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nuckolls.bob(at)aeroelect
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 Posted: Tue Oct 22, 2013 7:22 am Post subject: hypothetical question |
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Subject: hypothetical question
From: "user9253" <fransew(at)gmail.com>
To: aeroelectric-list(at)matronics.com
I recently had a conversation with a pal who's had a solid background
in marine electrical/electronic work...a long time successful
business after getting started in the USN. We discussed wiring a
hypothetical fiberglass airplane with a rear mounted battery. I
suggested running battery cables in integrated fiberglass conduit
with positive cables to starboard and negative to port side. He
claims that would be a big "no-no" as it would set up a powerful
magnetic field which would cause mayhem w/ radios. Unfortunately he
did not have time to elaborate on the mayhem.
I disagreed with his statement. I think there could be some noise on
the radio while cranking the engine, but that is of short
duration. Is there a real disadvantage to running positive wires on
one side of the fuselage and negative wires on the other side?
Joe
He's right. Noise is a dynamic thing. I.e.
AC or at least modulated DC currents which
generate in turn, modulated magnetic fields.
An up-the-right, down=the-left configuration
of fat wires forms the primary of
a transformer. All potential victims
situated in the 'core' of that primary WIll
be influenced. Whether it's enough to be
noticed is a guess. But we DO know that
devices that measure static magnetics like
your whisky compass and/or magnetometers
are the most vulnerable.
The management of noise-bearing conductors
is simple-idea science irrespective of the
conductor's role as potential antagonist
or victim. Designing for cancellation of
influence or vulnerabilities using the
benefits of twisted pairs or at least
parallel conductors is inarguable.
That's why the earliest single-wire telephone
systems working against earth ground would
pick up static from local thunderstorms and
became unusable if the poles were shared by
AC power transmission lines.
When the phone company adopted end-to-end,
parallel/twisted pair topology, your phone wires
could traverse the worst of noise environments
with little or no effect. This is why audio
systems with parallel-concentric signal
paths would probably operate well within
such a loop. But the compass instrumentation
is another matter.
Bob . . .
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Eric M. Jones
Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts
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| Posted: Wed Oct 23, 2013 6:45 am Post subject: Re: Magnetic fields of conductors (hypothetical question) |
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Let's take a loop 4 meters in diameter and put 200A through it. The magnet field created is about 0.63 Gauss. This is about the same as the Earth's magnetic field itself. So I wouldn't be too worried about it.
It is telling that your friend was a Navy man. The US navy knows more about the Earth's magnetic field than anyone else. Reading the published naval papers on the subject is fascinating. They degauss ships hulls (Philadelphia Experiment!), and even know how the pounding of rough seas changes the magnetism of ships' hulls over time.
Aircraft steel tubes magnetize over time and compass compensation is periodically figured in. But of course, the Earth's magnetic field and its consistency is not great either. For local use this is okay. No pilot ever used a compass for long distances.
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user9253
Joined: 28 Mar 2008
Posts: 1924
Location: Riley TWP Michigan
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| Posted: Wed Oct 23, 2013 8:35 am Post subject: Re: hypothetical question |
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That reminds me of a story that a college professor, who was also an electrical inspector, teaching the national electrical code told the class about an electrician who did a very neat job. At the service entrance panel, he put all of the white wires in one conduit and all of the black wires in another conduit. It looked very nice and the inspector passed it. After the business moved into the building, they noticed that the conduit was getting hot. They called the inspector to complain about it. The inspector then realized that when both white and black wires are in the same conduit, their magnetic fields cancel each other out. But if they are in separate conduits, then the magnetic fields are not cancelled. The rising and falling 60 Hz magnetic fields generate heat in the metal conduit.
But that was alternating current. We are dealing with DC in an airplane. The magnetic field around a wire carrying DC does not change unless the current is changing. Albeit, the DC is not pure but has an AC ripple.
I agree that the magnetic field around a wire can affect a compass. The instructions that come with a new compass say to twist the wires feeding the compass light. But will magnetic fields cause mayhem with the radios?
The magnetic fields surrounding two wires, carrying opposite current, oppose each other. I saw a film of a service panel whose load was faulted with hundreds of amps. The parallel conductors were forced apart by the large magnetic fields developed by the fault current. Since the magnetic fields of parallel conductors oppose each other, that makes me think that they do not cancel each other, at least not in the area right next to a wire.
Twisting of wires is what cancels induced currents in nearby conductors. Induced current caused by one twisted wire is cancelled by the opposing induced current caused by the other twisted wire.
Now consider a metal airplane where electrical currents from various loads all flow though the airframe back to the source (alternator or battery). If anything is going to cause interference, many currents flowing though the same conductor will. Also consider a bundle of wires carrying positive current to various loads. In that bundle are antenna coax and exterior lighting wires and fuel pump wires and etc. But the radios work just fine. There could be reasons to separate wires of opposite polarity. For instance, in a metal airplane, the airframe can be used as one conductor. In a plastic airplane, there could be mechanical situations where there is only room for one large conductor. Or perhaps it is desired to keep always hot wires separated to prevent a short circuit in case of a crash.
I am not disagreeing with anything that Bob said. Keeping positive and negative wires together and twisted is good practice. I am asking if positive and negative wires are separated in homebuilt aircraft, will there be significant interference to radios?
Joe
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user9253
Joined: 28 Mar 2008
Posts: 1924
Location: Riley TWP Michigan
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| Posted: Wed Oct 23, 2013 4:59 pm Post subject: Re: hypothetical question |
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I searched the internet for magnetic fields surrounding parallel conductors and found out that if current is flowing in the opposite direction in two conductors, the magnetic fields repel each other. The magnetic fields do not cancel each other out. Instead they become distorted. The fields extend out less in the direction of the other wire, and they extend out farther in the opposite direction. Here is a picture of the distorted magnetic fields:
http://www.physchem.co.za/OB11-ele/graphics/magnetic1_f8.gif
From this I conclude that running wires of opposite polarity close together and in parallel does not reduce the magnetic field surrounding the wires. If devices are sensitive to magnetic interference, they should be kept far away from wires carrying large currents.
If the magnetic fields do not cancel, how to explain the phenomenon in my previous post about conduit becoming hot when white and black AC wires are segregated, but the conduit remains cool with both white and black wires contained within one conduit? The current induced in the conduit by white wires is opposed by current induced by black wires. Thus, no eddy current flows and the conduit remains cool. But magnetic fields that surround each conductor are not cancelled.
A steady DC current will not induce voltages in adjacent wires.
A DC current with a large AC ripple can induce voltage in adjacent wires.
Twisting the positive and negative wires of a circuit together will minimize induced voltage in adjacent wires. If the positive and negative antagonistic wires are too big to be twisted together, and they are causing interference, then they should be isolated.
Other views and comments are welcome.
Joe
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uuccio(at)gmail.com
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| Posted: Wed Oct 23, 2013 10:14 pm Post subject: hypothetical question |
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| Quote: | > I searched the internet for magnetic fields surrounding parallel conductors and found out that if current is flowing in the opposite direction in two conductors, the magnetic fields repel each other. The magnetic fields do not cancel each other out. Instead they become distorted. The fields extend out less in the direction of the other wire, and they extend out farther in the opposite direction. Here is a picture of the distorted magnetic fields:
> http://www.physchem.co.za/OB11-ele/graphics/magnetic1_f8.gif
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Hi Joe
It's a question of scale. The fields do not cancel out exactly because the two wires carrying current in opposite directions are not in exactly the same spot. But If you move sufficiently far away from the pair of wires then the magnetic field effectively cancels out. What does sufficiently far mean in this context? Well if one goes through the calculations the critical distance d is the distance between the center of the two conductors. So if they're kept very close together (eg by twisting) then as long as you are at a distance D away from the pair, where D is much greater than d, then the magnetic field becomes very small. Typically d is of the order of a few millimeters so if you're eg a meter away, then D is hundreds of times greater than d and the magnetic field is for all intents and purposes zero.
Sacha
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livingjw(at)earthlink.net
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| Posted: Thu Oct 24, 2013 7:10 am Post subject: hypothetical question |
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Perfectly stated.
On 10/24/2013 2:13 AM, Sacha wrote:
| Quote: |
>> I searched the internet for magnetic fields surrounding parallel conductors and found out that if current is flowing in the opposite direction in two conductors, the magnetic fields repel each other. The magnetic fields do not cancel each other out. Instead they become distorted. The fields extend out less in the direction of the other wire, and they extend out farther in the opposite direction. Here is a picture of the distorted magnetic fields:
>> http://www.physchem.co.za/OB11-ele/graphics/magnetic1_f8.gif
Hi Joe
It's a question of scale. The fields do not cancel out exactly because the two wires carrying current in opposite directions are not in exactly the same spot. But If you move sufficiently far away from the pair of wires then the magnetic field effectively cancels out. What does sufficiently far mean in this context? Well if one goes through the calculations the critical distance d is the distance between the center of the two conductors. So if they're kept very close together (eg by twisting) then as long as you are at a distance D away from the pair, where D is much greater than d, then the magnetic field becomes very small. Typically d is of the order of a few millimeters so if you're eg a meter away, then D is hundreds of times greater than d and the magnetic field is for all intents and purposes zero.
Sacha
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user9253
Joined: 28 Mar 2008
Posts: 1924
Location: Riley TWP Michigan
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| Posted: Thu Oct 24, 2013 7:18 am Post subject: Re: hypothetical question |
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Thanks for clarifying that, Sacha.
Joe
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rob(at)hyperion-ef.com
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| Posted: Sun Oct 27, 2013 10:23 am Post subject: hypothetical question |
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Are you suggesting that New York to Paris is not a long distance? Some
folks thought it was in 1927.
Do not archive
Best regards,
Rob Housman
Europa XS A070
Rotax 914
Airframe complete
Avionics a work in progress
--
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Eric M. Jones
Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts
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| Posted: Mon Oct 28, 2013 4:33 am Post subject: Re: hypothetical question |
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Rob,
If you tried to fly from NY to Paris using a magnetic compass in 1927, you'd probably not make it. Lindbergh's primary navigation device was an "Earth Inductor Compass" see: http://en.wikipedia.org/wiki/Earth_inductor_compass
I stand by what I said. A magnetic compass is a pretty weak tool for long-distance navigation. On aircraft, the pilot steers compass courses, but aerial navigation depends on making corrections to account for all sorts of disturbances from the straight line course.
As you approach the magnetic poles, the magnetic compass becomes worthless, when you cross the equator, the compensators have to be flipped. Then there are the dozen compass errors they teach in ground school.
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Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net |
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