The difference of series inverter and parallel inverter is that they use different oscillation circuits. The series inverter connects L, R and c in series and parallel inverter connects L, R and c in parallel.
Series Inverter vs. Parallel Inverter
- The load circuit of series inverter has low impedance. The voltage source power supply is required and the large filter capacitor shall be connected at the DC power terminal in parallel. If the inverter fails, it is difficult to provide protection because of large surge current. The load circuit of parallel inverter shows high impedance and the current source power supply is required. The large reactor shall be connected at the DC power terminal in series. If the inverter fails, it is easy to provide protection because the large inductor can limit the current so as to avoid large impact.
- The output voltage of the series inverter shows a rectangular wave and the output current shows an approximate sinusoidal wave. When the current on the thyristor shows zero crossing, the current inversion will be conducted. This is why the current always leads the voltage. The output voltage of the micro inverter in parallel connection is approximately a sinusoidal wave and the output current shows a rectangular wave. The commutation is carried out before the zero crossing of the voltage on the resonant capacitor. Thus, the load current always leads the voltage. Both the series inverter and the parallel inverter work under the capacitive loading status.
- The series inverter adopts the power supply with constant voltage. To avoid the short circuit caused by the simultaneous conduction of the upper and lower arm thyristors of the inverter, the switch shall be turned off first and then be turned on in the current inverting process. The parallel inverter adopts the power supply with constant current source. To avoid the large induced potential caused by filter reactance, the current shall be continuous. It shall be ensured that the upper and lower arm thyristors of the inverter shall be turned on first and then be turned off in the current inverting process.
- The work frequency of the series inverter shall be lower than the oscillation frequency of the load circuit. The work frequency of the parallel inverter shall be slightly higher than the oscillation frequency of the load circuit.
- There are two methods to adjust the power of series inverter. That is to change the DC voltage or the thyristor trigger frequency. Normally, the power of parallel inverter can only be adjusted by changing the DC voltage. In addition, the power can also be increased by adjusting the inverter leading angle but the adjustable range is small.
- In the current inverting process of the series inverter, the thyristor is turned off naturally and the off time is very short, so small loss is caused. But in the current inverting process of the parallel inverter, the thyristor is forced to be turned off for long time and the great loss will be caused. Thus, the series inverter can be used in the heating device with high-frequency induction.
- The thyristor of series inverter does not need to bear high voltage. When adopting the 380V power grid, the thyristor of 1200V can be used. However, the thyristor shall load all the current, including the active and reactive current. When the inverter thyristor loses the pulse, the oscillation will stop but the inverter will not be overturned. The voltage loaded by the thyristor of parallel inverter is relatively high and the voltage is increased rapidly with the increasing of power factor angle. The load constitutes an oscillating current circuit. When the active current passes by the thyristor and the thyristor accidentally looses the trigger pulse, the oscillation can stilled be maintained and can work stably.
- The series inverter can work under both self-excitation and external-excitation. When it works under external-excitation, it is only necessary to adjust the trigger pulse of the inverter. Normally, the parallel inverter can only work under the self-excitation status.
- If the trigger pulse of the series inverter thyristor is asymmetry, the DC will not be introduced and the normal operation will not be affected. However, if the trigger pulse of the parallel inverter is asymmetry, the DC will be introduced and the failure will be caused.
- The series inverter can be started easily and can be applied in the workplace that requires frequently power starting. Meanwhile, the parallel inverter cannot be started easily because the additional starting circuit is required for starting the parallel inverter.
- As the series inverter thyristor bears the rectangular wave voltage, the voltage loaded on the thyristor will increase greatly. The absorption circuit plays an important role in this process. It is not necessary to have high current rise rate. The current passing by the thyristor of the parallel inverter is the rectangular wave. Thus, the high current rise rate is required but it is not necessary to have high voltage rise rate (du/dt).
- When the induction heating coil and the inverter power supply (include the tank capacitor) of the series inverter are distant from each other, the output power will not be greatly affected. If the coaxial cable is used or the forth-and-back cables are set closely together (it is better to twist them together), basically the output power will not be affected at all. The heating coil and the power supply (especially the tank capacitor) of the parallel inverter shall be set together as close as possible. Otherwise, the power output and the work efficiency will be greatly reduced.
- The voltage on the induction coil of the series inverter and the parallel inverter is Q times of the inverter output voltage. The current passing by the induction coil is equal to the output current of the inverter.
