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Z-13 Alternator and Pmag Question

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drill_and_buck

Joined: 28 Sep 2006
Posts: 2
Location: Bridgewater, MA

Posted: Thu Sep 28, 2006 9:37 am Post subject: Z-13 Alternator and Pmag Question

I am using Z-13 as a baseline to design my electrical system for my RV-8.   I have read the Aeroelectric book and attended one of Bob's weekend seminars, but haven't yet quite grasped all the concepts.  

 

I would appreciate it if someone could help me better understand the rationale behind the alternator field circuit.  In particular..

 

1.  What is the rationale behind using a fuselink AND a 5A breaker on the alternator field circuit?   Wouldn't the breaker eliminate the need for a fuselink?     

 

2.  If the fuselink is optional, would it be practical to just use a 5A fuse rather than a breaker?

 

On the P-Mag (self powered mag) circuit...

 

1.  What is the benefit of wiring the P-Mags to the battery buss.  Why not just wire the P-Mags to the main power bus.  

 

Thanks in advance for your replies..

 

Mike Draper

RV-8

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jgswartout(at)earthlink.n
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Posted: Sun Oct 01, 2006 1:59 am Post subject: Z-13 Alternator and Pmag Question

I'll try to jump-start this thread, Mike, but I don't have all the answers:  

Starting with your last question first, one reason is so that if you have to shut down the main bus due smoke in the cockpit, or it shuts itself down due to a crowbar trip, you still have power to the P-mags.  "P-mags, unlike E-mags, don't need power," you say.  Well, true, as long as their internal power is working, and as long as engine rpms are high enough.  But the theory is that your ignition system should always have power available.  This is explained on page Z-6 of Appendix Z (revision 11J) to the AeroElectric Connection, under "Figure Z-29.  Always-Hot Battery Bus."  

The other reason is, if you use Bob's Z-33 circuit for maintenance/hand propping, you need battery power available.  Sure, you could run it through the Main Bus, but if you're just working on the Pmags on the ground, having them on the battery bus means you don't have to power anything else up--even the 1-amp-drawing Battery Contactor.  

   

As for the next-to-last question, the archives are full of material on this question.  Here's a sampling to get you started, but you can do your own key-word search at              

Select the "AeroElectric" archive, and be sure to fill out the boxes as to how you want the results to appear. Select "Newest First" and "All Messages, No Index," and type "crowbar ov protection" into the Search String box.  Look at messages with the following numbers:  

   

#25386, #14884, #14895, and this one, which doesn’t come up with that search string:  

   

Match:  #1 Message:  #25588 Date:  Jul 27, 2005 From:  "Robert L. Nuckolls, III" <nuckollsr(at)cox.net> Subject:  Re: overvoltage protection false trigger fix   

>  

>Well that made my day! We are going to have to start using the spell  

>checker if such prestigious awards are on the line now though   

   

    oops! is there a spelling error on the certificate? I checked  

    again and didn't see one.  

   

>Glad I could return something useful to the group. I'm humbled - as no  

>one has been more generous with his time and efforts here than Bob and  

>it is Bob who has put his designs in the public domain for us...  

>  

>There was also a one liner buried in that post. When I realized that the  

>OVM with a 2.5 amp C/B was not blowing a 10 amp fuse despite numerous  

>activations, it put to rest any minor concerns I might have had about  

>extraordinary crowbar trip currents. I rather expected that the fuses  

>would blow but I was using 18 awg wire and before making up fuselinks I  

>thought I'd try feeding it off fuses and using the 2.5 amp breakers. I  

>will leave it that way.  

   

    That's another simple idea that I'd not had time to touch on.  

    I recommend the fusible link upstream of a crowbar breaker because  

    very early tests on a variety of breakers showed that SOME products  

    had a I-squared*T constant much larger than a fuse several times the  

    size.  I've had builders report opening a 20A fuse upstream of a  

    5A breaker. The fusible link seemed the best one-solution-fits-all  

    approach. I'm not surprised that your 10A fuse upstream of the  

    2.5 amp breaker is stable . . . a typical 2.5A miniature breaker  

    has about .28 ohms max internal resistance while the 5A is only  

    0.07 ohms max and typically .04 ohms.  

   

    It should be noted that not EVERYONE can expect the same results  

    given variability of breakers offered but if you're using the  

    miniature devices common to modern aircraft fabrication, odds  

    are that your experiences will parallel Ken's.  

   

    Bob . . .  

   

Be advised that these message numbers do change over time, and some disappear.  But as of today, those are the numbers that will give you some insight into the fusible link/breaker combination.  

   

Hope this helps.  

   

John  

   

   

--

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drill_and_buck

Joined: 28 Sep 2006
Posts: 2
Location: Bridgewater, MA

Posted: Sun Oct 01, 2006 11:34 am Post subject: Re: Z-13 Alternator and Pmag Question

John:



Thanks for the reply and insight.

The lightbulb went off when you mentioned OV crowbar protection.   I completely overlooked that factor.   The fuselink now makes sense on the alternator circuit when an OV situation arises and the 5A breaker doesn't trip.



-Mike Draper

RV-8 finish

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nuckollsr(at)cox.net
Guest

Posted: Sun Oct 01, 2006 2:22 pm Post subject: Z-13 Alternator and Pmag Question

At 12:34 PM 10/1/2006 -0700, you wrote:



 	  Quote: 	
		  



John:



Thanks for the reply and insight.

The lightbulb went off when you mentioned OV crowbar protection.   I 

completely overlooked that factor.   The fuselink now makes sense on the 

alternator circuit when an OV situation arises and the 5A breaker doesn't trip.


	


  Closer . . .



  The fusible link isn't in there to back up the 5A breaker.

  It's there to protect the length of wire between the fuseblock

  and breaker. Normally, we'd simply mount the breaker at the bus,

  but since the bus is the fuseblock . . . and probably remote

  from the panel . . . now there's a small wire connected to FAT

  wires that cant' use a fuse for protection. This is because we

  expect the fuse to stay in place during a crowbar event . . . which

  is difficult except when you use the fusible link.



  This is a classic rationale for use of fusible links for smaller

  wires and is simply a scaled down rationale for ANL limiters

  on large wires.



  Bob . . .

      ---------------------------------------------------------

      < What is so wonderful about scientific truth...is that >

      < the authority which determines whether there can be   >

      < debate or not does not reside in some fraternity of   >

      < scientists; nor is it divine. The authority rests     >

      < with experiment.                                      >

      <                            --Lawrence M. Krauss       >

      ---------------------------------------------------------

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