The big RPC 180 deg vs 120 deg myth......

As there seems to regularly be a controversy about this issue on RPCs, let the current crop of skeptics check the scope photo found a few posts into this thread.


http://www.practicalmachinist.com/vb/transformers-phase-converters-vfd/rpc-phasing-question-209463/

Or, just open up your RPC (or static for that matter) circuit box (depower it first ) and see that the 2 "1-phase" input lines pass directly through to 2 of the 3-phase output lines via short thick copper wires. It seems to me that there is no reasonable way to shift anything anywhere with this arrangement. So, if we agree that 3-phase is 3 60 Hz oscillations spaced 120* apart (which I think most folks agree on by now, and with solid empirical data from Jim's O-scope trace), you must conclude that the incoming "1-phase" lines are spaced 120* apart.

Jim, if you're reading this, I'd be curious to see an O-scope trace of the "1-phase" going into your converter. Seems to me that would either corroborate the above, or shoot it down .

Regards.

Mike

The incoming single-phase feeder (branch circuit) consists of L1 and L2, with a potential difference of 240 volts.

The neutral is almost never supplied to machinery. If it was, then there would be measured 120 volts from L2 to N and 120 volts from N to L1, with the sum being 240 volts.

As the sum adds to 240 volts (120 + 120 = 240), the two sources, L2-N and N-L1 MUST be in-phase (zero degrees). They cannot be out of phase (180 degrees) otherwise they would add to zero volts (120 - 120 = 0).

The sole purpose of the RPC is to generate a "quadrature" voltage (90 degrees from either of L2-N or N-L1).

Then, taking only L2, L1 and what I will call L3, there will be found 240 volts from L2 to L1, 240 volts from L1 to L3 and 240 volts from L3 to L2.

The input single-phase system is still referenced to true ground, N, which is also G.

The output three-phase system is referenced to N', the artificial neutral which is 69 /_ 90 (69 volts at a phase angle of 90 degrees) with respect to the true neutral, which is 0 /_ 0 (zero volts at a phase angle of zero degrees) .

So, the output three-phase system is really a 138 volt Wye system (138 /_ 90, 138 /_ 210, 138 /_ 330), which, by the well-known Delta-Wye transformation, is equivalent to a 240 volt Delta system.

peterh5322 said:

The incoming single-phase feeder (branch circuit) consists of L1 and L2, with a potential difference of 240 volts.

The neutral is almost never supplied to machinery. If it was, then there would be measured 120 volts from L2 to N and 120 volts from N to L1, with the sum being 240 volts.

As the sum adds to 240 volts, the two sources, L2-N and N-L1 MUST be in-phase (zero degrees). They cannot be out of phase (180 degrees).

Click to expand...

Eh, I beg to differ. The voltage difference between L1 and L2, both cycling at 60 Hz, has everything to do with their phase separation. If L1 and L2 were "in-phase" such that their peaks and valleys coincided, there would be 0V potential between them, even though they would both measure (peak) 120V potential (RMS considerations notwithstanding) to neutral/ground. At each instant in time that L1 was at its peak 120V, L2 would also be at its peak 120V, the difference being 0V.

Now, if you shift L1 by exactly 180*, then the peaks of L1 coincide with the valleys of L2, and you get a net sinusoidal potential difference, peaking at 240V (again, ignoring RMS).

So, L1 and L2 must be out-of-phase to some degree, else there is no potential between the two. The debate at-hand here is whether L1 and L2 are separated by 120* or 180*.

I think it's 120* .

Regards.

Mike

"I'd be curious to see an O-scope trace of the "1-phase" going into your converter."

You're looking at it, if you are seeing the traces from that screen shot. It's exactly one
of the three. Don't ask which one, as I never documented that fact!

You're getting hung up on phase difference - the definition of that term.

Two wires connecting to the scope. You see a sine wave on the display. What's
the phase?

Nonsense question. There's a single waveform there, there's nothing to provide
any independent phase reference, so the "phase angle" is strictly undefined. Does
not exist. Does not matter. Cannot be used for any calculations.

(remember the neutral is never connected in that converter)

NOW, you make a third wire. How, does not matter. Just a third wire, with
some other voltage on it, that varies with time.

NOW you can display three waveforms. Those would be, voltage between the first
two wires - but you already had those, pretty boring. But we'll leave them on the
screen. Next the voltage between the new third wire and one of the originals, and
of course to be fair, the voltage between the new third wire and the OTHER original
one.

Three waveforms, NOW you can discuss the phase angles between the various
waveforms.

Three wires = three phase angles.
Two wires = NO phase angles!

"Eh, I beg to differ."

I'm not going to be dragged into another useless debate on this subject.

I have stated that which I KNOW to be fact.

You may certainly choose to believe otherwise.

Tomorrow I will attempt to post photos of the traces I got on my Tektronix 2246 scope and a Hitachi V-1000A. I'd do it now except my camera is at home. I get the typical 2 input wires at 180 degrees not a 120 degree shift. If it is as you say 120 degrees then all the power in the shop will have 120 degree power since the two wires on the RPC are connected directly to the 240vac line. If that's the case and it is 240VAC then the line to neutral must be about 150vac.

Be *careful* when doing this. Peak voltages are a lot higher than the 240 volt
rms values typically used. A 10X probe really doesn't do it. In my case I
strung a bunch of 10 megohm resistors in series with the probe tip. If you go that
way, you HAVE to be sure the ground for the probe is good or you'll wind up
with an fried input on the scope. Another approach is to build an off-board
10X voltage divider to put the probe on, best of all is a 100X high voltage probe.

Do NOT attempt to lift the scope ground.

Powerco voltage is universally agreed to be "real" 3 phase.

It is 240V (or 208, etc) @ 180 deg*between phase wire A and phase wire B

it is also 240V @180 deg between phase wire B and phase wire C

and it is also 240V @180 deg between phase wire C and phase wire A.

You can measure those, "so there can't be any doubt"

180 + 180 + 180 = 540 deg, slightly more than are in a circle......

So, there you have the same sort of dilemma which exists for the RPC.

The key issue is where the reference is..... looking at the footnote** I am regarding the "3 phase" from the point of view of a delta load for the purpose of determining if it is "real" or not.

For those "can't be doubted" voltages, the reference is one of the phase wires. any COMMON voltage between the phases is left out of the measurement. By definition the only thing measured is the "180 deg voltage".

but with 3 phase Y, it can be proven that a "common mode" voltage exists on any two phase wires vs neutral. 3 phase Y has no net common mode voltage, so the third wire must carry a voltage that offsets that. Delta is the same, but lacks a built-in reference, although there is a "virtual neutral" that can be created.

What that means is that the "true" 3 phase neutral must be at a voltage different from the center tap of one phase winding.

Now, a type of "true" power company 3 phase** that is exactly similar to RPC output does exist.

That is "stinger" 3 phase, aka "lighting tap" 3 phase.

In that, a regular single phase transformer produces 240V center tapped standard house voltage. the center tap is grounded.

Another transformer (sometimes 2) is added which provides the third phase wire, which is definitely and truly at 120 deg to the first two, WITH REFERENCE TO THE TRUE NEUTRAL, OR "WYE POINT" OF THE 3 PHASE SYSTEM.

Now according to the "180 deg" theory, that first winding must "lock" the "phase" of those two wires at 180 deg, so the third wire "must" be at 90 deg, and therefore that isn't "real" three phase.

But it is genuine 120deg 3 phase**, with no magic, and no difference from "normal", aside from being wired "open delta" in most cases.

An RPC makes "stinger" 3 phase just like that. It HAS to, it can't avoid it.

And Jim's photos prove it.

* In this, I am using the convention of 180 deg referring to a voltage measured vs some third point, such as the single phase neutral..... and meaning only that there is a voltage difference of the "180 deg voltage" between wire 1 and wire 2.

Looking at it as a system, one can also call it an in-phase circuit.

** true 3 phase means that a delta load cannot determine a difference between the system under discussion and a reference 3 phase system of the same voltage.

What 180 + 180 +180 is 540????? What does that have to do with anything! 180 degree phase shift means that one wire is going + at the exact time the other is going - and 90 degree shift will mean that it is at peak while the other two are at 0vac In other words the peak to peak time of 90 degree shift is 4.16ms while peak to peak time of 120 is 5.55ms and all complete one cycle in 16.66ms since we are talking about 60 Hz!!! What we are saying is that the phase difference is 90 degrees not 120! The input single phase is 180 and the 3rd wire from the RPC is 90. In reality the issue is 90 degrees vs 120 degrees! Degrees is nothing more than the voltage vs generator shaft rotation. So no matter what the phase shift is it will all happen in 360 degrees! 90 degree phase shift was first created by Westinghouse by connecting 2 single phase generators together 90 degrees apart! A sine wave is the plotting the sine of an angle and its the same a plotting a voltage in a coil if a magnet was put on a shaft and rotated, the voltage will be max at 90 degrees and 270. One will be positive as the one pole is at 90 and at 270 the opposite pole will now cause it to be max negative. The phase shift is how far the second coil is from the first and the frequency is the speed of the shaft!

Peterh your logic is wrong!! In voltage +120vac and -120vac is 240vac! Yes if you you connect them together you will have 0 because you just blew the fuse! 180 degree single phase means that one line (L1) is going + at the exact time the other line (L2) is going -. They are mirror image of each other!!! In voltage or math the difference between +10 and -10 is 20 not 0 What we are measuring is the difference so a voltage of +120 and +120 is 0!!! And the phase angle is 0 also Furthermore 120vac on the power line is measured at 45 degrees and known as RMS!! What ever your meter reads it's at 45 degrees! Unless you have a special peak to peak meter!

Not sure who "we" is......... the "we" who are saying the difference between phases is 180 and 90 and 90.................... as far as I know it is "you"

Froneck said:

The phase shift is how far the second coil is from the first and the frequency is the speed of the shaft!

Click to expand...

Yup, and the mechanical difference is 120 deg in a motor.


*
*
*
Let's redefine this in a way that makes it relevant to a motor.

Connect a motor stator to the RPC output.

Now, look at the relative electrical phase of the magnetic fields created by the coils.

are you saying that:

1) fields are 120 deg apart in time. (mention how this comes from input that is NOT @120 deg intervals)

OR

2) fields are NOT 120 deg apart in time (mention how this comes from a motor with a fixed mechanical spacing)

Which are you claiming?

You can neglect the lagging current due to inductance, that is the same for all windings, and ignore minor variations, we want the general trend here.

It all depends on the "neutral" reference.
For single phase it is the center tap of a 240 Volt winding with each end 180° out of phase with that neutral, otherwise if they were in phase a center tapped bridge rectifier using 2 diodes has no advantage over a half wave using 1 diode.

For 3 phase it is the center of the Y as a virtual neutral. It is the 3 leads connected together on a 9 lead 3 phase motor wired for 240 Volts.

The correct relationships are shown here:

The attached image is a textbook diagram of the relationship of single phase to 3 phase. As in ELT-102 from 40+ years ago.

Thread here --->> http://www.practicalmachinist.com/vb/transformers-phase-converters-vfd/wild-leg-149745/

" It all depends on the "neutral" reference. "

Not for my converter. No neutral used at all!

You need the neutral reference to understand the 120° relationship between the phases. The neutral is not needed for operation of the equipment. It just takes your single phase power and generates the correct phase relationship for 3 phase.

To really confuse the "experts" how does a 6 Phase star transformer develop the 6 phases from a 3 phase power line???? Yes they are 60° apart.

"You need the neutral reference to understand the 120° relationship between the phases."

Nope. Absolutely not.

Did you read my post above? No discussion of a neutral.

Two wires means there's no defined phase relationship.

Three wires (and here, I mean the three hot legs - two from the utility and the
manufactured leg from my converter) and then you have three phase relationships.

No neutral.

I would go further and say, if you DO discuss the neutral here, you will NOT
understand the 120 degree phase angle situation.

First of all neutral reference can't be ignored because the input power is neutral referenced! Second in the scope connection shown in the link to "RPC Phasing Question" it's neutral referenced because I see only 3 probes connected to what is supposed to be an RPC thought it does not look like an RPC. The Scope inputs are connected to ground or neutral and if there is a requirement to separate the neutral you would need 6 probes and 6 inputs or a input isolator to look at the output of the RPC and not reference neutral. My Tektronix Isolator couples the input to the scope with photo optics. Also the scope trace shown is out of a text book and not a photo! However I do have an isolator but unfortunately it is only 2 input so tomorrow I will connect it to two of the lines and the trace will not be neutral referenced for 2 lines, I can't show 3 however I can show 2 of the 3 connected L1-L2 L1-L3 and L2-L3 if there is a 4.16ms peak to peak time or 8.34 pp then the two are 90 or 180 and not 120 degrees phase shifted Simply put the connection of the scope shown IS Neutral referenced!!!
I do understand that the pole separation in a 3 phase motor is 120 degrees! There are 4 poles for each phase in a 1800rpm motor. And that the coils are short pitched or overlap one another and I assume the input 180 degrees in inducted into the windings of the 3rd phase coils! I also believe that the 3rd line is generating a voltage 120 degrees from the other 2 inputs, and that the input will lead and lag the 3rd wire so that the end result is similar to a Scott T type connection! I also believe that if the motor was connected as a motor generator and there was no input connection to the motor the "generated output" would be 120 degrees But the fact that 2 of the inputs are connected to a strong 240vac 180 degrees it effects the output! And if as you say the shift effects 3 lines it must also be on the two! I want you to show me any power connection that is 120 degree separation where the voltage does not indicate 120 degrees! I did measure the neutral reference of the motor, since it is Y connected the motor has a neutral connection and low voltage is connected so that the 2 Y's are parallel. 4,5 and 6 is the neutral connection of the one Y and I will take the motor apart , find the common connection for 7, 8 and 9 and check that also! However I doubt that it will be different than 4, 5 and 6!
What your saying is similar to connecting 2 wires of a Wye output transformer to the output of a 180 degree single phase transformer. Because the spinning motor is generating 120 degrees yet 2 of its legs are connected to 180 degrees! You expect me to believe the output of the one transformer will not effect the other especially if the single phase transformer is much larger as in this case it is the full input power!
Hitandmiss; 6 phases are easy since the connection is to a double delta output transformer. The connection looks like the Star of David! Also a Scott T will change 3 phase 120 degree to 2 phase 4 wire 180 degree 90 degrees apart!

"I see only 3 probes connected to what is supposed to be an RPC thought it does not look like an RPC."

Very good eyes sir. I'm not sure how this topic crawled out of the woodwork but
there's a lot of information in the original thread, linked at the top of this one.

Apparently you have not gone back and spooled through it - and I don't blame you as
it went on for some time.

The short version that answers the question above:

1) the scope probes (yes three of them) are connected to the power panel on my
ESM-59 hardinge lathe. It was the easiest way to hook all three probes on at once,
short of making up another twistlock cordset. The scope is indeed tied to all
three output lines of my converter.

2) the converter itself lives in a small crawl space in the back of the shop. It's
started with a small 1/8 hp motor via a belt, and consists solely of a 5 hp
allis chalmers motor. No capacitors, no contactors, no nothing. Just an idler motor.

3) the scope in question is pretty sophisticated. It has the ability to compute and
display waveforms that are mathmatically related to any of the four input channels.
Obviously I am using three of them for this measurment, the fourth is turned off.
The three waveforms you see on the scope screenshot (this particular scope allows
a direct jpg screenshot that can be downloaded via a zip drive, very handy) are
set up the following way, before the measurement is started:

4) Here the W's are the waveform definitions, and the V's are the actual input voltages:

W1 = V1 - V2
W2 = V2 - V3
W3 = V3 - V1

For a purely analog scope, you are absolutely correct, you basically need SIX
inputs with the ability to analog difference pairs. Tough animal to find!

This instrument (borrowed from the lab where I work, WITH permission from the boss!)
is, I think a 5 GHz digitizing sampling scope. So it can measure all the signals,
process them, and display the results in real time, keeping track of all the information.
Because of the speed and bandwith of the scope, it isn't even aware it's busy when
doing this sort of test!

So to recap:

a) voltages are measured against ground
b) displayed waveforms are computed from voltages.
c) only the differences between hot legs is displayed
d) no neutral information is shown on the screen, nor was the neutral voltage measured.

Not sure where this was going:

"Also the scope trace shown is out of a text book and not a photo!"

You will find no textbook with that photo in it sir. I downloaded that jpg
from the scope personally, and posted it on the site.

It is similar to the ones I did later, showing the power factor for this same converter,
using thes same scope:





Here I needed to use all four probes, to show voltage and current feeding the
incoming line to the converter.

This originated as a hijack in another thread..... Jim, you probably never looked at it.

By looking at it "delta", the issue of ground reference is avoided...... hence the reference to the fields created in a motor.

Mr "Froneck" is still being hung up by the bonded neutral of the single phase vs the "true" neutral of the 3 phase, which are NOT the same.

the difference is a common mode voltage which exists between the two "theoretical" neutrals. the amplitude and phase of that "neutral voltage" distorts the measured amplitude and phase of the 3 phase side, WHEN MEASURED FROM THAT REFERENCE POINT

A "delta" measurement avoids that issue.

the main point being that when examined in "delta", the RPC output is essentially indistinguishable from somewhat imperfect powerco 3 phase.

And the delta output is what the motor responds to.

Jim; You are right 240vac rms is .707 X the peak voltage. So the peak to peak voltage is 1.414 X 240vac rms equals 339vac pp. My scopes are limited to 400volts. I have 1X1, 10X1 and 100X1 probes. I also have an input isolator that couples the input to the scope that is totally isolated being photo coupled made by Tektronix for the purpose of isolating inputs. BTW no amount of resistance can isolate the "neutral" reference. Resistors can attenuate the voltage and the current but can not isolate and any signal will be power line "neutral" referenced! Since all one side of the inputs are connoted to the chassis and the chassis is grounded (connected to the input power "neutral" via a non current carrying conductor) Any inputs that require one leg to be hot as in 240vac can not be connected unless it's connected to both input 1 plus input 2 and the selection is made to add the inputs. Some fools used isolation transformers to separate the "neutral" reference made by the ground connection and if connected to 240vac the scope is hot to ground by 120vac rms. Though every scope manual cautioned against connecting it to an isolation transformer it was done and these isolation transformers were sold for that purpose. An isolation transformer will remove "Neutral" reference and all displayed signals will be referenced to what ever is connected to the chassis side of the inputs. If one was careful and connected the scope power thru and isolation transformer and care was taken knowing that the scope would become "Hot" to ground it would be possible to connect the scope to an RPC without referencing neutral of the single phase power. Connect the probe of trace 1 to L1 and the clip to L2 and the probe of trace 2 to L3. The voltage will be displayed in the form of a sine wave and the phase relation to L2. However isolation will not occure because L2 is "Neutral" referenced plus the problem will come if attempting to view the 3rd phase. Keep in mind that "neutral" is nothing more than a common connection. I don't do this because I have an isolator nor do I recommend doing it.
The "Neutral" of the 240vac single phase is the connection to where the two coils of the secondary are connected together. The motor also has a "Neutral" and it is the center of the Y as well as a "Neutral" of the secondary on a Wye transformer in the center connection of the 3 coils. Because being magnetically coupled the "Neutrals" have no relationship to each other except for the very small amount of leakage. Therefore voltage between input to output of any transformer is 0 ignoring any leakage. However if I were to connect the "Neutrals" there will be a voltage between Primary and Secondary. (I'm ignoring Auto transformers as they do not have Primary and secondaries) If there is no connection of the "neutral" of a secondary to ground and one "Hot" line (X1,X2 or X3) becomes grounded there will be situation where the "neutral" is 120vac to ground! (Assuming the transformer secondary output was 240vac single phase or 208vac 3 phase) Therefore attempting to ignore "neutrals" of a RPC output can not be done because they share a common connection and no isolation.
Frank