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1. The two types of parallel inverters and series inverters can be divided from the load resonance mode. The main technical characteristics of the series inverter and parallel inverter are listed below and their comparisons: Series resonant voltage withstand test devices and The difference in parallel inverters stems from the difference in the oscillating circuits they use. The former is connected in series with L, R, and C, and the latter is connected in parallel with L, R, and C.
(1) The load circuit of the series inverter presents a low impedance to the power supply and requires power supply from the voltage source. Therefore, a large filter capacitor must be connected in parallel at the end of the rectified and filtered DC power supply. When the inverter fails, the surge current is large and the protection is difficult.
The load circuit of the parallel inverter has a high impedance to the power supply and requires power supply from the current source. A large reactor must be connected in series at the end of the DC power supply. However, when the inverter fails, since the current is limited by a large reactance, the impact is not great and it is easier to protect.
(2) The input voltage of the series inverter is constant, the output voltage is a rectangular wave, and the output current is approximately a sine wave. The commutation is performed after the current on the thyristor is zero-crossed, and therefore the current is always ahead of the voltage by φ. The input current of the parallel inverter is constant, the output voltage is approximately a sine wave, and the output current is a rectangular wave. The commutation is performed before the voltage across the resonant capacitor crosses zero, and the load current is always in front of the voltage one φ angle. This means that both are working in capacitive loads.
(3) The series inverter is a constant voltage source power supply, in order to prevent the upper and lower bridge thyristors of the inverter from being turned on at the same time, causing a short circuit of the power supply. When the inverter is commutated, it must be turned off first and then turned on. That is, all thyristors (other power electronic devices) should be turned off for a period of time (t ). The stray inductance at this time, that is, the induced potential generated from the dc terminal to the device lead inductance, may cause the device to be damaged, and therefore it is necessary to select a suitable device for the surge voltage absorption circuit. In addition, during the off-time of the thyristor, in order to ensure continuous load current and protect the thyristor from the high voltage on the converter capacitor, fast diodes must be connected in parallel across the thyristor. The parallel inverter is a constant-current power supply. In order to avoid a large induced potential on the filter reactance Ld, the current must be continuous. In other words, it must be ensured that the thyristors on the upper and lower arms of the inverter are turned off and then turned off.
That is, all thyristors are in conduction during the commutation period (tγ). At this time, although the inverter arm is straight through, since Ld is large enough, it will not cause a short circuit in the DC power supply, but the long commutation time will reduce the system efficiency, and therefore, tγ needs to be shortened, that is, the Lk value is reduced.
Series resonance and parallel resonance difference 3
(4) The working frequency of the series inverter must be lower than the natural oscillation frequency of the load circuit. That is, it should be ensured that there is a suitable t time, otherwise the inverter will fail due to the direct connection between the upper and lower arms of the inverter.
The operating frequency of the parallel inverter must be slightly higher than the natural oscillation frequency of the load circuit to ensure that there is a suitable back pressure time t, otherwise it will cause thyristor commutation failure; but if too high, the thyristor during the commutation The reverse voltage is too high, which is not allowed.
(5) There are two ways to adjust the power of the series inverter: change the DC supply voltage Ud or change the trigger frequency of the thyristor, ie change the load power factor cosφ.
The power adjustment mode of the parallel inverter can generally only change the DC power voltage Ud. Although changing the cosφ can also increase the inverter output voltage and increase the power, the allowable adjustment range is small.
(6) When the series inverter is commutating, the thyristor is naturally turned off, and its current is gradually reduced to zero before being turned off, so the turn-off time is short and the loss is small. During commutation, the time during which the turned off thyristor is back pressurized (t + tγ) is longer.
Series resonance and parallel resonance difference 4
When the parallel inverter is commutating, the thyristor is forcibly turned off in the full-current operation. After the current is forced to drop to zero, a back-pressure time is added and the turn-off time is longer. In contrast, series inverters are more suitable for use in induction heating devices with higher operating frequencies.
(7) The thyristors of series inverters are required to have low voltages. When 380V power supply is used, a thyristor of 1200V is used. However, all currents in the load circuit, including active and reactive components, must flow through thyristors. Inverting thyristors lose pulse and only cause the oscillation to stop, which will not cause the inverter to reverse.
The thyristors of parallel-connected inverters are required to withstand high voltages, whose value increases with the power factor angle φ, and increases rapidly. However, the load itself constitutes an oscillating current loop. Only the active current flows through the inverting thyristor, and the inverting thyristor occasionally loses the trigger pulse, the oscillation can still be maintained, and the operation is relatively stable.
(8) The series inverter can be self-excited or he can work. When he works, only changing the frequency of the inverter trigger pulse can adjust the output power; while the parallel inverter can only work in the self-excited state.
(9) In the series inverter, the trigger pulse of the thyristor is asymmetric and will not introduce the DC component current to affect the normal operation; while in the parallel inverter, the trigger pulse of the inverter thyristor is asymmetric, it will introduce the DC component. Current causes malfunction.
(10) Series inverters are easy to start and are suitable for applications where frequent starting work is required. Parallel inverters require additional starting circuits and start-up is more difficult.
(11) Because the thyristors in series inverters are subjected to rectangular wave voltages, the value of du/dt is large. The absorption circuit plays a key role, and its di/dt requirements are lower.
In a parallel inverter, the current flowing through the inverter thyristor is a rectangular wave, thus requiring a large di/dt, while the requirement for du/dt is lower.
(12) When the distance between the inductive heating coil of the series inverter and the inverter power supply (including the tank capacitor) is far, the impact on the output power is small. If the coaxial cable is used or the future loop is laid as close as possible (twisted together better), there is almost no effect. For a parallel inverter, the induction heating coil should be as close as possible to the power supply (especially the tank capacitor), otherwise the power output and efficiency will be greatly reduced.
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