JPS5910153B2 - Frequency converter and its operating method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The invention comprises an inverter with bridge-connected thyristors, the output terminal of which is connected to a parallel oscillating circuit, and the input terminal connected to an alternating current intermediate circuit and a controllable rectifier. The invention relates to a method of operating a frequency converter in which the frequency converter is connected to a voltage source and the thyristors of the diagonal bridge branches of the inverter are fired alternately in synchronization with the oscillating circuit voltage of a parallel oscillating circuit.

Such a frequency converter and its method of operation are described in "Brown
"Boveri Mitteilungen" Volume 53 (1
966), pages 693 to 701.

Frequency converters with parallel oscillating circuits as load are used in particular in induction heating devices and melting devices, in which electrical energy is transferred inductively to the object to be heated. This frequency converter is load excited (load commutated) in a steady state, and the operating frequency of the inverter is determined by the resonant frequency of the oscillating circuit. In this case, the current is transferred by direct commutation from one conducting bridge branch of the inverter to the next conducting bridge branch, and the commutated reactive power is supplied by the capacitors of the parallel oscillating circuit. Problems arise with this frequency converter when it is necessary to operate at higher frequencies. This is because the upper limit of the frequency is limited by the characteristics of the thyristor of the inverter. Furthermore, special means are generally required for starting or establishing operation of the frequency converter. This is because in this operating state the charge on the oscillating circuit capacitor is not sufficient for direct commutation. Finally, during operation, the resistance of the load generally changes due to heating, resulting in a change in the oscillating circuit frequency, which often causes difficulties in matching the load. The object of the invention is to provide a frequency converter of the type mentioned at the outset, which solves the problems during the start-up phase and which can also be operated at even higher frequencies.

According to the invention, this object is achieved by connecting a capacitor in series with a parallel oscillating circuit between the output terminals of the inverter, the capacitor forming a series oscillating circuit with at least a coil of the parallel oscillating circuit. .

In the frequency converter according to the invention, the series oscillator circuit acts in conjunction with the inverter bridge during the starting phase like an auxiliary commutation device known, for example, from U.S. Pat. No. 3,599,078.

With the method as described there, therefore, the frequency converter according to the invention can be started without difficulty, in which case the capacitors of the parallel oscillating circuit are not inserted directly into the commutation. In contrast to known frequency converters, the frequency converter according to the invention is essentially simple in its design with regard to the required components and control. In an advantageous method for the operation of a self-excited, at least partially load-excited frequency converter according to the invention, a parallel oscillating circuit is provided between the firings of the thyristors of the continuously conducting bridge branches of the inverter. There are n half-vibrations or half-periods, where n is an odd natural number (n=1, 3, 5...).

For frequency multiplication, n>1 is chosen, and the clock frequency at which the thyristor is fired is smaller than the frequency of the parallel oscillating circuit. In this case, for optimal excitation of the vibrations of the parallel oscillating circuit, it may be advantageous to choose the frequency of the series oscillating circuit to be smaller than that of the parallel oscillating circuit. However, it is advantageous to choose the frequency of the series oscillating circuit to be greater than that of the parallel oscillating circuit, since this preserves the advantages of a self-excited inverter during operation. The thyristor is not conducting during the entire half-wave period during which it is fired, which generally has no effect. However, if the full output energy of the inverter is to be obtained, ie if the limit value of the thyristor is to be used completely, then the frequency of the series oscillating circuit is chosen approximately equal to the frequency of the parallel oscillating circuit. It is advantageous to vary the number n1 or the block frequency during operation.

Thereby, especially if the DC voltage in the intermediate circuit is constant,
Controlled by a controllable rectifier in the known frequency converter provides a simple possibility to rapidly control the extracted power. For this reason, it is advantageous to bridge the output terminals of the rectifier with another capacitor. It is advantageous to lead the output terminals of the inverter via a capacitor to the taps of the coils of the parallel resonant circuit.

Loads with induction coils are thereby matched, which leads to approximately equal oscillating circuit frequencies for high- and low-resistance loads, thereby compensating load resistance changes during operation. Next, an embodiment of the frequency converter according to the present invention shown in the drawings will be described.

FIG. 1 shows a circuit example of a frequency converter according to the invention.

Inverter 1 includes single-phase bridge-connected thyristor 2,
Consists of 3, 4 and 5. A DC current terminal 6 of the inverter 1 is connected to a three-phase AC system via a DC intermediate circuit 7 having a smoothing reactor 8 and a rectifier 9, and the three-phase AC system is applied to a terminal 10. The rectifier 9 is composed of three-phase bridge-connected thyristors 9a to 9f. Its output is bridged by capacitor 11. A parallel oscillating circuit 13 as a load is supplied with power through the output terminal 12 of the inverter 1, and the parallel oscillating circuit 13 includes a coil 14, a capacitor 15, and a damping resistor 16 connected in parallel. The parallel resonant circuit 13 is connected in series with a capacitor 17 between the output terminals 12 . In this embodiment, the connecting conductors are led to taps 14a and 14b of the oscillating circuit coil 14, and another reactor 18 is connected in series to the capacitor 17. Thyristors 2 to 5 of inverter 1 and thyristors 9a to 9 of rectifier 9
A pedestal device is provided for f, but these are not shown for clarity.

Such a control device may, for example, be a G.I. horse゜1
Netzgefih-RteStrOmr by tgen
ichtermitThyristOren”Siem
ensAG Publishing, 1967, pages 275 or 280, and for inverters, D. E-Rnst and D.
“Industrieelekt-RO” written by Strδ1e
nik” Springer-Verlag Publishing 1973
2007) on pages 54 and 55. A series oscillating circuit is formed by the capacitor 17 and a portion of the parallel oscillating circuit coil 14, to which in the embodiment shown in FIG. 1 also a smoothing reactor 8 and a reactor 18 belong.

It must be emphasized here that the smoothing reactor 8 and the reactor 18 are not required in the circuit and can be omitted in some cases. The above-mentioned components forming the series oscillating circuit are such that the oscillating frequency of the series oscillating circuit is greater than and equal to the oscillating frequency of the parallel oscillating circuit 13;
Or it can be selected to be smaller. Advantageously, the components are selected in such a way that the oscillating frequency of the series oscillating circuit is greater than the oscillating frequency of the parallel oscillating circuit. Under this assumption, the operating method will be explained in detail with reference to FIG. In FIG. 2, the pulsed currents 12 and 13 flowing through the oscillating circuit voltage USl thyristors 2 and 3 of the parallel oscillating circuit 13 when they are conducting are shown over time t.

To operate the inverter, the thyristors 2 and 5 or 3 and 4 of the diagonal bridge branches of the inverter 1 are fired alternately in phase with the oscillating circuit cage pressure Us. The pulsed currents 12 and 13 are thereby in phase with the oscillating circuit voltage Us.
is obtained, the beginning of the pulsed currents 12 and 13 being adjusted by firing the thyristors, such that the beginning is before, during or after each zero value passage of the oscillating circuit voltage Us. However, each activated thyristor 2 and 5 or 3 and 4 of the inverter 1 is extinguished only at the end of the relevant half-wave of the oscillating circuit voltage Us of the parallel oscillating circuit 13, as in known frequency converters. rather than capacitor 17
The series vibration circuit formed by the parallel vibration τ
τ
Half cycle of the circuit? Extinguishing occurs after a shorter pulse duration. This is because a series oscillating circuit vibrates at a higher frequency than a parallel oscillating circuit. For vibrations in the parallel oscillating circuit 13, there is no need to energize the circuit in each half-wave. Rather, as shown in FIG.
Or after supplying energy at 13, time T. The inverter is not fired for the number of half-oscillations n = T2.
τV2 remains, and in those half-oscillations the oscillating circuit 13
damped vibration occurs.

In this case, in order to energize the parallel oscillating circuit 13 with the correct clock, n is an odd natural number, i.e., n=1, 3, 5...
······Must. The current supplied from the DC intermediate circuit 7 (
In other words, when the polarity of the current supplied from the capacitor 11 and the polarity of the voltage of the parallel oscillating circuit 13 (that is, the voltage of the capacitor 15) are the same, power cannot be injected into the parallel oscillating circuit 13 as a load. can.

However, if these polarities are opposite to each other, power cannot be injected into the parallel oscillating circuit 13 as a load. However, when the half frequency n of the parallel oscillating circuit 13 is an odd number, as is clear from FIG. 2, even if the number increases, the polarity of the current supplied from the DC intermediate circuit 7 and the parallel oscillating Since it is in phase with the voltage of the circuit 13, the load voltage can be increased (excited) or decreased (attenuated), and power control becomes possible. On the other hand, when n is an even number, the polarity of the voltage of the parallel oscillating circuit 13 and the polarity of the current supplied from the DC intermediate circuit 7 cancel each other out, so the load voltage cannot be increased, and therefore the power is injected. Can not do it. For frequency multiplication, n>1 is chosen.

This makes it possible to operate at considerably higher frequencies, which are limited not only by the limiting characteristics of the thyristor but also by the damping of the parallel oscillating circuit. This is because it will be ensured that the oscillations of the parallel oscillating circuit between two successive firings of the thyristors 2 to 5 of the inverter 1 are not too strongly reduced due to the damping. In this case, it is advantageous to choose the oscillation frequency of the series oscillating circuit to be smaller than that of the parallel oscillating circuit;
larger, the time product of the pulsed currents 12 and 13 increases the oscillating circuit voltage UsO)'Wf. This is because the pressure-time product is completely covered, thereby obtaining optimal excitation of the parallel oscillating circuit 13.

The running number n is also a function of time and can vary.

Therefore, assuming that in the example embodiment the DC voltage at the input of the inverter obtained by the capacitor 11 is constant, the power drawn from the damping resistor 16 can be controlled in a simple manner. This control is simpler than the power control in known frequency converters performed by controllable rectifiers. In that case, if the vibration frequency of the series vibration circuit is approximately equal to that of the parallel vibration circuit 13, τ
If τ, i.e., 2t, holds, then the total output power given can be obtained by utilizing the limiting characteristics of the thyristor.

The taps 14a and 14b of the parallel resonant circuit coil 14 allow easy matching of the load to the inverter. Since the load is matched to the oscillating circuit coil, the oscillating circuit frequencies for high and low resistance loads are approximately equal, which is particularly important for melting processes where the net resistance changes. The frequency converter according to FIG. 1 is furthermore complemented by an overvoltage protection device which can be inserted in particular into the frequency converter according to the invention, where overvoltages can occur at the inverter.

This overvoltage protection device is shown in dashed lines. A thyristor 19 is connected in parallel to the smoothing reactor 10, and the thyristor 19 has a polarity such that a self-induced voltage generated in the smoothing reactor 8 when the current decreases is applied in the forward direction of the thyristor 19. The firing electrode of the thyristor 19 is connected to the output of a limit value exceedance detector 20, the input of which is connected to the direct current terminal 6 of the inverter 1. If an overvoltage occurs, the thyristor 19 is activated via the limit value exceedance detector 20, which bridges the smoothing reactor 8. Overvoltage is thereby removed. In this case, it must be emphasized that the frequency converter must not be cut off when the above-mentioned overvoltage protection device is activated.

[Brief explanation of drawings]

FIG. 1 is a connection diagram of an embodiment of the present invention, and FIG. 2 is a diagram for explaining the operation of the present invention. 1...Inverter, 2 to 5...Thyristor, 6...DC current terminal of inverter 1, 7...DC intermediate circuit, 8... ... Smoothing reactor, 9... Rectifier, 9a to 9f...
... Thyristor, 10 ... Three-phase AC system terminal, 11 ... Capacitor, 12 ... Inverter 1 output terminal, 13 ... Parallel vibration Circuit, 14... Coil, 14a, 14b...
・Coil 14 tap, 15... Capacitor,
16... Attenuation resistor, 17... Capacitor, 18... Reactor, 19... Thyristor, 20... Limit value excess detector.

Claims (1)

[Claims] 1. An inverter having a bridge-connected thyristor, the output terminal of the inverter is connected to a parallel oscillating circuit, and the input terminal is connected to an alternating current voltage source via a direct current intermediate circuit and a controllable rectifier. A frequency converter characterized in that a capacitor is connected in series with the parallel oscillating circuit between the output terminals of the inverter, and the capacitor forms a series oscillating circuit with at least a coil of the parallel oscillating circuit. 2. The frequency converter according to claim 1, wherein the frequency of the series oscillating circuit is higher than the frequency of the parallel oscillating circuit. 3. The frequency converter according to claim 1 or 2, wherein the output terminal of the inverter is connected to a tap of a coil of a parallel oscillating circuit via a capacitor. 4. The frequency converter according to any one of claims 1 to 3, wherein a capacitor is connected in series with a reactor between output terminals of the inverter. 5. A frequency converter according to any one of claims 1 to 4, in which the positive and negative output terminals of the rectifier are bridged by another capacitor. 6 comprises an inverter with bridge-connected thyristors, the output terminal of the inverter is connected to a parallel oscillating circuit, and the input terminal of a frequency converter is connected to an alternating current voltage source via a direct current intermediate circuit and a controllable rectifier. In an operating method in which the thyristors of the diagonal bridge branches of the inverter are fired alternately in synchronization with the oscillating circuit voltage of the parallel oscillating circuit, the thyristors of the continuously conducting bridge branches of the inverter are fired. A method for operating a frequency converter, characterized in that n (n is an odd natural number, n=1, 3, 5, . . . ) half-oscillations of a parallel oscillation circuit are present between them. 7. The method of operating a frequency converter according to claim 6, in which n>1 is selected in the case of frequency multiplication. 8. A method of operating a frequency converter according to claim 6 or 7, in which the number n is varied during the operation period for power control.