How can i measure voltages on each phases of a 3 phase system. The measurement is to be processed by a microcontroller.
Is there any special chips available, or do i need to use the ordinary Transformer-->ADC technique??
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If this is a grid connected mains voltage and frequency system then you could use 3 transformers from each phase to neutral to give you low voltages to process with an ADC.
If you are confident in working directly with high voltage you could use resistive scalers from high voltage to ADC inputs.
What do you expect a "special chip" to do for you?
Would a custom made PCB assembly qualify, as they are sure to exist, and what would you expect to pay?
The ICs below are aimed at energy meter applications and do far more than you have asked for, but almost certainly provide what you want as a subset of their capabilities.
Maxim make an IC (costs about $20/1) which may be almost exactly what you want:
The MAXQ3180 is a dedicated electricity measurement front-end that collects and calculates polyphase voltage, current, power, energy, and many other metering and power-quality parameters of a polyphase load. The computed results can be retrieved by an external master through the on-chip serial peripheral interface (SPI™) bus. This bus is also used by the external master to configure the operation of the MAXQ3180 and monitor the status of operations.
The MAXQ3180 performs voltage and current measurements using an integrated ADC that can measure up to seven external differential signal pairs. An eighth differential signal pair is used to measure the die temperature. An internal amplifier automatically adjusts the current channel gain
Connection is the easy part :-) -
Or $12 from analog devices. ADE7754 Datasheet
If this is not on grid or not mains frequency or not mains voltage then there may be special considerations to properly ensure that your measurements reflected what you wanted to determine. For exampl, a 3 phase alternator driven by a petrol motor, or a quasi sine waver inverter or a wind turbine output could present wavefors, frequencies, rates of change of voltage amd more that challenged a system which was based on grid based assumptions.
This sort of instrument will do the hard work for you, at a price, with data available via RS232. From here
Or this also with RS232 (optically isolated)