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1. 02:17 AM - Re: RotaxEngines-List Digest: Capacitor ()
2. 02:41 PM - Re: Capacitor (Gilles Thesee)
Message 1
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| Subject: | RE: RotaxEngines-List Digest: Capacitor |
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
>Time: 01:40:19 AM PST US
>From: Gilles Thesee <Gilles.Thesee@ac-grenoble.fr>
>Subject: Re: RotaxEngines-List: Capacitor
>>
>>NYTerminat@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
Message 2
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Max,
>
>
> The Ducati regulator is fairly simple
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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