JPH0218029B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a control device for a current source inverter, and in particular to a starting control device.

It is well known that a current source inverter, which is used as a power source for induction heating, utilizes the induced voltage of a load and further advances the gate phase to ensure a commutation margin angle to obtain inverter operation. In addition, a current source inverter requires a special starting circuit because no induced voltage in the load is generated for starting the current source inverter.

As shown in Figure 1, the current source inverter's starting method includes auxiliary thyristors SU _A and SX _A and a starting capacitor C _A , and is operated in the following four modes.

(1) Turn on thyristors SU _A and SX to close the capacitor.
Pre-charge C _A in the direction shown.

(2) Turn on thyristors SU _A and SX _A to convert the forward converter.
The output of CNV is short-circuited through a DC reactor DCL and a current limiting resistor R, and a preliminary current is passed through the reactor DCL.

(3) When the short-circuit current of reactor DCL increases to a specified level or reaches the short-circuit current time,
By alternately firing thyristors SV and SX _A and then firing thyristors SU _A and SY, the reactor current is passed through the capacitor C _A to the load tank circuit (capacitor C _L , reactor L _L ). supply At this time, each thyristor is commutated by the alternating reverse polarity charging voltage of capacitor C _A.

(4) After operating Mode 3 for a certain period of time and after the time when sufficient voltage is expected to be induced in the load tank circuit, replace thyristors SU _A and SX _A with thyristors.
Main circuit operation begins with SU, SX, SV, and SY.
In such a starting method using an auxiliary thyristor and a capacitor C _A , if the resonance frequency of the tank circuit and the cycle of the auxiliary operation in mode 3 do not match, a voltage is induced only in the capacitor C _A ,
No voltage is generated in the tank circuit, and commutation fails when mode 4 main circuit operation begins. For this reason, there is a restriction on the starting frequency relative to the resonant frequency of the load, and some load tank circuits may cause starting failures.

The present invention has been made in view of the above problems, and
It is an object of the present invention to provide a control device that enables reliable starting by automatically detecting the resonance state of a tank circuit and setting a starting frequency.

FIG. 2 is a circuit diagram showing one embodiment of the present invention. The inverter main circuit is the same as in Figure 1, with a starting capacitor C _A installed between the midpoint of the series circuit of auxiliary thyristors SU _A and SX _A and current limiting resistor R and the series connection point of main thyristors SU and SX. The components that apply one pulse voltage to the load tank circuit of this main circuit are connected from the insulating transformer TF to the rectifier DB, the resonance capacitor C _p , the coil L _p , and the switch thyristor.
A pulse generator PG consisting of S _S is provided. When starting the inverter, this pulse generator PG activates the capacitor C by igniting the thyristor S _S through the gate circuit G _S from the start pulse generating circuit MM which outputs one pulse when the tank frequency confirmation command switch SW ₁ is turned on. Discharge from _p through coil L _p into the tank circuit. The pulse generator PG, switch _SW1 , start pulse generating circuit MM, and gate circuit _GS described above constitute a pulse generating means for applying one pulse voltage to the load tank circuit.

By supplying energy from the pulse generator PG to the tank circuit, the tank circuit causes a damped oscillation as shown in FIG. 3b, and also turns off the thyristor S _S (FIG. 3a). The oscillating voltage in the tank circuit is transferred to the instrument transformer.
PT, and this detection signal is converted into a rectangular wave by a zero-cross comparator CP _p to obtain a pulse that matches the period of the tank circuit. The trailing edge of the output of the comparator CP _p (FIG. 3c) is detected by the differentiating circuit DFF (FIG. 3d).

The RS type flip-flop _FF1 is set by the output of the start pulse generation circuit MM mentioned above, and reset by the output of the differentiation circuit DFF mentioned above, so that
As shown in FIG. 3e, a logic "1" is obtained at the output Q for a time period corresponding to a half cycle of vibration of the tank circuit. A counter whose gate is opened only during this output Q period.
The CNT stores a count value proportional to the resonant frequency of the tank circuit by counting the pulses of the reference pulse generator CK. The digital-to-analog converter DA which receives this count value as an input obtains an analog voltage proportional to the input, and this voltage is input to the voltage-to-frequency converter VF to obtain an inverter operating frequency signal at the output of the converter VF. Instrument transformer PT up to the above
The circuits from VF to converter VF constitute resonant frequency detection means for detecting the resonant frequency of the load tank circuit from the period of the oscillating voltage generated in the load tank circuit.

In addition, the output of the digital-to-analog converter DA is determined by the window comparator CP ₁ to determine whether the inverter operating frequency deviates from the upper and lower limits limited by the main circuit switch turn-off time, etc., and also from the set frequency range due to load open or short. If the load condition is inappropriate, the alarm indicator LED will be lit to indicate that starting is prohibited and a load check is required. The comparator CP ₁ and the alarm indicator LED described above utilize the resonance frequency detection function of the resonance frequency detection means to determine whether or not the resonance frequency of the load tank circuit is out of a predetermined frequency range to determine the suitability of the load conditions; It is used as a means of determining unsuitability.

With this startup preparation, the inverter operating frequency signal that matches the load's resonant frequency is transmitted to the converter VF.
After obtaining the output of
By operating SW ₂ , start control is performed in modes (1), (2), (3), and (4) described above.

When switch SW ₂ is turned on, counter timer _TA is started and flip-flops FF ₂ and FF ₃ are initialized to the reset state. The timer T _A is set to the time required for the mode (1) described above, and its output (Fig. 4 a) is synchronized with the signal of the converter VF (Fig. 4 e) to the gates of the thyristors SU _A and SX. Circuit G _SUA ,
The G _SX ignition control signal (Fig. 4 h, m) is used to pre-charge the capacitor C _A. After the time limit of timer T _A , timer T _B is activated by its output, and thyristors SU _A and SX _A are activated for the duration of the output (Fig. 4b).
A short-circuit current circuit of the DC reactor DCL is formed by obtaining the ignition control signal (Fig. 4 h, i). After timer T _B expires, its output starts timer T _C ,
During the period of its output (Fig. 4c), the complementary outputs (Fig. 4f, g) of the output 1/2 frequency dividing flip-flop _FF3 of the converter VF are connected to the thyristors SU _A , SX and SX _A , SU.
The alternating ignition control signals (h, i, l, m in Fig. 4) are used to perform width assist operation. After the expiration of timer T _C , the flip-flop FF ₂ is set at its output, and from its output (FIG. 4d) on, the flip-flop FF ₃ is set.
The complementary outputs of thyristors SU, SY, SX, and SV are used as alternate firing control signals (j, k, l, m in Figure 4), and main circuit operation is started. Switches up to the above
Each circuit from SW ₂ to the gate circuit constitutes a gate control circuit that starts and operates the main circuit of the current source inverter using the resonance frequency detected by the resonance frequency detection means as the starting frequency.

As described above, according to the present invention, since the resonant frequency of the load tank circuit is detected and the frequency is automatically set as the inverter operating frequency, reliable starting is possible for loads within the frequency range allowable for the inverter main circuit. and enable driving. Furthermore, abnormalities in the load tank circuit can be clearly distinguished from abnormalities in the power supply to be detected, making it easier to search for the cause of the accident and preventing startup failures due to abnormalities in the load.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram for explaining the starting of a current source inverter, FIG. 2 is a circuit diagram showing an embodiment of the present invention, and FIGS. 3 and 4 are time charts for each part of FIG. 2. SU _A , SX _A ...Auxiliary thyristor for starting, C _A ...Capacitor for starting, SW ₁ ...Tank frequency confirmation command switch, MM...Start pulse generation circuit, G _S ...Gate circuit, PG...Pulse generator, CP _p ...Zero cross Comparator, CNT...Counter, _SW2 ...Inverter start command switch, T _A , T _B , T _C ...Counter timer.

Claims (1)

[Scope of Claims] 1. Pulse generating means for applying one pulse voltage to the load tank circuit of the current source inverter when starting the current source inverter, and an oscillating voltage generated in the load tank circuit by the pulse voltage. and a gate control circuit that starts and operates the main circuit of the current source inverter using the resonant frequency as a starting frequency. Control device for current source inverter. 2. In claim 1, the resonant frequency detection means is a current source inverter including means for determining suitability or unsuitability of load conditions depending on whether the resonant frequency of the load tank circuit deviates from a predetermined frequency range. control device.