RU2159499C1 - Frequency converter control process - Google Patents

Abstract

FIELD: converter engineering; control systems for transistorized frequency converters. SUBSTANCE: controlling frequency converter that has rectifier built around diodes, capacitive filter, and push-pull transistor inverter with each of its transistors shorted out by switching capacitor and opposing diode operating into inductive load involves generation of control pulses and their alternate supply to transistors shaping forward and reverse current half-waves across load. In the process, instant voltages across filter and across transistor that has stopped conduction are measured and compared to one another, control pulses are applied, and next transistor is driven in conduction not earlier than at moment when instant values are equal. Two rectangular-pulse trains of length 0.5T are generated, where T is ac output current cycle; turn-on moments of transistors shaping forward and reverse current half-waves across loads depend on trailing edge of mentioned pulses. EFFECT: improved reliability of frequency converter. 2 dwg

Description

 The invention relates to a conversion technique and can be used in control systems with transistor frequency converters for electrical technology.

 There is a method of controlling a frequency converter containing a rectifier on diodes, a capacitive filter and a transistor inverter operating on a load with inductance, through which control pulses are generated and alternately applied to transistors that form the forward and reverse half-waves of the current in the load (Japanese application 62-38386, MKI H 05 B 6/12 Culinary stove with induction heating / Matsushita Danki Sange K. K. - Announcement 06.08.86, Publ. 19.08.87).

 The disadvantage of the control method is the low reliability of the frequency converter for an electrotechnological load due to high switching power losses in transistors.

 A known method of controlling a frequency converter containing a rectifier on diodes, a capacitive filter and a push-pull transistor inverter, each transistor of which is bridged by a switching capacitor and a counter diode operating on a load with inductance, through which control pulses are formed and alternately fed to the transistors that form the direct and reverse half waves current in the load (application UK 2171567, MKI H 05 B 6/04. A device for cooking with induction heating / Balay SA - Declaration 06.04.85, Publ. 28.08.86).

 The disadvantage of the control method is the low reliability of the frequency converter for an electrotechnological load due to high switching power losses in transistors.

 The closest in technical essence to the invention is a method for controlling a frequency converter containing a rectifier on diodes, a capacitive filter and a push-pull transistor inverter, each transistor of which is bridged by a switching capacitor and a counter diode operating on a load with inductance (Russian patent 2061292, MKI H 05 M 5 / 44. A method of controlling a frequency converter / Silkin EM - Declaration of September 22, 1992, Publish. May 27, 96 - Bull. No. 15), which is considered as a prototype.

 The method of controlling the frequency converter is to generate and alternately supply control pulses to transistors that form the forward and reverse half-waves of the current in the load, measure the instantaneous voltage value at the turning on transistor, apply a control pulse and turn on the next transistor at the moment of equal voltage level 0-0, 2 from the maximum voltage level, removing the control pulse and turning off the transistor at a time instant of 0.15-0.30 period of the output current of the frequency converter.

 The disadvantage of the prototype is the lack of reliability of the frequency converter in control systems with an electrotechnological load, characterized by a change in parameters over a wide range. When changing the load parameters, an increase in switching losses in transistors is possible, which reduces the reliability of operation.

 The invention is aimed at solving the problem of increasing the reliability of the frequency converter as part of control systems with an electrotechnological load, which is the purpose of the invention.

 Improving the reliability of the frequency converter is achieved by the fact that in the method of controlling a frequency converter containing a rectifier on diodes, a capacitive filter and a push-pull transistor inverter, each transistor of which is bridged by a switching capacitor and a counter diode operating on a load with inductance, through which pulses are generated and alternately fed control transistors forming the direct and reverse half-wave current in the load, measure the instantaneous voltage value on the transistor, feed the control pulse is turned on and the next transistor is turned on, the control pulse is turned off and the transistor is turned off, instantaneous values of the voltages on the capacitive filter are measured and the transistor turns off, the instantaneous values of the measured voltages are compared, the next transistor is turned on no earlier than the moment of equality of the measured voltages, two pulse sequences of rectangular pulses of duration 0 are formed , 5T, where T is the period of the output alternating current, the slice of which sets the turning off times of the transistors forming the direct and The half-wave current in the load.

 A significant difference characterizing the invention is to increase the reliability of the frequency converter to the electrotechnological load by reducing switching losses in transistors when the load changes over a wide range. Measurement of the instantaneous voltage value at the turning off transistor provides higher accuracy of monitoring the moment of switching on the next transistor, because control is carried out at a high voltage level, which ensures the inclusion of transistors with minimal switching losses. Measurement of the instantaneous voltage value on the filter capacitor provides the same conditions for switching on transistors with minimal switching losses while increasing the amplitude of voltage ripples on the capacitive filter during load surge.

 Improving the reliability of the frequency converter is a technical result due to new actions and the order of their implementation in the control method, i.e. distinguishing features. Thus, the distinguishing features of the proposed method for controlling a frequency converter are essential.

 In FIG. 1 shows a diagram of a device for implementing a method for controlling a frequency converter; in FIG. 2 - time diagrams of processes on circuit elements that explain the control principle.

 The control method of the frequency converter is implemented by the following steps. Measure the instantaneous voltage values on the capacitive filter and on the transistor that turns off. The instantaneous values of the measured voltages are compared. Control pulses are generated and alternately applied to transistors, which form the forward and reverse half-waves of the current in the load. They give a control pulse and turn on the next transistor no earlier than the moment of equality of the measured voltages. Two pulse sequences of rectangular pulses of duration 0.5 T are formed, where T is the period of the output alternating current. The control pulse is removed and the transistor is turned off by cutting a rectangular pulse of the pulse sequence.

 A device diagram for implementing a method of controlling a frequency converter includes a rectifier on diodes 1-4, a capacitive filter 5 and a push-pull transistor inverter on transistors 6-9, each transistor of which is shunted by a switching capacitor, respectively, 10-13 and a counter diode 14, is connected to the output terminals -17, the diagonal of the alternating current of which includes a load with inductance 18, a control unit 19, the outputs of which are connected to the control electrodes of the transistors, a voltage sensor across the capacitor pa 20, the outputs of which are connected to first inputs of the comparison device 21, 22, comparing units outputs are connected to inputs of the control unit, voltage sensors on transistors 23, 24 homonymous inverter group outputs are connected to second inputs of the comparison device.

The device operates as follows. The control unit 19 generates two pulse sequences of rectangular pulses u ₁ , u ₂ (Fig. 2) with a duration of 0.5 T, where T is the period of the output alternating current, the cut of which sets the turning off times for the pairs of transistors 6, 9 and 7, 8. The alternating supply voltage the network is converted to an alternating sign rectifier on diodes 1-4. The rectified voltage is filtered by a filter capacitor 5 and fed to the inverter input on transistors 6-9, shunted by switching capacitors 10-13 and counter diodes 14-17. The transistors of the diagonals (pairs) 6, 9 and 7, 8 are turned on alternately. For the full cycle of operation of transistors 6, 9, a complete period T of the output alternating current is formed. At the moment of switching off t _{2 of} transistors 6, 9, the instantaneous voltage values of u ₇ at transistors 7, 8 are maximum and equal to voltage u ₅ at the output of filter 5. Due to the energy stored in the electromagnetic field of load 18, the discharge of switching capacitors 11, 12 occurs along the circuit : 18-12-5-11-18. When the capacitors 11, 12 are discharged, the voltages u ₇ on the transistors 7, 8 are reduced to zero. In this case, the capacitors 10, 13 are charged to a voltage of u ₅ on the capacitive filter 5. At time t _{3, the} instantaneous voltage values on the transistors 6, 9 are equal to the voltage on the capacitive filter 5, a control pulse is applied and the transistors 7, 8 are turned on. The current through the load 18 changes direction to the opposite. During operation of transistors 6, 9, the first one is formed, and during operation of transistors 7, 8, the second half-wave of the alternating current output at load 18. Transistors 7, 8 are turned off at time t ₄ . Further, the electromagnetic processes in the circuit are repeated. The instantaneous voltage values on the capacitive filter 5 are measured by a voltage sensor 20, on transistors 6, 9 - by a sensor 24, on transistors 7, 8 - by a sensor 23. Since the circuit is symmetrical, it is enough to measure voltages on transistors of diagonals 6, 9 or 7, 8 measure the voltage at one of the transistors of the pair, for example, at transistors 7 and 9.

 Thus, the moment of turning on the next pair of transistors 6, 9 or 7, 8 is determined by electromagnetic processes in the switching circuit, and the turn-off is determined by the time specified by the pulse sequence.

Timing diagrams of FIG. 2 illustrate the operation of the device and the principle of controlling the frequency converter. In the diagrams, pulses i ₉ , i ₇ represent the control current of transistors 6, 9 and 7, 8. The supply of control pulses and the inclusion of transistors 6, 9 is carried out at time t ₁ , t ₅ , transistors 7, 8 - at time t ₃ . The removal of control pulses and turning off the transistors 6, 9 is carried out at time t ₂ , t ₆ , transistors 7, 8 - at time t ₄ .

 The voltage sensors 20, 23, 24 can be implemented on the basis of diode optoelectronic switches. Comparison devices 21, 22 are performed on operational amplifiers. The control unit 19 is performed according to any one of the options for digital-to-analog circuits for a similar application.

 Compared with the prototype, the use of the proposed method for controlling the frequency converter allows to increase the reliability of its work on the electrotechnological load with parameters varying over a wide range. This is ensured by a more accurate determination of the moments when the next pair of transistors is turned on due to the voltage control on the transistors at a high voltage level and voltage control on the capacitive filter, which ensures switching on with minimal switching losses. When changing the parameters of the electrotechnological load (discharge, surge), the moment of switching on the next pair of transistors changes optimally with respect to the moment of switching off as a function of the state of the switching circuit and the load parameters. Improving the reliability of the frequency converter can be estimated by the mean time between failures according to the results of trial operation. When working as part of the control system of an ozonator installation, the time between failures of a frequency converter with control by the claimed control method can be increased by 3-4 times in comparison with the prototype.

Claims (1)

 A method of controlling a frequency converter containing a rectifier on diodes, a capacitive filter and a push-pull transistor inverter, each transistor of which is shunted by a switching capacitor and a counter diode operating on a load with inductance, through which control pulses are formed and alternately fed to the transistors forming the direct and reverse half-wave current in the load, measure the instantaneous voltage value at the transistor, apply a control pulse and turn on the next transistor, control pulse is removed the wiring and the transistor are turned off, characterized in that the instantaneous values of the voltages on the capacitive filter are measured and the instantaneous values of the measured voltages are compared on the turned off transistor, the next transistor is turned on no earlier than the moment of equal measured voltages, two pulse sequences of rectangular pulses with a duration of 0.5 T are formed, where T - the period of the output alternating current, the slice of which sets the turning off times of the transistors that form the forward and reverse half-waves of the current in the load.

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