Today's Message Index:
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1. 05:20 AM - Re: Starter contactors (Jan de Jong)
2. 07:48 AM - Re: Starter contactors (Robert L. Nuckolls, III)
3. 07:53 AM - Re: Starter contactors (Robert L. Nuckolls, III)
4. 10:08 AM - Re: Purpose of Diodes on Relays (Noel Loveys)
5. 01:28 PM - Re: Starter contactors (user9253)
Message 1
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| Subject: | Re: Starter contactors |
A solid state loadswitch as starter contactor could also be an option
(attached).
I would be inclined to make my own though by parallelling some BTS555.
Jan de Jong
Message 2
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| Subject: | Re: Starter contactors |
At 09:05 PM 12/17/2011, you wrote:
>I've considered the "Little Switch" on this page:
><http://www.flamingriver.com/batterydisconnect>http://www.flamingriver.com/batterydisconnect
This appears to be the same switch offered by Harbor Feight
at a fraction of the cost . . .
http://www.harborfreight.com/heavy-duty-battery-cutoff-switch-66789.html
Bob . . .
Message 3
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| Subject: | Re: Starter contactors |
At 02:47 PM 12/17/2011, you wrote:
>
>My aircraft has a Rotax 912ULS engine with an 18 amp alternator. I
>am considering adding lighting and additional avionics that would
>load the alternator to near maximum.
Can you share your load analysis with us?
>Option 3 is the simplest and lightest weight and least
>expensive. But I am concerned about the starter contacts welding
>themselves shut with no way to shut off the power.
> Is the likelihood of the starter contactor failing closed, great
> enough to warrant the use of two contactors in series with the starter motor?
Not if you can climb out quickly and disconnect the battery
without undue hazard. But the incidences of sticking starter
contactors of the S704 variety are exceedingly rare.
How about a battery switch and S704-1 style starter
contactor.
When you say your alternator is loaded to "near max" . . .
under what conditions and for how long? Keep in mind
that legacy design goals for engine driven power sources
is to keep 20-25% of alternator capacity in reserve
for battery replenishment.
A load analysis of ENERGY requirements for various
phases of flight might yield some less stressing
facts about your night flight capabilities . . .
ESPECIALLY with LED nav lights. Landing lights are
a toss up . . . their current demand is high but
the duty cycle can be exceedingly low. You don't
really NEED light until you're in the flare.
Bob . . .
Message 4
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| Subject: | Purpose of Diodes on Relays |
One thing that really surprises me on your lower trace is the fact there is
no variance in the lower trace as the engine cranks. I know in my plane the
lights in the panel dim as the engine is cranked just because of the current
drawn by the starter motor really loads down the battery. This could be
because of the speed of the trace which I make out to be around 1.500 sec
left to right.
The only place I've actually seen a diode on a switch is on turbine engines
where the starter motor is also the generator (old 206). The MM specifies
the diode be in good working order to protect from reverse EMF. I was told
that from time to time I would see the diodes again but haven't
Noel
From: owner-aeroelectric-list-server@matronics.com
[mailto:owner-aeroelectric-list-server@matronics.com] On Behalf Of Robert L.
Nuckolls, III
Sent: December 16, 2011 6:27 PM
Subject: RE: AeroElectric-List: Purpose of Diodes on Relays
At 01:28 PM 12/16/2011, you wrote:
A starting motor is essentially a coil. When you activate the starter (read
coil) draws down on the current of the battery. The more load on the
starter the greater the current drawn by the starter. When you release the
starter button, key or whatever, all the energy stored up in the coil (or
starter motor) is immediately dumped into the circuit, with extremely high
voltage in reverse polarity. The diode allows this reverse potential to be
drained off safely without frying all your expen$ive avionics and other
digital equipment on the buss.
A few years back (about 11) when we were
exploring the behavior of energy stored on
inductive devices, we discovered (and demonstrated
on the bench) that un-suppressed spikes from
inductive devices are almost totally dissipated
across the spreading contacts of the switching
device. For example, here's one of many plots
we gathered while stirring the Contactor-Spike-
Stew pot:
http://www.aeroelectric.com/Pictures/Curves/CH_Bus_Noise_w_0p1_Cap.gif
There are two traces. The top one shows voltage
developed across the contacts of the controlling
switch . . . the lower trace shows voltage that
made it out to the bus . . . barely perceptible.
Know that electronic devices designed to run directly
from the bus of any vehicular DC system is (1) easily
designed to stand off the (2) minuscule packet of
energy that makes it across the opening switch.
In short, there is no risk of "frying" anything. The
major risk is shortened service life of the switch
that controls the contactor.
The same conditions apply to energy dumped from
a starter motor . . . it's the starter contactor
that takes the hit, not electro whizzies running
off the bus. One might think that an inductor
charged to hundreds of amps represents the grand
dragon of spike generators. Motors have a unique
feature called counter-emf. A 12v motor really runs
on the DIFFERENCE voltage between applied voltage
(battery) and counter-emf due to rotation. The
real free inductance model of a motor is rather
small in comparison with contactor coils having
hundreds of turns of wire. This is why you never
see spike suppressors on a motor . . . they're
just not a potential threat.
Bob . . .
Message 5
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| Subject: | Re: Starter contactors |
> Can you share your load analysis with us?
Aircraft load at present time
0.3A SL 40 com
1.6A GTX 327 X-PNDR
0.2A 496 GPS
1.5A Dynon D-180 EFIS
2.6A Autopilot w/ 2 servos
0.1A Intercom
0.5A Misc
0.7A Battery Contactor
1.5A Fuel Pump (continuous)
----------------------------------------
9A Total
3.5A Proposed Dynon SkyView (additional 1.5A during backup battery charging)
3.2A Proposed 35W HID Landing light (continuous for collision avoidance)
-----------------------------------------------------
15.7A New Total
The voltage regulator requires dedicated cooling at higher loads.
The Rotax 912ULS starts so quickly that main battery charging load is negligible.
The autopilot will not be turned on until after climbing to cruise altitude.
The battery will be charged by then.
I never fly at night, thus no position lights.
The electric fuel pump runs continuously per Van's RV-12 design.
> How about a battery switch and S704-1 style starter contactor.
I assume that you meant to type S702-1 starter contactor. I am reluctant to use
a manual battery disconnect switch. It would add another failure point to the
electrical system. In the event of a failed closed contactor, it would take
me a minute or more to exit the plane and operate the battery switch. The cost
to buy a second starter contactor is not much more than a quality manual battery
switch.
> But the incidences of sticking starter contactors . . . are exceedingly rare.
That answers my question.
If it is not a safety of flight issue, I am inclined to keep the electrical system
simple with one starter contactor connected directly to the battery and add
an indicator light showing starter energized.
Thanks Bob, for the reply.
Joe
--------
Joe Gores
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=361150#361150
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