Discussion on DC Reactance and Power Factor Improvement

Sep 04, 2023

DC reactor is not a standard configuration for frequency converters, of course, this is for some brands. Therefore, when choosing a frequency converter, targeted instructions should be made, either directly stating that a DC reactor should be installed in the frequency converter, or limiting its power factor. At the very least, its power factor should not be less than 0.90.

In this way, the manufacturer will understand their intention and configure corresponding models of DC reactors.

A DC reactor is a large coil installed at the top of a frequency converter and connected to a filtering capacitor in the DC circuit.

So, what role does this DC reactor play in the frequency converter? Continuing, of course, it is to improve the power factor. Usually, the power factor of the frequency converter can be mentioned to be very high, above 0.95, indicating that this DC reactor is working.

A while ago, a 400KW low-voltage frequency converter had a short circuit in its built-in DC reactor, which collapsed quite severely. The DC bus was deformed and dried, so it seems to be unusable. There are no spare parts on hand, so can we directly short-circuit it? That's not feasible to specify!

Another implicit function of a DC reactor is to filter out high-order harmonics of the current. Without this large coil, the high-order harmonic current will pass through the filtering capacitor and generate high voltage. The capacitor is the most afraid of high voltage, for example, the voltage withstand value of the filtering capacitor of this frequency converter is 1000V. If it exceeds this value, it will be directly discarded. Similarly, filtering out high-order current components is necessary to ensure the pulsation of DC voltage.

The principle of reactor filtering is very simple, that is, through the inductance coil, the current cannot suddenly change, and through the reactor, the high order current can be directly filtered out.

However, higher harmonic currents are all reactive components, so without filtering, the reactive component is relatively large, which leads to a lower power factor. After DC reactance, the higher harmonic currents are filtered out, and the power factor naturally increases.

Therefore, the most important function of a DC reactor is to filter out high-order current components, which results in a natural increase in power factor and ensures the stability of DC voltage fluctuations.

For some low-voltage frequency converters, the installation of a three-phase incoming reactor plays the same role as filtering, removing the influence of high current and improving the power factor. However, the three-phase incoming reactor is a large inductance, which will produce inductance. After passing through it, it will produce a certain voltage drop, which is not good because it directly affects the voltage of the DC bus.

Due to its influence, the improvement of power factor is also limited, and compared to DC reactors, there is naturally no convenience in using DC reactors.

For frequency converters, a large coil is built in, which greatly increases the total mass, especially for high-power equipment, which can also bring a lot of trouble to equipment installation.

This article will talk about the improvement of DC reactance and power factor. If there are any issues, please leave a message to correct them.

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