becomes zero and the diode D2 ceases to conduct.

The voltage V1 at the terminals of the capacitor C1 is then a little lower than UE because the oscillation of the resonant circuit is not complete. The capacitor C1 completes its charge between the moments t7 and t8, by means of the constant current L1, up to a voltage of value UE.

From the moment t8 on, the current L, which has remained constant and equal to I1 between the moments t7 and t8, tends to decrease. Since the current I1 should remain constant, the difference (1i - I4) then passes through the diode D3 which becomes conducting.

The resonant circuit consisting of the capacitor C1, the inductance coil L1, the diode D1 and the inductance coil L2 can then be regarded as closed through the diode D3 and the feed source.

An oscillation of the current 14 therefore becomes established through this circuit and consequently transfers the energy W = E (L1 + L2) I2, present in the inductance coils L1 and L2 at the moment ts into the capacitor C1 in the form of an over-voltage AV1 such that W = C1 A V21. At the moment till, the capacitor C1 would therefore be charged to a voltage equal to:

On the other hand, if the "commutation power" of the regulator, that is to say the maximum current through the thyristor Thl which the extinction circuit is capable of extinguishing, is expressed in simplified form by C1 V1 IM = k to Vl to in which to is the time taken by the thyristor Th1 to return to its blocked condition and k is a dimensioning parameter including, in particular, margins of safety, it is possible to write: k = k Cl 1M = to (UE + Avl) Now in the current regulator according to the invention, and shown in Figure 2, the commutation power is made independent of the input voltage UE by maintaining (UE + A V1) constant when the input voltage UE varies. This is achieved by means of the thyristor Th3 Let it be assumed, as is generally the case, that the current I1 passing through the inductance coil L3 can only vary slowly in relation to the time separating two successive conduction periods of the thyristor Thl.

When the current I1 increases, the over-voltage AV1 of the capacitor C1 increases proportionally to Il. The maximum voltage V1 appearing at the terminals of the capacitor C1 therefore increases likewise.

When V1 reaches the desired voltage, as a maximum value, generally selected equal to a value close to the maximum voltage which the input voltage UE can assume, the thyristor Th3 is fired and rendered conducting. This takes place at the moment-t9.

The current 14 which was still circulating in the circuit consisting of the inductance coil L2, the diode D1, the capacitor C1, the inductance coil L1, the inductance coil Lg, the load 1 and the input voltage supply, is then branched off and circulates in the circuit consisting of the inductance coil L1, the inductance coil Lg, the load 1 and the thyristor Ths.

The rate of rise of the current I I passing through the thyristor Th3 when it is fired is equal to the speed of decrease of the current 14 passing through the inductance coil L2. At the moment tlo, the current 14 passing through the inductance coil L2 having been cancelled, the capacitor C1 ceases to be charged and the voltage V1 at its terminals is maintained equal to the value reached at the moment tlo because the diode D1 prohibits the passage of a discharge current of the capacitor C1 towards the input supply. Between the moments tg and tlo, the energy stored in the inductance coil L2 passes into the capacitor C1 in the form of a small overcharge above the voltage UE, which is not very inconvenient.

The energy stored in the inductance coil L1 at the moment tlo and equal to + L1 Is2 is preserved because of the circulation of the current I5 through the circuit consisting of the inductance coil L1, the inductance coil L3, the load 1 and the thyristor Th3, between the moments tlo and t' l so that I1 = Is + 16.

During the next cycle of operation of the regulator, the thyristor Thl is fired at the moment t' l. Between the moments t' 1 and t' 2, the current 12 increases up to avalue equal to that of the current 16 which was circulating through the diode D3; then between the moments t' 2 and t"2, the current 12 continues to increase up the value I1. At the moment t"2, the currents Ig and I6 are cancelled and the energy 9 L1 Is previously stored in the inductance L1 is then transmitted in its entirety to the load 1, the energy having been stored in the inductance coil L1 between the moments tlo and t'2.

Thus the current regulator of Figure 2 enables the capacitor C1 to be charged to a voltage which is independent of the input voltage UE of the regulator, hence enabling a constant "commutation power" to be maintained, using a variable portion of the energy stored in the inductance coils L1 and L2, the remainder of the energy being stored and subsequently discharged to the load 1, thereby minimising energy losses in the regulator.

When the current I1 is of too low a value for the energy i (L1 + L2) 12 to permit an over-charge AV1 of the capacitor C1 such that (UE + A V1) reaches a value close to the maximum value which the input voltage UE can assume, the thyristor Th3 is not fired. Such a situation does not involve any inconvenience because, although the capacitor C1 is not charged to the maximum voltage, the current to be commutated is low and the extinction of the thyristor Thl is thus assured.

In order to maintain constant the voltage V1 developed across the capacitor C1, the moment of firing of the thyristor Th3 is arranged to be between the moments tg and tll. The moment of firing is determined by a device shown schematically in Figure 7. The voltage developed across the capacitor C1 is measured by a measuring device 2 and applied to one input of a comparator 3 which receives, at its other input, a reference voltage produced at 4.

The output signal of the comparator 3 is applied to a monostable trigger circuit 5, the output pulse of which is amplified by an amplifier 6 and applied to the gate of the thyristor Th3.

Thus, so long as the voltage V1 developed across the capacitor C1 remains lower than a predetermined value, generally selected close to the input voltage UE, the thyristor The is not fired. As soon as the voltage V1 reaches the desired voltage, the thyristor Th1 is fired, thus preventing the voltage V1 from exceeding the desired voltage.

The current regulator illustrated in Figure 2 may, in certain cases, require high values for the inductance coil L1 which results in a long oscillation period of the oscillating circuit consisting of the inductance coil L1 and the capacitor C1. A long period is sometimes a disadvantage because it increases the minimum duration of conduction of the regulator and does not allow a regulator to be produced which reduces the input voltage sufficiently to provide an acceptable voltage to the load 1.

This disadvantage is overcome by the circuit arrangement of Figure 4, in which an inductance coil L1 of value allowing an accpetable period of oscillation of the resonant circuit, is used, together with a further inductance coil L which does not form part of the resonant circuit but contributes, by the energy W = i L 121 which it sotres, to obtaining an adequate overcharge A V1 of the capacitor C1.

The modified embodiment of the regulator is illustrated in Figure 5 and includes a resistor R in series with the thyristor Th3 in order to obtain an extinction of the current in the branch of the inductance coil L for example.

In the other modification shown in Figure 6, the inductance coil L2 is connected in series with the thyristor Th3 and the resistor R. This inductance coil L2 is of low value because the application of voltage to Th3 takes place at low voltage. The inductance coil L2 ensures the limitation of the current in the thyristor Th3 on firing. This arrangement enables the small over-charge of the capacitor C1 to be eliminated, which occurs between the moments tg and tlo in the case of Figure 2. It therefore enables a more precise control of the voltage at the terminals of the capacitor C1 to be assured. Moreover, the extinction of the current I5 due to the presence of the dissipation resistor R causes the inductance coil L to ensure the limitation of the current in the thyristor Thl on firing.

WHAT WE CLAIM IS: 1. A current regulator for prociding a direct current for application to a load, comprising

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. the speed of decrease of the current 14 passing through the inductance coil L2. At the moment tlo, the current 14 passing through the inductance coil L2 having been cancelled, the capacitor C1 ceases to be charged and the voltage V1 at its terminals is maintained equal to the value reached at the moment tlo because the diode D1 prohibits the passage of a discharge current of the capacitor C1 towards the input supply. Between the moments tg and tlo, the energy stored in the inductance coil L2 passes into the capacitor C1 in the form of a small overcharge above the voltage UE, which is not very inconvenient. The energy stored in the inductance coil L1 at the moment tlo and equal to + L1 Is2 is preserved because of the circulation of the current I5 through the circuit consisting of the inductance coil L1, the inductance coil L3, the load 1 and the thyristor Th3, between the moments tlo and t' l so that I1 = Is + 16. During the next cycle of operation of the regulator, the thyristor Thl is fired at the moment t' l. Between the moments t' 1 and t' 2, the current 12 increases up to avalue equal to that of the current 16 which was circulating through the diode D3; then between the moments t' 2 and t"2, the current 12 continues to increase up the value I1. At the moment t"2, the currents Ig and I6 are cancelled and the energy 9 L1 Is previously stored in the inductance L1 is then transmitted in its entirety to the load 1, the energy having been stored in the inductance coil L1 between the moments tlo and t'2. Thus the current regulator of Figure 2 enables the capacitor C1 to be charged to a voltage which is independent of the input voltage UE of the regulator, hence enabling a constant "commutation power" to be maintained, using a variable portion of the energy stored in the inductance coils L1 and L2, the remainder of the energy being stored and subsequently discharged to the load 1, thereby minimising energy losses in the regulator. When the current I1 is of too low a value for the energy i (L1 + L2) 12 to permit an over-charge AV1 of the capacitor C1 such that (UE + A V1) reaches a value close to the maximum value which the input voltage UE can assume, the thyristor Th3 is not fired. Such a situation does not involve any inconvenience because, although the capacitor C1 is not charged to the maximum voltage, the current to be commutated is low and the extinction of the thyristor Thl is thus assured. In order to maintain constant the voltage V1 developed across the capacitor C1, the moment of firing of the thyristor Th3 is arranged to be between the moments tg and tll. The moment of firing is determined by a device shown schematically in Figure 7. The voltage developed across the capacitor C1 is measured by a measuring device 2 and applied to one input of a comparator 3 which receives, at its other input, a reference voltage produced at 4. The output signal of the comparator 3 is applied to a monostable trigger circuit 5, the output pulse of which is amplified by an amplifier 6 and applied to the gate of the thyristor Th3. Thus, so long as the voltage V1 developed across the capacitor C1 remains lower than a predetermined value, generally selected close to the input voltage UE, the thyristor The is not fired. As soon as the voltage V1 reaches the desired voltage, the thyristor Th1 is fired, thus preventing the voltage V1 from exceeding the desired voltage. The current regulator illustrated in Figure 2 may, in certain cases, require high values for the inductance coil L1 which results in a long oscillation period of the oscillating circuit consisting of the inductance coil L1 and the capacitor C1. A long period is sometimes a disadvantage because it increases the minimum duration of conduction of the regulator and does not allow a regulator to be produced which reduces the input voltage sufficiently to provide an acceptable voltage to the load 1. This disadvantage is overcome by the circuit arrangement of Figure 4, in which an inductance coil L1 of value allowing an accpetable period of oscillation of the resonant circuit, is used, together with a further inductance coil L which does not form part of the resonant circuit but contributes, by the energy W = i L 121 which it sotres, to obtaining an adequate overcharge A V1 of the capacitor C1. The modified embodiment of the regulator is illustrated in Figure 5 and includes a resistor R in series with the thyristor Th3 in order to obtain an extinction of the current in the branch of the inductance coil L for example. In the other modification shown in Figure 6, the inductance coil L2 is connected in series with the thyristor Th3 and the resistor R. This inductance coil L2 is of low value because the application of voltage to Th3 takes place at low voltage. The inductance coil L2 ensures the limitation of the current in the thyristor Th3 on firing. This arrangement enables the small over-charge of the capacitor C1 to be eliminated, which occurs between the moments tg and tlo in the case of Figure 2. It therefore enables a more precise control of the voltage at the terminals of the capacitor C1 to be assured. Moreover, the extinction of the current I5 due to the presence of the dissipation resistor R causes the inductance coil L to ensure the limitation of the current in the thyristor Thl on firing. WHAT WE CLAIM IS:

1. A current regulator for prociding a direct current for application to a load, comprising

an input for receiving an input voltage, an output for applying the current to a load, a first thyristor arranged so that when it is fired it passes current from the input to the output, an extinction circuit including charge storage means for storing an electrical charge and means for producing a discharge of the charge stored by said storage means after the thyristor has been fired such that the discharge reduces the current flowing through the thyristor and permits the thyristor to assume a blocked condition, and means for limiting the charge stored by said storage means to a maximum value independent of said input voltage.

2. A regulator in accordance with claim 1 wherein the extinction circuit is connected in parallel with the thyristor and includes a capacitor for storing said charge, an inductance connected in series with the capacitor, and a second thyristor connected in parallel with the inductance and the capacitor, the arrangement being such that, in use, before said first thyristor is fired the capacitor is charged to a first polarity and upon subsequent firing of said second thyristor, the capacitor is charged to a second inverse polarity and subsequently the inverse charge is discharged and causes the first thyristor to assume said blocked condition.

3. A regulator in accordance with claim 2 wherein the means for limiting the charge includes a switching means connected to the capacitor for limiting said inverse charge.

4. A regulator in accordance with claim 3 wherein said switching means comprises a third thyristor.

5. A regulator in accordance with claim 4 and including means responsive to the voltage developed across said capacitor and arranged to fire said third thyristor when the voltage across the capacitor produced by said charge of inverse polarity exceeds a predetermined value.

6. A regulator in accordance with claim 4 or 5 including animpedance connected in series with the third thyristor to limit current flow therethrough.

7. A regulator in accordance with any one of claims 2 to 6 and including a diode connected in parallel with the first thyristor such as to provide a current return path to the capacitor for some of the current produced upon said discharge of charge of inverse polarity.

8. A regulator in accordance with any one of claims 2 to 7 and including an inductance connected in series with and between the input and said first thyristor.

9. A regulator in accordance with any preceding claim wherein the output of the first thyristor is connected to a diode arranged to be connected in parallel with the load, and to an inductance arranged to be connected in series with the load.

10. A current regulator substantially as hereinbefore described with reference to Figures 2, 3 and 7 or as modified by Figures 4, 5 or 6 of the accompanying drawings.