Analysis of a non-returning AC frequency conversion main circuit - Power Circuit - Circuit Diagram

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0 Preface

The current AC frequency conversion technology mostly adopts the voltage type PMW control mode, and uses the feedback rectifier bridge connected in parallel to the three-phase inverter bridge to perform reactive current feedback. This method of simultaneous frequency and voltage control to keep the air gap flux constant, whether using parallel or series capacitors for reactive power compensation, will be proportional to the capacitive reactive power and frequency. Full reactive power compensation is achieved within the frequency adjustment range. The reactive current generated by an inductive load such as an asynchronous motor not only increases the capacity of the power supply and the power distribution device, but also inevitably causes a large waste of power in the transmission of the non-functional amount reciprocating. Trying to overcome the current phase lag caused by the step-down and the magnetizing inductance of the stator leakage inductance and the reactive current feedback in the frequency conversion have great practical value for further energy saving.

1 Main circuit wiring and its control method

The wiring circuit of Figure 1 is used to form a non-returning AC frequency conversion main circuit, in which: three thyristors and three rectifier tubes are used to form a three-phase half-controlled rectifier bridge, or six thyristors are used to form a three-phase full-controlled rectifier bridge; End series L1, L circle and L3 3 reactors for current limiting and filtering of capacitor bridges; C11 and C41, C31 and C61, C51 and C21 three pairs of power capacitors of the same capacity form a secondary capacitor bridge, and use T7, T8 and T9 3 bidirectional thyristors are connected to the main capacitor bridge; C1 and C4, C3 and C6, C5 and C2 3 form the main capacitor bridge for the same capacity power capacitor, the midpoint of the 3 bridges and the asynchronous motor three-phase stator winding The three ends are connected; the common thyristors T1 and T4, T3 and T6, T5 and T2 are respectively connected in series to form a three-phase inverter bridge, and the midpoints of the three bridges are connected with the three head ends of the three-phase stator windings. The main circuit is characterized in that: the three-phase inverter bridge and the three-phase main capacitor bridge together form a variable current main circuit of the three-phase winding, and form a double circuit of discharging a capacitor and charging another capacitor when an thyristor is turned on; The main and auxiliary capacitor bridges are connected to the AC terminals by three bidirectional thyristors. In the phase shifting of the trigger phase angle, the capacitor of the subcapacitor bridge can be gradually incorporated into the main capacitor bridge, so that the total capacitance is 50%~100. The main and sub-capacitors are voltage-filtered to the DC link in series or two, and the current is resonated and commutated by the bridge midpoint and the motor winding; the resistors connected in parallel on the inverter bridge thyristor The RC protection circuit in series with the capacitor not only delays the sudden change of voltage to achieve protection of the thyristor, but also forms a reverse voltage in its reverse current to ensure reliable shutdown of the thyristor.

The control method of the thyristor of the inverter bridge is as follows: the conduction cycle is triggered in sequence according to the order of the thyristors T1—T2—T3—T4—T5—T6, and the frequency is smoothly adjusted in the range of 5 ray 60 Hz.

When the trigger phase angles of the three triacs T7, T8 and T9 are advanced or shifted back, the total capacitance is increased or decreased accordingly. The trigger phase angle of the three-phase half-controlled or full-controlled rectifier bridge is the same as the conventional control mode, and the capacitance bridge is used to adjust the amplitude of the output pulse current.

2 Inverter half bridge and capacitor bridge combined flow principle

In order to make the current of the inductor coil rise rapidly and advance its phase, it is effective to dynamically apply the spike voltage and make the capacitance value close to the inductive reactance value in the power frequency range. The converter circuit combined with the inverter half bridge and the capacitor bridge can form a capacitor discharge and alternately charge the other capacitor in the alternating current flow of the two thyristors of the same bridge. For example, during the thyristor T1 conduction to make the winding Wa forward, the charge stored on C1 discharges the winding Wa via T1 while charging the capacitor C4 via the winding Wa; when the thyristor T4 triggers the conduction, the voltage on C4 A reverse discharge loop is formed for the winding Wa, and C1 is simultaneously charged. The combined current conversion mode utilizes the characteristic that the inductive potential of the inductive winding becomes a negative value after the current pulse wave passes through the amplitude, so that the charged capacitor voltage value changes from negative to positive and rises to exceed the voltage value of the DC loop; 1.3 The capacitor with 2 times the DC voltage value will quickly pass the inductive winding with a sharp pulse voltage when switching to the discharge path. The first half-wave current waveform of the discharge current approximates a sine wave; After the sinusoidal half-wave current crosses zero, the through-flow thyristor is naturally turned off. The double-capacitor combined current-passing process causes the capacitors to withstand the voltage of the DC-stacked alternating current, and also allows the current of the inductive load to supply only about 1/2 of the current value during the commutation.

The secondary capacitor bridge and the three bidirectional thyristors form a regulation loop of the capacitance. When the trigger phase angle of the triac T7, T8 and T9 is shifted forward, it is equivalent to incorporating a larger capacitor capacity into the main capacitor bridge, and vice versa. When the corners are moved back, the incorporated capacitance is relatively reduced until it is completely disconnected. This circuit can also be used to classify and control the load current by dividing the sub-capacitor bridge divided into 1 groups by 1 group of 4 solid-state relays.

In the sequential triggering of the thyristors of the inverter bridge, the pulse currents of the forward and reverse directions and separated by a certain electrical angle are combined into an alternating current, and the stator windings generate a corresponding rotating magnetic potential.

This circuit adopts the speed regulation method in which the pulse wave is constant and the interval is changed, that is, when the output thereof changes to the low frequency, only the distance between the pulse wave currents increases correspondingly, and the change of the current amplitude and the pulse width is small. . In the low-frequency operation phase, the width between the sinusoidal half-wave current waveforms is much larger than the width of the half-wave itself, and the rotational potential is still a sine wave closely connected by the positive and negative half-waves, which forms a difference from the conventional frequency conversion method. This fast-rising pulse current waveform can reduce the anti-bucking effect of the stator leakage reacting on the one hand, and avoid a series of losses caused by the reactive current feedback in the forward shift of the phase.

3 Double-winding without feedback inverter and electromagnetic vibration control

When the three-phase stator winding is decomposed into two ends connected in parallel, the first ends of the six windings are connected to the six outputs of the three-phase inverter bridge connected to the midpoint of the bridge in series with one reverse rectifier. ,as shown in picture 2.

The double winding method is adopted, especially when the stator winding adopts a single layer structure and the two phases of the same phase are arranged at intervals of 360° slot potential phase, the magnetic coupling between the two windings is relatively weakened, and the two winding ends are combined. The connected rectifier can realize self-feedback of reactive current. For example, when T1 is turned on and a positive half wave is conducted to the winding Wa1, when the pulse current decreases from the amplitude to make the potential become a negative value, the negative potential forms an inductive discharge to the winding Wa2 and the rectifier D7. The circuit causes the inductance energy stored in the winding Wa1 to be transferred to the winding Wa2 by itself; when the pulse current decreases, the winding of the winding Wa1 and the rectifier D7 is inductively discharged to form an inductive non-functional transfer process. The effect of releasing or absorbing the inductance energy from each other is premised on the fact that the magnetic coupling relationship between the two is weak, especially when the group of electromagnets is controlled to perform the displacement vibration molding, and the self-feedback effect is more remarkable.

When the double-winding non-return frequency conversion control is extended into 6 yoke 18 bridge inverter bridges and main capacitor bridges and sub-capacitor bridges, 12 yokes and 36 output terminals are connected to 12 ya 36 electromagnetic coils, which can be sequentially In the control of the 12-ray 36 beats through the flow, the electromagnets and springs arranged in a matrix form a displacement vibration process. The double-winding circuit structure composed of 6-ray 18 pairs of electromagnetic coils has a remarkable energy-saving effect of self-feedback through the rectifier, and can form an artificial quartz stone sheet in the vibration forming process of the LC resonance. Better densification effect.

4 Calculation of converter circuit parameters

Take the three-phase inverter bridge of Fig. 2 to control the six electromagnetic coils as an example to calculate the parameters of the main circuit. Specifically, the thyristor T1 controls the electromagnet coil Wa1 and calculates the loop formed by the C1 and C4 of the main capacitor bridge. Since the initial voltage values ​​of the two loops composed of C1 and C4 are nearly the same, the Wa1 can be decomposed into two parallel circuits, and the resistance and inductance are R and L, respectively. The simplified equivalent circuit is shown in Figure 3. According to the principle of capacitor charging and discharging of electrician foundation, and using Laplace transform and inverse transform, the current equation of the two loops can be derived as

The current actually flowing through the electromagnetic coil is a composite value of ic1 and ic4, and the first pulse current is approximately a sine wave. When calculating the commutation parameters of an asynchronous motor, the factors affecting the rotating potential should be considered. The choice of capacitor capacity should take into account both the current amplitude and the resonant angular frequency.

5 Conclusion

The double-capacitance resonant current flow method formed by the non-feedback inverter not only realizes the turn-off of the ordinary thyristor, but also realizes the rapid current flow of the inductive winding in the sudden action of the capacitor voltage, thereby generating in the forward phase of the current phase. The power saving effect of the reactive power is fully compensated. Adjusting the capacity of the secondary capacitor bridge is the dominant adjustment method for maintaining the speed and current stability. In the adjustment of maintaining the current amplitude and the pulse width near constant, the frequency is reduced by increasing the width between the pulse waves. This circuit is easy to manufacture high-voltage and large-capacity speed control devices, and can be easily converted into 6-phase or 18-phase to expand the control applied to other devices.