Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

[SOLVED] Measuring 3-phase AC voltage using voltage divider circuit

Status
Not open for further replies.
H

Heavenlyrider

Newbie level 5
Newbie level 5
Joined
Mar 9, 2016
Messages
10
Helped
1
Reputation
2
Reaction score
0
Trophy points
1
Location
India
Activity points
88
Hi,

i want to measure AC voltages of 3-phase supply using micro-controller through ADC. i have used three separate voltage divider circuits. when i measure any one of phase-neutral connection, the result of voltage is same as in external voltmeter. when measuring two or all phase-neutral supplies, i am getting mixed voltage values like 350V,440V respectively. how can i isolate the three phase values so that i can get 240V for each phase-neutral connection. i have attached the schematics of voltage divider circuit. Could any one help me with this problem?




Thanks & Regards.
 

The circuit can't work with bridge rectifiers due to common ground connection. You either need isolation transformers, or can use half-wave rectifiers with one diode per channel.
 
Better still, use transformers with a center-tap, which will be your common across all phases.

And use two diodes on each to obtain full wave rectification.

The rectified voltage will be around half though.
 

hi,

Thanks for your suggestion, i have used half wave rectifiers, now the problem is solved. But still i cannot understand the problem you have quoted.

FvM said:
The circuit can't work with bridge rectifiers due to common ground connection.
Click to expand...

could you explain?
 

Hi,

all "-" outputs of the rectifiers are connected to "GND". But all AC input voltages are different and refer to the same start point.

A simple drawing of the three voltages before the rectifers, and the rectifier outputs will show you why.

Klaus
 

A simple drawing of the three voltages before the rectifiers, and the rectifier outputs will show you why.
Click to expand...
Yes.
1. Sketch complete circuit.
2. Think.

8508061400_1479389811m.jpg
 

Hi,

I have designed the voltage divider with half wave rectifier as shown in below image. I have fixed the trim pot for a particular AC voltage (230V) with respect to a VOLTMETER. But, the voltage varies after several hour when comparing with voltmeter at the rate of +3V to -3V. how can i fix it?

 

hi,

FvM said:
The circuit can't work with bridge rectifiers due to common ground connection. You either need isolation transformers, or can use half-wave rectifiers with one diode per channel.
Click to expand...

I have used half wave rectifier with above mentioned circuit, but the result varies time to time. please help me to rectify this problem.
 

Too little information.
- Which kind of measurement is used by the reference voltmeter (Vrms, Vpeak, averaged rectified value)?
- Do you see constant mains voltage in the reference measurement and only variation of your ADC?
 

hi,

FvM said:
Too little information.
- Which kind of measurement is used by the reference voltmeter (Vrms, Vpeak, averaged rectified value)?
- Do you see constant mains voltage in the reference measurement and only variation of your ADC?
Click to expand...

1) I have used fluke meter to measure Vrms.
2) yes i see minimum variation in mains. variation in ADC values are much higher, when compared with mains.
 

Hi,

you are using some kind of "peak" voltage measurement with your half wave rectifier design.
This is the most inaccurate and the most prone_to_error measuring method.

With the adjust of your trimpot you change the time constant --> changing ripple voltage, too.

***
There are a lot of threads here where mains voltage measurement has been discussed.
I´ve participated some discussions of them. Telling always the same.

Klaus

- - - Updated - - -

Hi,

there are free simulation tools available.
(like LTspice)

If you use one of them you see that your reaction time is very slow.
With the pot at 50k it takes more than 20s to get below 5% error, while the ripple still is about 8mV.

With a lower pot value it becomes faster, but ripple increases.

Klaus
 
The measurement implemented in post #7 is not a peak measurement, by working of the large series resistors it's near to averaged rectified value.

The ratio to Vmrs will change slightly with varying waveform, but no much, But the observed errors might be caused by this effect. Another possible cause is instable ADC reference voltage, die you check it? You should also check the ADC input voltage with a multimeter and see if it changes along with the ADC readings.

Can you check the mains voltage waveform with an oscilloscope?
 

Hi,

can any one give me the circuit for measuring 3 phase voltage with reference to Neutral?. i tried all changes that mentioned above. but still i am not getting a stable output.
 

Hi,

Post#7.

If it gives no stable readings than most probably you do something wrong.

***
If you need help with "not stable readings" then provide useful information.
Input voltage, output voltage measured with DVM. ADC readings.
What do you expect? What is not like expected?
and so on


Klaus
 

Make a table like he following and measure with a DMM. Make sure that you include all decimals. 4.05 is not equal to 4.0 :

ACVolts measured DCVolts measured ADC counts
230 4.05 1012
220 3.87 967
and so on......

Repeat for all three phases.
Do you have access to a Variac? Do you have access to a scope?
 

FvM said:
The circuit can't work with bridge rectifiers due to common ground connection. ..
Click to expand...

But this is the most common way single phase power is supplied to domestic customers. The main transformer is a three phase four wire output and the customer gets one phase and a neutral. Each customer is free to use bridge rectifier between the phase and the neutral.

Can you please explain in more detail? In fact the neutral cable is relatively thin if the three phases are equally loaded then the neutral carries very little current.
 

Hi,

the problem is not how the power comes into the house.

The problem is that in the circuit of post#1 all three minus outputs of the three rectifers are connected... and this causes the problem.

You may draw current (power) form the rectifiers, but you don´t get a meaningfull voltage measurement.

--> see post#6

Klaus
 

KlausST said:
The problem is that in the circuit of post#1 all three minus outputs of the three rectifers are connected... and this causes the problem...
Click to expand...

The mistake is to call the "minus outputs"- they are the neutral.

The neutral has the voltage sin(t)+sin(2*pi/3+t)+sin(4*pi/3+t) and this is identically equal to zero.

The three phases has voltage equal to the three terms (just ignore the sign)- they are 120 degree apart with amplitude of 1.

The three phases after rectification becomes abs(sin(t)); abs(sin(2*pi/3+t)); abs(sin(4*pi/3+t)) (full wave; without filtering)

(there may be some small problems if the load becomes unbalanced and/or the phase shifts become different than 120 deg)
 

Hi,

Never heared of an rectifier with "neutral" output.
The connetions are: AC, AC, plus and minus.

Neutral often is the star point of three phase AC. But this is not what we talk about.

Klaus
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top