JPS60249875A - Controller of frequency converter - Google Patents

Abstract

PURPOSE:To enable to stably operate without intermittently flowing a DC current by no increasing a DC reactor even at a load communication time by controlling a gate pulse of a load commutation inverter in response to a load impedance. CONSTITUTION:The outputs of an AC/DC converter 11 and a current/voltage converter 12 are input to a divider 13 to obtain a load impedence. When the obtained impedance is high, the output (g) of the divider 13 increases, and the phase of a gate pulse (d) is advanced from the normal state time. Thus, a DC current is not intermittently flowed even at a light load time by advancing the gate pulse phase of a load commutation type inverter, thereby performing a stable operation.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention provides a frequency converter that improves control of commutation margin time or commutation margin time of an inverter device of a frequency converter configured with a load commutation type inverter. The present invention relates to a control device.

[Technical background of the invention and its problems] Frequency converters composed of load commutated inverters are generally used as power sources for induction furnaces, induction heaters, etc., and a schematic block diagram of such devices is shown below. Shown in Figure 1.

A load commutation type inverter 1 is connected to a forward converter 3 via a DC reactor 2. f; The output of the load commutation type inverter 1 is performed by varying the DC voltage by phase control of the forward converter 3. On the other hand, the load commutation type inverter is controlled by detecting the zero point of the voltage of the output voltage detection transformer 4 and using the reference voltage that determines the output of the circuit 5 that generates a triangular wave and the gate pulse phase of the load commutation type impark 1. 6 is input to a comparator 7, and its output is divided by a distributor 8 to generate gate pulses for the load commutating inverter 1.

Further, a tank load circuit 9, which can be equivalent to a parallel circuit of R and C, is connected as a load of the frequency converter.

The frequency converter used as a power source for induction heating generally uses a current type inverter, and the tank load circuit 9
Load commutation type inverter 1 at a frequency corresponding to the resonant frequency of
is being driven.

〔problem〕

However, there are times when it is necessary to operate an induction furnace with a small amount of molten metal in the furnace, and a heater with no heating effect. This corresponds to a state where the resistance R of 9 is increased,
A current source inverter has a high output voltage, but the thyristors that make up a load commutation type inverter have a voltage rating, so the output voltage cannot be used beyond that rating. limited to a value determined by the voltage rating of the

As described above, when the output voltage is limited, the output current becomes smaller as the load resistance R increases, and finally the DC current becomes intermittent due to the DC current ripple F, and the load commutating inverter becomes unable to operate and stops. There was a drawback to it. However, when trying to reduce the ripple of the DC current in order to widen the intermittent limit of the DC current, there was also the disadvantage that the DC reactor 2 would become larger, resulting in an economical disadvantage.

[Object of the Invention] One object of the present invention is to provide a q+ control device for a frequency converter that eliminates the above-mentioned problems and can operate normally even under light loads.

[Summary of the Invention] The present invention achieves this object. First, the impedance of the tank load circuit is measured by the input voltage t& of the forward converter and the output voltage, and the load commutation type impark is determined according to the load impedance. If we control the gate pulse of t, then,
To provide a load commutation type inverter capable of stable operation without intermittent DC current without increasing the size of a DC reactor even when the load is light.

[Embodiment of the Invention] FIG. 3 shows an embodiment of the present invention, and parts having the same functions as those in FIG. 1 showing a conventional example are denoted by the same reference numerals and explanations thereof will be omitted.

AC/D that outputs DC voltage commensurate with the output voltage.
Input the respective outputs of the C conversion 11 and the current-voltage converter 12 to the divider 13, take the current as the denominator, the voltage as the numerator, calculate v + I, calculate the load impedance, and when the load impedance is high, That is, when the denominator current is smaller than the numerator voltage, the output g of the divider 13 becomes large, so that the gate pulse d shown in FIG. 4 has the phase 1. It comes out at a more advanced phase t2. In other words, the signal from the voltage reference 6 that determines the phase of the gate pulse becomes larger and its phase advances. On the other hand, if the impedance is negative and the impedance is small, the output of the divider 13 becomes small for the opposite reason to the above, and it settles to a value close to the phase determined by the reference voltage.

Next, using the vector diagram in FIG. 5, it will be explained that by advancing the gate pulse phase of the load commutation type inverter, stable operation can be achieved without intermittent DC current even during light loads.

Output voltage V. In contrast, the output current is ■. operates in a steady state with the commutation margin angle β advanced, and the capacitor current IC, coil current L, and current I flowing through the resistor have the relationships shown in FIG. 5, respectively.

Here, if the phase of the gate pulse is advanced, the commutation margin angle becomes β
On the other hand, it is generally known that increasing the commutation margin angle increases the output frequency of a load commutation type inverter, and as the output frequency increases, the coil current increases from I, to L'1 slightly increases the capacitor current from ■○ to IO2, so the output current, which is the sum of these vectors, becomes ■. Since the current increases from IO2 to IO2, by advancing the gate pulse of the θ IC type inverter by 1 at light load, the DC current increases and stable operation is possible without interruption.

In order to obtain the above effect, the input current is DC′ current, and the output′
It is obvious that the output voltage may be changed to a current, and the output voltage may be changed to a DC voltage after the DC reactor.

〔Effect of the invention〕

As described above, according to the present invention, stable operation without intermittent DC current is possible even under light load without increasing the size of the DC reactor.

[Brief explanation of drawings]

Figure 1 is a block diagram showing a conventional frequency conversion device, Figure 2 is a block diagram showing a conventional frequency conversion device.
1 is a time chart for explaining the operation of FIG. 1, FIG. 3 is a block diagram showing an embodiment of the present invention, FIG. 4 is a time chart for explaining the present invention in detail, and FIG. 5 is a time chart for explaining the operation of FIG. FIG. 2 is a vector diagram for detailed explanation of the present invention. 1... Load commutation type inverter 2... DC reactor 3... Forward converter 4... Transformer 5... Triangular wave generator 6... Reference voltage generator
7... Comparator 8... Distributor 9... Tank circuit 10... Transformer 11... AC-DC converter 12.
・・IV converter 13...%>'I device (7317)
Agent Patent Attorney Kensuke Chika (#1 or 1 person) Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] In a frequency converter configured with a load commutation type inverter, a load impedance detection means for outputting a signal according to the load impedance is provided, and the firing phase of the load commutation type inverter is corrected and controlled by the means. A control device for a frequency converter, characterized in that: