leaving alternator breakers on all the time

Hi -

My PA23-250 POH uses strong wording to insist that whenever the
engines are running, the alternator breakers should be on. This
contradicts some checklists for this plane (and those of a few other
single-engine Cessnas) that suggest that those breakers be open during
engine start and after shutdown. On the twin, one checklist includes
an alternator load check during runup that includes opening one then
the other breaker, to ensure that the electrical load can be carried
by either unit - that sounds valuable but appears to go against the POH.

Can someone offer an explanation of the relative wisdom of two
approaches?

- FChE

If you open the alternator output breaker while the alternator is
generating power then you get what is known as a "Load Dump".
this will generate a short inductive voltage spike of several hundred
volts followed by a steady +90 volts. This really stresses the diodes
in the alternator.
Most 12 and 24 volt circuit breakers that are used in GA aircraft are
not designed to open 90 volts DC. (There is a big difference between
the AC and DC voltage ratings of circuit breakers, switches and
fuses.) The breaker either burns up due to the constant arc, the
contacts melt and short or the breaker finally opens but the contact
surfaces are damaged.
The more current that is flowing when the breaker opens the worse the
result. Some part of this voltage will feed through the breaker while
it is opening and put a several hundred volt spike into your expensive
radios.

On my C172 when the old tired 60 amp alternator breaker opened one
night it burned up the alternator field, the regulator, the over
voltage warning light, and the over voltage relay. The alternator
stator and diodes were ok. This cost me about $500 because the old
tired 60 amp breaker would open at 35 amps after a few minutes.
Cessna screwed up when they wired the alternator and the regulator on
the "M" model but that is another story.
John



On 20 Oct 2003 16:53:18 -0400, (Frank Ch. Eigler)
wrote:


Hi -

My PA23-250 POH uses strong wording to insist that whenever the
engines are running, the alternator breakers should be on. This
contradicts some checklists for this plane (and those of a few other
single-engine Cessnas) that suggest that those breakers be open during
engine start and after shutdown. On the twin, one checklist includes
an alternator load check during runup that includes opening one then
the other breaker, to ensure that the electrical load can be carried
by either unit - that sounds valuable but appears to go against the POH.

Can someone offer an explanation of the relative wisdom of two
approaches?

- FChE

wrote:

If you open the alternator output breaker while the alternator is
generating power then you get what is known as a "Load Dump".

So, Based on this (and the rest of the info in your post), it sounds like it
would be perfectly ok to start the aircraft with the alternator out of the
circuit. I'm not sure it would be a good idea to cut one out in a twin. Does
this load dump occur if there are two alternators feeding the system and you
cut one out?

George Patterson
To a pilot, altitude is like money - it is possible that having too much
could prove embarassing, but having too little is always fatal.

The correct way to take an alternator off line is to open the field circuit, not
the output circuit. Electro-mechanical regulators do exactly that many times a
second. The alternator can only produce an output if the field is energized.
Basically the field (which is actually the rotating part of the alternator) when
energized is a rotating magnet which then induces currents in the stator
windings. Open field means no induced currents, which mean no output. This also
avoids the load dump problem.

"G.R. Patterson III" wrote:

wrote:

If you open the alternator output breaker while the alternator is
generating power then you get what is known as a "Load Dump".

So, Based on this (and the rest of the info in your post), it sounds like it
would be perfectly ok to start the aircraft with the alternator out of the
circuit. I'm not sure it would be a good idea to cut one out in a twin. Does
this load dump occur if there are two alternators feeding the system and you
cut one out?

George Patterson
To a pilot, altitude is like money - it is possible that having too much
could prove embarassing, but having too little is always fatal.

--
--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
401/884-7930 Fax 401/884-7950
email
http://www.andraka.com

"They that give up essential liberty to obtain a little
temporary safety deserve neither liberty nor safety."
-Benjamin Franklin, 1759

The correct way to take an alternator off line is to open the field circuit, not
the output circuit. Electro-mechanical regulators do exactly that many times a
second. The alternator can only produce an output if the field is energized.
Basically the field (which is actually the rotating part of the alternator) when
energized is a rotating magnet which then induces currents in the stator
windings. Open field means no induced currents, which mean no output. This also
avoids the load dump problem.

You can help extend the life of your alternator by not having the alternator field

energized while starting the airplane. The starter presents a load of over a
hundred
amperes when it is cranking the engine. If the alternator field is energized, the

alternator is trying to supply current into a load that far exceeds its capacity,
which
stresses the diodes in the alternator. If the alternator field is open during
starting,
the alternator produces no current, therefore the diodes remain unstressed. Your
car charging circuit is similar to an airplane's circuit except that in a car,
turning the
key to the start position deenergizes the alternator field so that it does not
produce
any output while the starter is being cranked.

If you have a split master, you can approximate the action of the automotive
starter
lockout by leaving the alternator side of the master off until the engine is
started, then
turning it on. This also has the benefit of not putting the field load (up to a
couple of
amperes) on the battery before and during the start. If you don't have a split
master,
I wouldn't advise using the field circuit breaker to turn it off, as those are not
designed
for the amount of operations you'd subject them to pulling it on every engine
start.
The other thing, is make sure you turn the alternator side of the master on once
you do
have the engine running, otherwise you'll find yourself with a dead battery before
long.


"G.R. Patterson III" wrote:

wrote:

If you open the alternator output breaker while the alternator is
generating power then you get what is known as a "Load Dump".

So, Based on this (and the rest of the info in your post), it sounds like it
would be perfectly ok to start the aircraft with the alternator out of the
circuit. I'm not sure it would be a good idea to cut one out in a twin. Does
this load dump occur if there are two alternators feeding the system and you
cut one out?

George Patterson
To a pilot, altitude is like money - it is possible that having too much
could prove embarassing, but having too little is always fatal.

--
--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
401/884-7930 Fax 401/884-7950
email
http://www.andraka.com

"They that give up essential liberty to obtain a little
temporary safety deserve neither liberty nor safety."
-Benjamin Franklin, 1759

Ray Andraka wrote:
: If you have a split master, you can approximate the action of the automotive
: starter
: lockout by leaving the alternator side of the master off until the engine is
: started, then
: turning it on. This also has the benefit of not putting the field load (up to a
: couple of
: amperes) on the battery before and during the start.

Another little tidbit on this idea is to wait for a little bit
before energizing the alternator field (with split master). The
alternator actually presents a fairly significant amount of torque load
when putting out full output, which it does briefly when flipped on after
starting. I generally wait until the oil pressure has built up and
stabilized for a bit before flipping on the alternator and watching the
ammeter spike.

I'm not familiar with the PA-23 electrical topology, but someone
who does feel free to correct this thinking. It's probably just like the
PA-24 and older PA-28's, which have a single toggle 'Master' switch. In
this case, that master switch turns on the main contactor relay at the
battery. The problem with this is that if the field stays energized while
the master is turned off, the main power sink and buffer in the system
(battery) is now disconnected. If there wasn't much current going into
it, it's not a problem. If the battery was low, had just been started,
etc, etc, there could be 50A going into the battery that disappears from
the bus. This will cause the "load-dump" transient voltage spike
mentioned before. If the field(s) can be turned off individually, I would
think that's the appropriate technique. Kinda like power
increase/decrease is mixture/prop/throttle and throttle/prop/mixture,
respectively. Electrical startup would be master/starter/alternator field
and shutdown the reverse.

-Cory

--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

In article ,
wrote:

Another little tidbit on this idea is to wait for a little bit
before energizing the alternator field (with split master). The
alternator actually presents a fairly significant amount of torque load
when putting out full output, which it does briefly when flipped on after
starting. I generally wait until the oil pressure has built up and
stabilized for a bit before flipping on the alternator and watching the
ammeter spike.

In planes with a split master, I generally leave the alternator off
during starting. I figure between the field drawing current and the
alternator shaft presenting a torque load, having the alternator on
while cranking will just make it harder to start.

Of course, I have no real numerical data to quantify either of these
effects, so the above is all pretty unscientific.

Hi -

papenfuss wrote:

[...] I'm not familiar with the PA-23 electrical topology, but
someone who does feel free to correct this thinking. It's probably
just like the PA-24 and older PA-28's, which have a single toggle
'Master' switch. [...]

The PA23-250 is the Aztec, a twin. My model has a split master, but
it's a Left-Right split. There doesn't appear to be a normal setting
that corresponds to a C172's battery-power-only configuration, just
the dual-alternator-failure emergency case (masters on, alternators off).

- FChE

Ray Andraka wrote:

You can help extend the life of your alternator by not having the alternator field

energized while starting the airplane.

The field breaker in my aircraft cannot be pulled. The only way to disable the
alternator is with the split master.

George Patterson
To a pilot, altitude is like money - it is possible that having too much
could prove embarassing, but having too little is always fatal.

If you want to take an alternator off line remove the field current to
that alternator. Do NOT open the output of the alternator. The
alternator is a rotary current amplifier. Most 12 volt 50 to 60 amp
alternators have about a 2 amp field current which implies the current
GAIN is around 30 amps per amp.
On a piston engine the cranking speed is much too low to generate
enough output voltage to get much current generated until the engine
starts. No output current means no torque required to spin it.
However you will be drawing full field current of 2 amps or so which
is about 1% to 2% of the cranking current. This additional current
must be supplied by the battery which does not help but don't hurt
much either. At idle speed and full field current ,which the
regulator will give since the battery voltage is low, if the output is
not connected to the battery the alternator voltage will rise to 50
volts or more. The open circuit alternator output voltage depends on
RPM and field current. The voltage is proportional to the magnetic
flux and the rate of change of that flux. which is the alternator
field current and RPM. This extra voltage will not help extend the
life of the diodes in the alternator or the circuit breaker contacts
as you will get a good arc when the breaker contacts bounce open upon
closing of the breaker

A turbine engine is an entirely different story. At the end of a
turbine start cycle the turbine has to be running at about 50% - 66%
RPM before the engine loop gain exceeds one. At this speed the
alternator would be spinning fast enough to be generating a lot of
current if it had field current. If the alternator were 100%
efficient it world not matter if it was generating or not however I
have yet to see one that even approaches this efficiency in an
aircraft. This means that if the alternator field is enabled the
starter may not be able to spin the engine past the point where the
engine gain is at least one. This will yield a hung or hot start.
Some turbines have separate starters and alternators. However in
many cases the alternator/generator on a turbine is also the starter
motor so this is a moot point as it is impossible to have both
functions at the same time.

On Mon, 20 Oct 2003 23:33:16 -0400, "G.R. Patterson III"
wrote:



wrote:

If you open the alternator output breaker while the alternator is
generating power then you get what is known as a "Load Dump".

So, Based on this (and the rest of the info in your post), it sounds like it
would be perfectly ok to start the aircraft with the alternator out of the
circuit. I'm not sure it would be a good idea to cut one out in a twin. Does
this load dump occur if there are two alternators feeding the system and you
cut one out?

George Patterson
To a pilot, altitude is like money - it is possible that having too much
could prove embarassing, but having too little is always fatal.