Single Phase Field Coil Question - Page 2

 

alternator

excitation

depends

Actually I was thinking of rewiring an alternator but that gets me thinking
about using a 3-phase motor as an alternator.  tI would be easier to find
couplings for a 1/2" or 5/8" shaft than for the Delco shaft.  Is there some
easy way to determine the output from the specs on the motor?  I don't care
much about frequency as I will be charging batteries directly.

 

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.

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.



Big coil around iron is a great inductor :-)


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.

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.


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.

daestrom

 

alternator

excitation

depends

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.


stator,

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.


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.

 Ulysses wrote:

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.

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

 

polarity

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alternator

battery.

instead

By "watts" do you mean the rated output of the alternator?