nuckolls.bob(at)aeroelect
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 Posted: Fri Jul 31, 2009 9:27 am Post subject: Z-20: Fuse Links, Master Switch, Alt OV Disc. and OVM |
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At 07:41 PM 7/29/2009, you wrote:
Hello 
First, thanks to Bob his dedication to helping build dependable
electronic systems for homebuilts. I am building a Sonex powered by
a Jabiru 3300. I've read through the book and have some questions
below. Unless noted otherwise, I am referring directly to your Z-20 diagram.
18awg and 16awg Fuse Link to Starter Contactor
On Z-20, connecting to the starter contactor is a 14awg wire from
the Alt OV disconnect relay that has an 18awg fuse link inline with
it. Although it meets the criterion of being 2 steps smaller than the
14awg, it also say to contact Bob about making them any bigger than
with a 22awg line (protececting 18awg). Same issue occurs above this
in the diagram with a 16awg fuse link called out to protect the 12awg
master power line.
Fusible links called out in the Z-figures have been evaluated
for appropriate application of this protection philosophy. Use
in OTHER places should be discussed before proceeding. Consider
that fusible links fall in the same protection category as
ROBUST device (ANL, MANL, etc. current limiters).
Are these fuse links of a different type? On the diagram, it says to
refer to Note 4, which doesn't shed any light on this issue.
No, a fusible link can be crafted from wire that is
4AWG or more wire steps SMALLER than the wire being
protected. Note that fusible links called upon to do
their job will burn. The legacy design goal for TC
aircraft (and I presume automobiles) is that in the
rare but stressful situation that might open that
protection, the designer installs them such that
damage to adjacent wires is minimized (fiberglas
sleeving over wire) and what ever smoke is expected
will be minimized (keep link on the order of 6"
and out of the cockpit).
DC Master Power Switch (S1)
"Up" (keyway) position: As I understand it, the 2-10 switch is a
3-position dpdt switch. The top position (oriented according to the
diagram) would connect terminal 2 to 1, connecting the main power
distribution bus to the battery via a starter contactor terminal. At
the same time, terminal 5 would connect to 4, where it connects both
to the crowbar OVM and the alternator OV disconnect relay, and if
then eventually, if all is well in the world, to the PM alternator.
This would be the 'normal' position for regular flying, right?
Down is all power OFF. Mid position is intended to
bring the battery only on line. The up position adds
the engine driven power source. Full up (both sources
on) is the normal position for flight.
"Center" position: Toggling the switch to the center position would
still connect terminal 2 to 1 (I had originally thought it made it
open), and terminal 5 goes to 6, disconnecting the OVM and cutting
power to the alt OV disconnect relay. The alt OV disconnect magnetic
field collapses, causing the NO switch to move to the NC position.
Moving the toggle to the center position manually takes the
alternator offline.
yes
"Down" (opposite) position: Moving this switch to the down (opposite)
position disconnects terminal 2, cutting off the connection to the
battery. And terminal 5 is the same as the 'center' position,
disconnecting the alternator.
yes. this is explained in the section on switches where
the functionality of progressive transfer switches is
discussed.
ALT 5A Fuse: Current will flow through the 22awg wire between
terminals 2 and 5 only when the switch is in the "Up" (keyway)
position, i.e. both the alternator and battery are online. The fuse
will blow when more than 5A passes through it, caused by an
overvoltage condition.
OV conditions DO NOT increase current demands expected
to open ANY STYLE of circuit protection. Fuses and breakers
protect wires. OV conditions are managed by millisecond-fast
detection devices specific to the task of taking an alternator
off line.
But if there is an OV condition, wouldn't the crowbar OVM take the
alternator offline? Is this just redundancy, or am I missing
something? Maybe so when the OV condition returns to normal it will
keep the alternator permanently offline, instead of going off and on
and off, etc, caused by the crowbar OVM cycling off and on?
Yes, ALL ov protection devices qualified for aviation are
latching. I.e., when an ov condition is detected, the
alternator shuts down and stays down. Keep in mind that
the condition immediately following an OV trip is a LOW
VOLTS situation that should cause your LV warning system
to become active.
Crowbar OVM
In Z-20, the crowbar OVM is shown sharing terminal 4 of the master
power switch with the line to the alternator. For me, the crowbar OVM
is a black box. All I need to understand is what the 'net' function
of it is. When it senses a voltage above a certain value, does it
allow a current path to be completed through it to the panel ground?
If so, then there would be no current going to the alt OV disconnect
relay, which would then take the alternator offline. Is that how it works?
Yup, it opens the UPSTREAM circuit protection (in this
case a circuit breaker) thus latching the alternator OFF.
Alternator OV Disconnect Relay
Am I right in assuming that in the normal engine operation with the
power master switch in the 'up' position, the power is applied
through the switch to the alternator OV disconnect relay, whose
magnetic field then pulls down the alt OV switch from the N.C.
position to the N.O. position? That had me confused at first, cuz I
figured 'normal' operation is when both the alternator and battery
are connected. I think I can interpret 'Normally Open, Normally
Closed' as being the 'unenergized' switch position, right?
Yes, this is a vernacular common to relay-speak
since about day-one. Further, you may find the
"COM", "NO" and "NC" nomenclature molded into
the plastic housing of the relay. See . . .
http://aeroelectric.com/Pictures/Relays/Plastic_Relay_2.jpg
Dynamo, aka PM Alternator
Z-20 shows just two wire coming off the alternator. One connects to
the voltage regulator at terminal 3, the other at terminal 1. There
is also a tach signal wire spliced into the terminal 1 wire. So both
of these wires are carrying current generated by the alternator to
the voltage regulator? If so, why have two leads instead of one, and
shouldn't there be a ground someplace?
The Dynamo is an AC power generating device that
needs both leads brought out to a rectifier/regulator
for conversion and management.
The classic "alternator" has the rectifiers
built in and usually take power (-) to case
ground thus eliminating the need for one of
the power output wires.
Capacitor
Is the capicitor after the regulator for filtering out noise to the
radio? I know of someone who wired according to Z-20, but left out
the capacitor. I'm not sure why.
The capacitor reduces alternator noise an enhances
alternator stability during battery-off operations
(contactor failed). If left off, you're not likely
to notice the difference for 99.9% of your operations.
Bob . . .
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