GB1586872A

GB1586872A - Apparatus for controlling the dc current supplied to a load circuit

Info

Publication number: GB1586872A
Application number: GB28611/77A
Authority: GB
Inventors: Jean Gouthiere
Original assignee: ACEC; Ateliers de Constructions Electriques de Charleroi SA
Current assignee: ACEC; Umicore NV SA
Priority date: 1976-07-16
Filing date: 1977-07-07
Publication date: 1981-03-25
Also published as: AU514748B2; FR2358775A1; ES460748A1; BR7704685A; NL7707895A; BE844198A; YU172377A; FR2358775B1; IT1079319B; AU2697077A; DE2731529A1; ZA774160B

Description

(54) APPARATUS FOR CONTROLLING THE D.C.

CURRENT SUPPLIED TO A LOAD CIRCUIT (71) We, ATELIERS DE CONSTRUCTIONS ELECTRIQUES DE CHARLEROI (ACEC), of 54 Chaussee de Charleroi, Saint Gillies, Brussels, Belgium, a body corporate organized according to the laws of Belgium, 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:- The present invention relates to apparatus for controlling the D.C. Current supplied to a load circuit.

Belgian Patent No. 704,419 discloses apparatus for regulating D.C. current designed to feed a load circuit from a fixed D.C. voltage source. This apparatus includes:-- in series with the D.C. voltage source, a chopper assembly comprising at least one thyristor and a diode in antiparallel; an extinction circuit for the thyristor of the chopper element, comprising a series oscillation circuit and a second element constituted by a thyristor and a diode in anti-parallel; a diode and a resistor connected in series with the coil of the oscillating circuit, said extinction circuit being arranged in parallel with the load circuit. The second element comprising a thyristor and a diode in anti-parallel is omitted in devices which do not require very precise regulation, for example in devices feeding low power auxiliaries.

The present invention relates to apparatus controlling D.C. current of the type described, capable of feeding, via an isolating transformer, low voltage apparatus when the feed network is at high voltage, for example 3000 volts unidirectional.

According to the present invention there is provided apparatus for controlling the D.C. current supplied to a load circuit from a D.C. voltage source, said apparatus including: a chopper circuit constituted by a thyristor and a diode connected in antiparallel therewith, one side of which is connected to the D.C. voltage source; an extinction circuit for said thyristor and constituted by an oscillating circuit comprising a series oscillating circuit consisting of two elements, a diode and a resistor in series across one of said elements of the oscillating circuit; a further diode; and an isolating transformer having primary and secondary windings, said further diode being connected in series with the primary winding of the transformer, and the load circuit to be supplied being connected to the secondary winding of the transformer.

The present invention will now be described in greater detail by way of example with reference to the accompanying drawings, wherein: Figure 1 is a circuit diagram of one preferred form of apparatus for controlling D.C. current; and Figure 2 shows a set of graphs which assist in explaining the operation of the circuit shown in Figure 1.

Referring to Figure 1, a chopper element comprises a parallel combination of a thyristor 1, and a diode 2. The chopper element is connected at one side to a terminal 3 of a source of unidirectional voltage E through a choke 20, and at the other side to a second terminal 3' through a diode 4 and the primary winding 5 of an isolating transformer, the second terminal 3' being the other terminal of the source of unidirectional voltage E. An extinction circuit for the chopper element is constituted by an oscillating circuit comprising a choke 6 and a capacitor 7.

The capacitor 7 is connected to a terminal 7' which may be connected to either the terminal 3' or 3. If, as indicated by the dotted line, the terminal 7' is connected to the terminal 3, the oscillating circuit is connected in parallel with the thyristor 1. If, on the contrary, as indicated in full lines, the terminal 7' is connected to the terminal 3', then the oscillating circuit is connected in parallel with the diode 4 and the primary winding 5. A series circuit comprising a resistor 8 and a diode 9 is connected in parallel with the capacitor 7. A capacitor 10 is connected between the terminals 3 and 3'.

The isolating transformer comprises, apart from the primary winding 5, a secondary winding 11 to which is connected a load circuit including two diodes 12 and 13 and a smoothing choke 14.

The operation of the D.C. controlling device shown in Fig. 1 is now described with reference to the graphs of Fig. 2.

At the instant to when the thyristor 1 fires, the voltage E across the terminals 3 and 3' is applied via the diode 4 to the primary winding 5 and simultaneously to the oscillating circuit consisting of the choke 6 and the capacitor 7 (waveform b). From the instant to, the thyristor passes a current (waveform a) consisting in the case of an essentially resistive load of a constant mean current 1m and a sinusoidal current I, (waveform b) passes through the oscillating circuit. Due to the current I, a voltage Uc (waveform c) is set up across the terminals of the capacitor 7 which is accordingly charged. This voltage varies between 0 and 2E (left-hand ordinate) in the case where the terminal 7' is connected to the terminal 3' and between -E and +E (right-hand ordinate) in the case where the terminal 7' is connected as indicated by the dotted line, to the terminal 3. For reasons which will be explained hereinafter, the variation of the voltage Uc follows the waveform c and passes slightly beyond the limits of 0 and 2E or -E and +E. At the instant tithe current I, passes through zero and is inverted; the voltage Uc has reached the maximum value; and the current carried by the thyristor 1 has the momentary value 1m At instant t2 the current carried by the thyristor 1 falls to zero; the current I, exceeds the value 1m and a current I,Im circulates in the diode 2 for as long as I,Im is positive. At the instant t3 the current I, falls to the value Im. During the interval of time between t2 and t3 the thyristor 1 has time to extinquish, in such a manner that at the instant t3 the chopper assembly comprising the thyristor 1 and the diode 2 is completely extinquished. The voltage UT across the terminals of the winding 5 which had the value E (waveform d) at the output of the chopper assembly as long as either thyristor I or diode 2 were conducting, suddenly reduces to the voltage across the terminals of the capacitor 7 which discharges and at the instant t4 the voltage Uc across the terminals of the capacitor 7 is inverted and all circulation of current in the series oscillating circuit dies down in accordance with a critical attenuation curve or, in general, a damping curve which exceeds the critical damping curve due to the fact that the value of the resistor 8 (equal in the case of critical attenuation to

L being the self-inductance of the choke 6 and C the capacitance of the capacitor 7), is selected to be substantially higher than

in order to permit demagnetization of the transformer 5. Consequently, the waveform remains below zero (left-hand ordinate) or below -E (right-hand ordinate) until the instant when the thyristor is refired, this being the reason for exceeding the maximum value 2E or +E respectively as referred to above. Starting from the instant t3 means are provided in the order lo allow magnetizing current of the isolation transformer to flow subsequent to the extinction of the thyristor 1. In the circuit shown in Fig. 1 the magnetizing current flows through the diode 4 and the extinction circuit (6,8,9) of the chopper element. To prevent this magnetizing current from degenerating into oscillations, the diode 4 must necessarily be connected in series with winding 5. As shown by waveform dthe voltage UT appearing at the output of the thyristor 1 between the instants to and t3 is fixed to voltage source E and predetermined. For the remainder of the period between two firings of the thyristor 1, this voltage is zero or slightly below zero. If it is desired to vary the mean output voltage, it will be necessary to vary the firing frequency of the thyristor 1, this being something which is very readily carried into effect.

WHAT WE CLAIM IS: 1. Apparatus for controlling the D.C.

current supplied to a load circuit from a D.C. voltage source, said apparatus including: a chopper circuit constituted by a thyristor and a diode connected in antiparallel therewith, one side of which is connected to the D.C. voltage source; an extinction circuit for said thyristor and constituted by an oscillating circuit comprising a series oscillating circuit consisting of two elements, a diode and a resistor in series across one of said elements of the oscillating circuit; a further diode; and an isolating transformer having primary and secondary windings, said further diode being connected in series with the primary winding of the transformer, and the load

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

Claims

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

parallel with the capacitor 7. A capacitor

10 is connected between the terminals 3 and 3'.

The isolating transformer comprises, apart from the primary winding 5, a secondary winding 11 to which is connected a load circuit including two diodes 12 and 13 and a smoothing choke 14.

The operation of the D.C. controlling device shown in Fig. 1 is now described with reference to the graphs of Fig. 2.

At the instant to when the thyristor 1 fires, the voltage E across the terminals 3 and 3' is applied via the diode 4 to the primary winding 5 and simultaneously to the oscillating circuit consisting of the choke 6 and the capacitor 7 (waveform b). From the instant to, the thyristor passes a current (waveform a) consisting in the case of an essentially resistive load of a constant mean current 1m and a sinusoidal current I, (waveform b) passes through the oscillating circuit. Due to the current I, a voltage Uc (waveform c) is set up across the terminals of the capacitor 7 which is accordingly charged. This voltage varies between 0 and 2E (left-hand ordinate) in the case where the terminal 7' is connected to the terminal 3' and between -E and +E (right-hand ordinate) in the case where the terminal 7' is connected as indicated by the dotted line, to the terminal 3. For reasons which will be explained hereinafter, the variation of the voltage Uc follows the waveform c and passes slightly beyond the limits of 0 and 2E or -E and +E. At the instant tithe current I, passes through zero and is inverted; the voltage Uc has reached the maximum value; and the current carried by the thyristor 1 has the momentary value 1m At instant t2 the current carried by the thyristor 1 falls to zero; the current I, exceeds the value 1m and a current I,Im circulates in the diode 2 for as long as I,Im is positive. At the instant t3 the current I, falls to the value Im. During the interval of time between t2 and t3 the thyristor 1 has time to extinquish, in such a manner that at the instant t3 the chopper assembly comprising the thyristor 1 and the diode 2 is completely extinquished. The voltage UT across the terminals of the winding 5 which had the value E (waveform d) at the output of the chopper assembly as long as either thyristor I or diode 2 were conducting, suddenly reduces to the voltage across the terminals of the capacitor 7 which discharges and at the instant t4 the voltage Uc across the terminals of the capacitor 7 is inverted and all circulation of current in the series oscillating circuit dies down in accordance with a critical attenuation curve or, in general, a damping curve which exceeds the critical damping curve due to the fact that the value of the resistor 8 (equal in the case of critical attenuation to

L being the self-inductance of the choke 6 and C the capacitance of the capacitor 7), is selected to be substantially higher than

in order to permit demagnetization of the transformer 5. Consequently, the waveform remains below zero (left-hand ordinate) or below -E (right-hand ordinate) until the instant when the thyristor is refired, this being the reason for exceeding the maximum value 2E or +E respectively as referred to above. Starting from the instant t3 means are provided in the order lo allow magnetizing current of the isolation transformer to flow subsequent to the extinction of the thyristor 1. In the circuit shown in Fig. 1 the magnetizing current flows through the diode 4 and the extinction circuit (6,8,9) of the chopper element. To prevent this magnetizing current from degenerating into oscillations, the diode 4 must necessarily be connected in series with winding 5. As shown by waveform dthe voltage UT appearing at the output of the thyristor 1 between the instants to and t3 is fixed to voltage source E and predetermined. For the remainder of the period between two firings of the thyristor 1, this voltage is zero or slightly below zero. If it is desired to vary the mean output voltage, it will be necessary to vary the firing frequency of the thyristor 1, this being something which is very readily carried into effect.

WHAT WE CLAIM IS: 1. Apparatus for controlling the D.C.

current supplied to a load circuit from a D.C. voltage source, said apparatus including: a chopper circuit constituted by a thyristor and a diode connected in antiparallel therewith, one side of which is connected to the D.C. voltage source; an extinction circuit for said thyristor and constituted by an oscillating circuit comprising a series oscillating circuit consisting of two elements, a diode and a resistor in series across one of said elements of the oscillating circuit; a further diode; and an isolating transformer having primary and secondary windings, said further diode being connected in series with the primary winding of the transformer, and the load

circuit to be supplied being connected to the secondary winding of the transformer.
2. Apparatus according to Claim 1, wherein said element across which the series diode and resistor are connected is a capacitor, and wherein the value of resistance of the resistor is selected to be substantially higher than

L being the inductance of an inductor constituting the other element of the oscillating circuit and C being the capacitance of the capacitor.
3. Apparatus for controlling the D.C.

current supplied to a load circuit from a D.C. voltage source constructed and arranged to operate substantially as herein described with reference to and as illustrated in the accompanying drawing.