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nuckolls.bob(at)aeroelect
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 Posted: Sun Sep 15, 2013 1:07 pm Post subject: Why does a master switch turn off devices? |
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At 02:17 AM 9/14/2013, you wrote:
--> AeroElectric-List message posted by: Dan Charrois <dan(at)syz.com>
I've never quite understood something about aircraft electrical systems and I'm hoping someone here can enlighten me.
I've always flown under the impression that a primary purpose of the master switch is to effectively connect/disconnect power from the electrical bus, which in turns provides for the flight instruments and other electrical devices in the cockpit. However, my question is that since the alternator is typically connected on the bus side of the master contactor (not the battery side), why doesn't the alternator continue to provide electrical power to the bus while the engine is running when the master is off? Or does it? I have to admit to not ever having tried to switch off the master in flight, but I've been under the impression that doing so should turn off the electrical equipment - it's after all, a recommendation to turn off the master in a forced approach.
To achieve a solid understanding of how and why various
are placed on the ship's control panel, it's useful if
not a mitigation of risk, to understand what the controls
do and why they're included.
The legacy short answer to your question comes out of FAR23
as follows:
Sec. 23.1361 Master switch arrangement.
(a) There must be a master switch arrangement to allow ready disconnection
of each electric power source from power distribution systems, except as
provided in paragraph (b) of this section. The point of disconnection must be
adjacent to the sources controlled by the switch arrangement. If separate
switches are incorporated into the master switch arrangement, a means must be
provided for the switch arrangement to be operated by one hand with a single
movement.
Okay. In times gone by, separate switches for battery(ies) and
generator(s) were located right next to each other such that a
downward stroke of two fingers or a 'crash bar' makes the electrical
system go to max-dark.
The infamous 'split-rocker' switch was birthed during the transition
from generators to alternators on the single engine aircraft in the
1960s time-frame. It was discovered that alternators often would not
come on line without having a battery on also to excite the field. It
was also common wisdom that unlike generators (that self-excited
and happily ran sans battery), it was good practice to make sure
that a battery was ON any time an alternator was ON.
Hence the infamous 'split rocker' switch seemed like a good idea . . .
it could be fitted with mechanical interlocking of the two rockers
such that you can operate battery only but any attempt to turn
the alternator ON would add the battery also . . . further, any
attempt to take the battery OFF would also shut down the alternator.
This bit of mechanical slight-of-hand is easily accomplished with
a progressive-transfer, ON-ON-ON toggle switch wired as shown
http://tinyurl.com/kdqwahq
and in many of the wiring diagrams in Appendix Z of the 'Connection.
Do you have a copy of the book?
(b) Load circuits may be connected so that they remain energized when the
master switch is open, if the circuits are isolated, or physically shielded,
to prevent their igniting flammable fluids or vapors that might be liberated
by the leakage or rupture of any flammable fluid system; and
(1) The circuits are required for continued operation of the engine; or
(2) The circuits are protected by circuit protective devices with a rating
of five amperes or less adjacent to the electric power source.
(3) In addition, two or more circuits installed in accordance with the
requirements of paragraph (b)(2) of this section must not be used to supply a
load of more than five amperes.
(c) The master switch or its controls must be so installed that the switch
is easily discernible and accessible to a crewmember.
From the electrical diagrams I've seen, it would seem as though that switching off the master while the engine is running should do nothing other than disconnect the battery from the bus, which would not prevent the alternator from continuing to power electrical devices . . .
It depends on who's diagrams you've seen and what
the designer calls a "master switch". Bottom line is
that for every significant power source there should
be a pilot controlled switch properly labeled as
to which source is being controlled.
. . . but instead merely remove the battery's ability to absorb transients, etc.
Batteries are poor mitigators of bus noises and are
effective only in mitigation of LARGE transients like
energizing a hydraulic pump motor . . .
Wouldn't the alternator continue to supply its own field current once it is already running, so long as the alternator switch remains on?
Yes, unless your MASTER is wired like many a single engine
airplane and the alternator is forced off with the battery.
The modern Bonanzas have separate battery and alternator
switches. It seems that the Bonanza alternators WILL self
excite so ganging the alternator and battery switches together
is not necessary.
I think I'm missing something pretty basic here. Any help would be greatly appreciated!
Do you have a copy of the AeroElectric Connection? Electrical
system architecture and design goals are discussed at length.
Also, check out hte articles on http://aeroelectric.com for
further enlightenment.
Bob . . . [quote][b]
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Sun Sep 15, 2013 2:52 pm Post subject: Why does a master switch turn off devices? |
|
|
At 02:17 AM 9/14/2013, you wrote:
--> AeroElectric-List message posted by: Dan Charrois <dan(at)syz.com>
I've never quite understood something about aircraft electrical systems and I'm hoping someone here can enlighten me.
I've always flown under the impression that a primary purpose of the master switch is to effectively connect/disconnect power from the electrical bus, which in turns provides for the flight instruments and other electrical devices in the cockpit. However, my question is that since the alternator is typically connected on the bus side of the master contactor (not the battery side), why doesn't the alternator continue to provide electrical power to the bus while the engine is running when the master is off? Or does it? I have to admit to not ever having tried to switch off the master in flight, but I've been under the impression that doing so should turn off the electrical equipment - it's after all, a recommendation to turn off the master in a forced approach.
To achieve a solid understanding of how and why various
are placed on the ship's control panel, it's useful if
not a mitigation of risk, to understand what the controls
do and why they're included.
The legacy short answer to your question comes out of FAR23
as follows:
Sec. 23.1361 Master switch arrangement.
(a) There must be a master switch arrangement to allow ready disconnection
of each electric power source from power distribution systems, except as
provided in paragraph (b) of this section. The point of disconnection must be
adjacent to the sources controlled by the switch arrangement. If separate
switches are incorporated into the master switch arrangement, a means must be
provided for the switch arrangement to be operated by one hand with a single
movement.
Okay. In times gone by, separate switches for battery(ies) and
generator(s) were located right next to each other such that a
downward stroke of two fingers or a 'crash bar' makes the electrical
system go to max-dark.
The infamous 'split-rocker' switch was birthed during the transition
from generators to alternators on the single engine aircraft in the
1960s time-frame. It was discovered that alternators often would not
come on line without having a battery on also to excite the field. It
was also common wisdom that unlike generators (that self-excited
and happily ran sans battery), it was good practice to make sure
that a battery was ON any time an alternator was ON.
Hence the infamous 'split rocker' switch seemed like a good idea . . .
it could be fitted with mechanical interlocking of the two rockers
such that you can operate battery only but any attempt to turn
the alternator ON would add the battery also . . . further, any
attempt to take the battery OFF would also shut down the alternator.
This bit of mechanical slight-of-hand is easily accomplished with
a progressive-transfer, ON-ON-ON toggle switch wired as shown
http://tinyurl.com/kdqwahq
and in many of the wiring diagrams in Appendix Z of the 'Connection.
Do you have a copy of the book?
(b) Load circuits may be connected so that they remain energized when the
master switch is open, if the circuits are isolated, or physically shielded,
to prevent their igniting flammable fluids or vapors that might be liberated
by the leakage or rupture of any flammable fluid system; and
(1) The circuits are required for continued operation of the engine; or
(2) The circuits are protected by circuit protective devices with a rating
of five amperes or less adjacent to the electric power source.
(3) In addition, two or more circuits installed in accordance with the
requirements of paragraph (b)(2) of this section must not be used to supply a
load of more than five amperes.
(c) The master switch or its controls must be so installed that the switch
is easily discernible and accessible to a crewmember.
From the electrical diagrams I've seen, it would seem as though that switching off the master while the engine is running should do nothing other than disconnect the battery from the bus, which would not prevent the alternator from continuing to power electrical devices . . .
It depends on who's diagrams you've seen and what
the designer calls a "master switch". Bottom line is
that for every significant power source there should
be a pilot controlled switch properly labeled as
to which source is being controlled.
. . . but instead merely remove the battery's ability to absorb transients, etc.
Batteries are poor mitigators of bus noises and are
effective only in mitigation of LARGE transients like
energizing a hydraulic pump motor . . .
Wouldn't the alternator continue to supply its own field current once it is already running, so long as the alternator switch remains on?
Yes, unless your MASTER is wired like many a single engine
airplane and the alternator is forced off with the battery.
The modern Bonanzas have separate battery and alternator
switches. It seems that the Bonanza alternators WILL self
excite so ganging the alternator and battery switches together
is not necessary.
I think I'm missing something pretty basic here. Any help would be greatly appreciated!
Do you have a copy of the AeroElectric Connection? Electrical
system architecture and design goals are discussed at length.
Also, check out hte articles on http://aeroelectric.com for
further enlightenment.
Bob . . . [quote][b]
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
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