Page 13 - PDF_Flip_Book

Rod Machado’s Private/Commercial Pilot Handbook
4-10

Figure 19 represents a water model
of how a voltage regulator might work.
Most turbine pumps (similar to that used
as the model for our alternator) need to
be primed before they will start pump-
ing water. In other words, they need to
have a little water running through
them before they can create enough
suction to pump more water.
Alternators operate in a similar man-
ner. They need a little bit of electricity
running through them before they’ll
start producing more electricity. This
small amount of electrical prime is
called the alternator field current. Take
away this alternator field current
(prime) at any time, and the alternator
stops producing electricity.
Water prime flows directly into the pump through
tube F. Like alternators, our water pump needs to be Fig. 19
continuously primed if it is to keep on pumping. The
greater the prime flowing through tube F, the greater the
output of the pump. Alternator’s operate similarly; vary-
How does the brain know whether to open or close the
ing the alternator field current varies the alternator’s valve? It senses the pump’s output water pressure, or in
voltage output.
an alternator, its voltage. By comparing the pump’s
When the engine is not running, prime for the pump intake pressure at point G, with its output pressure at
comes from the battery. In Figure 19, water flows from
point B, the regulator’s brain can compute the precise
the battery to the primary bus. It then goes through line amount of prime needed to produce the required water
S and the alternator field circuit breaker T to the left
pressure. In electrical terms, these sense lines allow the
hand side of a split-rocker master switch A. By turning voltage regulator to restrict an alternator’s output to 14
on (pushing) the alternator side of the master switch (the
or 28 volts of electrical pressure.
left side), water flows into the voltage regulator V and
through valve W which is inside the voltage regulator. When switch A is turned on during engine start, valve
Prime (alternator field current) must pass through valve W starts in the full-open position, thus providing the ini-
W and through tube F, to get to the pump for priming. tial prime. After engine start, the pump commences to
produce water pressure (voltage) and brain V decides how
Valve W is controlled by the hand attached to the volt-
age regulator’s brain (those electronic components men- much to close valve W to bring this output into regula-
tioned earlier). The brain tells the hand to open or close tion. It is in this initial startup period that the pump’s
Valve W. This controls the amount of prime (alternator output is momentarily unregulated and you DO NOT
field current) reaching the pump. want your electrical equipment turned on. A sudden
surge of pressure could damage this equipment—despite
Machado’s Believe It or Not #1 each piece of equipment having protective circuit
breakers.
Sometimes brains in a voltage regulator, just like in
humans, behave errantly. I recall a student pilot who was
flying solo near the Southern California coast when he
looked down and saw an aircraft carrier. A little devil on
his left shoulder said, “Do it! Do it!!” A little angel on his
right shoulder said, “Yeah, do it!” He did it—a carrier
landing (actually, more like a touch-and-go). The captain
of the ship was not amused. Neither was the FAA. It was
a clearcut case of a brain gone astray, and it occasionally
Oh boy, happens to voltage regulators as well. When the brain in
I must have flown a voltage regulator goes south, it usually presents definite
into an electrical
storm. symptoms. You can expect either an over or underproduc-
tion of voltage by the alternator.