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Measuring 3 Phase voltages through a PIC micro controller

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prabhukaran3

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Measuring 3 Phase voltage through PIC micro controller

Hi everyone,

Currently I am Dealing a project with 3 Phase motor.

I need to measure phase to phase voltage with 5% accuracy without any transformers. I tried with Transformer less power supply like attached image. But its not really working. Anyway I am having one PCB board Its giving exact output what I am looking for.While I do reverse engineering I cant understand really what they did, but I could understand they did something with capacitors and resistors. I have seen 47nK capacitors and 1W resistors here and there. Apart from this everything is embedded thing like LCD, PIC MCU, Buttons and LEDs

Also I want to do forward reverse and control the speed of the motor. power-supply-9vdc-no-transformer1.jpg

I am looking all your help
 

Re: Measuring 3 Phase voltage through PIC micro controller

Hi,

Peak, rectified average or true RMS?

Did you see the "similar threads" in the box below?

Klaus
 
Re: Measuring 3 Phase voltage through PIC micro controller

You can surely use resistive voltage dividers for the three phases -AB, BC and CA. You can rectify the AC from the voltage divider with a low drop diode and use a RC based 'sample and hold'. You will need three of them.

Do not mix topics- make another thread on forward, reverse and control the speed of the motor.
 
Re: Measuring 3 Phase voltage through PIC micro controller

More questions.

- Does "without any transformers" refer to voltage measurement only or also to processor power supply?
- Do you connect neutral line or 3 phases only?
 

Re: Measuring 3 Phase voltage through PIC micro controller

Only for measurement. I need to display the present voltage in the each phase.

I wont use Neutral line
 

Re: Measuring 3 Phase voltage through PIC micro controller

If the supply is delta (ie Neutral is available) then you could measure all three phases wrt neutral and then subtract one phase voltage from another to get the difference. The difference represents the phase-phase voltage. This would have to be done three times obviously. Not trivial but it does allow you to eliminate three voltage transformers.
 

Re: Measuring 3 Phase voltage through PIC micro controller

"with respect to" is an open statement in this case because it implies you have a common point to measure the voltages from. Prabhukaran3 states there is no neutral so the problem reverts back to Klaus's comment in post #2. Is an RMS, peak or rectified average needed? Most likely one phase will have to be taken as the reference and the other two measured against it but the phase difference plays a big part in the calculation. Converting to DC removes the instantaneous phase problem but the result would have to be mathematically scaled afterwards.

Brian.
 

Re: Measuring 3 Phase voltage through PIC micro controller

I wont use Neutral line

Ok so I assume the above means no neutral is available.
 

Re: Measuring 3 Phase voltage through PIC micro controller

Not using neutral means that the phase-to-neutral voltages are unknown. You can only measure phase-to-phase voltages and calculate nominal phase-to-neutral voltage, if necessary.

Devices with three phase supply, e.g. VFD inverters are usually working with phase-to-phase voltages only. Typically they are measuring two voltages and are calculating the third as difference of the two measurements. This can be performed in the digital domain if the instantaneous voltage is sampled fast enough.
 

Re: Measuring 3 Phase voltage through PIC micro controller

Only for measurement. I need to display the present voltage in the each phase.

I wont use Neutral line

You cannot measure a single point voltage; you can only measure a voltage difference. So you must put two probes at two different points and measure the potential difference.

If the three phases are not properly balanced, the potential at the neutral depends on how the neutral is formed.

If you do not have a defined neutral, all you can show are the phase-to-phase voltages (three of them).
 
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Re: Measuring 3 Phase voltage through PIC micro controller

Hi,

Maybe the three phase signals need to be low pass filtered to attenuate all the overtones generated by the inverter.

Then - since you don't want to use transformers - you will have no galvanic isolation.
This is dangerous and you need additional effort to make it safe to use.
(Or do you want us to find an isolated solution?)

If you are sure the three phases are with exact 120° phase shift (are you?) then it is sufficient to measure only two voltages and you can calculate the third.

Klaus
 

Re: Measuring 3 Phase voltage through PIC micro controller

If you are sure the three phases are with exact 120° phase shift (are you?) then it is sufficient to measure only two voltages and you can calculate the third.

If three phases are exactly symmetrical, it is sufficient to measure only one of them; the remaining two can be determined from symmetry.

In fact, for three phase power supply, only one voltage is specified because that is necessary and sufficient.
 

Re: Measuring 3 Phase voltage through PIC micro controller

Hi,

Three phases sine waves may vary in
* phase shift
* amplitude

I asked the OP if he could confirm that the phase shift is exactely 120° each.

Klaus
 

Re: Measuring 3 Phase voltage through PIC micro controller

Hi everyone,
Thanks for your valuable reply. I will tell you the real application of my project. I need to show the incoming voltage. If there is any mismatch(Low voltage or Only two phases are available means) accordingly I need to make some alarm.. I cant say with technical words. Embedded part is over. Only thing remaining is measuring voltage. If anyone having Circuit???

I have tried with Voltage divider Rule. But problem is much heat in resistors.
I opened one product called Digital Phase Prevent-or. It will display the phase to phase voltage and existence of all the three phase. there I have seen 3 number of 1 Watt resistors and only 1 Film capacitor(470nF/400V). I cant get the logic behind the operation
 

Re: Measuring 3 Phase voltage through PIC micro controller

Casually asking why re-invent the wheel? You'll get a lot of panel meters which will display 3 phase voltages and those which are not using any transformers except those for the power supply SMPS section.
 

Re: Measuring 3 Phase voltage through PIC micro controller

Yep,
I am having working sample of the same. But I could not understand how it is working. That is the problem here.
 

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