Nathan Young wrote:
: Scenarios:
: #1. Most planes have an over-voltage protection that opens the
: circuit to the alternator field windings. Let's say that while the
: alternator is delivering about 30amps (lights, avionics, pitot heat,
: etc) - the OVR gets tripped, causing the alternator field to be
: dropped immediately to 0 volts.

Aside from the magnetic field breakdown of the field's windings, this will be
fairly harmless. The regular is designed to deal with this, as it's a small amount of
current and the inductance isn't that big.

: #2. Assume scenario #1 happens (due to a transient condition - not an
: alternator failure)... It takes the pilot some time to realize the
: alternator is offline, so the avionics and lights drain the battery
: for several minutes. Anyway, the pilot cycles the alternator field
: current to bring the alternator back online. The battery is run-down
: a bit, so the current delivered by the alternator spikes from zero to
: around 30-40 amps, and then gradually tapers off as the battery is
: charged.

Again, it might not be the best on the system, but it's designed to deal with
it. The field current will "spike," but it still doesn't draw more than a few amps.
As long as the battery isn't too gone and "squishy," or have bad connections, the
current overshoot should be absorbed harmlessly by the battery with a negligible
voltage overshoot.

: Question: Can either one of these dramatic swings in the alternator
: field voltage/current, and/or the alternator output current damage the
: alternator?

Both of those are fairly normal designed-in operations of all the components,
so it should be fine. The one that has a tendency to cook alternator diodes is the
old, "disconnect the battery and see if it still runs" automotive trick. The
self-inductance of the armature windings (read: stator 3-phase coils) is very high for
a typical alternator. They are also designed to push out lots of current... the
back-emf of an idling (or slightly higher than idling) alternator shoving out 50A is
generally on the order of 60-90V. That gives you an indication of how much inductance
there is (and consequently how much 1/2*L*I^2 energy is stored there). If that load
suddenly goes away (like disconnecting a heavily chargining dead battery), that "load
dump transient" of the stator inductor energy can't do anything but spike the bus to
dissipate.

As long as the battery and connections are in good shape, this shouldn't
happen in an aircraft. If your battery is weak and you flip off the pitot heat,
landing light, and nav lights just as the electric landing gear stops drawing
40A, you could easily spike things high enough to trip the overvoltage relay (if it's
fast enough to catch it).

Hope that helps,

-Cory


************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
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