Most of the inverter manufacturers produce traditional inverters, and their control systems and other power supplies are directly provided by single-phase or three-phase power grids. However, due to the increasing use of VFDs and servo motor amplifiers, a new power supply may emerge: powered by the central DC bus of the VFD. The advantage of this power supply is that the "free" kinetic energy stored in the running motor can be used to power the control system. If this possibility materializes, it will greatly increase the robustness of the power supply to the fluctuations of the opposing grid, without the need for a battery buffer system that requires frequent protection.
To understand this application, we can take the crane as an example: what happens if the power supply network suddenly stops when the crane has just lifted the goods? Often, the control system must be equipped with a battery buffer so that the raised goods can be safely lowered. If the control system is powered by the central DC bus, when the goods are dropped, the crane's motor will act as a generator, insisting on the voltage on the central DC bus and the control system. This forced synchronization between the actuator and the control system makes system planning easier and safer.
So, what are the requirements that must be met for a power supply that is suitable for use with a central DC bus? Some people may say that there is no requirement, because from the basic principle, the switching power supply is always rectified internally, so the switching power supply can be AC and DC power supply. Moreover, the product description of many switching power supplies will clearly indicate a certain DC input range, such as 450... 750Vdc, etc. So, why is it that in practice, the direct access of the switching power supply to the central DC bus presents a problem?
One reason for this is that the voltage on the central DC bus is often a high-frequency communication voltage with fluctuations of several hundred volts to ground, which is called "common-mode noise". The symbol "DC voltage" actually refers only to the voltage between the positive and negative electrodes, not to ground, as the entire central DC bus changes to ground at the same rhythm. This effect is due to the fact that fast switching (IGBTs) in frequency conversion periodically ground the positive and negative electrodes at high frequencies through motors and other capacitors.
Although the inverter has its own filter, it only operates externally, that is, it faces the input power grid; Internally, that is, facing the center of the DC bus, it is not possible. And because this is locally internal to the device, there is no clear EMC specification. However, the electromagnetic compatibility specification of the power supply does not include this application, because the input terminal of the general power supply will not have long-term disturbance in the frequency range of 600V and several thousand Hz, which exceeds the input reliability value required by the general power supply by dozens of times. Therefore, the power supply requirements of the central DC bus are completely different from those of the general power supply.
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