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Posted by Ulysses on June 12, 2009, 6:57 pm
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> Ulysses wrote:
> >>>>
> >>>>> Anyone happen to know if the poles on a single phase alternator
field
> >>> coil
> >>>>> (rotor) are opposite such as they are on a 3-phase alternator? I
> >>> applied
> >>>>> *some* voltage to it but don't know how much is OK and don't want to
> >>> burn
> >>>>> it
> >>>>> out but it seems, so far, that the poles are the same on both sides
> >>> (i.e.
> >>>>> N
> >>>>> and N and not N on one side and S on the other). Also, would the
> > field
> >>>>> coil
> >>>>> be getting 120 volts on a 120/240 volt generator? I have this idea
> > but
> >>>>> before I waste my time I need to find out why it won't work ;-)
> >>>>>
> >>>> There should be one N and one S if it only has two poles. If more
than
> >>>> that, it should alternate N, S, N, S and so on. The voltage applied
to
> >>> the
> >>>> field winding is seldom anywhere near the same as the output voltage
> > and
> >>>> it's always DC. (although once in a while the rectifier is mounted
> > right
> >>> on
> >>>> the rotor so you can apply AC to the rotor's slip rings).
> >>>>
> >>>> Typical car alternator uses DC on the field winding, applied via two
> > slip
> >>>> rings. Although it is only one coil, the metal pieces on each side
> > have
> >>>> inter-woven fingers so that alternating 'fingers' are opposite
polarity
> >>> (N,
> >>>> S, N, S as I mentioned above). One alternator has as many as 28
> >>>> 'fingers'
> >>>> so that's 14 N poles and 14 S poles, interwoven.
> >>>>
> >>>> Surprisingly, a rotor for a three-phase machine and a single-phase
> >>>> machine
> >>>> have the same number of poles and winding. The only difference
between
> >>>> three-phase and single-phase is in the stator (armature) winding, not
> > the
> >>>> rotor (field) winding.
> >>>>
> >>>> daestrom
> >>>>
> >>>>
> >>> What I have in mind is to use a field coil from a single-phase
> > alternator
> >>> to
> >>> produce three-phase (or more) power by winding three coils (or six
> >>> perhaps)
> >>> for a new stator. This particular field coil has a tapered mount so
it
> >>> would be direct-drive. It has slip rings so I could apply the
> > excitation
> >>> current directly and control it for the desired output. That all
> > depends
> >>> upon whether or not the coils are still OK. The generator I saved it
> > from
> >>> had a melted stator but the field coils appear to be OK but I'm not
sure
> >>> if
> >>> it's one coil wound on two poles or two seperate coils.
> >> It may be either. The important question is just how many magnetic
poles
> > it
> >> has. If it has one N and one S, it is a two-pole rotor. Two N's and
two
> >> S's means four pole and so on. Call this number N. It must be an even
> >> number or you've got a consequent pole issue and I really, REALLY doubt
> > that
> >> for an alternator rotor.
> >
> > There are two windings (not sure yet if it's two seperate windings or
one
> > continuous wire) and they are wound around two iron cores. With *some*
> > voltage applied to the slip rings I was unable to determine the pole
> > directions by holding a permanant magnet up to each core. Not enough
> > voltage yet, I guess.
> >
> >> If you want to wind a three-phase stator, you need 3*N number of coil
> > groups
> >> (we call them pole-phase groups). Then the next thing to do is then
count
> >> the slots in your stator (call it S) and hope that it is divisible by
3*N.
> >> If it is evenly divisible by 3*N then you are almost ready. You need
to
> >> figure out how wide to make your coils (the coil pitch).
> >>
> >> Commercial machines we would put all the coils in with the leads
extending
> >> out one end. The first few you leave one side of the coils 'up' out of
> > the
> >> slot until you work your way around the stator. Put one side of each
coil
> >> in the 'bottom' of a slot and move on. After you've put in one coil
pitch
> >> worth, you can put one side in the bottom of the next slow and the
other
> >> side in the 'top' of the first slot you started with, on top of the
first
> >> coil you laid.
> >>
> >> After you have them all installed and wedged suitably tight in the
slots,
> >> you figure out how many coils connect together to form one phase group.
> > If
> >> you have a nice even stator, it comes out to S/(3*N). So a 72 slot
> > stator,
> >> for a four pole machine, you have 72/(3*4) = 6 coils that you connect
> >> together into one phase group. When you're done with that step you
should
> >> have 3*N (in my example 12) phase groups.
> >>
> >> Count off by 3 to see which ones are the same phase. If you imagine
the
> >> rotor in the middle of the stator and one pole directly over one phase
> >> group, the other poles of the rotor will be directly over the other
phase
> >> groups of the same phase.
> >>
> >> Connecting the individual phase groups together into each of the phases
is
> >> not hard, but it's much easier to show with a diagram than to explain
in
> >> words. But basically you can connect them either in series (more
voltage)
> >> or parallel (more available current).
> >>
> >> BUt considering this is a seat-of-the-pants design, you might be best
off
> > by
> >> insulating the leads and assembling it first one time, spin it up and
see
> >> how much voltage you get from one phase group. Then you can get an
idea
> >> what it's capable of.
> >>
> >> If you want more current per coil but don't want to buy wire that is
> > double
> >> in size, you can use an old trick we used of winding your coils
> >> 'two-in-hand'. Simply use two strands of the same size wire and wind
both
> >> strands at the same time making half as many turns. You'll get half
the
> >> voltage but by soldering the two strands together at each end you get
> > double
> >> the cross-section.
> >>
> >>
> >>> Like I said I tried
> >>> applying some DC voltage and didn't get any definate answers but I did
> > get
> >>> quite a zap when I disconnected the DC source so something is still
> >>> working.
> >> Big coil around iron is a great inductor :-)
> >>
> >>> All I would need to do is to make a brush holder, mount the non-engine
> > end
> >>> in a bearing, and one way or another position the stator coils around
> > it.
> >>> And rectify it, of course. Since there is some residual magnetism in
> > the
> >>> field coil I think it will self-excite just like my Delco car
alternator
> >>> does. But I don't know how much DC voltage it will need to excite and
> > end
> >>> up producing, say, 80 volts at 3600 rpm, but I think that will be
> >>> controlled by the number of turns on the stator coils.
> >> The first thing you want to figure out is what is the max DC current
you
> > can
> >> push through the rotor. Take a look at the wire size to get an idea.
> > Then
> >> start out with your trials using only about 25% of what you think the
max
> >> is. This would be about the 'no-load field amps' operating point.
> >
> > Yes, I don't want to melt the wires ;-) I tried using a "dead" 12 volt
car
> > battery that only put out about 9 volts with no load and couldn't tell
for
> > sure what the poles were. I guess it should be safe to try a good
battery.
> >
> >> As the load is applied to an AC alternator, the DC field current has to
be
> >> increased quite a bit to keep the voltage constant.
> >>
> >>> To answer Jim
> >>> Wilkins the coil was originally for a single-phase, 120/240 3600 rpm
> >>> generator but I would be running it at 3600 maximum and slowing down
the
> >>> engine to control my output voltage (and adjusting the field current
as
> >>> needed too). Before I get started I just want to be pretty sure of
what
> > I
> >>> have to work with. It sounds like, from what everyone said, that the
> > pole
> >>> should be correct for my purposes. :-D
> >> Okay, if it originally was 3600 rpm for 60Hz, then it is definitely a
> > 2-pole
> >> rotor. So for three-phase you'll need six pole-phase groups.
> >
> > From reading about making "axial flux alternators" for wind generators I
> > have a pretty good idea about placing the coils and wiring them for
either
> > Wye or Delta plus I may even have the option of making it six phase
instead
> > of just three. Neon John recently explained to me how to test one coil
and
> > figure what the total output would be at a given RPM. I know what the
field
> > coil voltage normally is for a 12 volt, 3 phase alternator but couldn't
find
> > any such information on a 120 volt, single phase field coil. Maybe it
> > should be obvious, but it's a mystery to me. Once I learn this it will
> > probably be obvious.
> >
> >
> >> daestrom
> >>
> >
> >
> The field coil voltage is entirely up to the designer of the alternator.
> There is no 'standard' solution for an AC generator as there is no
> convenient DC line to feed it from. The simplest design in most cases
> is to use the rectified AC output, but it rather depends on the original
> application. Measuring the coil's resistance should give you a clue, if
> you calculate the voltage (sqrt(Watts*Ohms)) that will dissipate around
> 5% of the alternator's rated output power, that shouldn't cook it and
> should give plenty of field.
>
By "watts" do you mean the rated output of the alternator?
> Of course we are talking about *maximum* voltage here, in operation the
> regulator should back off the field current to maintain the correct
> output voltage. Full field current only occurs at high load and low
> RPM: in automotive terms, charging a flat battery on idle. At all other
> times the field current will be reduced, quite a lot.
>
>
> Tim Jackson
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