Measuring Phase to Phase voltage using Multimeter

[PhysicsTest]

TL;DR

Measuring the phase to phase voltage using multi meter

I want to understand the concept of measuring the phase to phase voltage using multi meter. For example if i want to measure R-Y voltage, i connect the positive probe of the multi meter to the R phase and the negative probe to the Y phase. But both phases will be sinusoidal, so how does it measure? Does it take difference? Will it consider the phase angle, is it RMS value? Please advise.

 

[trurle]

All multimeters do reject common mode voltage, and their output in AC mode is RMS voltage unless specified otherwise. In particular, digital multimeter measure momentary voltage difference between two phases, sampling it several times per AC period. Then the onboard MCU calculate the RMS voltage difference. Analog multimeter frequency response is slower, and RMS readings of these my be inaccurate if frequency is over 100 Hz.

 

[Rive]

i connect the positive probe of the multi meter to the R phase and the negative probe to the Y phase. But both phases will be sinusoidal, so how does it measure?

In a 'clean' three phase system between phases there will be also a sinusoidal voltage. With an average multimeter it measures the same way that the single phase (to ground). Just the voltage will be higher.

In general sinusoidal voltages measured in 'AC' mode. Multimeters can use several different trick to measure AC voltage. Usually, with DC component rejected, but that's not always happens, sadly...
Also, AC measurement is expected to measure the equivalent DC value (also not always the case...)

Since you have only two points of reference, you can't measure angle (directly, with a common multimeter).

A simple example: if you have 230V AC between phase and ground, you will measure 400V AC between phases. These values are the equivalent DC voltages for a sinusoidal voltage with 325V and 565V peak.

 

[Baluncore]

Here is one cycle of three-phase voltage, with the difference between red and yellow.
The individual phases are each 230 VRMS to Neutral.

 

[PhysicsTest]

Could you please help me to replicate the same what you have created, i think it is done is Ltspice? Could you please share the model? This is the circuit i have created.

There is a related question i wanted to ask before that i want to clear this, to get difference of the sine signals, do i need to use opamp circuit in Ltspice?
The output is simple sine waves with 120 Deg phase shift.

 

[Baluncore]

Could you please share the model?

There is no op-amp needed. To plot a differential voltage, click and hold the node, then drag to your choice of reference voltage. You can also edit the graph titles and equations in the plot window.

Attached are .asc and .plt files. Remove the extra .txt extension that allows the text files to be attached to the post, then they will run as LTspice.

Note that I used Run, then Tools, Copy Bitmap to Clipboard, then I edited the image to relabel millisec to degrees, using the degree symbol from my Neutral label.
Also, it may be needed to set the horizontal axis to 30 deg ticks.

 

[Averagesupernova]

Considering that a voltage is thought of as a DIFFERENCE in potential you can think of a voltmeter as a subtractor. Apply the probes to the points in question and it tells you the voltage between.

 

[PhysicsTest]

Thank you for the help, the main question i am facing is, i am involved in a testing and the setup has knobs to change any of Phase - Phase voltages ex: RY, YB, BR. The corresponding voltages are displayed on the same unit, so it is power supply and a display unit. Now i have taken few readings which i am not able to understand,
RY YB BR
481V 478V 482V -> Initial set voltages
452V 478V 452V -> Changing the RY voltage to 452V changes BR to 452V automatically, YB remains same.
400V 478V 400V -> Changing the RY voltage to 400V changes BR to 400V automatically.

Similar phenomenon happens for other phases if i change one particular phase, one other particular voltage changes and one voltage does not change.
I don't see the 3 phase voltages add up to 0 in the 2nd case and 3rd case. Does it mean the 3 phase voltage is imbalanced or anything that i am missing? The RY, YB, BR are assumed notations.

 

[Baluncore]

Does it mean the 3 phase voltage is imbalanced or anything that i am missing?

Draw a triangle with all internal angles equal to 60°. Label the corners R, Y and B. Make the edge lengths equal to the phase-phase voltages. Since the triangle must close and the phases or angles are fixed, there is a dependence between the edge lengths or voltages.

The neutral will be near the centre of the triangle. If you change one neutral to phase voltage, then two associated phase-phase voltages must also change.

 

[PhysicsTest]

Sorry for delay, i tried hard to understand how do i approach, when i have to maintain 60 degrees,

if i shift the Neutral point to Neutral1 point, the lengths of L_RY, L_BR, L_YR will still remain the same. Can you please give one hint to proceed further, I am struck.

 

[DaveE]

Sorry for delay, i tried hard to understand how do i approach, when i have to maintain 60 degrees,
View attachment 302009
if i shift the Neutral point to Neutral1 point, the lengths of L_RY, L_BR, L_YR will still remain the same. Can you please give one hint to proceed further, I am struck.

While 60^o is the nominal phase difference, it won't always be that in unbalanced cases. A triangle with 3 60^o angles must be equilateral. You have a triangle with 400, 478, and 400 sides, that one will have angles of 53^o, 53^o, and 73^o.

https://www.calculator.net/triangle...&vy=400&va=&vz=478&vb=&angleunits=d&x=69&y=16

 

[Baluncore]

Can you please give one hint to proceed further, I am struck.

I may have confused you with the deviation from the ideal equilateral triangle.

Start with neutral at the origin. The reference phasor is drawn along the +x-axis. The other two phasors, lie on radial lines from the origin, spaced 120° from the x-axis.

Measure the three phase-neutral voltages, then plot each along the appropriate 120° radial. Join those three points to make a triangle. The edge lengths should then equal the phase-phase voltages.

If you have only the phase-phase voltages, you can draw a triangle with those edge lengths. Then drop that triangle, so the corners fall on the 120° radials. The distances from the origin to the points are then the phase-neutral voltages.

 

[Baluncore]

Thank you for the help, the main question i am facing is, i am involved in a testing and the setup has knobs to change any of Phase - Phase voltages ex: RY, YB, BR.

It appears that when you change the RY voltage, it is actually changing the RN voltage, which is why the BR voltage also changes in the same direction as RY.

Maybe the knob on the power supply that you thought changed RY, actually changes RN only, with implications to both the displayed RY and BR.

 

[awaygood]

All multimeters do reject common mode voltage, and their output in AC mode is RMS voltage unless specified otherwise. In particular, digital multimeter measure momentary voltage difference between two phases, sampling it several times per AC period. Then the onboard MCU calculate the RMS voltage difference. Analog multimeter frequency response is slower, and RMS readings of these my be inaccurate if frequency is over 100 Hz.

Your terminology is incorrect. There is NO SUCH THING as a 'phase to phase voltage. The correct term is 'line to line'. Phase voltages are measured ACROSS phases; line voltages are measured BETWEEN lines. The same applies to labelling. Lines and line terminals are labelled A,B,C (etc.); phases are labelled A-B, B-C, and C-A.

 

[Baluncore]

The correct term is 'line to line'.

Or "phasor to phasor", since the electrical conductor is a phasor in the mathematics.

 

[Baluncore]

Your terminology is incorrect. There is NO SUCH THING as a 'phase to phase voltage. The correct term is 'line to line'.

Electrical energy is commonly distributed using a three-phase transmission line. The balanced 3Φ transmission line has three conductors, called phases. The system is specified by “line voltage” which, for an ideal line, is the voltage difference measured between any two of the three phases.
https://en.wikipedia.org/wiki/Three-phase_electric_power#Principle

Since there is only one "transmission line", the concept of a line-to-line voltage does not seem to be meaningful.

 

[Rive]

As it's used here, the 'line voltage' is the voltage of a phase relative to ground (so it's about everyday appliances).
The 'phase voltage' usually refers to a phase-to-phase voltage (thus clarifying that it's usually some serious machinery with all three phase present).

 

[Baluncore]

As it's used here, the 'line voltage' is the voltage of a phase relative to ground (so it's about everyday appliances).

That use is restricted to the domestic world of a single phase.

The 'phase voltage' usually refers to a phase-to-phase voltage (thus clarifying that it's usually some serious machinery with all three phase present).

I disagree with that statement.
A phase voltage is a phasor, measured relative to the neutral origin.
That is the mains voltage as measured by a single phase residential contractor.

 

[Rive]

I disagree with that statement.

The 'here' above refers to the country (where I live).
May be right or wrong, but the down to earth wire cutter people will give you only the funny look, you know...

Well, the point actually was that before a general 'terminology is incorrect' statement it may wort to clarify the actual environment/context it's based on, since terminology do vary - while the meaning not so often.

 

[Baluncore]

I agree that the terms are to some extent flexible and colloquial. In this postmodernist world, there are as many truths as there are observers, and no one is wrong.

But the mathematical basics and terms are reasonably well-accepted, and are used in the Wikipedia article.
https://en.wikipedia.org/wiki/Three-phase_electric_power#Principle

 

[awaygood]

If you examine a three-phase line diagram, delta or wye, it should be obvious that while you can measure the voltage across phase (phase voltage), you cannot measure a 'phase to phase' voltage. The expression, 'phase to phase' is widely used but completely incorrect... the correct expression is 'line voltage' or 'line to line voltage'. I suggest visiting www.professorelectron.com for an article on the correct terminology for describing three-phase measurements.

 

[awaygood]

Electrical energy is commonly distributed using a three-phase transmission line. The balanced 3Φ transmission line has three conductors, called phases. The system is specified by “line voltage” which, for an ideal line, is the voltage difference measured between any two of the three phases.
https://en.wikipedia.org/wiki/Three-phase_electric_power#Principle

Since there is only one "transmission line", the concept of a line-to-line voltage does not seem to be meaningful.

No, the three conductors are called 'lines'. The 'phases' are the three windings of a generator or transformer and the three loads which are connected in wye or delta. Which is why you CANNOT measure a 'phase to phase' voltage -it simply doesn't exist. See article on this subject www.professorelectron.com

 

[Averagesupernova]

No, the three conductors are called 'lines'.

Or legs. I at one time was very guilty of incorrectly using the word phase instead of leg or line.
-
@Baluncore line-to-line voltage is phase voltage.

 

[DaveE]

If you examine a three-phase line diagram, delta or wye, it should be obvious that while you can measure the voltage across phase (phase voltage), you cannot measure a 'phase to phase' voltage. The expression, 'phase to phase' is widely used but completely incorrect... the correct expression is 'line voltage' or 'line to line voltage'. I suggest visiting www.professorelectron.com for an article on the correct terminology for describing three-phase measurements.

OK, I won't argue with the pedantry, because I don't know or care what's precisely correct, nor do I even know who would decide. But, in my experience as a practicing EE with 3 phase fed power supplies, everyone in the lab will know exactly what you are referring to if you say "line to line", "phase to phase", "leg to leg", etc.

Part of our job as EEs is to communicate well with others. Some of those people learn at different schools, different times, different countries, in different languages, etc. Sometimes it's a rectifier, sometimes a diode, sometimes a valve. Sometimes it's a phase, line, leg. Whatever, I'll do my best to adjust and communicate as needed, even with the grammar police.

 

[Averagesupernova]

OK, I won't argue with the pedantry, because I don't know or care what's precisely correct, nor do I even know who would decide.

Yet you felt the need to reply. Start dealing with rotary phase converters and you may change your opinion.

 

[DaveE]

Yet you felt the need to reply. Start dealing with rotary phase converters and you may change your opinion.

Spec'd some, but never really worked with them. But, no, I don't think I will change my opinion. In fact, I think it adds to my argument. It's not enough for you to know the correct phrase, you have to know that the other guy knows it too. So, in my experience the answer is either obvious, like a 4-wire WYE where different styles can be accommodated; or you must have a more detailed description like "H1-H3 Voltage" presumably with a drawing associated with it. The grammar pedantry is either overkill or insufficient.

However, you are correct that I tend to overreact when I think people stress learning jargon over understanding concepts and then communicating as necessary. I spent some time working with really good engineers that learned their material well, but in different countries with different phrases. We did not waste time on who was right, but we did ask for clarification from time to time.

 

[Averagesupernova]

It's not enough for you to know the correct phrase, you have to know that the other guy knows it too.

Well that's generally how good communication works.
-
Single phase requires 2 conductors. In the USA we have split phase so there are two hots and a neutral but that is irrelevant. Two lines or legs, whichever is prefer, gets you one phase. A phase converter manufactures one extra line/leg but in doing so gains two phases. The original single phase source provides two lines/legs (one phase) for the three phase motor load. The final output looks like a high leg delta. Doubt you've ever heard it called a high phase delta. A high leg delta has one winding of a transformer center tapped and grounded. This allows single phase 120 or 240 volt loads the same way we have single phase in most residential services.
-
Then we also have corner grounded delta. This type of service is exactly as it sounds. Three transformer windings are configured in a delta with one corner grounded. We still have voltage between A-B, A-C, and B-C. But we only can measure voltage between ground and two legs. More than one newb electrician has scratched their heads over that configuration.
-
If you prefer to mix and match your vertices with the sides of a triangle I guess that's your business. You or your approach will not be taken seriously by those who work with three phase routinely.

 

[DaveE]

Well that's generally how good communication works.
-
Single phase requires 2 conductors. In the USA we have split phase so there are two hots and a neutral but that is irrelevant. Two lines or legs, whichever is prefer, gets you one phase. A phase converter manufactures one extra line/leg but in doing so gains two phases. The original single phase source provides two lines/legs (one phase) for the three phase motor load. The final output looks like a high leg delta. Doubt you've ever heard it called a high phase delta. A high leg delta has one winding of a transformer center tapped and grounded. This allows single phase 120 or 240 volt loads the same way we have single phase in most residential services.
-
Then we also have corner grounded delta. This type of service is exactly as it sounds. Three transformer windings are configured in a delta with one corner grounded. We still have voltage between A-B, A-C, and B-C. But we only can measure voltage between ground and two legs. More than one newb electrician has scratched their heads over that configuration.
-
If you prefer to mix and match your vertices with the sides of a triangle I guess that's your business. You or your approach will not be taken seriously by those who work with three phase routinely.

Oh yea, I know those monstrosities. OK for air conditioners in west Texas. We absolutely forbade it for our Ion Lasers, Applied Materials and such forbade it for their semiconductor processing equipment. I trained our field service engineers to first off always measure 6 voltages under load, 3 line to line, plus 3 line to ground (we were Δ, so they could ignore the neutral). Then if they weren't balanced, tell us the numbers. No special names, no trusting the facilities guy, just 6 measurements for starters. I don't care what they called it, I want the data, which was really easy to get, often by just asking the customer to measure it for us.

You did a nice job of describing a couple of power systems in this case. It was clear and didn't take too long to do. I didn't have to learn what a "high leg delta" was or wonder if we agreed. It was more efficient in this case than using slightly obscure names. Good engineers know lots of stuff without knowing what it's called by the other guy.

 

[awaygood]

Or legs. I at one time was very guilty of incorrectly using the word phase instead of leg or line.
-
@Baluncore line-to-line voltage is phase voltage.

No. A 'line to line' voltage is a 'line voltage'. There is NO SUCH THING as a 'phase to phase voltage'. See explanation at www.professorelectron.com

 

[DaveE]

There is NO SUCH THING as a 'phase to phase voltage'.

"Two fundamental terms which frequently cause confusion, when studying three-phase alternating-current systems are: ‘phase‘ and ‘line‘. This is hardly surprising, because the terms are often used quite incorrectly, not only in the field, but very often, unfortunately, in textbooks too!" - professorelectron

But, you see, those terms do exist, you and this professor guy just don't like them and wish they would be used "properly". Good luck with that. I, for one, won't say you're wrong. I also won't help you fix it.

Kind of like how I hate the word irregardless. I doubt that I'll fix that either.