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a.s.elliott(at)cox.net
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 Posted: Mon Dec 15, 2008 8:27 am Post subject: Update on alternator whine problem. |
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Gang:
I previously reported an alternator whine problem in my recently completed OBAM aircraft. The plane has an 18A permanent magnet alternator (John Deere) and a motorcycle-type electronic voltage regulator (Crane). The wiring is very similar to Z-19, but with dual batteries. The whine problem was so bad that at cruise power, the radio was useless.
[1] Following the advice from the Connection and on this list, I tested the system with the radio hooked to an isolated battery, which confirmed it was a power problem, not a ground loop or radiation problem.
[2] Noting a difference between the motorcycle wiring diagram and the Z diagram, I moved the regulator output from feeding directly to the bus to the manual battery selector switch output. (No master relay.) This has the effect of using the two batteries as a *big* capacitance and reduced the whine to a "manageable" level. At least, with this change, I could hear and understand the radio at all power settings, but it was still not good.
[3] I installed an automotive-type whine filter (Peripheral PNF-12) on the + lead to the radio. This is a classic coil-cap type filter that I got on-line from HiFi Sound Connection (http://tinyurl.com/5ehsal) for $7.18 with free shipping. The whine problem is now *gone*.
Identify - Analyze - Hypothesize - Test - Confirm
(Repeat as required)
Thanks to group members who helped, and of course to Bob!
Andy Elliott, Mesa, AZ
N601GE,601XL/TD,Corvair
[quote][b]
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nuckolls.bob(at)aeroelect
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| Posted: Mon Dec 15, 2008 8:49 am Post subject: Update on alternator whine problem. |
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At 10:22 AM 12/15/2008, you wrote:
| Quote: | Gang:
I previously reported an alternator whine problem in my recently completed OBAM aircraft. The plane has an 18A permanent magnet alternator (John Deere) and a motorcycle-type electronic voltage regulator (Crane). The wiring is very similar to Z-19, but with dual batteries. The whine problem was so bad that at cruise power, the radio was useless.
[1] Following the advice from the Connection and on this list, I tested the system with the radio hooked to an isolated battery, which confirmed it was a power problem, not a ground loop or radiation problem.
[2] Noting a difference between the motorcycle wiring diagram and the Z diagram, I moved the regulator output from feeding directly to the bus to the manual battery selector switch output. (No master relay.) This has the effect of using the two batteries as a *big* capacitance and reduced the whine to a "manageable" level. At least, with this change, I could hear and understand the radio at all power settings, but it was still not good. |
Which confirms a recently modified theory as to the
value of batteries as noise filters. Given the heft
and obvious stiffness of a battery both as an energy
load and supply device, it's easy to assume that it
would behave sorta like a super-capacitor. I preached
that doctrine for years even when evidence to the contrary
was well known to me and others.
Batteries deliver energy below 12.5 volts or so and accept
energy at 13.5 and higher. As both sources and loads, the
dynamic impedance of the battery (read ability to play
super-cap) is pretty good. But once it's charged and floated
on a "happy bus" voltage, it's relative stiffness wanes
and in fact, it's a rather poor filter.
This is why Mil-Std-704 and similar documents advise
us that in a perfectly functioning system, the battery
has very little effect on the magnitude of ripple voltage
one can expect on the bus from a rectified 3-phase machine
(alternator). EXPECT about 1.5 volts pk-pk trash from
the alternator. It's even worse for a single phase machine.
| Quote: |
[3] I installed an automotive-type whine filter (Peripheral PNF-12) on the + lead to the radio. This is a classic coil-cap type filter that I got on-line from HiFi Sound Connection (http://tinyurl.com/5ehsal) for $7.18 with free shipping. The whine problem is now *gone*.
Identify - Analyze - Hypothesize - Test - Confirm
(Repeat as required)
Thanks to group members who helped, and of course to Bob! |
Pleased that it worked out. I'm going to order one
of those filters and deduce it's characteristics.
Radio Shack quite handling a line of filters that
builders used to find useful some years back. It would
be helpful to have an alternative to recommend. Thanks
for doing the trade search!
Bob . . .
----------------------------------------)
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
----------------------------------------
[quote][b]
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