Why? What would be gained? As I see it, all that would happen is that both
the core and the conductors would be longer -not an advantage. Mechanically
it would be a real pain in the ass to build. Simply put, a magnetic core is
used to direct the flux to where you want it by providing a good magnetic
path compared to air, etc. The shape of the core is of minor concern
(smooth corners rather than square ones are nice, but... ) except that it
should preferrably be as short as possible.
I suggest that you think about the cross section of the strip (rather than
its sides)- where the current would flow. You will not get two turns from
one. Another problem is that the current and the field are mutually
perpendicular so the same wire cannot deal with that no matter how it is
Don Kelly email@example.com
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So, a question for you. All the large generators I've worked with have a
'saturation curve' listed in the documentation. Open circuit voltage (P.U.)
versus field current. Nice and linear, up to about AFNL (Amps, Field,
No-Load), then from there the line curves off as the field iron saturates.
(they also have a line for short-circuit operation (short-circuit current
P.U. versus field current) and AFFL (Amps, Field Full-Load) that is always
quite a straight line. Between these, I can get a pretty good idea of the
syncronous impedance of the machine. )
But like you say, the iron doesn't saturate during operation. I've been
modeling steady-state generator operation as an ideal voltage source, and an
inductance (equal to the synchronous impedance) in series. But I've had
some 'discussions' with others trying to model these machines about what to
use for the 'ideal voltage source'. I've been using a linear calculation of
the field current times the slope of the linear portion of the 'saturation
line'. Another bloke as been trying to argue that his results are 'better'
because he uses a curve fit of the saturation line that 'flattens' as field
I maintain that the MMF of the field current is being countered by the MMF
of the armature current and the iron doesn't saturate. What you've said
here seems to support that idea.
Yes, this was my thinking. Reactive load currents (lagging) create an mmf
that is exactly along the direct axis, but displaced 180. There is a 90
shift between the field winding and the maximum induced voltage in the
stator, and another 90 shift between the voltage in the stator and the
lagging pf load current. This puts the mmf associated with lagging load
current in direct opposition to the field mmf, resulting in poor voltage
The text I've been using ("Standard Handbook for Electrical Engineers",
Knowlton) gives the vector diagram for regulation and field excitation,
taken from an old ASA Standard. That, along with a 'Blondel diagram'
('two-reaction diagram')have been my primary tools/sources of data. Doesn't
mention 'Park' by name though...
As I said, I've been using the linear, no-saturation model to calculate the
steady-state excitation requirements at various load levels, and they seem
to agree with the actual operating unit data. But some of it is supposition
on my part. I felt that the air-gap flux *must* be the net result of these
two mmf's (field and armature), and therefore saturation couldn't be
occurring under normal operation. Another clue is the much larger field
currents that are normally seen when operating near rated conditions.
Interestingly, I found one of our units operating with the field current
showing *below* the AFNL value, yet it was carrying enough MVAR to be
running about 0.95 lagging. So I concluded that the field current
transducer input to the logging computer must be defective. The computer
tech swore that his computer was right, and the I&C tech swore that it
really was reading the number of milliamps that corresponded to the field
current shown. But when we checked the field-current to mA transducer, lo
and behold it was way off, reading low. I told them that my 'calculations'
predict the field current should be 'X', and the 'as-found' testing of the
transducer showed I was within 3%. They were suitably impressed :-)
I haven't tried to tackle transient or sub-transient features in my model
yet though. Still working on the steady-state. I am working on the
transient rise in voltage with a load-reject sceanrio. It *seems* like I
could just drop the load current to zero while maintaining rated field
current. But that gives some pretty incredible voltages (as in three times
Yes, I've found that the synchronous reactance is really the major player in
voltage regulation of the unit. In my calculations of generator, main
transformer and 'infinite bus', I've found that from the 'ideal voltage'
source viewpoint, it carries quite a large 'reactive load', most of it right
inside the machine in the 'synchronous reactance' component. And yes, when
I compare the synchronous reactance with the winding inductance, there is a
Haven't had to model one, but was thinking I could use the exact same
components, just swap the torque angle to the opposite side. Sort of just
what happens in the machine. This reverses the real component of the
current. And what was a 'lagging' reactive current is now 'leading' (the
reactive current vector doesn't change, just its relationship to the real
current vector that is now 180 from where it was in a generator). But as I
say, haven't tried it yet, so.....
Not sure what you mean about "the bulk of synchronous reactance lives" on
Your previous statements also help explain why a constant V/Hz is desirable
in variable speed applications. As frequency is reduced, the voltage must
be reduced concurrently to avoid saturation. I've also seen some small
isolation transformers used for attaching instruments to AC servo-motor
machinery. Along with other ratings, the label listed frequency as ">
V/1.5". As long as you connected to a system with a frequency higher than
that, no problem. When it was inadvertently hooked up to a servo-motor
incorrectly, the output was nasty 'spikes' every half-cycle as the iron
within went from saturated in one direction, to saturated in the opposite
(fortunately, the thing wasn't damaged, must have had enough impedance to
limit the primary current)..
P.S. Not to worry, these models aren't being used for design/analysis, I'm
not a PE. Just what you might call, 'professional curiosity'. If I can
calculate/predict behavior, then I figure I've got a better understanding.
(getting too old to actually crawl inside the things, besides carbon dust is
hard to wash off :-)
P.P.S. Although we have some salient pole units, MG-sets (~5000 hp) and
diesel generators (nice EMD 4500kW units), I'm still just working through
round-rotor machines (tubine driven).
Why, yes they are. 7000 hp drive motor, 5000 hp pump motor.
Been having some discussions about the losses in the fluid coupling. The
losses are related to the torque/speed curve of the pump being driven and
can peak out about 800 kW. So modern reactor changes that allow operating
with lower core flow end up using almost as much power as rated flow. Just
being wasted in the coupling instead of the pump power.
I did a lot of DC machinery repair at sub-base. The DC equipment can be
rather 'fun'. Take your average civilian electrician and show him the
starter/controller for a 50 hp DC motor and they're amazed at the size of
stuff needed, versus a simple MCC 'cubicle' controller.
Having worked at a PWR not a BWR, I didn't realize the MG sets were variable
speed. I had thought it was a "multi" speed setup with set frequencies out,
with fine adjustments using throttle valves. Do you know if other BRW
plants run like that?
Are the MG sets part of 1E systems? If not, has someone evaluated
converting them to VFD for energy savings?
All GE BWR's use variable recirculation flow through the reactor as part of
the power control. BWR 3/4/5 all use variable speed pumps (the pump motors
are ~5000 kW). BWR-6 designs have used flow control valves and two speed
Surprisingly, no they are not 1E. The only safety-related function is an
upper limit on speed (limits power excursions). Unlike a PWR, a complete
loss of recirculating pumps is not an 'accident', several plants have
experienced this without SCRAM.
I had heard rumor of at least one BWR that went to VFD, but can't find out
who it was or any OE about it. The biggest concerns are *reliability*. As
you know, unplanned shutdowns (or even downpower to 25%) to recover a lost
pump is a 'hit' on generation as well as WANO. For our 'unregulated'
generators, being able to boast significant reliability allows us to command
a much better price than most 'unit-contingent' without the liability of
'replacement-power' contracts. Not to mention the improved 'quality of
life' not getting call outs to deal with equipment failures.
So the system has to reliably operate for 2+ years with zero downtime. And
of course it cannot put excessive EMI/RFI in the building or on the supply.
Combined with a desired life of +30 years, it's a bit of a challenge.
That's probably what I head about. I had a co-worker who previosly worked
at Perry, and that may be where this tidbit came from.
That's what I suspected since forced circulation is just needed to crank up
I'm not sure you would use 'accident' in a PWR either. Yes it's a trip for
most plants (I think there are some units with TS allowance with 3 of 4
pumps). It most likely would also be a Unusual Event classification since
the bulk of the time it would occur on loss of offsite power.
Try posting on nukeworker.com. If that identified the right plant the
chances of finding a EE to talk to go way up.
I'm sure it is. Harmonics of course would be a big concern. Add to that,
frying the motor is REAL expensive. I just wonder how much savings are?
You're in the unusual position where power costs you what, 2 cents / kWHr?
Yes, but it's even 'less' of an issue with BWR. Unless you're not licensed
for single-loop ops (most are), you just stabilize at around 50% power, fix
the MG, start it back up and recover. No 'cold water' interlock or other
No, that's the wrong way to look at it. It's not what it cost to produce
that energy, it's what I could have sold it for if I hadn't wasted it. Say
it 'costs' me 200 kW. That's 1752 MWHr each year. I could have sold that
1752 MWHr for something like $45 / MWhr (unit contingent, long-term power
purchasing contract). So that's a lost opportunity of $78840 / year.
But you're right about the pump motor. No way we want to risk damaging
I was just taking deference to the term 'accident'
The more I learn about BWR, the more they seem the way to go.
No question in my mind that availability and safety are better.
I also note that there has yet to be a big BWR shut down - they all have
been PWR plants.
That's the "cost" I was figuring in, lost sales. $45/MWHr is good, at least
from the perspective of a resident of the northwest where there is lots of
A VFD risks the motor in medium voltage applications due to partial
discharge induced by the rapid rise and fall times. Output filtering is
becoming the norm on medium voltage drives to reduce the rate of failure.
I'm assuming the pumps are 4160 or 6900.
Well, 'transient' that is anticipated to occur several times in the life of
the unit. The recent issues with PWR's seem to be the RPV head issues.
BWR's are dealing with RPV internal inspections (core shroud cracking, jet
pump/ ram's head cracking). BWR's are easier to start up and operate (no
worry about 'flux tilt', just manipulate the rod pattern a bit; start up
cold and use reactor heat for heating up the plant;). But issues with
off-gas treatment, and personnel exposure during maintenance (just about
*everything* is contaminated to some degree). I look forward to GE's ESBWR
design, if it ever gets built.
That's also a good price for long-term PPA's here in the northeast. But
that's 4.5 cents/kWh, not 2. We could get better if we could 'get' the
power to NYC, but the transmission systems are pretty well maxed. A new
line was proposed to run north-south down towards NYC, but all the NIMBY's
have been protesting it. So I say, "Fine, pay your congestion fees to the
NYISO and watch your rates climb."
Well, actually they're *rated* for 56.5 Hz @ 3920V. But that works out to
4160V/60Hz, same V/Hz ratio. Yeah, I'd heard that about some VFD systems,
even some motor vendors de-rating their motors based on VFD drive. I've
also been hearing about VFD that go straight from AC to AC without the
conventional DC-Bus. But I don't know much details about them, the only
type I've worked with are the AC-DC-AC type.
I used to see something similar with DC motors. If run on 'pure' DC (such
as a station battery), they may be X hp. But if run on full-wave,
unfiltered AC, then they had to be derated to some fraction of X.
I was refering to the fact that the big units shut down - San Onofre 1,
Rancho Seco, Trojan, Zion 1&2, TMI 2, Yanke Rowe, Conneticut Yankee - they
are all PWR units. I'm not aware of a big BWR shut down, just the BWR 1 & 2
Well, Big Rock, which wasn't really a 1 or 2. NMP1 and Oyster Creek are the
only BWR 2's I know of, and they're still running.
You're right, I can't think of a BWR that's been shutdown, only PWR (and the
HTGR at Fort St. Vrain). But IIRC, many of the PWR's that were shutdown it
was the high cost of steam generator replacements. Something I thought was
supposed to last the life of the unit (40 years), but haven't (nasty sorts
of corrosion on tube/tube-sheet and tube/supports). Lately, the RPV heads
with their numerous nozzles (of materials that are susceptable to certain
corrosion-cracking) have also been 'door closing' challenges.
BWR's are facing some of their own challenges though. In-vessel inspections
of shrouds and jet-pump assemblies are raising concerns. Various in-vessel
repairs of shroud cracking have been acceptable so far, but if one needed to
*replace* the shroud, well that could be a 'door-closing event'. BWR lower
heads have large number of nozzles, much like PWR's upper head. But there
is no feasible replacement process (entire RPV). So we'll see I guess.
Of course, any *new* plant would have the benefit of the advances in
metallurgy and better understanding of many of the corrosion processes.
I thought BRP, Pathfinder and Humboldt bay were the true BWR1 units.
I also thought Lacrosse and Elk River were the 'not BWR 1' early plants.
Weren't Shoreham and Millstone 1 also BWR 2?
And the other oddballs of Saxton, Shippingport, Piqua, Fermi 1, Bonus and
I would argue that was a pretext. Until ~ '92, utilities shut the plants
down rather than face the cost of replacing the SG. After that, SG
replacement became a 45 to 60 day turn around project. I doubt we will see
any further early shutdowns from SG corrosion.
Actually, the predicted life up front was less than the unit life - it is
stated in more than one FSAR. What happened in many cases is that they
didn't even hit the expected 25 year life.
The NRC has a good page on the SG issues.
Is the shroud removable, or is it welded in? The core internals on a PWR
are replaceable, but I'm not sure how often it has been needed.
Millstone was a -3 (I think). BRP, Pathfinder and Humboldt Bay are too
early to really be considered 'turn-key' designs. They were all
one-of-a-kind, proof-of-something type plants. The -2/3/4/5/6 were/are from
the 'turn key' power plant generation ('69 + ) where a utility would buy a
nuclear plant 'turn key' from the NSSS vendor and AE. Many didn't really
appreciate what they were getting in to and did it more for the prestige of
having an "Atomic Power" plant in their portfolio.
But those early units didn't really fall into a BWR-XX category, they were
Well, yes, you're right about there being a wide variety of early designs
that have long been shutdown. Fermi 1 was liquid metal, Shippingport was a
'quick' adaptation of Naval design for political motives, and a host of
I think it's a combination of better turn around and higher capacity
factors. Back in the '80's, if it took 200 days to do a SG job, and you
only had a capacity factor in the 60-70 % range, the 'bean counters' would
sharpen their pencils and say, "We'll never pay it back." But now, with a
much higher capacity factor (most in the 90+ range), and shorter turn
around, you're right, it is more economic to do the work and keep operating.
Well, for a $$PRICE$$, anything can be done :-)
They are welded in. A sort of flat 'ring' around the bottom head / belt-line
hold the circular arrangement of jet-pumps and the shroud. But depending
where you want to try and cut it off, you'd have to either a) reach in a
very tight place on the outside between the shroud and jet-pumps (or remove
jet pumps??) or b) remove the lower core plate that directs the flow into
the fuel bundles. And of course even with the fuel off-loaded, it's pretty
'hot' around all that steel that has been irradiated for many years.
Problem is, nobody's done it yet and the uncertainty/cost is a big
Mark I BWR containments (those that look like an inverted light-bulb sitting
in a doughnut) are now reviewing some issues with the torus steel developing
fatigue cracks. A manageable issue, but just one more part of the plant
that needs another inspection program. "Like my daddy always used to say to
me, 'Little Rosanna Danna, If its not one thing its another' "
Pressurized Water Reactors. The reactor is cooled by water (called primary
loop water) maintained at high enough pressure not to boil at the operating
temperature. The primary loop water flows to a heat exchanger where water
at a lower pressure is boiled to steam to run a trubine.
2/3 of the US reactors and most of the French reacotrs are PWR
About 1/3 of the US reactors are BWR. Japan appears to favor the BWR on new
On higher voltage AC systems, the local electric field can exceed the
breakdown potential of the insulation, but the gap is large enough to
prevent a arc from forming. The circuit is completed through the stray
capacitance of the air or other insulation. In air, it forms a blue glow,
and is often called corona. The crackling sound heard on high voltage power
lines is from this phenomenon. In solid insulations it is usually
destructive to the insulation and will eventually lead to a full arc.
Because the current flow though a capacitor is proportional to frequency,
partial discharge can worsen as the frequency rises.
The synchronous reactance includes the relatively small leakage reactance of
the stator winding as well as the larger effect of armature reaction-(the
mmf produced by the armature).
a) You have correctly recognised the effect of armature reaction as
essentially demagnetising and simply assume no saturation. This is
considerably closer to the truth than your colleagues approach which
overestimates drastically the effect of saturation.
b) There is no way that one can realistically get any more than an estimate
of saturation under load. You can do it by putting on a load at a given open
circuit voltage and measuring the terminal voltage, current and phase and
calculating the effect -which would be true for that one load condition but
not true for any other condition- just not practical.
I have just been checking a couple of references and trying to adapt the
process for estimation of saturation. A correction curve could be made.
This would involve what you have intuitively recognised - subtract the
armature reaction mmf from the field mmf- essentially look for the voltage
behind leakage reactance and base saturation effects on that - this is not
quite true but is better than what your colleagues are doing. Then a
corrective factor can be determined and, guess what- it requires an
interative approach to get a solution.
An older approach to the estimation is through use of what is called a
Potier triangle- still a guestimate.
As for a synchronous motor- note that a motor is simply a generator
producing negative power. Simply stick a minus sign on the "generator"
current and all is well. As to vars, they will work out as well. Under
excited motor sucks vars -lagging pf as motor or leading as generator.
Have to go now, time is out for now.
Don Kelly firstname.lastname@example.org
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