GB2062988A

GB2062988A - Arrangement for determining the resonant frequency of load- commutated converters having an oscillatory load circuit

Info

Publication number: GB2062988A
Application number: GB8032857A
Authority: GB
Original assignee: Andritz Hydro GmbH Austria
Current assignee: Andritz Hydro GmbH Austria
Priority date: 1979-10-10
Filing date: 1980-10-10
Publication date: 1981-05-28
Also published as: GB2062988B; AT365384B; CH653823A5; DE3036955A1; ATA660079A

Abstract

An arrangement for determining the resonant frequency of load- commutated converters having an oscillating circuit by means of a starting pulse for triggering the loaded oscillating circuit, only one starting pulse being required to enable the converter to be reliably started. The sinusoidal voltage applied to the load can be transferred to a discriminator to be transformed into an in-phase square-wave voltage (b), which square-wave voltage can be transformed by means of an integrator into a voltage ramp (c) which can be transferred to a sample/hold circuit which stores the ramp voltage amplitude (e) at the end of the positive half-wave of the square-wave voltage so that the said stored voltage is inversely proportional to the natural frequency of the loaded oscillating circuit. <IMAGE>

Description

SPECIFICATION Arrangement for determining the resonant frequency of load-commutated converters having an oscillatory load circuit The invention relates to an arrangement for determining the resonant frequency of load-commutated converters having an oscillating circuit, particularly for induction heating by means of a starting pulse for triggering the loaded oscillating circuit.

The loaded oscillating circuits of load-commutated converters have resonant frequencies which are often extremely variable during operation.

Banks of capacitors can be connected or disconnected, or, in the case of induction heating devices, the inductance of the load varies depending on the quantity, state and temperature of the material to be heated. In practice this causes variations in the resonant frequency of the loaded oscillating circuit in a ratio of up to 1:20.

If, however, the electronic control system of the converter does not receive a pre-set value of the approximate resonant frequency, commutation failure occurs in the converter during the starting tests.

Therefore, in the case of extreme load variations, it was hitherto necessary to estimate the approximate resonant frequency from empirical values and then feed in the value by means of manual input. In this case it was often necessary to carry out several tests, which was very time-consuming and, moreover, did not exclude the risk of damage to the thyristors of the converter.

It is therefore the object of the circuit, according to the invention, to avoid the disadvantages specified and to carry out a starting test, in which the resonant frequency of the converter is determined automatically, and to impress a voltage on the electronic control system of the converter as a pre-set value of the resonant frequency to ensure that the following starting tests on the converter are successful.

The invention is characterised in that the sinusoidal voltage applied to the load can be transferred to a discriminator to be transformd into an in-phase square-wave voltage, which square-wave voltage can be transformed by means of an integrator into a voltage ramp which can be transferred to a samplel hold circuit which stores the delta voltage generated at the end of the positive half-wave of the squarewave voltage so that the said stored voltage is inversely proportional to the natural frequency of the loaded oscillating circuit.

As a result of this first automatic starting test, it is possible to determine automatically the pre-set value of the resonant frequency of the converter, thereby making it possible for the circuit to be also operated by unskilled personnel. Rapid and reliable starting of the load-commutated converter is therefore possible, irrespective of the load characteristics.

In one embodiment of the invention the maximum value of the delta voltage ramp can be stored at the output of the integrator by means of a peak value detector. Peak value detectors are commercially available as integrated or hybrid modules, thereby making it possible to reduce the number of soldered joints on the printed circuit board and to advantageously reduce the installation costs, and thereby also increasing reliability.

In a further embodiment of the invention the voltage to be stored is stored for an unlimited length of time by means of a digitally operating memory circuit. A feature of all analog sample/hold circuits is the slow discharge of the storage capacitor, as a result of which the voltage drops and the pre-set value of the frequency is no longer correct. Digital sample/hold memory circuits avoid these problems by converting the analog value into a digital signal which can be stored for any desired length of time and therefore advantageously always stores the exact pre-set value of the frequency.

In a further embodiment of the invention the digitally operating memory circuit consists of a successively approximating analog/digital converter and receives the converting pulse for conversion after the end of the positive half-wave. Successively approximating analog/digital or digital/analog converters are available in the form of complete hybrid units or integrated circuits, thereby permitting a particularly compact construction having a reduced number of components. The advantages are increased reliability and monotonic conversion, thereby avoiding discontinuities in the characteristic of the converter.

The invention is explained in more detail in Figures 1 and 2 with the aid of the basic circuit diagram and in Figure 3 with the aid of the time diagram. In Figure 1 the voltage of the load converter a, which voltage is electrically separated and transformed, if appropriate, via a voltage transformer, is applied to the input of the discriminator 1. The discriminator 1 switches over whenever the zero passage of the sinusoidal input voltage a is changed and transforms this voltage into an in-phase squarewave voltage b, which can be transferred to the integrator 2 and is converted into a delta voltage ramp c. The value of the delta voltage ramp cat the time of zero passage IZI is stored either, as shown in Figure 1, by means of a sample/hold circuit 3 or, as in Figure 2, by means of a peak value detector 5.The sample/hold circuit 3 can either charge the internal storage capacitor during the positive square-wave voltage edge b and store the voltage at the time of zero passage m, or can be controlied by means of a short pulse from the high/low edge of the squarewave voltage b.

The digitally operating memory circuit 4 consists of an analog/digital converter which concerts the analog voltage into a digital value. The digital value can thus be simply stored for any desired length of time and can be converted back into an analog value.

It is possible for the digitally operating memory circuit to be simplified by means of a successive analog/digital converter. In this case the digital values of a counter are applied to the digital inputs of a digital/analog converter, the analog outputs of which are compared by a comparator. A clock generator continues to increase the value of the counter until the voltage dto be stored corresponds to the analog output voltage e of the digital/analog converter. Then the conversion cycle is completed and the output voltage e of the digital/analog converter can be stored for any desired length of time and corresponds to the maximum value of the voltage dto be stored, shortly after m passage.It is also possible to combine the digital-operating memory circuit 4 with the sample/hold circuit 3 or with the peak value detectorS. A prerequisite is very rapid conversion during the rise in the delta voltage ramp so that the said voltage is followed, with hardly any time lag, by the analog output voltage of the digital/analog converter.

In operation, the load-commutated converter feeds a trigger pulse to the load circuit causing it to resonate and produce the wave forms shown in Figure 3, and such as to produce the analog output voltage e which is a function of the load circuit resonant frequency at start up. The control signal e is fed back to a control system (not shown) which controls the commutation of the switching devices (typically thrystons) of the load-commutated converter. Thus the switching timing is set up automatically in dependence upon the resonant frequency of the load circuit at start up, thereby setting the circuit in a condition in which it can continue oscillating. Thereafter the commutation of the switching devices will be determined by the impedance parameters of the load circuit itself, in such a manner that it will continue to oscillate.

Claims

1. Arrangements for determining the resonant frequency of load-commutated converters having an oscillating circuit, characterised in that the sinusoidal voltage applied to the load can be transferred to a discriminator for conversion into an in-phase square-wave voltage, which square-wave voltage can be transformed by means of an integrator into a voltage ramp which can be transferred to a sample/ hold circuit which stores the delta voltage generated at the end of the positive half-wave of the squarewave voltage so that the said stored voltage is inversely proportional to the natural frequency of the loaded oscillating circuit.

2. Arrangement according to claim 1, characterised in that the maximum value of the delta voltage ramp can be stored at the output of the integrator by means of a peak value detector.

3. Arrangement according to claim 1 or 2, characterised in that the voltage to be stored is stored for an unlimited period of time by means of a digitally operating memory circuit.

4. Arrangement according to claim 1, characterised in that the digitally operating memory circuit consists of a successively approximating analog/ digital converter and receives the converting pulse for conversion after the end of the positive halfwave.

5. A load-commutated converter having an oscil latory load circuit, switching means for cyclically switching current in the load circuit, control means for controlling the timing of operation of the switch ing means in such a manner as to maintain an oscillatory current in the load circuit, an arrange ment responsive to the oscillatory voltage in the load circuit upon start up thereof said arrangement being adapted to produce a control signal indicative of the time duration between successive instances at which the oscillatory voltage achieves a predetermined value upon start up, said control means being adapted to control the switching timing as a function of the value of said control signal so as to set the switching timing at start up in dependence upon the resonant frequency of the load circuit.

6. A converter according to claim 1 including a pulse generator for applying a start up pulse to the load circuit, said arrangement being responsive to the oscillatory load circuit voltage produced by said start up pulse.

7. A load commutated converter or an arrangementfor determining the frequency of the load circuit thereof, substantially as herein described with reference to the accompanying drawings.