T&T: 110vac

[Jim Healy]

JHWard asked: "Has anyone ever tried setting up an engine-mounted alternator
to feed the 110vac system directly (throw over a switch)? I understand the
cycle control may not be perfect, but with the mass, torque, and excess
power my engines have at cruise settings (typically 1600 rpm or so), I feel
it would be pretty steady."
 
JHW,
 
Not sure exactly what you mean by "engine-mounted alternator," but...   
 
Alternators on home and boat ac gensets are built as single-phase ac
machines and are designed to spin at 1800 or 3600 rpm.  The RPM at which the
machine spins determines output frequency.  
 
An automotive style alternator as found on most small engine gasoline and
diesel boat engines is, beneath it all, really a 3-phase AC machine, NOT a
single-phase machine.  There are three diode packs built into them.  Two of
the diode packs work together as a full wave bridge rectifier that changes
the internally-generated 3-phase ac into dc.  The third is a blocking diode
that protects the charging system when it's turned off.  The diode packs are
why an automotive alternator is rated as if it were a dc machine; i.e., a
12vdc, 100a device, or a 24vdc, 85a device, or whatever.  A voltage
regulator maintains the output at it's dc set point.  A battery is required
as a reference voltage to keep this system running in its nominal range.
Automotive alternators do not care about internal frequency of the generated
ac voltages, since they are converted to dc before they exit the machine.
In fact, it's easier to filter 3-phase than single phase, which is why
3-phase machines are used in this application.  And in that same vein,
automotive alternators turn at speeds of 5000 - 6000 rpm, making the dc
ripple voltages less important.
 
If this is what you meant by your question, yes, it is possible to get rid
of the diode packs and bring the 3-phase ac out of the alternator body.  To
do it, you have to know how 3-phase wye connected machines are wired,
because the wye connection point becomes your ac neutral, but yes, it can be
done.  When I was a kid, years ago, I did just that to power ham radio gear
on field days.  (That was in the dark ages, when homemade radio equipment
still had vacuum tubes and before inverter technologies that weighed less
than cars had been invented.)  HOWEVER, it's much more challenging and
temperamental than you might think to get this to work at a 110v, 60 cycle
design point.  (Well, let's say between 57 and 63 cycles).  And, at high
loads like those air conditioning or refrigeration present, not likely to
work with any reliability.  
 
First, for single phase 110vac, you'd only be using one of the three
armature windings in the alternator frame, so a machine with a nameplate
rating of 100 amps would be overloaded when drawing (theoretically)
33.33333334 amps (RMS) from any one of it's armature windings (which is what
you'd have with a 110vac, single phase load, like an air conditioner).
Maybe that'd be enough for you, but then, you could not, in practice, get
that much current from the machine in this modified configuration.  The
capacity of the machine modified like this would have to be be substantially
de-rated.  (Because of the field winding, not the armature windings.)
 
Then, you would definitely have to build a voltage regulator that would
drive the machine's field at many times higher-than-normal voltage (and
current) in order to produce 110 VAC RMS from the armature windings.  The
OEM field winding is not designed for that amount of increased current, so
you would certainly shorten its service life, if not fry it outright, trying
to draw 30+ amps from an armature winding.  
 
As to the idea of a switch to turn the field on and off, not probably so
good.  Perhaps a rheostat....    Let's say you had the air conditioning
running, and were drawing 25 amps to do that, and you flipped that switch
off.  At that instant, field current would be removed, and the mammoth
magnetic field established by the rotating field winding would collapse,
with no place to go (electrically).  That would create spike voltages of
several thousand volts, which would arc in the field winding, and probably
fry it in that instant.  So, you definitely need ways to manage the collapse
of that magnetic field.  And, a similar kind or problem occurs in the
armature windings when the a/c compressor, or other ac loads, cycle on and
off.  
 
So, you would have a number of challenging - but not insurmountable -
problems to solve in making this work.  As an economic exercise, it's
clearly not worth it.  As a technology solution, unless you are very
familiar with automotive alternators (AC and DC machines, in general),
modifying one to produce single phase 110 VAC is very much a laboratory
exercise.  And again, as a technology solution, regulating this hummer is
going to require electrical/electronic design knowledge and will be very
challenging to achieve.
 
As another poster suggested, there are machines made to produce 110vac, belt
driven, mounted off-engine.  I would think that would be the way to go in
pursuit of this idea.
 
Jim
 
Peg and Jim Healy aboard Sanctuary,
currently at Rock Creek, Pasadena, MD
AGLCA # 3767
MTOA # 3436