(1) Rectifier. As one of the most important components of the inverter, the rectifier can convert the AC power into direct current. Its working method is more common to that of a battery charger or arc welder. It uses a diode bridge to restrict the communication sine wave from moving in only one direction. The result is a fully rectified communication waveform interpreted by the DC circuit as a local DC waveform, while a three-phase inverter takes three independent communication input phases and converts them into a single DC output. In addition, most three-phase inverters can also withstand a single-phase (230V or 460V) power supply. However, since there are only two input branches, we need to derate the output so that the DC current that occurs decreases proportionally. On the other hand, a true single-phase inverter can use a single-phase input and produce a DC output proportional to the input. When it comes to variable speed operation, there are two main reasons why three-phase motors are more common than single-phase counter components. First, they have a wider power scale, and secondly, single-phase motors usually require some external interference to start spinning.
(2) DC bus. The second component of the DC bus, which is not seen in all inverters, does not directly affect the inverter operation. But it has always been found in high-quality universal frequency converters. The DC bus uses capacitors and inductors to filter out the communication "ripple" voltage in the converted DC current, and then enters the inverter part after this process. In addition, it prevents harmonic distortion from the filter and can be fed back to the inverter power supply.
(3) Inverter. The inverter uses three sets of high-speed switching transistors to create all three phases of DC "pulses" that mimic the communication of sine waves. These pulses can determine the voltage change and frequency of the wave. Modern frequency converter inverters use a technique called "pulse width modulation" to regulate voltage and frequency. Let's give a brief introduction to IGBT, which refers to the "insulated gate bipolar transistor", which is the switching element of the inverter. The transistor can act as an amplifier and add a signal, or it can act as a switch and simply turn the signal on and off. Modern IGBTs can provide higher switching speeds (3000 - 16000 Hz) and reduce heat generation. Higher switching speeds can significantly improve the accuracy of communication radio wave simulations and reduce motor noise. The heat reduction that occurs means a smaller heat sink, which in turn cuts the footprint, increasing equipment operating expenses and reducing costs.
(4) The concept of useful voltage. Let's understand a very important concept in the principle of frequency converters, that is, 'useful voltage'.
A DC voltage and an alternating voltage are applied separately to two identical resistive devices. If the amount of heat generated by them is equal within the phase, then the value of the DC voltage is regarded as the useful value of the alternating voltage, which is also called the useful voltage. The size of the communication electricity changes with time, and the size of the instantaneous value changes between zero and positive and negative peaks, and its maximum value is only a temporary value, which does not reflect the ability of the communication electricity to do work.
For example, two 460V communication sine waves, one is a 60hz curve and the other is a 50hz. Both have a peak voltage of 650V, however, 50hz is much wider and it is easy to see that the area within the first half of the 50Hz curve (0 - 10ms) is larger than the first half of the 60Hz curve (0 - 8.3ms). Also, because the area under the curve is proportional to the useful voltage, the useful voltage is higher. As the frequency decreases, the increase in useful voltage becomes more dramatic.
Suppose we let the 460V motor work at these higher voltages, then its lifespan will be greatly reduced. Therefore, the inverter must constantly change the "peak" voltage relative to the frequency to adhere to a stable useful voltage. The lower the operating frequency, the lower the peak voltage.
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