On Nov 5, 7:30 am, TheSmokingGnu
wrote:
SockPuppet wrote:
In article cc1Xi.198$Cb.81@trnddc08, anonymityisavirtue@
1111011010011.com says...
Alternators have to be supplied with a sufficient current from the
battery in order to generate the larger currents required by the
aircraft systems.

This is a new concept to me, can you elaborate?

My understanding of alternators is that mechanical energy from the
engine causes the alternator to spin, which then powers aircraft
components and (when regulated correctly) can recharge the battery.

First, recall that electricity induces magnetic fields and magnetic
fields induce electricity and vice versa, and vis a vis and so forth.

A simple generator is a wire coil rotating in opposite poles of two
permanent magnets. The magnetic field is provided by the magnet, and the
faster the coil spins, the more often the magnetic field cuts through
the coil (or more properly, the coil cuts through the magnet's field),
the more current is generated.

Alternators use a different method to generate current. There is a
wire-wound stator, within which spins a drum coil (with soft iron
cores). The drum coil (or rotor) has to be energized from an outside
source in order to generate the magnetic fields, necessary to induce
current in the stator windings. While the process is self-sustaining
once begun, alternators cannot "bootstrap" themselves into current
generation (since you can't generate the current to generate the fields
to generate the current, so to speak). Much like the magnetos need the
impulse coupling to push the engine over with a hot spark, the
alternator needs the external "kick in the pants" to begin outputting
current, which is the battery.

Now, as I recall, we use alternators instead because they're lighter,
more efficient, and higher output than can be achieved with a
bog-standard generator.

TheSmokingGnu

The alternator has diodes in it, and those cause a slight voltage
drop in the output process. If the battery is too weak to produce a
sufficiently strong magnetic field in the rotor to generate sufficient
voltage in the stator to overcome diode resistance, bootstrapping
cannot occur. The old generators had no diodes, and residual magnetism
in the field pole shoes would start the thing generating. There's a
bit of residual magnetism in the alternator's stator but not enough to
do any good with those diodes in the way.
Alternators in aircraft should be taken off, opened up and
inspected every 500 hours. The brushes wear and if they get short
they'll fall out of the holder and their springs will gouge up the
slip rings. Gets a lot more expensive than the 500 hour check.
Airplanes that don't fly much have alternator bearings in which the
grease dries out, and bearing failures happen.
Regulators, especially the newer electronic replacements for the
electromechanical-switch types, give us plenty of grief. Regulators
fail more often than alternators. And as the OP found, those dumb
crimp terminals breal through vibration fatigue. Alternator output
breakers age, their contacts corrode and cause resistance heating and
they start popping off,causing the naive mechanic to spend lots of
time looking for a short.

Dan