filloux

Text translated with Google:

Hello, I manufactured a "railgun" electromagnetic miniature functioning with
a pile of 9V according to the diagram found on the following Web site:
http://www.scitoys.com/scitoys/scito...n/railgun.html Somebody
could it explain me in detail the exact reason of the displacement of the
metal carriage; I have a small idea but I wish to be sure for it at 100%.
Thank you in advance!

Jean-Christophe FILLOUX
Professor de Physique from Poitiers FRANCE

jm albuquerque

"FILLOUX" <jc.filloux@wanadoo.fr> escreveu na mensagem
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Bonjour,
I've read all and I've been trying to understand.
I'm not an expert, but I'm doing my best.

Concerning your explanation, which helps a lot, I've found that in a
mechanical balance forces doesn't hold good. If the forces F(L1) and F(L2)
points to the right, then the railgun must go to the right. You can put
gravity force and friction force on the drawing and their resultant doesn't
balance the "magnetic impulsive force" F(L1&2).

Since you have the toy, I had the following idea:
Why don't you get rid of the rails and the friction force?
You can suspend the gail gun by some thin flexible cooper wires. If you can
make a good electrical contact with the perimeter of the magnets, the all
set up will look like a pendulum.

In a pendulum set up what happens ?
Does it go to the right, or to the left ?
Do we have levitation ?

If it doesn't move at all, as I suspect, then forget your idea and we could
begin to think on another.

Best Regard,
JM Albuquerque

jm albuquerque

"FILLOUX" <jc.filloux@wanadoo.fr> escreveu na mensagem
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Well,...
.... maybe it could seem right because basic magnetism rules apply, but in
fact you have created a force in space coming from...? Where?
I can see no coil.
Even if I push hard, and I can see a coil, I never seen a coil moving
without an external magnetic field directing it.

Therefore, I insist, if the idea of the pendulum set up doesn't levitate,
then there is no force applied to the axis of the iron, and so...

.... the cooper rails could have something to do with it. In general,
considering the other experiences in your initial URL, maybe the conducting
cooper wires play a roll in the magnetic balance.

By levitation I mean any movement of the pendulum when energy is applied.
Because pendulum has a circular movement, the mass will go higher in
potential energy, against gravity.

Another hypotesis is:
Maybe the magnetic field created around the iron conducting rod (between
magnets), that must have a circular magnetic field by the rules, could
created a torque on the magnet-wheel system. A torque will not create
levitation on the pendulum system.
I believe that the proposed force (on the axis) must creat levitation.

After a close analysis, maybe one can consider the air coil inside the blue
circular line. Since that's the current line, there is in fact an air coil,
with B pointing up.
This new B will oppose the circular magnetic field around the iron
conducting rod. So the railgun has in fact a torque which pull him away and
correctly.

Then you might ask. If so, what are the two magnets for?
They are to make the circular magnetic field around the iron rod bigger
(more diameter), thus creating more torque. How? Magnetic fields don't like
to cross.

Air coils can melt cooper bars in generators damping windings, that's for
sure.

Also, I believe that big generators don't have coils in stator. The only
way a can understand stator windings is forgetting the coil idea. One must
think of a big copper bar, inside a slot, with a huge magnetic field
perpendicular. When there is movement, we have MW and hundred Amps, and no
coil at all. If we look at the real existing coil, then current must be
flowing both ways in that coil, both starting from the opposite side of the
coil. I believe that's why experts talk about "coil-side", and "windings"
instead of real coils. Can anyone confirm this?

If so, a given length of wire will have a magnetic field equal strong as the
same length of wire in a closed loop coil. The coil idea is not needed for
nothing. Can anyone confirm this also?


Best Regards,
JM Albuquerque

jacques lavau

Sorry for the others, I stick with french this evening, for Pr Fillioux.

Du point de vue des symetries, il faut considerer trois elements de courant :
2 - Dans le barreau,
1 et 3 - dans les rails.

Intensite dans le barreau. Aucun effet theoriquement previsible, pour deux
raisons.
Raison 1 : Les deux aimants sont opposes, et leurs effets se retranchent, voire
s'annulent s'ils sont exactement de meme force.
Raison 2 : direction du courant incapable de produire une force de Laplace. Dans
votre langage : E et B sont des vecteurs colineaires.
Dans le mien : la projection de E sur le plan de B est nulle.

Intensite dans les rails d'amenee du courant. La, la symetrie est la meme que
celle des aimants-roues, avec renversement droite-gauche.
Il suffit donc de regarder en elevation une seule arrivee de courant, et un seul
aimant.
Le courant arrive de la gauche, jusqu'a la roue. Je veux que la roue suive la
meme direction, vers la droite. Il doit donc avoir le sens d'une spire de
courant, telle que sa portion la plus proche de l'amenee du courant, soit
repoussee, donc circule en sens oppose. Autrement dit, le B de l'aimant vu
devant moi, tourne comme la spire qui le figure, dans le sens horaire. Donc je
vois une face Sud.

En arriere plan, le deuxieme rail qui remmene le courant vers la gauche. Pour
obtenir le meme mouvement, le second aimant doit me presenter sa face Nord.

Si mon explication est correcte, nul besoin de faire passer le courant par le
barreau.
Variante 1 : le courant passe le long d'un seul rail. Ca doit marcher aussi bien.

Variante 2 : le courant passe dans le meme sens dans les 2 rails, aimants
opposes.

Variante 3 : Le courant passe en sens oppose dans les 2 rails, mais aimants
paralleles.
Variante 4 : Aimants paralleles, vous alimentez un rail a son extremite droite,
l'autre a son extremite gauche. Ca doit marcher aussi bien. Et le courant passe
a nouveau par le barreau (ce qui realise un interrupteur, apres tout).

Desole, pas de dessin...

Et ce n’est PAS un moteur homopolaire !

Sorry for french accents, gone away !

--
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don kelly

------------
Big (and small) generators definitely DO have stator coils. The coils are
laid so that one side of the coil is under the N pole of the field while the
other side is under the S pole of the rotating field- the two sides are
additive with regard to voltage and have the same current. Your copper bar
and a bar 180 (+/-) electrical degrees away from would be connected to form
the two sides of a coil. Hence the term "coil side" of real coils. In
small machines the coils will be wound, in others they are preformed and may
have one turn or multiple turns per coil. In any case there will be a
number of these coils distributed over part of the periphery of the stator
and these form the "winding" Look at any engineering text on
electromagnetic machines.
As far as your statement about current flowing in both directions in the
coil... you have left me confused. There will be only one current in a coil-
after all it is simply a loop. It is possible to have a single bar but as
the rotor field has pairs of poles, it makes economic and engineering sense
to have two such bars in series (forming a coil) and all external
connections at one end. This is, in fact, what is done ---------------------------------------


--------------
In a coil, the two sides will produce an additive mmf in the region inside
the coil and a subtractive mmf external to this region- the actual field
distribution will be quite different from that obtained with a single wire.

--
Don Kelly
dhky@peeshaw.ca
remove the urine to answer

filloux

Hello,

First of all thanks for helping me to understand this phenomenon of the
railgun. My railgun functions with a pile of 9 V under less 100mA!!!

You can see who I am here: http://perso.wanadoo.fr/jc.filloux/mapage/jc.htm

You can see my railgun here:
http://perso.wanadoo.fr/jc.filloux/mapage/Diaporama.pdf (diaporama 1.63 Mo)

You can see a short video of my railgun here:
http://perso.wanadoo.fr/jc.filloux/mapage/DSCN1283.MOV


A +

JC FILLOUX from FRANCE

don kelly

--


-----------------
What you are describing appears to me that you are assuming the coil sides
are connected at each end - why- then you have a short circuited coil and
that will do nothing but generate heat due to the high circulating current.
As for connection at one end (only?) what is the point of this.?
In an actual machine you will have something like this:
|___N_____| |___S_____| ------> o-----------------------o
| --->I | Basic 1 turn coil.
Each phase winding will have several
| | such coils in
series to form a phase winding
|_____ ________|
| V |
The current is physically opposite in each coil side as seen from above but
there is only one current in the two "coil sides" as shown -------------

---------
If you are saying that the current flows from front to back in one side of
the coil and back to front in the other coil side- Yes -provided that there
is a closed circuit. However, this is a single current electrically as the
coil sides are in series. See drawing above.
Also please note that the rotor magnetic field produces a voltage not a
current and this voltage exists whether or not stator current flows. The
current, when and if it flows will produce an mmf which generally is
opposite to the rotor mmf, weakening the field (Generally as phase shift comes into play). ---------------


----------
Current is not created or destroyed in the bars. What is produced is a
voltage. when the coil is connected to an external load (i.e. a closed
current path is made) then this voltage will cause a current to flow. If the
coil is open circuited or a bar is not connected at one end- no current. -------------------


----------------
If you look at an electrical machines text (engineering - physics texts
won't deal with this ) you will see drawings and explanations of the
windings and the mmf and flux distribution around the periphery of the stator. --------------


----------------------
You can connect the three phases in a delta or a wye formation.
Delta: A1------A2 B1-----B2 C1-------C2 where A, B and C are the
three phases
then one connects A1 and C2 to output a
A2 and B1 to output b
B2 and C1 to output c
This gives 3 output leads. The connection gets its name from the usual way
of drawing it like a Greek letter delta.

Wye
A1 to a, B1 to b, C1 to c (again 3 leads)
A2, B2 and C2 connected together. This neutral point is usually brought out
as a separate lead but this need not be done. The name of the connection is
based on the usual way of drawing this -as aY -------------------


-------------------
I have quite a few University texts (including one for which I was a
co-author) which do not agree with what you have indicated. Perhaps you are
confused by the fact that with a coil, the two terminals are at the same end
of the physical coil as a matter of typical design. I have also seen the
preformed coils of large generators and each coil definitely does have two
terminals. In fact, what good would a coil with only one electrical connection be? -----------------

------------
1)The pole pitch for a phase will not be 60 degrees but 180 degrees- look at
each phase by itself and see what coil distribution will produce a
sinusoidal voltage in that phase. A 60 degree pitch will not do it.
2)Not parallel connected- As the coil sides are not in the same location but
distributed as you say- the field seen by each side will differ from that of
adjacent bars (i.e. the voltage in each individual coil side will be
slightly out of phase with that of the adjacent coil sides) bars so there
will be a diffferential voltage between bars- in parallel this would cause
circulating currents which would do nothing but quickly overheat the coils.
A series connection is used. In a 4 pole or higher machine, the coil "group"
under one pair of poles can be paralleled with a symmetrically corresponding
group under another pair of poles as there will not be a differential
voltage between such groups.
------------------


---------------
In the real world phase poles will always span 180 electrical degrees
(ideally), not 60 degrees. If the coils could be wound on the surface of the
stator the individual turns would be distributed sinusoidally. That is not
physically possible so the coils are in discrete slots. Often the coils may
have a short pitch (<180 degrees) to reduce some of the harmonics (usually
the 5th and 7th - the 3rd, 9th etc harmonics do not cancel in a 3 phase
machine.) produced by the discrete distribution.
The coils of the individual phases definitely do overlap. The stator slots
hold 2 coil sides each- some will have coils from two different phases.
Eg:
top of slot ccccccaabbbbbbccaaaaaabbcc...
bottom of slot bbccaaaaaabbccccccaabbbbbb....

Don Kelly
dhky@peeshaw.ca
remove the urine to answer

don kelly

-----------------
You can have several parallel groups per phase for machines wih more than 2
poles. All practical windings will have double layers -top and bottom of slot configurations. ---------


---------
I did not assume or imply any monopoles. Consider a simplified phase winding
as shown
+ current into screen

N <----------- S field produced

o current out of screen
This is the situation I intended to show for the one phase.
Every AC machine has the same number of stator poles as there are on the
rotor. Otherwise the machine will be rather useless (and if big enough will
simply shake itself to pieces.
In a 3 phase machine with each phase designed for 2 poles and the phases
carrying balanced 3 phase currents, there will result a stator field with 2
poles and this field will rotate at synchronous speed. The poles are due to
all the windings acting together. This rotating field is the basis of the
induction motor and is of great concern in a synchronous generator as, in
the latter, the stator and rotor fields must be moving at the same speed
except for momentary transients- if not- blackout time.
You call it a 6 pole machine but the industry and engineering texts call it
a 2 pole, 3 phase machine. >

-------
No- look at the drawings- even yours doesn't indicate this. ----------------


-------------
NO. If you energised the windings and measured the fields you would find a
magnetic field centered on the winding axis. This is similar to what happens
in a solenoid. The output voltage is not in this virtual vector -it is due
to the rotor field and is in time quadrature to this rotor field. The
electrical phasor diagram is based on an mmf diagram -as seen by an observer
on the rotor- hence references to direct and quadrature axes.

---------
except that the Ea vector that you are referring to is rotating -this
complicates the analysis greatly- that is why all quantities are referred to
ficticious rotor windings which produce the same fields as the actual
stator windings and satisfy equal power constraints.


--------
I disagree. Note that the mmf produced by the stator winding is not related
to the voltage generated. It is an armature reaction effect>


-------
I disagree and you have not backed up this contention.


--------
No- as it is the same as a normal coil. Reomove the iron from the stator
and rotor and consider a phase winding in free space. It is a normal coil.
Now put the iron back to get a good magnetic path (as that is all it is
there for) and the direction of the field and the relationship between the
coil and the field will not change. ----------

---------
The bar length is important but that is because only the bar lengths are in
the active field, the end turns are not. Essentially they are normal but
elongated coils.
Aslo, in repaly to a previous comment of yours - the field due to a coil and
the field due to a bar are quite different in terms of total flux and the
flux distribution. In addtion please refer to Maxwell's equations in the
integral form where the voltage induced in a closed path is due to the time
rate of the flux within that path and normal to it. and see how it applies
to a bar.>

--------
No- they cancel in the machine output- a virtue of 3 phase..

-----
See Fourier analysis -hal wave symmetry- no even harmonics.


--
Don Kelly
dhky@peeshaw.ca
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jm albuquerque

"Don Kelly" <dhky@peeshaw.ca> escreveu na mensagem
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- snip -

- snip -

- snip -

Dear Don Kelly,
This time you really destroy all my previous reasoning about conventional
generators.

My mistakes:
1 - The mmf produced by the stator winding is an armature reaction effect.
I know there is a reaction and I know that stator poles and rotor poles work
engaged, like teeth of a gearwheel. But I couldn't perform the all mental
exercise. I was thinking about a direct action from the rotor against the
stator.
2 - The poles are due to all windings acting together.
I haven't realise this...

....and this changes all my previous thinking.

I'm afraid you will tell me next that small three-phase generator (with lots
of wire-turns in each coil) are wire-wound like big generators?

I have two more questions, please:
1 - Have you ever heard about a synchronous generator with 6 real coils in
the stator (3 N poles + 3 S poles) and one N pole plus one S pole in the
rotor?
2 - In a set of 3 phase coils (aa',bb',cc'), when looked separately and
placed side by side, doesn't the central phase coil (bb') need to be
wire-wounded opposite to the other two coils, which means that the magnetic
field produced by the central coil is opposite to the other two coils ?

I'm afraid you never heard!
Until Yesterday I believe that any conventional 3-phase generators have 6
coils in the stator, for each pair of poles in the rotor.

It happens that, some time ago, I needed to conceive what I believed to be a
conventional 3-phase generator stator winding. The stator was to apply in a
given rotor, I had in mind, for a special application. So, as usual, I start
reading all the information I could and, simultaneously, constructing
mentally the all set up with the expected working mechanism.
So I came up with a 6 poles machine that actually produces only two
revolving magnetic poles in the stator, against two opposite magnetic poles
in the rotor, like any conventional generator does.

So far, I believe that my big mistake was to call it "a conventional
stator", because in fact it is totally different. I found it to be the right
stator for the application.
So far a conventional stator doesn't seem to work.

Aren't there any old variants of 3-phase stator windings? (for a
conventional rotor, I mean).

Would you like to see the drawing I made for the 3-phase stator with 6 coils
(delta and star connected)? When external 3-phase voltage is applied, all
the 6 coils act together to perform a revolving magnetic field with one N
pole and one S pole spaced 180 electrical degrees.

Thank you very much.

Best Regards,
JM Albuquerque

don kelly

-----------
Small generators will have what are called "random wound" coils- i.e. the
wires are wound into the slots as coils. For large units this is not
practical so pre-formed "coils" are made and these are then placed into the
slots with one side at the top of the slot and the other at the bottom. This
has advantges in making the coils (particularly at higher voltages) and also
in replacing a damaged or defective coil. This is a matter of engineering
design rather than a fundamental principle.

-----
six real stator coils producing a 2 pole 3 phase winding - yes.
6 poles on the stator and 2 on the rotor- no.
This seems to be a point of difficulty in our relative perceptions ---------


------------
There are 6 coil sides but this translates to 3 coils . For a 4 pole machine
1
4 2 1 and 2 are the two sides of one
coil 1 is in and 2 is out
3 3 and 4 "
2 in and 4 out.
The field is in to the centre between 1 and 2 and between 3 and 4 and out
between 2 and 3 and between 4 and 1 - 2 N poles and 2 S poles
Developing this in a typical linear diagram gives the following appearance.

1 \/ 2 /\ 3 \/ 4 /\ (1) <--as we are back to
beginning)
N S N S
2 windings - 4 poles
For a single phase winding excited with AC the poles will be stationary but
will alternate in direction
This can be modelled as two sets of 4 poles of half strength rotating in
opposite directions.
For a 3 phase winding , one must consider the interaction as below whic, in
this case would reuslt in a rotating 4 pole field.

---------
If you are looking at a 3 phase machine, with, according to your
visualisation 2 poles per phase - the result of energising the 3 phase
stator windings with balanced 3 phase is a 2 pole rotating field.

Common terminology is that this is a 2 pole stator winding, not a 6 pole
winding. The actual mmf produced by the stator windings is due to all the 3
phases not each in isolation. .
If the windings were distributed sinusoidally in space on the stator (ideal)
then the mmfs are
Fa =NI(coswt)(cos a)
Fb=NI(cos (wt-120)) cos (a-120) (a is the mechanical position of the
winding)
Fc=NI(cos (wt+120))cos(a+120)
adding these results in an mmf
F=(3/2)NI cos (wt-a)
which is a rotating (travelling) wave of constant peak magnitude and speed
da/dt =w -that is synchronous speed. The field, to an observer, will be a 2
pole field, which is revolving at synchronous speed. This rotating field is
the basis of operation of polyphase machines (i.e induction motors/generators and synchronous motors/generators)

---------
I would be very happy to see this but what you describe is just what
happens in a conventional machine. This goes back to Tesla. It appears that
nomenclature/wording is the major problem.
Please send this to me removing the pee from the address.
--
Don Kelly
dhky@peeshaw.ca
remove the urine to answer