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Max Johansson
Joined: 09 Jan 2006
Posts: 25
Location: Helsinki, Finland
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 Posted: Fri Feb 16, 2007 2:17 am Post subject: RotaxEngines-List Digest: Capacitor |
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Hi Gilles et al
The Ducati regulator is fairly simple and works by firing the
controlled rectifiers in the negative half of the full bridge
rectifier when the terminal C voltage is under the firing treshold,
in other words, terminal C is the voltage sense for the unit.
But the current that is needed for the firing will also come
from the C terminal. So if terminal C is isolated, no charging
will take place (just like the Rotax manual says).
A potential problem will arise if the wiring is such that the
capacitor is still somehow attached to the C terminal when
the battery is disconnected but the engine is still running.
The rectifier/regulator then tries to output full voltage for
a few revolutions and there is no battery to limit the voltage
as this is disconnected. The capacitor again is filled within
milliseconds and will not really limit this kind of voltage
surge that contains hundred of times more ampere-seconds of
charge than a typical noise spike.
On the other hand, moving the capacitor behind the battery
switch might slowly burn the switch or relay with its
inrush current.
There is one fundamental difference between firing into
a battery or into a capacitor. The capacitor voltage will rise
linearly with the current but the battery just a little depending
on its charge state. But when the battery is close, the capacitor
voltage will always be the same as the battery voltage.
Well, this is a controversial subject, but in my opinion
the capacitor is not needed if a battery is close to the
Ducati rectifier/regulator. Only for longer cable runs is
a capacitor a must, giving the additional benefit to limit
the voltage of short spikes. Any capacitance does help in this
and the capacitance value of a 22mF capacitor anyhow typically
has a large +-20% variation (and might be damaged by cold).
So, I feel safer with the battery close and no capacitor.
Rotax is an excellent company but sometimes makes life
unecessary complicated, a recent example is the Evans change...
If possible, the simplest solution is always the best.
Does anybody know if Harley-Davidson uses any capacitor
in their similar style PM-generator regulator ?
regards
Max
| Quote: | Time: 01:40:19 AM PST US
From: Gilles Thesee <Gilles.Thesee(at)ac-grenoble.fr>
Subject: Re: Capacitor
>
>NYTerminat(at)aol.com a crit :
> *e ignition. It has nothing to do with keeping the engine running.
> The capacitor is there to alleviate any spikes from the charging
> dynamo. *
> ...
>
> *The battery is not needed to keep the engine running.*
Hi all,
True for the 912 and 912S, but the Rotax 914 is electrically
dependant, and does need an electrical source to keep the
engine running.
One needs two independant electrical sources to have a
reasonable engine redundancy.
At the time, I made some lengthy experiments with the Rotax
alternator and regulator. The capacitor is definitely needed
to ensure proper working of the regulator.
Some info here :
http://contrails.free.fr/elec_ducati_en.php
The main failure mode for the regulator, is from to high current draw.
It does seem reasonable to expect more than about 12 amps continuous.
The regulator doesn't come to life if a sufficient voltage is
not applied to the sense wire, or if the alternator stalls,
and the battery or regulator is depleted.
As already stated, Bob Nuckolls has designed remarkable
circuits for the Rotax installation, including OV protection,
which I think is a must because of the miserable service
record of the Rotax regulator.
Due to its shortcomings, I ended up bench testing and
installing a Schicke GR 4 regulator.
Regards,
Gilles Thesee
http://contrails.free.fr
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_________________
Max
Max Johansson, Helsinki, Finland
Building a CH701SP-912S
on CZAW amphibian floats |
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Max Johansson
Joined: 09 Jan 2006
Posts: 25
Location: Helsinki, Finland
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| Posted: Sat Feb 17, 2007 5:00 am Post subject: RotaxEngines-List Digest: Capacitor |
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Gilles,
I am not saying that the capacitor is not needed,
only that it does not have any significant role
in the regulation process if the battery is close,
and can in this case as well be left out, with
the further advantage that overvoltage will not
be possible if the battery is switched off
while the engine is still running.
Of concern is the overvoltage that might occur
in two situations. The first (1) is handled by
an overvoltage relay and the other (2) is not.
(1) Constant overvoltage will occur if one of the
silicon controlled rectifiers will die shorted out
for example because the module gets too hot during
flight. In this case the overvoltage relay will
save the day (and the avionics) after the battery
has taken the brunt of the initial attack.
(2) If the wiring is such that when the battery
accidentally is switched off and the capacitor charge
is still available, then the controlled rectifiers
will fire because the voltage on the sense terminal
is under the treshold. For how many cycles they will
fire I do not know, they anyhow are always fired
simultaneously but only the one having the G voltage
forward biased will actually conduct.
The information above and in my previous message
is based on the schematic you have provided yourself.
To my knowledge nobody else has come forward with
another reasonable explanation for the overvoltage
experienced by some 912 users, so I will still not
install a capacitor if the charged battery is close.
Regarding the Harley-Davidson alternative - no, I have
912 or 912S in all my aircraft, but the majority of
all PM-generator rectifier-regulators in the world
are used in snowmobiles and certain motorbikes like
the Harley-Davidson, so we should not discount the
experience these users have gained. To my knowledge
they do not need a capacitor for their regulator
if a battery is fitted.
best regards
Max
RE
| Quote: | As you know, our perception of the working of the regulator
is not similar to yours. It took us several bench test sessions
to get a clear notion. Is your opinion based on data we are not
aware of, or did you make deductions from existing data ?
> On the other hand, moving the capacitor behind the battery
> switch might slowly burn the switch or relay with its
> inrush current.
Not sure I clearly understand the circuit you are referring to.
Bob Nuckolls' figure Z provides anything that is needed to
design a safe and efficient Rotax circuit with OV protection,
and no damage to the ship's circuits, whatever the action
on the master switch in flight.
> Well, this is a controversial subject, but in my opinion
> the capacitor is not needed
The electricity experts I submitted the regulator to do not agree
with this statement. I would be really willing to publish any
additional study or data you have collected to support this.
> Does anybody know if Harley-Davidson uses any capacitor
> in their similar style PM-generator regulator ?
>
You really trying to design your aircraft circuits the
Harley-Davidson way ?
Regards,
Gilles
http://contrails.free.fr
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_________________
Max
Max Johansson, Helsinki, Finland
Building a CH701SP-912S
on CZAW amphibian floats |
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Roger Lee
Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.
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| Posted: Sun Feb 18, 2007 6:50 pm Post subject: Re: RotaxEngines-List Digest: Capacitor |
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Hi All,
Just spent the last three days in a Rotax 912 class. The capacitor is only necessary if you don't want to fry your avionics/circuts from overcharging. If the regulator/rec. fails then there is nothing to stop severe over charging from the engine and this will trash your avioncs/circuts. The capacitor acts to help protect your investment. Those who have had reg/rec failures have paid without capacitors. Your right it is not needed to run the engine, but if you have a reg/rec. failure then you have no one to blame, but yourself for trashing your system. Pay a little now or pay big$ later. Think of it like insurance. You don't usually need auto insurance, but for those of you that have been involved in an accident then I bet you were happy you didn't have to pay the whole thing out of pocket. Pay a little now for piece of mind, well that can go a long way. I have had reg/rec failures over the years and if you use them long enough then you may also. In other words why gamble with your big dollar investment for a couple of bucks. Doesn't seem like a hard choice to me.
Check with Bob at aero electric, he is on top of things.
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_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056 |
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