JPH0753314B2 - Power control device for inverter type resistance welding machine - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply control device for automatically suppressing a magnetic bias phenomenon in a welding transformer of an inverter resistance welding machine.

(Prior Art) Recently, a resistance welding control device using an inverter in a power supply circuit has been developed and has become widespread. Since such an inverter resistance welding machine has a higher operating frequency than the conventional AC resistance welding machine, the cross-sectional area of the core of the welding transformer can be reduced, and the welding transformer can be made smaller and lighter. It is advantageous to

However, there are variations in the collector-emitter voltage (VCE) and switching speed of each of the transistors that perform the switching operation of the inverter, and the forward voltage (VF) of the two rectifying diodes provided on the secondary side of the welding transformer. Due to variations etc., the magnetic flux flowing in the core of the welding transformer becomes unequal depending on the direction of flow, and by making the core smaller depending on the operating frequency, magnetic saturation is reached with either the positive or negative polarity of the welding transformer. The phenomenon was easy to occur. If the voltage continues to flow in the state where this demagnetization phenomenon occurs, it becomes as if a direct current component had been superimposed on the welding transformer, causing the demagnetization phenomenon to progress further due to the accumulated residual magnetic flux, resulting in excessive magnetic saturation. It was extremely dangerous because an electric current flowed to destroy the transistors and diodes and burned out the welding transformer.

Therefore, conventionally, as a method of preventing or suppressing this demagnetization phenomenon, the size of the core is increased and the saturation point is raised so that magnetic saturation does not occur in the transformer core even if residual magnetic flux is generated due to superposition of direct current components. Or a gap is provided on the cut core connecting surface to prevent residual magnetic flux from being generated, or an unbalanced magnetic flux phenomenon is observed while observing the primary current waveform of the welding transformer in anticipation of the imbalance of magnetic flux between positive and negative polarities. The power controller was manually adjusted so that it would not occur.

(Problems to be solved by the invention) However, increasing the size of the transformer core in order to raise the magnetic saturation point results in a conflict with the size and weight reduction of the welding transformer, and prevents the generation of residual magnetic flux. Providing a gap in the cut core has the disadvantage that the magnetic resistance increases, iron loss increases, and heat generation in the transformer is likely to occur.

In addition, even if the magnetic flux between both polarities is balanced by manual adjustment, it is only temporary, and the operating point of the inverter / transistor changes with the change of the ambient temperature, causing the balance to be lost, or the welding transformer, the inverter.・
If the balance is lost by modifying or replacing parts such as transistors and rectifying diodes, or elements as necessary, it has to be readjusted, which is troublesome.

The present invention has been made in view of the above problems, and provides a power supply control device that suppresses, when a magnetic bias phenomenon occurs in a welding transformer of an inverter resistance welding machine, with a quick and precise control. The purpose is to

(Means for Solving Problems) In order to achieve the above object, a power supply control device for an inverter-type resistance welding machine according to the present invention rectifies commercial alternating current into direct current, and converts the direct current to a predetermined frequency by an inverter. Converted to pulsed high-frequency alternating current, passed the high-frequency alternating current through a welding transformer and then made into a direct current again through a rectifier, in an inverter resistance welding machine adapted to supply this direct current to the material to be welded via a welding electrode, Current detection means for detecting the primary current of the welding transformer, and a predetermined point in the first half position in the intermediate portion between the rising portion and the falling portion of each electromotive pulse of the primary current based on the output signal of the current detecting means. The difference between the current value of the current value and the current value at a predetermined point after that is calculated as the amount of change in the current in the intermediate portion, and each of the two pulses having different polarities before and after each other is calculated. The amount of change is compared and the output signal representing the comparison error is generated as a magnetic bias detection signal, and the magnetic bias detection signal from the magnetic bias detection means is received. And an inverter control means for controlling the switching operation of the inverter.

(Operation) FIG. 4 shows a primary current waveform for one cycle. This pulse-shaped current waveform has a similar shape between positive and negative polarities, and can be divided into three parts, a rising part, a middle part, and a falling part. When the demagnetization phenomenon does not occur, the intermediate portion becomes a straight line with a certain inclination as shown by the solid line. However, when the magnetic bias phenomenon occurs, in one of the polarities, for example, in the positive polarity pulse as shown in the drawing, the rising portion and the falling portion hardly change, but the middle portion has a corner or a protrusion as shown by a broken line. After that, the slope is not constant and changes.

According to the present invention, the primary current is constantly monitored by the current detecting means, and the current value at the predetermined point in the first half position in the intermediate portion between the rising portion and the falling portion of each of the positive and negative pulses and the subsequent values. The difference from the current value at the predetermined point is obtained as the current change amount in the intermediate portion.

Then, if there is no eccentricity in the welding transformer,
The amount of change in current in the intermediate portion of both positive and negative pulses that follow each other is almost equal to each other, and the comparison error is almost zero. The bias detection signal indicating this state is given to the inverter control means, but this does not cause any particular change in the switching control of the inverter.

If the magnetic bias phenomenon occurs, the amount of change in the current in the middle portion of both pulses of one polarity becomes larger than that of the other polarity, so a comparison error occurs and the bias detection signal shifts. In response to this, the pulse width control means controls the switching operation of the inverter so as to return the deviation (comparison error) to zero. That is, switching control is performed so that the pulse width of the pulse of the polarity in which the amount of change in current in the middle portion of both pulses is large is made relatively smaller than the pulse width of the pulse of the other polarity. Demagnetization of the transformer is weakened and suppressed.

(Embodiment) An embodiment of the present invention will be described below with reference to FIGS. 1 to 3.

FIG. 1 shows a main configuration of an inverter type resistance welding machine system according to an embodiment of the present invention.

The input terminal of the rectifier circuit 12 is connected to the three-phase commercial AC power supply terminal 10, and DC is obtained at the output terminal of the rectifier circuit 12. This direct current is smoothed by a smoothing circuit composed of the coil 14 and the capacitor 16, and then input to the inverter circuit 18. The inverter circuit 18 is a well-known one using GTO or GTR as a switching element, and converts the input DC into a pulse-shaped (rectangular wave) high-frequency AC by a high-frequency switching operation. The switching of the inverter circuit 18, and thus the pulse width of its high frequency AC output, is controlled by a pulse width control circuit 32 via an inverter drive circuit 30, as will be described later.

The high frequency alternating current output from the inverter circuit 18 is supplied to the primary coil of the welding transformer 20, which is smaller than the conventional one.
High-frequency alternating current with a reduced voltage is obtained in the secondary coil,
This is rectified into a direct current by a rectifier circuit 24 including a pair of diodes 22a and 22b. Then, this direct current I is supplied to the materials to be welded 28a, 28b via the welding electrodes 26a, 26b.

The pulse width control circuit 32 controls the pulse width of the high frequency AC output of the inverter circuit 18 by a pulse width modulation (PWM) method. When the start signal ST is received from the sequence circuit 36, the pulse width control circuit 32 outputs, for example, a constant frequency signal from the frequency generator 34. The triangular wave signal SF is input as a modulation wave to generate a PWM signal SPWM, and this signal controls ON / OFF of each GTO or GTR of the inverter circuit 18.

By the way, according to the present embodiment, there is provided a device for automatically variably controlling the pulse width of the inverter output and suppressing the bias magnetism when the bias magnetism phenomenon occurs in the welding transformer 20.

First, in order to detect the magnetic saturation of the welding transformer 20, a current sensor 38 such as a toroidal coil or a current transformer is provided in the wiring (primary conductor) between the primary coil of the welding transformer 20 and the output terminal of the inverter circuit 18. To be The output signal of the current sensor 38 is supplied to the primary current detection circuit 40, and the circuit 40 supplies the output signal SI corresponding to the primary current IP to the magnetic bias detection circuit 42. As will be described later, the eccentricity detection circuit 42 generates a detection signal MS indicating the eccentricity state of the welding transformer 20, and in response to the signal MS, the pulse width control circuit 32 causes the primary current IP to occur when the eccentricity phenomenon occurs. The switching operation of the inverter circuit 18 is controlled so as to adjust the pulse width of.

FIG. 2 shows a circuit configuration example of the bias magnetic detection circuit 42.

In this magnetic bias detection circuit 42, a sample and hold circuit (S / ₁ ) 44, an error circuit 46, and a sample and hold circuit (S / ₁ )
₂ ) 48 forms a positive pulse change amount calculation circuit, and the sample hold circuit (S / ₃ ) 50, error circuit 52, sample hold circuit (S / ₄ ) 54 forms a negative pulse change amount calculation circuit .

Detection circuit 40 representing the primary current Ip as shown in FIG.
The output signal SI from the sample and hold circuits 44 and 50 and the error circuit 4 of the positive and negative pulse change amount detection circuits, respectively.
Input to 6,52.

In the positive pulse change amount detection circuit, the sample and hold circuit 44 uses the gate signal GATE1 as shown in FIG. 3 (B).
Value IPO at the first half position of the positive pulse middle part in response to the falling edge of
₊ Is sampled and the sampled value is held until the next sampling. The error circuit 46 has its sampled value IPO
_{With +} as the reference value, the amount of change in the positive pulse intermediate portion relative to that is output as an error signal with time. The sample and hold circuit 48 uses a gate signal GA as shown in FIG.
In response to the fall of TE2, the amount of change ΔIP ₊ (IPP ₊ − at approximately the end position (near the peak value) of the middle portion of the positive pulse
IPO ₊ ) is sampled and the sampled value ΔIP ₊ is applied to one input terminal of the comparison circuit 56. The same operation as described above is performed in the negative pulse change amount detection circuit, and the change amount ΔIP ₋ (IPP ₋ −IPO ₋ ) for the negative pulse is determined at the same timing as for the positive pulse. This change amount ΔIP ₋ is applied to the other input terminal of the comparison circuit 56. The comparator circuit 56 compares the two inputs ΔIP ₊ and ΔIP ₋ and outputs the difference (ΔIP ₊ −ΔIP ₋ ) as the magnetic bias detection signal MS.

The gate signals GATE1 to GATE4 are GATE2 and GA among them.
TE4 has pulse width corresponding to PWM signal SPWM, and GATE1, G
ATE3 may have a certain pulse width with respect to GATE2 and GATE4, respectively.

The pulse width control circuit 32 (FIG. 1) uses the magnetic bias detection signal MS (Δ
IP ₊ -ΔIP _-) in response to adjusting the pulse width of positive and negative pulses of the next cycle. If the magnetizing phenomenon does not occur in the welding transformer 20, MS (ΔIP ₊ −ΔIP ₋ ) is almost zero, which causes no particular change in the switching control of the inverter. However, if a magnetic bias phenomenon occurs and an angle or a protrusion appears in the middle portion of the positive pulse as shown in FIG. 3 (A), for example, the amount of change becomes larger than that of the negative pulse, so MS (ΔIP _{+ -ΔIP -)} is biased to a positive value, the pulse width control circuit in response to this 32 generates a PWM signal SPWM as return the shift to zero. As a result, switching control is performed so that the pulse width of the positive polarity pulse is relatively smaller than the pulse width of the negative polarity pulse with respect to the high frequency AC output of the inverter circuit 18, and the demagnetization of the welding transformer is weakened. Will be suppressed.

As the amount of change in each of the positive and negative pulses of the primary voltage, in addition to the values IP ₊ and IP ₋ as described above, the value at the end position of the pulse intermediate portion, that is, the fall of the gate signals GATE2 and GAT4, respectively. It is possible to select the values IPP ₊ and IPP ₋ to be sampled, and in this case as well, the difference IPP ₊ and −IPP ₋ between the two represents the biased state, and this can be used as the bias detection signal. When a magnetic bias phenomenon occurs, corners or protrusions appear in the waveform of the middle part of either the positive or negative pulse of the primary current.Therefore, compare the current change amount in the middle part of both positive and negative pulses with each other. All circuit configurations that generate the comparison error and control the switching operation of the inverter so that the comparison error becomes zero are included within the scope of the technical idea of the present invention.

(Effect of the Invention) As described above, according to the present invention, when the magnetic flux demagnetization phenomenon occurs in the welding transformer, the current in the middle portion of the positive / negative (or negative / positive) pulse that follows the primary current is generated. By detecting the change amount comparison and controlling the switching operation of the inverter so that the comparison error becomes zero, the bias magnetic phenomenon is suppressed quickly and in detail.
Even if a compact and lightweight welding transformer is used, it is possible to prevent damage to the inverter transistor, the diode, the welding transformer, or the like. In addition, since the adjustment is automatically performed, there is no need for manual adjustment, and there is an advantage that readjustment is not required even when various parts or elements are modified or replaced.

[Brief description of drawings]

1 is a block diagram showing a main configuration of an inverter type resistance welding machine system according to an embodiment of the present invention, FIG. 2 is a block diagram showing an example of a circuit configuration of the bias magnetic detection circuit 42 of FIG. 1, FIG. 3 is an iming diagram for explaining the operation of the demagnetization detection circuit 42, and FIG. 4 is a waveform diagram showing a primary current waveform for one cycle and a demagnetization phenomenon. In the drawing, 12 …… rectifier circuit, 18 …… inverter circuit, 20 …… welding transformer, 24 …… rectifier circuit, 26a, 26b …… welding electrode, 28a, 28b …… welding material, 32 …… pulse width control Circuit, 34 …… Frequency generator, 36 …… Sequence circuit, 38 …… Current sensor, 40 …… Primary current detection circuit, 42 …… Bias magnetism detection circuit, 44,50 …… Sample and hold circuit, 46,52 …… Error circuit, 48,54 …… Sample and hold circuit, 56 …… Comparison circuit.

Claims (1)

[Claims] 1. A commercial alternating current is rectified into a direct current, the direct current is converted into a pulsed high frequency alternating current of a predetermined frequency by an inverter, and the high frequency alternating current is passed through a welding transformer and then passed through a rectifier to be converted to direct current. In an inverter resistance welding machine configured to supply the welding target material through a welding electrode, current detection means for detecting the primary current of the welding transformer, and the primary current of the primary current based on the output signal of the current detection means. The difference between the current value at the predetermined point at the first half position and the current value at the predetermined point after that in the intermediate portion between the rising portion and the falling portion of each pulse is calculated as the current change amount in the intermediate portion, and the phases are sequentially changed. A bias detecting means for comparing the respective variations of the two pulses having different polarities and generating an output signal representing the comparison error as a bias detecting signal. Receiving said polarized 磁検 detection signal from Kihen 磁検 detecting means,
An inverter control means for controlling the switching operation of the inverter so that the comparison error becomes zero.