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JonSmith
Joined: 21 May 2010
Posts: 110
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 Posted: Mon Jan 17, 2011 11:25 am Post subject: Electrical Question (Basic) |
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Just a couple of basic questions (my electrics knowledge is very basic)
I'm wiring up my new secondhand Rotax - upgrading my plane from a previously installed car engine conversion. 95% of the aircraft wiring aft of the firewall is common to both thank goodness but I have a question about the alternator switch.
My panel currently has a combined double battery/ alternator switch unit - one of the red coloured double rocker type switch units exactly as found in Cessna 150's - in fact it has "Cessna" printed on the case. My previous set up had a battery solenoid (still keeping that) but the car-type alternator output was controlled by switching the field on/ off via the alternator switch. Thus both switches had very low load through them.
My limited understanding is that the Rotax does not have a separate field like a conventional alternator and so the output from the regulator must all go direct to the positive bus via the alternator switch. My question is - can anyone advise if this Cessna switch is man-enough for the job? - it has no rating markings on it and I can't find it in any catalogues to check it's rating. (It looks pretty chunky but size isn't everthing so to speak). Do other people use these or is it best to have separate switch units?
Second quick question - the manual advises to connect a diode across the starter solenoid coil. Do people bother to do this? If so, can anyone advise a neat and easy way of physically connecting it. I have one and can think of several ways of bodging it up but would much prefer a professional looking solution.
Thanks in advance - any help appreciated! Jon
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europa(at)pstewart.f2s.co
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| Posted: Mon Jan 17, 2011 12:07 pm Post subject: Electrical Question (Basic) |
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Jon
I was told not to bring the full alternator current to the panel to any sort
of switch. I have therefore used an engine bay mounted relay rated at 30amps
(I think) which I switch from an alternator switch on the panel.
Diodes I connected using AMP crimp ring terminals
Regards
Paul
G-GIDY
On 17/1/11 19:25, "JonSmith" <jonsmitheuropa(at)tiscali.co.uk> wrote:
| Quote: |
Just a couple of basic questions (my electrics knowledge is very basic)
I'm wiring up my new secondhand Rotax - upgrading my plane from a previously
installed car engine conversion. 95% of the aircraft wiring aft of the
firewall is common to both thank goodness but I have a question about the
alternator switch.
My panel currently has a combined double battery/ alternator switch unit - one
of the red coloured double rocker type switch units exactly as found in Cessna
150's - in fact it has "Cessna" printed on the case. My previous set up had a
battery solenoid (still keeping that) but the car-type alternator output was
controlled by switching the field on/ off via the alternator switch. Thus
both switches had very low load through them.
My limited understanding is that the Rotax does not have a separate field like
a conventional alternator and so the output from the regulator must all go
direct to the positive bus via the alternator switch. My question is - can
anyone advise if this Cessna switch is man-enough for the job? - it has no
rating markings on it and I can't find it in any catalogues to check it's
rating. (It looks pretty chunky but size isn't everthing so to speak). Do
other people use these or is it best to have separate switch units?
Second quick question - the manual advises to connect a diode across the
starter solenoid coil. Do people bother to do this? If so, can anyone advise
a neat and easy way of physically connecting it. I have one and can think of
several ways of bodging it up but would much prefer a professional looking
solution.
Thanks in advance - any help appreciated! Jon
--------
G-TERN
Classic Mono
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=327315#327315
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budyerly(at)msn.com
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| Posted: Mon Jan 17, 2011 8:27 pm Post subject: Electrical Question (Basic) |
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<?xml:namespace prefix="v" /><?xml:namespace prefix="o" /><![endif]--> Jon,
Note the electrical section in the build manual (Chapter 25) as well as the Rotax installation manual, brings alternator power to the panel through a 25 amp switch/switch breaker.
Like you, I am not comfortable with that.
The "Cessna" style split master switch is a low amperage switch. Normally, no more than 5 amps.
Here are the options.
Wire the C lead of the regulator to the alternator side of the split master switch and continue the wire to a 5 amp breaker on the main bus. When the switch is off, the control circuit input of the regulator is cut and the regulator shuts down. (See the Aeroelectric Connection for wiring techniques.) Now, Ducatti doesn't like this, as the circuit diodes still have power coming in from the dynamo. Normally, you fly with the alternator on all the time, so in a single alternator system, no problem. If you want the alternator power off, it is probably an emergency and who cares about the regulator then.
The alternative is to wire the alternator side of the split master switch from a 2-3 amp circuit breaker to a 30 amp automotive relay switch. The relay will keep you from having high amps coming to your finger. I use this method a lot, especially with the dual alternator setup. See www.customflightcreations.com accessories/techniques section.
Notice that, the split master switch has a trick. Most of us want the switch to be completely independent, that is, so one can select either the alternator on or off and the master on or off completely independent, but there is a small plastic tab that locks the two sides together. So that both come on, but only one shuts off independently, and the other shuts both off. I take a sharp Exacto knife and trim off the small plastic locking tabs off and have mine completely independent. Just a technique.
As for the diode, I have seen guys operate their planes for years without it. However, it is a handy device. When the start switch is disengaged, there is a slight chance that the field around the coil will collapse and cause a reverse current called a fly back, and the diode immediately neutralizes the fly back current. This allows the contactor to separate cleanly and not do a quick open and close during the disengage or hang-up which eventually will burn the start solenoid contacts. It is an oldie but goodie that started in the late '50s when the start solenoid and starter were in close proximity and field shielding and wires were not as good as they are today. Our solenoid has a really small coil and is well separated from other components so the chance of a fly back is diminished, but not eliminated. Use the diode and you should get a longer life out of your solenoid. Again, reference an Aeroelectric Connection article (http://www.matronics.com/aeroelectric/articles/spikecatcher.pdf)
Use a ohm meter or a low power test light to get the diode direction if you can't see the markings and figure out which way the power flows. You can make your own hookups and put a 3/16 inch terminal on the ground end of the diode and on the other end wire the diode end together with the start wire into a spade female connector which connects to the solenoid blade. Cover with heat shrink. Hook the 3/16 inch terminal to the grounded base of the solenoid (grounded to the battery or negative terminal of course). If you get it backwards, you'll know real quick. You'll fry the diode and perhaps see some smoke. My friend Mike says smoke is when the electric genie is released and the magic disappears.
Regards,
Bud Yerly
Europa Tech Assistance
Custom Flight Creations, Inc.
www.customflightcreations.com
(813) 653-4989
[quote] ---
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JonSmith
Joined: 21 May 2010
Posts: 110
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| Posted: Tue Jan 18, 2011 1:24 am Post subject: Re: Electrical Question (Basic) |
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Thank you for the info Paul and Bud. Very useful and much appreciated. Jon
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