nuckolls.bob(at)aeroelect
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 Posted: Tue Mar 24, 2009 9:37 am Post subject: Parallel operations of generators/alternators (Corrected L |
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At 11:44 AM 3/24/2009, you wrote:
[quote]At 08:36 AM 3/22/2009, you wrote:
| Quote: | Good Morning Chase,
Different Bob here, but it is my understanding that a properly operating alternator has diodes or other rectification devices in the circuit that allow us to have direct current, not alternating current, at our disposal.
If we hook two alternators together at the point where they are still producing alternating current, the current will need to be put in phase or "paralleled" as the big boys say.
Once the output has been rectified or smoothed out enough to be considered direct current, paralleling ceases to be a problem. The source that has the highest voltage will hog the load. The lower voltage source will just wait until the voltage gets down to its level before it adds anything to the mix. That is how we balance the load between two power output devices, we mess with the voltage and the resistance between the current producing entities.
Make any sense at all? |
Absolutely! With AC machines (like those that power
the national grid), the task of paralleling multiple
alternators is two-fold. (1) they need to be adjusted
for output voltage so as to limit their output below
rated value and not upset system voltage such that
other alternators shut down and (2) their phase angle
of alternating current must be in lock step with the
system. In DC machines there is no phase angle of
output power to consider, only output voltage
levels with respect to system requirements and
PARALLELING multiple machines such that they SHARE
appropriate proportions of total load.
Back in the good ol' days of carbon pile regulators
on DC generators, this was a relatively easy task.
Each generator already had a really handy feature for
measuring the generator's output current. Large
generators were fitted with "compensation" or
"interpole" windings that kept the axis of field
flux from "twisting" due to reaction flux in the
armature. Without these compensation windings.
Armature reaction flux moved the ideal commutation
point out from under the brushes as output load
increased. This caused an increase in arcing at the
brushes with commensurate increases in wear. See:
http://tinyurl.com/bkhzb5
This compensation winding had a rather large
voltage drop when the generator was producing
full load . . . something on the order of 5%
of system voltage. This offered a great opportunity
to make two regulators negotiate with each other
for the purpose of sharing loads between two
generators. See:
http://aeroelectric.com/Pictures/Schematics/Parallel_Aircraft_Generators.jpg
Alternators came along and our handy-dandy
paralleling signal went away.
The first two-alternator system integration
task I was aware of happened on the Cessna 337
while I was a tech-writer there. We struggled
with the task of getting two alternators to
balance their outputs. It seems that the smallest
change in voltage setpoint of one alternator would
cause the higher setting machine to pick up most
if not all of the total load. Regulators then
(and even today) are generally incapable of
precise balancing of two machines based solely
on the sensing of voltage. The "fix" was to
run BOTH alternators from the same regulator.
The pilot could switch between a main regulators
and a spare. Beech did the same thing on the
Baron.
I've designed two alternator-specific balancing
systems. One was offered to Cessna many moons
ago to balance alternators in the T303 Crusader.
In this approach we regulated voltage at the
alternator's b-lead and case ground. We depended
on the resistance of ship's wiring between b-lead
and the bus to "ballast" tiny variations in setpoint.
Another approach done later was an add-on box
that would take control of one regulator as
a "slave" forced to track variations in the
"master". Both approaches worked well but they've
never taken deep roots because alternators are
generally limited to piston engine aircraft.
Every manufacturer would be really pleased to get
out of the piston business if the market would
let them . . . so let's not spend a lot of $time$
upgrading alternator systems to the best-we-know-how-
to-do.
Years later we're presented with an opportunity
to install two alternators of significant capability
on one engine. Okay, what's a mother to do? One
of the most practical approaches is illustrated in
Z-12. One main alternator with a smaller stand-b7
machine held in reserve. The other was Z-14 where
both alternators are assigned normal operations
tasks commensurate with their size in totally
independent systems. Then provide cross-feed
capability for cranking and mitigation of
alternator failure events. This eliminates
the need for anything special in the way of
alternator paralleling regulators.
Bottom line is that figuring out a way to avoid
paralleling alternators is a good thing to do.
None of the Z-figures features feature paralleled
alternator operations.
Bob . . . [b]
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