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peter(at)sportingaero.com
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 Posted: Sun Nov 03, 2013 12:43 pm Post subject: Contactors |
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Sacha,
Apologies, I'm not very familiar with Rotax components. It would be good
to know how much current a Rotax starter pulls - I guess it is likely
that as the engine is only 1200cc the draw would rather less than a
starter for a 5/6 litre Lyc. I believe a Lyc starter pulls between 200
to 500A depending on starter type and compression ratio. If a Rotax
pulls substantially less the potential for the starter contactor to weld
itself shut would be lower (probably not a linear relationship), and so
the risk of a problem that much lower. You may decide you can tolerate
the risk without additional mitigations.
Peter
On 02/11/2013 21:10, Sacha wrote:
| Quote: |
>> If a modern starter with a piggy-back solenoid is fitted,
>> to avoid the master relay handling the starting current, the jumper that
>> normally energises the solenoid can be brought into a cockpit switch to
>> disable the starter if the the starter contactor does weld on. A small
>> additional parts count, but the switch never does any switching except in
>> emergency.
> You're speaking about the solenoid on the starter motor? I don't recall
> seeing any external jumper there (the engine is a Rotax 912). I'll have a
> check next time I'm down at the hangar.
I couldn't find anything there unfortunately. The starter motor just has one terminal for the fat positive wire.
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uuccio(at)gmail.com
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| Posted: Sun Nov 03, 2013 2:08 pm Post subject: Contactors |
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Thanks Peter
Actually I realized today that I can easily use the semi-useless alternator off warning light that I fitted on my panel as a "starter on warning light" with minimal rewiring. Since 99.9% of the time I'm going to be on the ground while starting my engine, I think I can live with the fact of not being able to disable the starter motor if the contactor gets stuck somehow since I can just shut down the engine.
Out of interest I will try to measure how many amps are drawn by the starter next time I'm down at the hangar. Though I suspect this would vary somewhat depending on temperature.
I've almost finished the CAD drawings of my electrical system so I'll be submitting them to the list for review and criticism soon.
Sacha
On Nov 3, 2013, at 21:42, Peter Pengilly <peter(at)sportingaero.com> wrote:
| Quote: | Apologies, I'm not very familiar with Rotax components. It would be good to know how much current a Rotax starter pulls - I guess it is likely that as the engine is only 1200cc the draw would rather less than a starter for a 5/6 litre Lyc. I believe a Lyc starter pulls between 200 to 500A depending on starter type and compression ratio. If a Rotax pulls substantially less the potential for the starter contactor to weld itself shut would be lower (probably not a linear relationship), and so the risk of a problem that much lower. You may decide you can tolerate the risk without additional mitigations.
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etienne.phillips(at)gmail
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| Posted: Sun Nov 03, 2013 11:28 pm Post subject: Contactors |
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Hi Sacha
Being able to pull the mixture control whilst on the ground will stop the engine from producing power, but if the start motor is still energised due to the failed contactor, won't it keep cranking until you pull a terminal off the battery? I'm not that familiar with the starters used in aviation (I hand-swing my engine), so am unsure if they have a disengage mechanism like they do on cars.
Thanks
Etienne
On 4 November 2013 00:07, Sacha <uuccio(at)gmail.com (uuccio(at)gmail.com)> wrote:
[quote]--> AeroElectric-List message posted by: Sacha <uuccio(at)gmail.com (uuccio(at)gmail.com)>
Thanks Peter
Actually I realized today that I can easily use the semi-useless alternator off warning light that I fitted on my panel as a "starter on warning light" with minimal rewiring. Since 99.9% of the time I'm going to be on the ground while starting my engine, I think I can live with the fact of not being able to disable the starter motor if the contactor gets stuck somehow since I can just shut down the engine.
Out of interest I will try to measure how many amps are drawn by the starter next time I'm down at the hangar. Though I suspect this would vary somewhat depending on temperature.
I've almost finished the CAD drawings of my electrical system so I'll be submitting them to the list for review and criticism soon.
Sacha
On Nov 3, 2013, at 21:42, Peter Pengilly <peter(at)sportingaero.com (peter(at)sportingaero.com)> wrote:
> Apologies, I'm not very familiar with Rotax components. It would be good to know how much current a Rotax starter pulls - I guess it is likely that as the engine is only 1200cc the draw would rather less than a starter for a 5/6 litre Lyc. I believe a Lyc starter pulls between 200 to 500A depending on starter type and compression ratio. If a Rotax pulls substantially less the potential for the starter contactor to weld itself shut would be lower (probably not a linear relationship), and so the risk of a problem that much lower. You may decide you can tolerate the risk without additional mitigations.
=
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nuckolls.bob(at)aeroelect
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| Posted: Mon Nov 04, 2013 11:06 am Post subject: Contactors |
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At 03:16 AM 11/2/2013, you wrote:
| Quote: |
<peter(at)sportingaero.com>
There is a potential problem in using only the starter contactor to
switch the starter motor power - those contactors are known to
occasionally weld themselves together. In that case, with only the
starter contactor switching power to the starter, there is then no
way of disengaging it from the running engine until the battery is flat
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This was a greater risk when starter contactors
were intermittent duty versions of the battery
contactors (large surface area, low pressure).
The modern automotive contactor is a low area,
very high pressure connection. These CAN stick
but probably not if the battery is good. Too
many batteries are flogged until they don't
crank the engine any more . . . they've been
unsuited for use on an airplane for some period
of time before a soggy battery makes a 'soft closure'
of a contactor and causes it to stick.
| Quote: | (if that ever happens as the starter is now a generator).
|
Doesn't happen. Every starter has a disconnect
mechanism that prevents a running engine from
back-driving the starter. Consider that a starter
armature runs at or near it's max horsepower rpm
while cranking an engine at ~200 rpm. An engine
running at 2000 rpm would drive a starter at 10x
it's rated running speed . . . throws windings
and/or tears up gearboxes.
| Quote: | In the UK the authorities insist that a light is installed in the
panel warning the pilot the starter remains energised, as there
have been several accidents. The engaged starter saps a large
amount of power from the engine and gets very hot, with unpleasant
consequences.
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Yes, the starter does continue to run from
due to applied electrical energy. These are
NOT efficient motors. They draw substantial
current even when no mechanically loaded. Given
their design goals for intermittent duty operation,
they simply cannot reject internal heat generated
while running unloaded
Keep the battery in good shape and starter
contactor sticking is very rare. Starter energized
light is a simple hedge against taxiing out with
a starter energized. Having a battery contactor
carry starter current is a pretty cheap insurance.
Bob . . .
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nuckolls.bob(at)aeroelect
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| Posted: Mon Nov 04, 2013 11:08 am Post subject: Contactors |
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| Quote: |
I couldn't find anything there unfortunately. The starter motor just
has one terminal for the fat positive wire.
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Rotax starter doesn't have a contactor/solenoid
combo. Rotax uses a simple external contactor
to control power to the motor. The pinion gear
is extended inertially on spiral grooves cut into
the motor shaft.
Recommend you have either a battery contactor
or manually operated battery switch.
Bob . . .
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uuccio(at)gmail.com
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| Posted: Fri Nov 08, 2013 5:31 am Post subject: Contactors |
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Recommend you have either a battery contactor
or manually operated battery switch.
Yup, sounds like a good idea. For what it's worth there is an aircraft at
my local field which uses the same automotive "cube" 70A relay that I'm
using as a battery contactor. His engine is a Rotax 912 (the certified
version although his a/c is not TC being of his own custom design). I
measured the peak startup current this morning and it came to 110A, but the
automotive relay seems to take this in its stride. The owner told me he's
been running that way for 500 hours and 8 years without any problem.
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nuckolls.bob(at)aeroelect
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| Posted: Fri Nov 08, 2013 6:47 pm Post subject: Contactors |
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At 07:29 AM 11/8/2013, you wrote:
| Quote: |
Recommend you have either a battery contactor
or manually operated battery switch.
Yup, sounds like a good idea. For what it's worth there is an aircraft at
my local field which uses the same automotive "cube" 70A relay that I'm
using as a battery contactor. His engine is a Rotax 912 (the certified
version although his a/c is not TC being of his own custom design). I
measured the peak startup current this morning and it came to 110A, but the
automotive relay seems to take this in its stride. The owner told me he's
been running that way for 500 hours and 8 years without any problem.
|
The plastic 'cube' relays I've seen all feature
fast-on tabs. The 70A versions too albeit larger
ones. How do you make wired connections to this
relay with robustness capable of carrying starter
current? Can you take some pictures of the installations
and post them?
Bob . . .
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uuccio(at)GMAIL.COM
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| Posted: Fri Nov 08, 2013 9:25 pm Post subject: Contactors |
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On Nov 9, 2013, at 3:46, "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com> wrote:
| Quote: | The plastic 'cube' relays I've seen all feature
fast-on tabs. The 70A versions too albeit larger
ones. How do you make wired connections to this
relay with robustness capable of carrying starter
current? Can you take some pictures of the installations
and post them?
|
Yes the Faston tabs are 3/8" instead of the regular 1/4". Here's a pic I took yesterday but its not easy to see how the connections are made on the cube.
Sacha
Note that he appears to be using something (on the left) that looks to me like a battery contactor in lieu of a starter contactor. The master contactor is the red "384" series cube on the right.
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