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Stuck starter questions

 
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roughleg(at)gmail.com
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PostPosted: Sun Oct 21, 2018 9:39 am    Post subject: Stuck starter questions Reply with quote

Many of the Z diagrams show the starter current passing thru the master contactor as well as the starter contactor, and I gather this is to allow for the possibility of the starter contactor welding itself shut. If that were to occur then the pilot can disengage the starter motor by turning the master switch off (having a "starter engaged" light and an e-bus alt feed would both be useful for this scenario).
However, I see that Z-20 shows the starter current direct from the battery to the starter contactor and not thru the master. Z-20 is for a Jabiru system, and I have a Jabiru engine so I figure (no pun intended) that I could use the Z-20 arrangement, although I don't know why Jabirus would be different in this respect. 
So, here are my questions: How unlikely is a welded contactor? How well does the diode prevent this from happening? Is there a way to tell if the diode has failed open-circuit?
Pat


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alec(at)alecmyers.com
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PostPosted: Sun Oct 21, 2018 10:59 am    Post subject: Stuck starter questions Reply with quote

Can’t speak for how common this is overall, but I’ve had a stuck starter solenoid on both my TC aircraft.

On Oct 21, 2018, at 13:38, Pat Little <roughleg(at)gmail.com (roughleg(at)gmail.com)> wrote:
Many of the Z diagrams show the starter current passing thru the master contactor as well as the starter contactor, and I gather this is to allow for the possibility of the starter contactor welding itself shut. If that were to occur then the pilot can disengage the starter motor by turning the master switch off (having a "starter engaged" light and an e-bus alt feed would both be useful for this scenario).
However, I see that Z-20 shows the starter current direct from the battery to the starter contactor and not thru the master. Z-20 is for a Jabiru system, and I have a Jabiru engine so I figure (no pun intended) that I could use the Z-20 arrangement, although I don't know why Jabirus would be different in this respect.
So, here are my questions: How unlikely is a welded contactor? How well does the diode prevent this from happening? Is there a way to tell if the diode has failed open-circuit?
Pat


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rickofudall



Joined: 19 Sep 2009
Posts: 1334
Location: Udall, KS, USA

PostPosted: Mon Oct 22, 2018 3:55 am    Post subject: Stuck starter questions Reply with quote

Going from memory here but as I recall the diode is there to prevent arcing of the switch contacts controlling the solenoid and not the solenoid itself. I'm sure that that info is in the archives. Try a search for diode or diode and contactor to be sure.

Rick
On Sun, Oct 21, 2018 at 2:04 PM Alec Myers <alec(at)alecmyers.com (alec(at)alecmyers.com)> wrote:

Quote:

Can’t speak for how common this is overall, but I’ve had a stuck starter solenoid on both my TC aircraft.

On Oct 21, 2018, at 13:38, Pat Little <roughleg(at)gmail.com (roughleg(at)gmail.com)> wrote:
Many of the Z diagrams show the starter current passing thru the master contactor as well as the starter contactor, and I gather this is to allow for the possibility of the starter contactor welding itself shut. If that were to occur then the pilot can disengage the starter motor by turning the master switch off (having a "starter engaged" light and an e-bus alt feed would both be useful for this scenario).
However, I see that Z-20 shows the starter current direct from the battery to the starter contactor and not thru the master. Z-20 is for a Jabiru system, and I have a Jabiru engine so I figure (no pun intended) that I could use the Z-20 arrangement, although I don't know why Jabirus would be different in this respect. 
So, here are my questions: How unlikely is a welded contactor? How well does the diode prevent this from happening? Is there a way to tell if the diode has failed open-circuit?
Pat




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user9253



Joined: 28 Mar 2008
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Location: Riley TWP Michigan

PostPosted: Mon Oct 22, 2018 4:29 am    Post subject: Re: Stuck starter questions Reply with quote

I agree with Rick.
One way to test a diode is to remove it from the circuit. Then put it in series with a test light and battery. The light should illuminate when the diode is orientated one way and not illuminate when reversed.
Trying to start an engine with a weak battery can lead to welded contacts.


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alec(at)alecmyers.com
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PostPosted: Mon Oct 22, 2018 4:32 am    Post subject: Stuck starter questions Reply with quote

>>Trying to start an engine with a weak battery can lead to welded contacts.

Yes… of the two occasions this happened to me, one was because of a weak battery.
The other was caused by an intermittent dead short (yes!) in the starter motor.


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yellowduckduo(at)gmail.co
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PostPosted: Mon Oct 22, 2018 6:57 am    Post subject: Stuck starter questions Reply with quote

Everything that I have newer than my 70 year old Ford has contacts built
into the starter and no separate starter contactor.  Both systems can
weld shut but it is pretty rare with the built in contactor IMO. 
Despite having integral contacts on my starter, I do run the starter
current through the battery contactor on my aircraft as it was
convenient to do it, it added miniscule resistance to the circuit, and
(in my case) it makes the heavy starter wire go dead when the master is
off.  I would recommend that scheme if a standard 200 amp battery
contactor is used or if the battery was inaccessible and could not be
disconnected within a minute or three.  I would not argue that it is
essential.  I would take another look at this if using a Li battery.

Either way the important thing is to make sure system voltage recovers,
or battery is charging after the engine starts (or at least before
takeoff)!   Phooey on starter engaged lights.

Even paralled with the coil, diodes can often be tested in circuit with
a nominal one volt low current source or ohmeter as they typically
conduct at 0.6 volt or less but I don't bother unless it is physically
damaged or suspect for some reason. They seem to last forever or
occasionally simply blow apart when shorted.
Ken

On 21/10/2018 1:38 PM, Pat Little wrote:
Quote:
Many of the Z diagrams show the starter current passing thru the
master contactor as well as the starter contactor, and I gather this
is to allow for the possibility of the starter contactor welding
itself shut. If that were to occur then the pilot can disengage the
starter motor by turning the master switch off (having a "starter
engaged" light and an e-bus alt feed would both be useful for this
scenario).

However, I see that Z-20 shows the starter current direct from the
battery to the starter contactor and not thru the master. Z-20 is for
a Jabiru system, and I have a Jabiru engine so I figure (no pun
intended) that I could use the Z-20 arrangement, although I don't know
why Jabirus would be different in this respect.

So, here are my questions: How unlikely is a welded contactor? How
well does the diode prevent this from happening? Is there a way to
tell if the diode has failed open-circuit?

Pat


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nuckolls.bob(at)aeroelect
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PostPosted: Mon Oct 22, 2018 8:12 am    Post subject: Stuck starter questions Reply with quote

At 01:58 PM 10/21/2018, you wrote:
Quote:
Can’t speak for how common this is overall, but I’ve had a stuck starter solenoid on both my TC aircraft.

It's one of those pesky FMEA thingys . . . to get
your product/design qualified onto a sophisticated
TC aircraft you need to demonstrate or at least calculate
that failure of the device presents ZERO safety of flight
implications because it's either (a) insignificant
or (b) is backed up. In the case of conducting
FMEA, you ASSUME that it will fail . . . another
way of acknowledging that the risk of failure
is not zero.

The other path to qualification Nirvana is the
so called reliability study. One puts on the
green eye-shade, digs out their Hogwarts-issue
magic wand and divines a failure rate for
EVERY component of the of device down to the
solder joints. Then you put them in this big
pot (computer) and it simmer to a predicted
failure rate for the constellation of components.

If one achieves failure rates on the order of
one per million flight hours, you can get onto
most flying machines, churn out a one-per-
billion-flight-hours design and you're good to go
to Pluto.

We lowly tin benders and electron herders find
it more practical to go the FMEA route . . .
especially when the not-zero=risk assumption
is validated with demonstrated anecdotes from
history. Hence the legacy policy of routing
starter current through the battery contactor
(assuming there is one).

There are special cases like Z20 where the
system's energy source is limited.
VariEz airplanes flying and SD-8 as sole
source of engine driven power is one example.
There are other examples of PM alternator
where we'd rather not toss off 0.8 Amps
of current just to hold the battery contactor
closed; hence no contactor.

It doesn't represent a terrible risk . . . but
if one would LIKE to emulate the battery contactor's
function with a zero energy budget, consider
a battery switch. The TriPacer in which I studied
back in the dark ages had no battery
-OR- starter contactor . . . they were both
manual switches.

Here's one example of a suitable switch

https://tinyurl.com/ycechd8f

I've updated Z20 to reflect this configuration.
Hmmmm . . . last time that drawing was updated
was 10 years ago . . .

http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z20M.pdf











Bob . . .


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PostPosted: Mon Oct 22, 2018 10:10 am    Post subject: Stuck starter questions Reply with quote

Quote:
How well does the diode prevent this from happening? Is there a way to tell if the diode has failed open-circuit?

The diode across the coil of a contactor has no
significant effect on the life of that device's
contacts. An often quoted document by Potter and
Brumfield claims that coil suppression techniques
can have a deleterious effect on contact life.

https://tinyurl.com/yayk835c

One of several exemplar figures is shown below:

[img]cid:.0[/img]

When this document was first cited on the AeroElectric List,
I went to the bench and captured some relay performance
traces of my own:

http://aeroelectric.com/articles/spike.pdf

https://tinyurl.com/yb6s6l7c

www.aeroelectric.com/articles/spikecatcher.pdf


I'm mystified by the trace pictures in the P&B
document. They fail to illustrate any drop-out-delay
caused by addition of the simple catch diode.
At the same time, the figures and text purport to
show how coil suppression techniques can affect
CONTACT SPREADING VELOCITY which is the critical
to contact life. They also fail to show any
contact bounce on closing. If I were a physics
prof grading these illustrations as part of a
student's work-product, I would ask them to
'show me' back in the lab.

I was able to show that contact spreading velocity
does not change appreciably with variations in
coil suppression technique (for reasons that
have to do with MAGNETICS and little to do
with coil current).

The short answer is, worrying about the diode
is not useful. They seldom fail and when they do
fail, they'll fail shorted . . . not open . . .
and may even blow apart as noted by another Lister.

The PRIMARY reason for contact welding is a soggy
battery. So if your starter fails to get the
prop whipping around right smartly . . . STOP
and charge/replace battery or plug in ground
power.


Bob . . .


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skywagon185(at)gmail.com
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PostPosted: Mon Oct 22, 2018 10:41 am    Post subject: Stuck starter questions Reply with quote

Joe,
You made a remarkable statement about weak batteries, starting, and contactor contact problems.
I hope everyone reads your remark and remembers this potential problem.  Too many try to do starts via a weak battery and cold engines.  This will eventually do contact damage in the starter contactor.

On Mon, Oct 22, 2018 at 5:34 AM user9253 <fransew(at)gmail.com (fransew(at)gmail.com)> wrote:

Quote:
--> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com (fransew(at)gmail.com)>

I agree with Rick.
One way to test a diode is to remove it from the circuit.  Then put it in series with a test light and battery.  The light should illuminate when the diode is orientated one way and not illuminate when reversed.
  Trying to start an engine with a weak battery can lead to welded contacts.

--------
Joe Gores




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