ontent.
We wish to thank Mr. Adkins and the Piper Owners Magazine for We wish to thank Mr. Adkins and the Piper Owners Magazine for
permission to reprint this fine article on aircraft electrical charging
systems. It will help you gain a general working knowledge of this
rather simple but often neglected and abused system. With this
knowledge you can avoid costly maintenance and troublesome failures.
-----*****-----
Know Your Charging System
By Robert M. Adkins
The following article was offered on the net to assist owner-operators
of certified aircraft (notably Pipers) live in harmony with their
electrical systems. There is much information that is accurate but some
important points that are not accurate. Further, the trouble shooting
procedures do not first look at most probable causes with diagnostic
measurements of major components. If major components are not implicated,
then one should divide and conquer; halve and then quarter the field of
investigation to narrow down the search for a faulty connection. But most
of what is suggested as possible/probable cause for investigation
practically never happens.
I have no doubt that the author would do a good job fixing his airplane
and he put a lot of work into crafting this piece. However, the
procedures cited would not be very useful to the neophyte technician. The
article was downloaded from
http://www.nflite.com/ChargingSystem.html
on
February 10, 2008
Robert L. Nuckolls, III
11 February, 2008 Light aircraft charging systems are similar to their automotive and
marine counterparts, and are quite simple in nature. There are a
couple of notable differences, however.
Unlike a car or boat, an aircraft electrical system is controlled by a
separate switch (the master), not the engine ignition switch. In
addition to the battery master switch, a separate switch is provided to
allow the charging system of an aircraft to be shut down while still
leaving the battery on. This separate switch is the cause of the
majority of the problems and the short life expectancy (high failure
rate) of light aircraft charging systems. There really is little
difference in all of the other parts of the charging system,
mechanically. Electrically, an alternator that is used in an aircraft
should last just as long, if not longer, than the same alternator used
in a car or boat.
If you have doubts about the statement just made, read on. I think by
the end of this article you will find substantial justification for
that statement, as well as proven suggestions that you can use in
virtually any single engine or light-twin aircraft to substantially
extend the life expectancy of your charging system. This might just
bring a little more peace of mind the next time you find yourself in
heavy IFR at night near freezing level, not to mention the savings in
maintenance costs.
I will explain the function of each of the major components found in
aircraft (and most all) charging systems. I will also describe the
most common failure of each of the individual components and the
symptoms that may be observed. In addition, I will show some basic troubleshooting tips for several of the most common charging system
failures and the likely causes for each.
Please note that I have taken great care to arrange the troubleshooting
and parts list in a logical and cost effective order. You may chuckle
a little when I suggest that you verify the operation of a switch,
circuit breaker or connection, but these checks take very little time
and effort to perform and will not cost you anything. These components
do play major roles in the charging system. We may not want to admit
it, but a lowly switch, circuit breaker or connection can completely
disable a charging system. If you don't believe me, you might want to
read the tale of David and Goliath -- and ask Murphy his opinion, while
you're at it.
Anyway, if a problem is found with one of these components, they will
be the least expensive to replace. And I assure you that they do fail,
especially in older aircraft or those exposed to damp or corrosive
environments. If after checking all of these basic parts you conclude
that the problem is elsewhere, at least you will feel a bit more
confident about dropping $100-$300 on voltage regulators and
alternators.
The Major Components
Aircraft charging systems consist of the following major components:
Alternator
Battery
Voltage regulator
Over voltage relay
Battery master switch
Master relay
Alternator master switch
Alternator field breaker
Alternator output breaker
The components of most light aircraft charging and starting systems
carry the same brand names as automotive models, and they appear to be
the same. Are they any different, or are they just automotive parts
sold at inflated prices? The answer is neither, Yes, they do appear to
be the same from an outside look, and many of the internal components
are the same. Yet there are many differences, and unless you know what
they are for your particular part, do not substitute automotive parts.
Brushes, for example, have different mechanical and electrical
specifications. In some alternators, the cooling fins are backward
compared to their automotive counterpart. Don't gamble that what looks
alike IS alike.
Not sure of the significance of this last statement. If were
talking about certified aircraft then the only legal replacement
for any part of the airplane is whats called out in the service
parts catalog. It matters not what visual similarities the part
has with a non-approved part of any brand.
The alternator is the business end of the charging system. Alternators
typically produce their rated output power at 5,000-6,000 RPM. In
automotive applications, the alternator drive is usually reduced 2 to 1 to achieve optimum power output at typical cruise speed RPM's. In an
aircraft installation, the drive ratio is typically 3 or 4 to 1. For
this discussion I will assume that the alternator is always turning at
the optimum RPM which would at any time allow it to produce its maximum
rated power output.
I think the author means that the alternator is turning about 2x
crank speed in cars such that for 2500 rpm cruise on the highway,
the alternator is running about 5000 rpm . . . fast enough to
carry rated loads. If the ratio on aircraft is 3x to 4x, then at
2400 rpm on the crank, the alternator will be running at 7200 to
9600 rpm. This is a wide span of speeds which speak more to
adequate than optimum.
Alternator and Battery
An alternator consists of three basic components: rotor stator
rectifier. The rotor and stator are windings made up of varnished
copper wires; the varnish acts as an insulator. The rectifier is made
up of six diodes, arranged in pairs. Each pair of diodes rectifies the
current from each of the fixed phase windings in the stator.
Most alternators do not have fixed magnets, and therefore do not
produce any power on their own. In effect, an alternator is a form of
power amplifier; it can turn a small amount of electrical power into a
large amount of electrical power by using mechanical force (the engine
drive).
Perhaps energy converter is a better term as opposed to
amplifier. Alternators convert mechanical energy into
electrical energy is and there there is no amplification. In
fact, the alternator is probably on the order of 60-70% efficient
. . . meaning that a portion of the input energy is tossed off as
heat. I.e, output power is less than input power.
An alternator requires a small amount of external power to produce a
magnetic field in the windings of the rotor. The strength of this
magnetic field determines the amount of power (current) that may be
sourced from the stator windings. The strength of the magnetic field
produced by the rotor is controlled by controlling the amount of
current that it draws. Most rotor field windings can draw up to 4
amps. The output of the stator windings is three phase AC. A three
phase, full wave diode rectifier (two diodes per phase) rectifies the
AC voltage produced by each winding of the stator to useable DC
voltage.
In this sense, one might consider the wiggling of a field winding
at up to 14v and 3A (42 watts) has control of as much as 14V at
100A (1400) watts to be a manifestation of gain . . . but then
a vacuum tube can be said to have power gains of perhaps 1000 or
more. Nonetheless, for total power in versus usable power out,
both alternators and vacuum tubes are much less than 100%
efficient.
The battery plays two main roles: 1)

It supplies current to the rotor field windings to produce a
magnetic field.
Sorta . . . many alternators require external excitation (usually
ships battery) to come on line but once started the alternator
does not depend on the battery for continued operation. I.e.,
once on line the alternator supplies its own field power.
2)

It acts as a capacitor to both draw and smooth the rectified power
(current) from the stator of the alternator.
I used to think that too . . . you know, it sorta made intuitive
sense; the battery is a huge energy source as a supply or energy
sink as a storage device. One can easily morph these facts into
some notion of batteries offering a kind of electrochemical
inertia that tends to smooth small perturbations in bus voltage.
Consider this: A battery becomes a source of energy at 12.5 volts
and below. It becomes a sink of energy at 13.8 volts and above.
There is a span of about 1.3 volts where the battery is neither
an effective sink nor source of energy. Any wiggles of bus
voltage that venture into this range are essentially unaffected.
The whole idea of the battery being any sort of filter falls
apart. In fact, put your oscilloscope on