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Single Phase Field Coil Question

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Posted by Ulysses on June 8, 2009, 3:27 pm
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 ;-)

Posted by harry on June 8, 2009, 7:01 pm

The excitation coil of  an alternator runs on DC and may be on the
stator or the rotor of an alternator.  (It can even be a permanent
magnet these days). It's almost invariably single pole for cost
reasons though theoretically it could be multi-pole.
The voltage applied to it controls the voltage output from the
The source of the DC is usually from the alternator itself so there
has to be a rectifier, a "smoothing circuit" & some sort of voltage
control system.
The whole bunch of tricks is usually in a little box somewhere on the
machine & is likely to be solid state these days.  You may be able to
deduce things if there is a label on it.
 The DC voltage is not likely to be the same voltage as the voltage
out from the alternator due to this control system.
The difference between a single phase and three phase alternator lies
entirely in the number, distribution and connection of the coils where
the power comes from, (which can be on the fixed or the rotating bit)
but is usually on the fixed bit, ie the stator)
This also depends on the running speed of the device.. If you double
the number of coils you can drive it at half the speed and get the
same frequency.
It's not practical to convert a single phase alternator to single
phase (or vice versa), the whole thing would need to be rewound.

Posted by daestrom on June 8, 2009, 10:14 pm

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.


Posted by Ulysses on June 9, 2009, 2:58 pm


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.  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.
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.   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

Posted by richard on June 9, 2009, 4:08 pm

3-phase motors already existed.  Why bother?   You can't increase more
voltage and current because there is no room left to add.  Bigger wire
would mean lower voltage, high current output, you need both.  You should
forget about modifying existing motors, the reason is they are all glued
with industrial Epoxy.

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