# 3 phase 3 wire measurement

1. ## 3 phase 3 wire measurement

Dear all,

I am trying to build 3 phase voltmeter that could measure L-L and L-N voltage during 3 phase 3 wire and 3 phase 4 wire connection. 3 phase 3 wire and 3 phase 4 wire connection are external connection.

as we know in 3 phase 4 wire we will get neutral reference so we can get accurate reading for L-L and L-N.
but in case of 3 phase 3 wire we dont have neutral reference .In this case it will be difficult to get gnd reference. I would like use both connection type in common circuit,

I have seen one of the product where they using V-X-M-H IC , can some one suggest me functionality of this iC. i have attached schematic for reference.

If any other suggestion with schematic will be appreciated. to differentiate 3 phase 3 wire and 3 phase 4 wireCadstar - Voltmeter.pdf

•

2. ## Re: 3 phase 3 wire measurement

The schematic DOES have a neutral so the IC isn't creating a virtual one.

With only three wires, your only option is to use Earth as a reference but that may not be reliable depending on where you are, or use one of the phases as reference and compare against the other two.

Brian.

3. ## Re: 3 phase 3 wire measurement

The circuit will work on both 3 phase 3 wire and 3 phase 4 wire. when i tested the device i found with neutral i could get 1v difference and without neutral connected means 3 phase 3 wire. i could see difference of 3V.(Neutral connection removed)

THe above attached circuit they using V-X-M-H IC i dont know to identify. its taking care of neutral absence.

can you explain the circuit when 3 phase 3 wire without neutral connected.

when neutral is not connected , unbalance voltage can be seen in neutral point due to (R,Y,B) what does V-X-M-H IC does as functionality.

4. ## Re: 3 phase 3 wire measurement

Here i have attached image for reference.

In circuit: Here i would like use both Y and delta connection for measurement. when delta connected it should connected to virtual neutral . if virtual neutral connected if 3 phase voltage are balance i would get 0 volt across virtual gnd. if 3 phase are unbalance i would get differential voltage across virtual ground . how can i take care if i wanna use Y and delta connection in same circuit.

How can i create virtual neutral for case 1.

In circuit 2:: I have modified 1 st thread circuit . i found few more people using same ckt with V-X-M-H or TA1G . is any one could able to rectify any one of IC.it is SOT 353 package

5. ## Re: 3 phase 3 wire measurement

Hi,

I don´t know how this "VXMH" device works....

But all you need to do is: make NEUTRAL = PIC_ADC_VREF / 2
I don´t know what the ADC uses as reference voltage, thus I can´t give a value.

With the DC shift of Neutral all the ADCs see just positive voltage. (This is important for the ADC to convert the values properly)
The absolute value of the DC shift is not that imortant.
All the rest can be done in software.

In software: NEUTRAL_value = (R + Y + P) / 3
I recommend to do heavy low pass filtering on NEUTRAL_value.

R_phase to star_point voltage = ADC_R_value - NEUTRAL_value
similar with the other phase signals.

For delta voltages just calculate: ADC_R_value - ADC_Y_value.
similar with the other phases. No need to care about NEUTRAL_value.

All this works with and without NEUTRAL connection to the grid.
But you need good isolation of system GND to EARTH. Best if you use a plastic box and battery powered.

Klaus

6. ## Re: 3 phase 3 wire measurement

There are few question which are running out of mind
I wanted to use star and delta connection as common. When i use star i will get proper neutral reference. so it wont be difficult to read the analog voltage with reference to neutral line

when i use delta connection there wont be neutral i will be using virtual ground it will be difficult to measure R,y,B

as you said in case of delta connection
NEUTRAL_value = (R + Y + B) / 3
this r,y,b is converted value from ADC_value?? or Adc value need to take.

R_phase to star_point voltage = ADC_R_value - NEUTRAL_value
IN delta connection there will be no neutral connection?? How can create it? or what will be Rphase value here

you have any recommend circuit to make it work.

https://datasheets.maximintegrated.c...ds/78M6631.pdf

https://www.digikey.com/reference-de...ing-for-ac/833

https://www.analog.com/en/products/a...-evaluationkit

There is no issue with star connection but facing when delta connected .
In software , when delta connected how calculation i need to do??
how MCU will know weather star connection or delta connection??
or Weather i need to use above formula itself weather it will work?? do you have suggestion

7. ## Re: 3 phase 3 wire measurement

Hi,

*******
You can not choose between star and delta connection.
If you wamt to choose then you need 6 wires, which you don't have.

You just have 4 wires...
Thus you need to connect the three phase wires....
And the only option is whether you connect the star point or not.

--> If you don't agree with the above, then please draw your two connection schemes.

********
as you said in case of delta connection
NEUTRAL_value = (R + Y + B) / 3
this r,y,b is converted value from ADC_value?? or Adc value need to take.
No, I wrote:
In software: NEUTRAL_value = (R + Y + P) / 3
I recommend to do heavy low pass filtering on NEUTRAL_value.
In software you never have analog values, thus, it's clear that they are ADC values.
****
IN delta connection there will be no neutral connection?? How can create it? or what will be Rphase value here
Use the math above.
****
you have any recommend circuit to make it work.
What circuit?
You just need to generate
NEUTRAL = PIC_ADC_VREF / 2
--> use two resitors. You may add a buffer, but no need to.

****
In software , when delta connected how calculation i need to do??
See post#5.

****
how MCU will know weather star connection or delta connection??
It can't know. And it doesn't need to.

****
--> If you still have any doubts, then I recommend to use a simulation software. Post your simulation for further discussion.

Klaus

8. ## Re: 3 phase 3 wire measurement

Friendly reminder : If you are making your own neutral point (Y connection) better you connect your neutral to earth with a dumping impedance. In my region Y neutral - earth difference may vary 2-5V.

•

9. ## Re: 3 phase 3 wire measurement

Originally Posted by Antor
Friendly reminder : If you are making your own neutral point (Y connection) better you connect your neutral to earth with a dumping impedance. In my region Y neutral - earth difference may vary 2-5V.
Can you show with small circuit how can i do it??

10. ## Re: 3 phase 3 wire measurement

Since I have no access to wikipedia I can't find the source. I can't exactly remember the reason I do this. But it might be for better measurement when there is unbalanced load or to avoid "a thing" if there is a fault on load side.

[Moderator's note: Member requests readers to see his clarification, post #16.]

11. ## Re: 3 phase 3 wire measurement

Hi,

Since I have no access to wikipedia I can't find the source.
Nobody wrote about wikipedia... so what do you refer to?

Klaus

12. ## Re: 3 phase 3 wire measurement

Originally Posted by nayakajit87
in case of 3 phase 3 wire we dont have neutral reference
Perhaps the 3 sources influence each other in such a way that you can detect. Possibilities:
* zero crossings
* voltage waveforms
* Ampere waveforms
* peaks versus troughs
* Does a supply's peak influence the other two supplies?
* sequence of Ampere peaks versus voltage peaks
* Do readings change when you disconnect one supply?
Etc.

Simulation of the delta arrangement. Each source has two LED's in anti-parallel, to indicate presence of current and direction. An led lights only once per peak of a supply, yet the other two supplies are influenced when that happens.

13. ## Re: 3 phase 3 wire measurement

Hi,

Especially with the picture of post#10 I get confused.

* The picture shows the "load" side, usually inside a device, where you don't have access to the internal circuitry and thus are not able to measure anything inside.
* The picture shows "currents" only, but you say you want to build a voltmeter.

Please use a pencil and a paper and draw the situations you want to measure.
All situations should include
* the source side
* the load side
* your measurement device
* the connections

Klaus

•

14. ## Re: 3 phase 3 wire measurement

Here i have attached for reference.
i tried to trace IC label printed on using these website
1) V-X-M-H http://www.s-manuals.com/smd/vx
2)T-A-1-G http://www.s-manuals.com/smd/ta

I am assuming in circuit using it.

http://www.s-manuals.com/pdf/datashe...93_richtek.pdf

http://www.s-manuals.com/pdf/datashe...vhc1g09_on.pdf

15. ## Re: 3 phase 3 wire measurement

Hi,

Instead of telling us what (and how) you want to measure .... you confuse with an even more uselss post...

Maybe it´s my fault, but I can´t find any useful information in post#10.

Klaus

16. ## Re: 3 phase 3 wire measurement

There are transformers called "measurement transformers". When there is no Neutral line you may have to create your own neutral by connecting the seconder of the this measurement transformer as star (for some reasons Line-Line voltage measurements may not be enough). And measure the phase-neutral voltage. But the neutral of the star is not always 0 volt with respect to ground because of unbalanced loads.

That's why you connect the new neutral to ground. But I can't remember why this grounding is done with an impedance. Nobody said wikipedia but me. The reason was explained on wikipedia but in my region we can't access to wikipedia now. That's an explanation of "why I can't refer to my source".

Same thing done with Y connected generators and distribution transformers also. Its called "earthing impedance" but this is out of our case. This is about measurement transformer earthing impedance.

17. ## Re: 3 phase 3 wire measurement

From white papers (1):

Resistance Grounding Systems are used in industrial electrical power distribution
facilities to limit phase-to-ground fault currents. IEEE Standard 142-1991 states: “The
reasons for limiting the current by resistance grounding may be one or more of the
following:
1. To reduce burning and melting effects in faulted electrical equipment, such as
switchgear, transformers, cables, and rotating machines.
2. To reduce mechanical stresses in circuits and apparatus carrying fault
currents.
3. To reduce electrical-shock hazards to personnel caused by stray ground fault
currents in the ground return path.
4. To reduce the arc blast or flash hazard to personnel who may have
accidentally caused or who happen to be in close proximity to the ground
fault.
5. To reduce the momentary line-voltage dip occasioned by the occurrence and
clearing of a ground fault.
6. To secure control of the transient over-voltages while at the same time
avoiding the shutdown of a facility circuit on the occurrence of the first ground
fault (high-resistance grounding).”

(1)
IEEE Standard 141-1993, “Recommended Practice for Electrical Power Distribution for
Industrial Plants” (Red Book)
IEEE Standard 142-1991, “Recommended Practice for Grounding of Industrial and
Commercial Power Systems” (Green Book)
GE Publication GEI-72116, “High Resistance Pulsing Ground Detection System”

--[[ ]]--