JPS647471B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an induction heating inverter. The induction heating inverter, as shown in Figure 1,
Forward converter that rectifies AC power and converts it to DC power
SC, DC reactor DCL, DC reactor DCL
An inverter that converts DC power into AC again.
INV, inductive load Z, power factor compensation capacitor C connected in parallel with it, forward converter, inverse converter, gate phase control circuit that controls the firing phase angle of each thyristor.
The main components are CNT and an initial excitation circuit SS for inverter startup. In other words, this induction heating inverter has a parallel resonant circuit as a load in which an inductive load Z for induction heating and a power factor compensation capacitor C are connected in parallel to the inductive load Z,
This is a parallel resonant self-control inverter that applies a constant current to this inverter and commutates each thyristor of the inverter INV in synchronization with the generated parallel resonant voltage oscillation. In addition,
Inverter INV Each thyristor is of course commutated according to a preset control advance angle γ, ie the advance phase angle of the output current with respect to the resonant voltage. By the way, this kind of parallel resonant self-limiting inverter is
As shown in Figure 2, the output current _{is 0} and the output voltage at resonance.
The relationship of V ₀ is a special state where the voltage is compared to the current ₀
In the case of region C where V ₀ is too large or conversely region A where current ₀ is too large compared to voltage V ₀ , stable operation could not be expected. In other words, in region C, the output current is small and the following current by the DC reactor DCL is impaired, causing the current to be intermittent, while in region A, the output current is too large compared to the output voltage, and the commutation energy is insufficient, making the system highly reliable and reliable. It is difficult to carry out such commutation, and in any case, in regions C and A, the inverter becomes unstable. Furthermore, in region C, not only stable operation cannot be obtained, but also an abnormal voltage is generated due to the current interruption, and the thyristor of the inverter may be destroyed. Note that this region C is a no-load or very light load range for the inverter, and in order to reduce this to a level that does not pose a problem in practical operation, it is considered to increase the inductance of the DC reactor DCL. However, the load of this type of induction heating inverter is a large capacity and efficient inductive load, and the no-load current is extremely small compared to the rated current, and the inductance determined by this no-load current corresponds to the rated current. This requires a DC reactor DCL with both current capacity and current capacity, which increases the size and cost of the equipment. In view of the above, this invention prevents the operation of the induction heating inverter within the range of region C. In other words, before operation, the load condition is predicted in advance and the inverter is operated within the range of region C. If the startup is stopped, and if it is running, the output current and output voltage will _{be 0} .
When the ratio of V ₀ is measured and the ratio reaches a certain value, the operation is immediately stopped to prevent an adverse effect on the inverter thyristor. FIG. 3 shows a block diagram of an embodiment of the present invention, and FIG. 4 shows a time chart for explaining the operation. In FIG. 3, the gate phase control circuit CONT,
The initial excitation circuit SS is the same as the main circuit components of the induction heating inverter shown in Figure 1, and supplies current pulses for starting the inverter to the parallel resonant circuit load, as well as thyristor firing signals for the forward converter and inverse converter, respectively. Output. The logic sequence circuit 1 receives the activation signal and outputs an initial excitation signal to the initial excitation circuit SS, and also sets the integration time of the integrator 2. The integrator 2 rectifies the voltage signal of the parallel resonant load, integrates it for a certain period of time using the output of the previous logic sequence circuit 1, and sends it to the next comparator 3. The comparator 3 outputs an inverter start-stop command or, conversely, an inverter start-up command depending on whether the predetermined time integral value of the parallel resonance voltage signal reaches a preset reference value. Inverter start command is gate phase control circuit
CONT is sent to continue normal inverter operation, but the start-up abort command can be, for example, connected to a warning lamp and turned on. The rectifier 4 rectifies the above-mentioned resonant voltage. Comparator 5 compares the output current ₀ of the inverter with the output voltage V ₀ and determines whether the load condition during inverter operation is within _region C. If the load suddenly decreases and enters region C, a load abnormality signal is immediately given to the gate phase control circuit CONT to stop the operation of the inverter. The time chart in FIG. 4 shows the resonant voltage waveform of the parallel resonant circuit, which is the load, at the time of initial excitation when a current pulse is applied, and shows how the degree of attenuation changes depending on the magnitude of the resonance sharpness Q. That is, in the main circuit configuration of FIG. 1, the equivalent series resistance including the inductive load reactor L of the load N and the heated object of the load Z is expressed as a resistance R,
If the power factor compensation capacitor connected in parallel to the load Z is represented by a capacitor C, then the Q of the L/R-C parallel resonant circuit is:

[Formula], and the transfer function can be expressed as (SL + R)・1/LC/S ² +R/LS+1/LC, so
The impulse response, that is, the attenuation characteristic of the resonant voltage due to the initial excitation of the current pulse is

[Formula], and the time constant τ becomes R/2L. In this way, Q is

[Formula] The decay time constant τ is expressed as R/2L, and since the load condition of the inverter is determined by this R, it is possible to determine the load condition by measuring Q. In other words, Q is inversely proportional to the equivalent series resistance R, and the decay time constant τ is inversely proportional.As shown in the figure, the larger Q is, the slower the attenuation is, and the smaller Q is, the steeper the attenuation is. . On the other hand, the output characteristics of the induction heating inverter shown in Figure 2 express the magnitude relationship of the resonant voltage V ₀ with respect to the output current _0. If the equivalent series resistance R is small and Q is high, the voltage V ₀ is large; If R is large and Q is low, the voltage V ₀ will be small, and in the former case it can be said to be in region B or A, and in the latter case it can be said to be in region C rather than B. In other words, measuring the Q of this parallel resonant circuit will give you some idea of whether the inverter's load condition is in region A, region B or C, and this Q is the attenuation of the impulse response of the circuit. closely related to characteristics. In view of the above, the present invention utilizes the initial excitation of the induction heating inverter to obtain the impulse response of the load circuit, determines the load situation based on the degree of attenuation, and estimates whether the load is in region C or B. If so, no signal is sent to the gate control circuit CONT, the activation of the inverter is stopped, and a warning lamp etc. is turned on instead. Specifically, as shown in the circuit example in Figure 3, the resonant voltage waveform during initial excitation is rectified and then integrated for a certain period of time, and whether or not the integrated value reaches a predetermined value is determined. If it is in region C, the alarm lamp is turned on, and if it is in region B, a start signal is sent to the gate control circuit CONT, and a regular start pulse is sent from the gate control circuit CONT for initial excitation. It is sent to the circuit SS, and also generates gate signals for each thyristor of the forward converter and inverse converter, and the inverter starts to start up and continues normal operation. In addition, when the transition from region B to region C occurs due to load reduction such as disappearance of heated objects during operation, the values of inverter output current ₀ and resonance voltage V ₀ are monitored, and the current/voltage ₀ /V ₀ is determined in advance. This can be determined based on whether or not it reaches a certain value. That is, in the circuit shown in Fig. 3, if the comparator 5 corresponds to this and the current/voltage ratio ₀ /V ₀ is less than the reference value, it sends a signal indicating that the load is abnormal to the gate phase control circuit CONT. , stop the inverter operation. As described above, the present invention provides an induction heating inverter that is a parallel resonant self-limiting inverter by preventing operation under no load or very light load, which is the region of intermittent current, and This prevents abnormal voltage caused by the intermittent current of each thyristor, and as a means of determining abnormal load conditions such as no load, the Q of the parallel resonant circuit is used before startup, and in reality, it is closely inseparable from this. This is done by monitoring the resonance voltage waveform attenuation characteristics of the impulse response, and by taking the ratio of the inverter output current and output voltage during operation, before an abnormal situation such as current interruption occurs. In addition, it becomes possible to stop or stop the inverter, and there is no need for a large reactor, which is indispensable for reducing the unstable operation region of conventional devices, making it possible to reduce the size, weight, and cost. Note that the determination of the load condition is not limited to the method using the resonant voltage attenuation characteristic of the impulse response of the embodiment, and any means for measuring the sharpness Q of resonance in a general parallel resonant circuit may be used.

[Brief explanation of drawings]

Figure 1 is the main circuit diagram of the induction heating inverter.
FIG. 2 is a region diagram of output current and voltage, FIG. 3 is a block diagram of an embodiment of the present invention, and FIG. 4 is a resonant voltage waveform diagram of an impulse response of a parallel resonant load. SC...Forward converter, DCL...DC reactor,
INV... Inverter, CONT... Gate phase control circuit, SS... Initial excitation circuit, 2... Integrator for resonant voltage, 3... Comparator for resonant voltage, 4... Rectifier for resonant voltage, 5... ...Output current/voltage comparator.

Claims (1)

[Scope of Claims] 1 Synchronized with the voltage oscillation of a parallel resonant load circuit consisting of a forward converter, a DC reactor, and an inverse converter as main components, and an inductive load including resistance and a power factor compensation capacitor connected in parallel. In the induction heating inverter of the resonant self-limiting inverter mentioned above, before starting, the sharpness of the resonance of the parallel resonant circuit is measured and compared with a predetermined reference value, and if it exceeds the reference value, the start-up is stopped. An induction heating inverter characterized by: 2 In the configuration described in claim 1,
As an equal means of measuring the sharpness of resonance, before startup, obtain the resonant voltage attenuation characteristic waveform of the impulse response due to the initial excitation of the parallel resonant load circuit, rectify it, and then integrate it for a fixed time period. An induction heating inverter characterized in that the induction heating inverter is characterized in that it compares with a predetermined reference value and stops starting if it exceeds the reference value. 3 The above-mentioned inverse conversion is performed in synchronization with the voltage oscillation of a parallel resonant load circuit, which has a forward converter, a DC reactor, and an inverse converter as its main components, and is made up of an inductive load including resistance and a power factor compensation capacitor connected in parallel. In the induction heating inverter of the parallel resonant self-limiting inverter that controls the firing of the thyristor, the ratio of the inverter output current and output voltage is constantly monitored during operation, and the operation is stopped if the value is less than a predetermined reference value. An induction heating inverter characterized by: