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1. 07:55 PM - Revmaster 'dual' alterantors (Robert L. Nuckolls, III)
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| Subject: | Revmaster 'dual' alterantors |
I've had a few weeks (and road trips) to apply some
'asphalt engineering' effort to a combination of
threads discussing relative fragility of the
Revmaster 'dual' alternators. The latest thread
explored the notion that changing from SLVA to
LiFePO4 was much more likely to burn an alternator winding.
This is not a new topic here on the List . . . found
a few other postings on the same problem.
Without talking to the designers, it's
difficult to KNOW the original thinking behind the
design. We also cannot know what efforts the factory
has applied to this issue over the years. But I
think we're on pretty firm ground to assert that
the current design is marginal with respect to
thermal robustness.
Alternator windings should be as reliable as
propeller bolts. You might smoke some regulators,
trash some batteries, find yourself wanting for a
few more amps of output . . . but suffering destruction
of the windings suggests the alternator is a weak
link in the system.
Based on the drawing that Dan supplied a few weeks
ago it seems that the designers intended that only
one of the two windings be used at the same time.
I'm guessing that this is a "normal-and-spare"
design philosophy. Therefore, any time a winding fails due
to exceedance of thermal limits, the other winding
was off line.
We don't know the internal configuration of the
recommended rectifier/regulator but it's almost
certain to included silicon controlled rectifier
and one diode in series on each conduction half-
cycle. That's 3, silicon junctions in series
that carry alternator output current.
My current hypothesis suggests that it would be
much better to use BOTH windings all the time. Reduce
the current in each winding by 50% or more. It seems
better to have one configuration that's bulletproof
than two relatively fragile configurations that 'back
each other up'.
Here's the line of reasoning supporting this design
goal. Recall the bits-and-pieces of design? (1)
properties of materials, (2) management of energy
and (3)refinement of process. In this case, our
weak link seems to center on an energy management
issue. Some copper windings are heating past
practical operating limits. This can be either
an insulation failure, wire failure or both.
Properties of Materials:
We know that copper has a pretty significant positive
resistance coefficient for temperature. We observed
this in the temperature vs. current observations
in battery contactors:
https://tinyurl.com/mpcgp3t
https://tinyurl.com/k6bwdqo
We also considered the physics of why an overloaded
wire tends to burn open at the center of a free-air
span.
As copper heats up, resistance goes up, voltage drop
goes up, dissipated energy goes up, temperature rises
some more . . . and you can see where this is headed.
Take this tid-bit of knowledge about copper
wire and consider how many times you've observed or
heard of the windings or lead wires of any system
failing due to overheat.
I've seen some windings burn up on alternators for
reasons OTHER than poor thermal management. I've never
had a winding burn on my car. I've had one short
and start popping fuses . . . but I doubt that it
was due to burned insulation or melted wires!
The point is, ANY configuration that demonstrates
repeated failure to do open wires or cooked insulation
is MARGINAL at best; hazardous at worst.
Okay, how to reduce the load in Revmaster alternator
wires . . . hopefully without compromising ability
to deliver ENERGY into the system. Here's the energy
management consideration:
We know that PM alternators have a checkered history
of performance . . . but mostly due to stone simple
regulators that DO NOT offer active current limiting.
Furhter, many versions use a full wave bridge consisting
of two pairs of junction diode and silicon controlled
rectifiers. Alternator output current flows through
two of these devices with total drop on the
order of 2 volts. 2 volts out of a 14v system is
a substantial proportion of energy . . . something
on the order of 15 to 25 watts that needs to be
managed in those cute little castings . . . but
that's another story.
Harking back to the earliest days of my studies
in electron herding, ac rectifiers were vacuum
tubes (not junctions) and ac power sources were transformers
plugged into the wall (not spinning magnets).
Except for the systems with very low energy
requirements (table top radios), the rectifiers
were dual diodes driven from either side of a
center tapped transformer secondary. This configuration
had some profound effects in thermal management.
EACH diode carries 1/2 the total current. These
devices have considerable resistance . . . watts
I(squared) x R so if you reduce current by 1/2,
watts goes down to 1/4. Same thing happens in the
transformer secondary . . . the secondary wire
size can be made smaller in a trade off between
energy lost and transformer size.
Refinement of process:
So take a peek at the simplified proposal diagram.
Hook the two Revmaster windings in series-aiding
and bring out the center-tap. Connect in full wave
configuration with only TWO junction rectifiers.
Feed this resulting energy off to a filter capacitor
size to be determined.
This will give us a 'supply voltage' of some value
ideally adjust to about 20VDC at cruise RPM and
alternators full load design point.
This voltage is going to be all over the place
depending on load and engine rpm . . . but that's
the nature of the PM alternator.
Now, let's power condition that energy with a constant
current, constant voltage, switch-mode regulator.
NOW we have a BIG difference in performance:
Voltage is adjustable and controlled by active electronics.
Current is adjustable and limited by active electronics.
I suggest this configuration offers
significant improvement in Revmaster's engine
driven power source. Energy being delivered
to the regulator is at significantly higher than
the output set point, CURRENT in from the alternator
will be LOWER than current delivered by the regulator.
Further, each winding works 1/2 of the time so it
follows that heating effects on each winding are much
reduced from the current configuration.
Silicon junctions in the rectifier are significantly
reduced off-setting some new losses introduced by
the electronic CCCV regulator.
It seems that we could craft an engine driven power
system that almost never fails in exchange for one
that fails too many times and for the wrong reasons.
I'm tied up in some house remodeling and we've
added another grandson to the population of Medicine
Lodge Jr High School . . . so refinement of this idea
will be slow. But I'm building an alternator drive stand,
power supply and load bank out in the mess-making shop. B&C has
provided me with both PM and wound-field test articles.
I have a couple of those CCCV regulators that we discussed
on hand right now. They may not be the right size in all
respects but satisfactory for proof-of-concept experiments. I
already have some LiFePO4 and SLVA batteries on hand.
The pieces are coming together. Comments and considered
critical review are most welcome.
Bob . . .
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