How internal 24V DC is generated in a AC Drive
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Some Equipments use an auxiliary power input from mains (230V), others uses directly the mains three phase power input (3x400).
Both of them rectify this input by means of a rectifier bridge and then convert it to a low level control DC voltage by means of a switched mode power supply.
Your question seems incomplete to me, you probably you want to know some other type of information regarding the 24VDC that is made available in the drive terminal blocks.
However, to give you a very general answer like your question, in the electronic regulation boards there are DC / DC converters, which use the DC source coming from the bus after rectification to realize the voltages used by the drive.
There are therefore the voltages for the microprocessors, the 10 V for the analogs circuits power supply lines and the 24 VDC which is normally used for external commands.
Some drives have terminals to power the logic even if there is no power at the input, in order to carry out presets even without power or to keep the drive logic active, if for example it is connected to a field bus. If you specify what interests you, you will probably receive a more detailed answer.