Phase angle question on generated 3phase power

I have been reading about the various ways to create three phase power from single phase, but I'm still puzzled about one aspect of this. True three phase power has its phases offset by 120 degrees, but other than a VFD (which I'm not sure about), it looks like the other conversions will produce three phases with 90-180-90 degree offsets. Even the mighty Phase Perfect looks to pass the 180 degree separation of the two input legs through to its output. First, do I understand this correctly, and second, why is this not an issue? I'm guessing that a motor's mass will absorb (dampen) the differences in phasing, but wouldn't this have an impact upon finish for example? I'm somewhat familiar with single phase power supplies for electronics, but know nothing about what a three phase power supply would look like--I'd assume that they might expect their input phases to be with equal 120 degree offsets, and thus the CNC machines would want to see "proper" phase offsets as well as voltages.

These are just curiousities for me as I only have manual machines, I'm just trying to sort out the puzzle.

Merry Christmas!

--Larry

"First, do I understand this correctly ..."

No.


"... and second, why is this not an issue?"

1) RPCs ensure the 120-120-120 displacement by using the 120-120-120 inherent in the idler; what the load motor actually sees is -30, 90 and 210 degrees, with the manufactured three-phase system being 69 volts above the neutral, and

2) Phase Perfect is simulating that behavior of an idler, but is doing it more perfectly, and with none of the shortcomings of an RPC.

Characteristics of an RPC include low cost and a relatively good source of three-phase power (power factor is low to moderate), but with an inability to sink three-phase power.

Characteristics of a Phase Perfect include moderate cost and an excellent source of three-phase power (power factor is essentially 1.0, which is perfect), including the ability to sink three-phase power.

An RPC system cannot be used where regeneration is used, as in some kinds of ac and dc drives.

A Phase Perfect can be used where regeneration is used, particularly were it is used in place of dynamic braking resistors.

Peter,

Thanks, I'm glad to have the clarification that I'm thinking about this incorrectly. Frankly a lot of the science behind electrical power is a mystery to me despite the fact that my Dad (a respected EE) tried to enlighten me many times. He passed away a few years ago, so sadly I can't ask him any more power questions.

I was basing my assertion of 90-180-90 phase offset on looking at the diagrams for RPCs and the block diagram of the Phase Perfect in this whitepaper. Referencing the Phase Perfect diagram, I see L1 and L2 being passed through to T2 and T1 respectively, while T3 is the generated leg. I then assumed that the phase offset between T1 and T2 would be the 180 degrees offset of the 240v input. I hear that this is incorrect.

When I envision the voltage graph of the 240v L1 and L2, I see a pair of sine waves offset by 180 degrees. Correct so far?

When I look at that block diagram (for example) I would then assume that the voltage graph for T1 and T2 would also show the same offsets. Is this where I'm going wrong?

--Larry

Is this where I'm going wrong?

Click to expand...

Hi Larry,

You're not going wrong. Your analysis is correct.

Peter's been spouting this nonsense for a while. But every time I ask him to explain how you can coerce a 60 degree phase shift back into the AC mains, he just gets mad and cusses me out and leaves.

Uh oh, sounds like I've stepped in a rats nest

I'm guessing that my assumptions are too simplistic and that while I may be correct when only looking at the voltage curves, there is more to AC power systems than just the cycling voltages. To my mind, this is where the mystery or magic enters & thus why I'm just asking for clarification of where I'm going wrong. I'll stay tuned and see what comes up.

--Larry

"I was basing my assertion of 90-180-90 phase offset on looking at the diagrams for RPCs and the block diagram of the Phase Perfect ..."

The Phase Perfect patent document is a better representation of what actually goes on within that device than its web-based sales literature.

A high voltage, generated voltage (phase) is added to, or is subtracted from the incoming L1 and L2, thereby deriving an X (L1), Y and Z (L2) which is displaced from L1/L2 by 30 degrees, thereby creating the -30, 90 and 210, or its equivalent, as I previously stated.


"Peter's been spouting this nonsense for a while."

Yeah, right, "The Fake Leigh". Why don't you "get real" for a change?

When I explained that 0 + 90 = 90, and (your claim) 180 + 90 = 270, and i remarked that 90 + 180 always adds to zero, it was you who ran and hid.

I already explained that 0 + 90 = 90 (L1/A case) and 0 + 90 = 90 (L2/C case), thereby the "manufactured" phase adds to a measurable voltage, 208, but you elected to ignore that post.

Afraid of the facts? Or, do you hold onto your fantasies?

I suspect the latter.


"I'll stay tuned and see what comes up."

Well, don't look to "The Fake Leigh" for facts. He's just spouting off, period. Or, overly concerned with his nonsense.

Hi Peter,

Yes, 0 + 90 = 90, 180 + 90 = 270, and anything else you choose to add to 90 will produce a sum which is 90 larger than the other number. So what.

You have yet to give any explanation at all of how you're shifting the phase of the voltage on a line coming in from the pole transformer.

I'm not talking about VFDs or other active voltage sources. They can manipulate their output phase easily, and control it without difficulty. I'm talking about a system in which two of the wires going to the running 3-phase motor come directly from the transformer on the pole. How are you back-feeding the power grid?

Larry:
See what I mean about his paranoid responses

BTW, Merry Christmas and Happy New Year.

The Phase Perfect patent document is a better representation of what actually goes on within that device than its web-based sales literature.

Click to expand...

Hello again, Peter,

I pulled down a copy of the patent (5,465,040 issued 7 Nov 1995). If anyone wants to look at it I put it up on one of my websites http://AtwaterKent.Info/Images/Patent5465040.pdf for download.

The patent doesn't relate to our discussion. It discloses a means of correcting the power factor of a load which is connected to an existing multi-phase system. It does not generate the third phase, which is the whole subject of the current discussion. The patented device operates on a system which already has the proper phase relationships.

Considering Peter has been one of the anchoring points of this forum,
and has helped many of us achieve a better understanding of theory etc,
and has never as far as I know said he knows everything while being pretty open to alternative ideas backed up by sound theory,
(look at his conversations with bnelson)

I find flaming him very distasteful.

This forum has very little of that, and either flaming him or baiting him into a response really reflects a level of disrespect that has no place here.

jhg

For anyone who doubts peter's analysis, I
challenge them to do what I did: simply
slap a scope on the output of a rotary converter
idler motor.

I can assure you all that when this is done,
the three legs are as the idler motor's
manufacturer installed the windings in the motor.
Namely, 120 degrees apart.

If any of you suggest this is a trick of the
light, or some kind of capacitor-induced
subterfuge, I again will say that the measurement
was done on my stone-age phase converter, with
nary a capacitor in sight.

Yep, 120 degrees between waveforms. For those
interested in doing this test, one handy
instrument is a Fluke Scopemeter, because the
common lead is isolated.

So it's just a matter of clipping the common
one one leg, one probe on a second leg, and the
second probe on the third leg.

It's three phase. No doubt about it - 120
honest degrees between 'em.

Jim

"The patent doesn't relate to our discussion ..."

The above cited patent refers to a power factor corrector.

U.S. Patent 6,297,971 refers to the Phase Perfect phase converter.

IPCs and RPCs are "prior art", and are as old as electrified railways, where these first found wide application in converting a high-voltage ac, often about 11,000 volts, single-phase, for distribution, into 600 volts, three-phase, for traction.


Incidentally, the electrified railroads do this without capacitors, primarily because their converters employ a wound rotor machine.

With a wound rotor, a source of +VARS (a capacitance) as well as a source of -VARS (an inductance) is available, all by varying the "excitation" of the machine.

In our shop-built converters, these being of the squirrel cage type, there can be no source of +VARS unless this is provided by capacitors external to the machine.

It is through the addition of capacitors that better control over the manufactured phase voltage is achieved, and the "regulation" of the converter is thereby improved.

U.S. Patent 6,297,971 refers to the Phase Perfect phase converter.

Click to expand...

Hi Peter,

OK. I put that one up on the site also http://www.AtwaterKent.Info/Images/Patent6297971.pdf I didn't find it because it doesn't contain the phrase "phase perfect".

I see what they're doing. They float the neutral on the motor and overdrive the third phase hard enough to shift the effective phase reference of the neutral. That will work if you drive it hard enough.

But unless I'm mistaken, the neutral of a wye-connected 3-phase motor normal returns to the AC distribution neutral. Is that not the normal connection? If the idler neutral and the running motor neutral are floated and connected together, then the scheme under discussion would work, but that was never mentioned.

"But unless I'm mistaken, the neutral of a wye-connected 3-phase motor normal returns to the AC distribution neutral. Is that not the normal connection?"

No!

The normal configuration is that the neutral point in the idler and load motors, what I have termed N', is always floating.

In practice, this is never returned to the real neutral, excepting in real three-phase systems, not manufactured ones, and only in those which are very large, say, large enough to be 4,160 volts.

That was the only case I can recall in the years I was an EE at this nation's largest municipal utility.

4,160 volts, not being one of our preferred voltages, was a customer installation, where he provided all of the supporting equipment, including the 4,800 to 4,160 transformer.


4,800, a Delta system, was our standard offering. Another was 34,500 volts, also a Delta system.

As you may recall, there is never a neutral in a Delta system.

(I acknowledge that certain small customers may have a composite single- and three-phase service, where there is in fact a wired neutral, but in these cases the neutral is never utilized except as a "return" for 120 volt single-phase loads within that premises).


"If the idler neutral and the running motor neutral are floated and connected together, then the scheme under discussion would work, but that was never mentioned."

Yes it was!

That fact was implicit in all my earlier discussions, particularly where I mentioned ... more than once ... that "zero sequence currents cannot flow in a Delta system because that system is not grounded".

Remember that?

So, return to my earlier presented diagrams, where I showed that there was an N, the premises' true neutral, and also a pseudo-neutral, N', the latter being wholly within the RPC system.

The manufactured three-phase system is 138/240 Wye, which is fully equivalent to 240 Delta using well-known Wye-Delta transformations, and, additionally, the manufactured three-phase system is floating 69 volts above the premises neutral.

That fact was also discussed several times, and is central to the way in which an RPC works.

I was always upfront about that fact.

Perhaps you did not understand the significance of that fact. And, if so, then also you don't understand how these RPC systems are engineered.

Or, at least you don't understand the fundamental principles upon which these systems are based.

But, I now sense that you're beginning to understand, so perhaps now you can dispense with the name-calling.

Hi Peter,

OK. As I mentioned earlier, the system with a floating neutral will work. That may have been mentioned in a thread before I joined the discussion, as I don't remember it.

I'm also an EE, and couldn't understand how the RPC would work with the wired neutral. The floating neutral certainly is "central to the way in which an RPC works".

Anyway, I'll concede that it was an erroneous interpretation on my part. My apologies for the antagonistic tone.

Thanks and Happy New Year.

"Thanks and Happy New Year"

And to you, Leigh.

When I looked in on the Forum last night I was a bit taken aback by the tone of the thread as it seemed counter to the spirit of my initial question and the season. I agree with jhg, Peter’s postings have always seemed well thought through (even if I do not always understand them!), and I have appreciated his strong contributions.

I pulled the Phase Perfect patent down (thanks Leigh) per Peter’s suggestion that it more clearly explains what is going on. With a brief perusal of the patent and the conversation between Peter and Leigh, I think I’m now seeing where my thinking was going wrong. Never having had access to a three phase system, I’ve always visualized AC power as being comprised of one or more sine curves relative to neutral . Taking this simplistic perspective and looking at these various diagrams, I kept seeing the 180 degree offset of L1 and L2 relative to neutral, and thus I was not understanding how the 120-120-120 was being created. The key bit I was missing is that the three phase sine curves reflect the voltage difference between the phases themselves (T1-T2, T2-T3, and T3-T1) rather than each phase to neutral. I’m sure I’m missing much more of the picture, but at least I’m a little farther down the road.

Jim’s empirical results help to solidify this understanding. Thanks all!

--Larry

"I pulled the Phase Perfect patent down ..."

Going from the drawing on Page 4 to the drawing on Page 5 requires a leap of faith.

In one case, the special, high-voltage source is being added to the incoming L1-L2; in the other case this source is being subtracted from L1-L2.

The math does work out, if you would describe it in analytical form.

For instance, write expressions for the A-, B- and C-phases, as a function of L1-L2 and the special, internal high-voltage source.

As the inventor elected to describe this process in graphical form, he elected to show L1-L2 and L2-L1 in the same Figure, with one obviously being the inverse of the other.

This could lead one to conclude there was a 180 degree difference, while there is not such a difference.

And, as I have repeatedly pointed out, the A-, B- and C-phases are floating with respect to the premises neutral, which isn't used in the Phase Perfect device, anyway.

Note that the Phase Perfect patent, Figure 1 shows a ground, but this ground is internal to the distribution system, that is, it is external to the Phase Perfect and is never propagated to the load, nor even to the Phase Perfect itself.

(Figure 2 shows a ground within the Phase Perfect, but this ground uses a symbol which is distinct from that which is used for the neutral, and this ground is really an internal ground return).

Details, details, details.

"I kept seeing the 180 degree offset of L1 and L2 relative to neutral,..."

This is quite common. You probably have an
engineering background, yes?

True electrical engineers will look at the
centertapped 240 service that is common in
US residences, and say that the neutral is
the common reference point. And that if one
measures one hot leg, one sees a sine wave which
one could say is a phase reference at zero
degrees. Then if one puts the other scope probe
on the *other* hot leg, one sees another sine
wave at 180 degrees out from the first.

But the electrician won't call that polyphase
power because he can't run rotating machinery
from it.

When thinking about a rotary phase converter
built to power machines in a home, it's really
best to image it as just two hot legs - and don't
think about the neutral at all. Don't measure
any voltages with respect to the neutral, and
don't measure any currents in it either.

The closest thing to a home-made RPC in the
industrial world would be 240 volt three phase
delta, with one side of the delta centertapped.

If one defines residential centertapped service
as "two phase" then this causes all kinds of
semantic difficulties and all sorts of problems
making the explaination about why it really
*isn't* "two phase."

The short answer is, it really is from an
electronics engineer's viewpoint. But from
an electrican who wants to wire up rotating
machinery then it isn't. Because, like a
bicycle where the pedals are stuck straight up
and down, and the feet are constrained to only
run straight up and down, the crank can't start
turning on its own!

Jim

"Because, like a bicycle where the pedals are stuck straight up and down, and the feet are constrained to only run straight up and down, the crank can't start turning on its own!"

My brother, a specialized bicycle rider and builder, claims he has a bike which has a slightly offset "crank", and, if that wasn't enough, the sprocket is not round, but is "egg shaped".

Apparently, these alterations provide a measurable benefit to the sophisticated owner/rider.

This is quite common. You probably have an
engineering background, yes?

Click to expand...

Jim, well sorta , professionally I'm a computer guy focusing on infrastructure and software design, but I lack the discipline of the true engineer. Some label me more of an artist, particularly since I use a lot of intuition in my solutions. The engineer relies upon the language of mathematics to describe the world, but math is not my first choice when it comes to explaining what I see.

Letting go of the voltages being referenced to neutral is the key bit that helps me to picture what is happening here. Now to do some reading about three phase wiring arrangements...

--Larry