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Z11 field supply circuit protection

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draker

Joined: 30 Jun 2020
Posts: 2

Posted: Mon Jul 27, 2020 3:22 pm Post subject: Z11 field supply circuit protection

Hi, this is my first E-mail to the list. I've done some searches but came up empty for answers. I'm finalizing the electrical schematic for my RV-7 build, which is very loosely based on the AeroElectric Z11 example. Single alternator, LR-3 regulator. A few questions:



1. Z-11 shows the field supply circuit running from the fuse block stud through a 22AWG fusable link, through the battery/alt switch, through a 5A circuit breaker, and ending up at the voltage regulator (pin 6). Since the circuit is protected by the 5A breaker, what is the purpose of the fusable link, and what are the negative consequences of omitting the fusable link?



2. Should current drawn through the above circuit be considered for the purpose of load analysis to help me choose the right size alternator? If so, what range of currents should I expect through it?



3. Same question for the bus voltage sense (pin 3) on the regulator. Does that draw significantly current?



Thank you!

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user9253

Joined: 28 Mar 2008
Posts: 1913
Location: Riley TWP Michigan

Posted: Mon Jul 27, 2020 5:47 pm Post subject: Re: Z11 field supply circuit protection

1. The purpose of the fuselink is to protect the wire between the main power 

bus and the circuit breaker.  If that wire is very short and installed with good 

workmanship and the chances of shorting to ground are slim, then you could 

eliminate the fuselink.

2. Yes, the field current needs to be considered.  The current depends on the 

alternator size and the alternator load.  3 amps is a guess.

3. Voltage sense current is insignificant.

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Joe Gores

ceengland7(at)gmail.com
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Posted: Mon Jul 27, 2020 6:22 pm Post subject: Z11 field supply circuit protection

On 7/27/2020 6:22 PM, draker wrote:

 	  Quote: 	
		   



 Hi, this is my first E-mail to the list. I've done some searches but came up empty for answers. I'm finalizing the electrical schematic for my RV-7 build, which is very loosely based on the AeroElectric Z11 example. Single alternator, LR-3 regulator. A few questions:



 1. Z-11 shows the field supply circuit running from the fuse block stud through a 22AWG fusable link, through the battery/alt switch, through a 5A circuit breaker, and ending up at the voltage regulator (pin 6). Since the circuit is protected by the 5A breaker, what is the purpose of the fusable link, and what are the negative consequences of omitting the fusable link?



 2. Should current drawn through the above circuit be considered for the purpose of load analysis to help me choose the right size alternator? If so, what range of currents should I expect through it?



 3. Same question for the bus voltage sense (pin 3) on the regulator. Does that draw significantly current?



 Thank you!



I had similar questions, back when I first started studying the Z drawings.
	




The key thing to remember is that fuses (links, etc) always protect the 

downstream wire from the current source. Without the link, the path from 

buss to breaker is unprotected.



Your next question (or at least mine was) is the why of the various wire 

sizes. The link is the smallest wire size that's reasonable for physical 

ruggedness. Wire steps up 4 sizes so the link can do its job. After the 

breaker, it can step back down to what the 5A breaker can protect.



The alternator ultimately must 'pay back' the energy used to drive its 

field, so, yes.



There should be negligible current on the sense wire.



Have fun,



Charlie



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johnbright

Joined: 14 Dec 2011
Posts: 165
Location: Newport News, VA

Posted: Mon Jul 27, 2020 6:53 pm Post subject: Re: Z11 field supply circuit protection

Bob speaks briefly about alternator field current in the fourth post at http://forum.matronics.com/viewtopic.php?t=16775302



BTW: Are you aware the latest Z dwg revs are in the Adobe... folder at http://www.aeroelectric.com/PPS/

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_________________
John Bright, RV-6A, at FWF, O-360
Z-101 single batt dual alt SDS EM-5-F.
john_s_bright@yahoo.com, Newport News, Va
N1921R links

nuckolls.bob(at)aeroelect
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Posted: Mon Jul 27, 2020 10:28 pm Post subject: Z11 field supply circuit protection

 	  Quote: 	
		  



 The alternator ultimately must 'pay back' the energy used to drive its field, so, yes. 	


   Alternator nameplate ratings account for internal

   demands of the field winding and are not part

   of your load analysis.



   At low rpm and high loads, the field CAN be

   on the order of 3A. At cruise on a lycoming

   I've seen it as low as 1A. But it's not

   a part of your energy management concerns.



 

   Bob . . .

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draker

Joined: 30 Jun 2020
Posts: 2

Posted: Tue Jul 28, 2020 6:36 pm Post subject: Re: Z11 field supply circuit protection

Thank you, very informative answer. I erroneously assumed that a breaker or fuse protected the whole circuit, not just what's downstream from it. Rookie mistake.



About the field supply current: It makes sense that if the field current draw is accounted for by the alternator's advertised output, then you wouldn't include it in your energy management calculations.  Question is, how do you know if a particular alternator accounts for it? Is this standard alternator marketing practice? Do B&C alternators account for it? Will I get 40A out of a 40 or will I get 40A minus field current?

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user9253

Joined: 28 Mar 2008
Posts: 1913
Location: Riley TWP Michigan

Posted: Wed Jul 29, 2020 5:52 am Post subject: Re: Z11 field supply circuit protection

An alternator rated output is a nominal value, not an exact value.  As the load

is increased, the alternator will increase its output to supply the demand.  If 

the load demands more than the alternator's rated output, the alternator 

might still supply the current, but the voltage will start to sag as the load gets 

larger.  When the alternator voltage sags down to battery voltage, then the 

battery will also supply current to the load.

 Modern avionics and LED lights do not draw much current.  My RV-12 needs 

less than 10 amps.  Of course heated pitot tubes and electric seats use more.

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Joe Gores

nuckolls.bob(at)aeroelect
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Posted: Wed Jul 29, 2020 12:41 pm Post subject: Z11 field supply circuit protection

At 09:36 PM 7/28/2020, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "draker" <ryan-ae(at)existens.org>



 Thank you, very informative answer. I erroneously assumed that a breaker or fuse protected the whole circuit, not just what's downstream from it. Rookie mistake.



 About the field supply current: It makes sense that if the field

 current draw is accounted for by the alternator's advertised

 output, then you wouldn't include it in your energy management

 calculations. 	


  Consider the internally regulated alternator

  wherein field current is supplied INTERNALLY.

  The nameplate rating would be that current

  available at the b-terminal irrespective of

  that which is consumed internally. So bringing

  the field lead out to accommodate an external

  regulator doesn't change the overall energy

  picture.



  	  Quote: 	
		  Question is, how do you know if a particular alternator

 accounts for it? Is this standard alternator marketing practice?

  Do B&C alternators account for it? Will I get 40A out of a 40

 or will I get 40A minus field current? 	


  Standard practice. You're guaranteed the nameplate

  rating under worst case operations (lowest rpm for

  full output combined with max rated internal temperatures). 



 

 

   Bob . . .

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