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Individual phase voltage measurement in 3-phase system

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ravi1488

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Hello All,

How to measure individual phase voltages in the 3-phase unbalanced system. I dont want to use any kind of transformer. Also the system Ground and Neutral should not be common.

Please suggest.

Thanks
 

Hi,

there are power metering devices with serial interfcace.

spend an extra supply for it and transmit the data with optocouplers.


For better information it woud be helpful to give us technical data like:
voltage, data rate, resolution, frequency, desired values like "RMS"...
and what you want to do with the values: for example: feed to a ucontroller and do some regulation loop

Klaus
 
Thanks for reply.

I want RMS values of the voltage. I want to measure the RMS voltage with Microcontroller. I dont want to use any external ADC for that purpose. Because I am using this RMS voltage for closeloop control, So I external ADC is not reliable option as interface communication(I2C or SPI) may break.

- - - Updated - - -

The RMS voltage range should be around 0-5V.
 

Differential amplifiers with voltage dividers are the usual solution. You can either use precision rectifier or RMS measurement circuits and feed a DC voltage to the ADC. Or sample the AC voltage and perform the RMS calculation inside the µP.
 
Just a suggestion

If the wave form of the 3 phase is a sinusoidal
Then its easy, all you would need is a 3 phase
Rectifier and a voltage divider to reduce the voltage.
You can then calculate the RMS value
using your micro. (= 0.707 x PK)
However if the wave is not a sine wave, you would have
To do some samples then calculate the RMS value.
(average the samples, square them, take the root)
This is only possible if the sample time of you
ADC is fast it needs to be at least 2X the Max frequency
 

Thanks for reply.

The main problem is that the system is 3-phase unbalanced type. As we are not connecting neutral to the ground, there is some potential voltage at neutral which depends on the voltage of R, Y and B. So the Voltage of R phase with reference to Neutral will change if there is voltage in Y phase or B phase.
 

The main problem is that the system is 3-phase unbalanced type. As we are not connecting neutral to the ground, there is some potential voltage at neutral which depends on the voltage of R, Y and B. So the Voltage of R phase with reference to Neutral will change if there is voltage in Y phase or B phase.
Click to expand...
You did mention this in post #1. Differential amplifiers (or measurement transformers) are usual solutions to handle this situation.
 
Just a suggestion.

Un-balanced ? do mean that the voltages
May not be the same on each phase due to loading.
As there is not neutral point then you can either create one
By using 3 resistors and use the resistor connection junction
as the reference point for your measurements.
The instrumentation should not draw any significant current
To up set the false reference point.

If the phases needs to be completely isolated then
a voltage rectifier across each phase could be used.
It would require multiplex ADC input (or 3 ADC), to acquire
Each phase peak voltage to calculate RMS
Value using the micro computer.
You can find many procession full wave and half wave AC to DC
Rectifiers circuits design suggestions on the WWW, but if it is for power applications, Whereby the phase voltage is much larger than a diode Voltage drop, then maybe a simple diode peak voltage rectifier Would suffice.

To maintain the phase isolation small transformers (one connected to each phase) Could be used before the rectifiers Or an isolated front end instrumentation amps could be used.
 
Last edited:
Hi,

So I external ADC is not reliable option as interface communication(I2C or SPI) may break.
Click to expand...

Why especially should it break? there have to be a lot of other components connected to your ucontroller. Don´t you think these don´t break too?

In my eyes a broken serial connection can be easily recognized by software... but for sure you have to design it for high reliability - not only the communication, but the whole circuit.


Klaus
 

THINKER_KAM_MAN said:
Just a suggestion.

Un-balanced ? do mean that the voltages
May not be the same on each phase due to loading.
As there is not neutral point then you can either create one
By using 3 resistors and use the resistor connection junction
as the reference point for your measurements.
The instrumentation should not draw any significant current
To up set the false reference point.

If the phases needs to be completely isolated then
a voltage rectifier across each phase could be used.
It would require multiplex ADC input (or 3 ADC), to acquire
Each phase peak voltage to calculate RMS
Value using the micro computer.
You can find many procession full wave and half wave AC to DC
Rectifiers circuits design suggestions on the WWW, but if it is for power applications, Whereby the phase voltage is much larger than a diode Voltage drop, then maybe a simple diode peak voltage rectifier Would suffice.

To maintain the phase isolation small transformers (one connected to each phase) Could be used before the rectifiers Or an isolated front end instrumentation amps could be used.
Click to expand...


Yes. Unbalanced means R, Y and B voltages are not equal due to loading. There is common neutral.

- - - Updated - - -

KlausST said:
Hi,



Why especially should it break? there have to be a lot of other components connected to your ucontroller. Don´t you think these don´t break too?

In my eyes a broken serial connection can be easily recognized by software... but for sure you have to design it for high reliability - not only the communication, but the whole circuit.


Klaus
Click to expand...

Thats true other circuits also need high reliability. The communication break with ADC is not acceptable because, there is a close loop control working based the sensing of ADC. So it may leads to whole system failure.
 

Hi,

what about isolation amplifier like AD202?


Klaus
 

I think, I should try with Isolation Amplifiers.
 

I think, I should try with Isolation Amplifiers.
Click to expand...
This will surely work. For cost-sensitive applications, voltage dividers with differential amplifiers should be consdered as well. If you can tolerate a finite isolation resistance against ground (e.g. several MOhms) and a respective leakage current, it's an effective solution.
 
just a suggestion

all you need are voltage transforms for isolation and voltage drop.
you can then sample the waveform of each phase with you fast
ADC and uP and work out the RMS value,Ofsett, voltage, frequency
and any thing else you need.
 
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