Matronics Email Lists :: View topic

Matronics Email Lists
Web Forum Interface to the Matronics Email Lists

Get Email Distribution Too! FAQ Search Memberlist Usergroups Register

Profile Log in to check your private messages Log in

math

Matronics Email Lists Forum Index -> AeroElectric-List

View previous topic :: View next topic

Author Message

jessejenks(at)hotmail.com
Guest

Posted: Fri Jan 15, 2010 3:14 pm Post subject: math

OK C I wanted to at least try to understand the math for my starter fat wire. Here's what I came up with using Bob's example of wiring a 110V 1500w heater from page 8-7 in the AEC as a model.

On the schematic from Skytec I got 200A (at) 12V which is 2400w. At 10v (battery voltage) that will be 240A. I calculated starter resistance to be .042 Ohms. The wire resistance will be .0023 Ohms for 15' of #2 wire (assuming 0 resistance from airframe ground). This gives a total circuit resistance of .0443 Ohms and a circuit current of 227.27 amps. Voltage at the starter will be 9.55V C and power at the starter will be 2 C170.43 Watts C for a power loss of 9.6% and a Voltage drop of .45 Volts. I did the same calculations for #1 wire C and the difference seems minor: .02V and 9.52W C or .6% power.

So I have two questions; Did I do that right? If so C is #2 wire acceptable C and what standard are we using for power loss in the starter wire?

That's actually 3 questions I guess.

Also C I realize this only addresses the power loss problem. Wire insulation heating is the other problem C but I assume power loss will be the more restrictive of the two.

Thanks.

Jesse



 		 	   		  Your E-mail and More On-the-Go. Get Windows Liv' target='_new'>Sign up now.  [quote][b]

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

nuckolls.bob(at)aeroelect
Guest

Posted: Fri Jan 15, 2010 5:50 pm Post subject: math

At 05:05 PM 1/15/2010, you wrote:



 	  Quote: 	
		  OK, I wanted to at least try to understand the math for my starter 

fat wire. Here's what I came up with using Bob's example of wiring a 

110V 1500w heater from page 8-7 in the AEC as a model.

On the schematic from Skytec I got 200A (at) 12V which is 2400w. At 10v 

(battery voltage) that will be 240A. I calculated starter resistance 

to be .042 Ohms. The wire resistance will be .0023 Ohms for 15' of 

#2 wire (assuming 0 resistance from airframe ground). This gives a 

total circuit resistance of .0443 Ohms and a circuit current of 

227.27 amps. Voltage at the starter will be 9.55V, and power at the 

starter will be 2,170.43 Watts, for a power loss of 9.6% and a 

Voltage drop of .45 Volts. I did the same calculations for #1 wire, 

and the difference seems minor: .02V and 9.52W, or .6% power.

So I have two questions; Did I do that right? If so, is #2 wire 

acceptable, and what standard are we using for power loss in the starter wire?


	


   Don't worry about "getting it right" by calculation

   for cranking circuits. Temperature rise is not part

   of the concern for VERY light duty cycle loads . . . i.e.

   seconds per flight cycle. Voltage drop studies at very

   high currents demand that you also consider internal

   resistance of the battery and voltage drops across

   contactors.



   This is mostly an academic exercise unless every

   system drop is accounted for.



   2AWG has be PLENTY big for cranking with rear

   mounted batteries in light airplanes for decades

   and hundreds of thousands of aircraft. The overall

   system losses due to resistances is more than

   you really want to contemplate . . . fortunately

   it's not significant in the grand scheme of

   things. 4AWG jumpers to batteries for ALL installations

   is to accommodate ease of maintenance and reduction

   of stresses on battery terminals.



   Bob . . .

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

jessejenks(at)hotmail.com
Guest

Posted: Fri Jan 15, 2010 9:06 pm Post subject: math

 	  Quote: 	
		  2AWG has be PLENTY big for cranking with rear   mounted batteries in light airplanes for decades   and hundreds of thousands of aircraft. The overall   system losses due to resistances is more than   you really want to contemplate . . . fortunately   it's not significant in the grand scheme of   things. 4AWG jumpers to batteries for ALL installations   is to accommodate ease of maintenance and reduction   of stresses on battery terminals.    Bob . . . Thanks Bob. I need to start looking in my inbox before I hit send.Jesse 	


[quote] 

 		 	   		  Hotmail: Trusted email with powerful SPAM rget='_new'>Sign up now.   	  Quote: 	
		  [b]

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

jessejenks(at)hotmail.com
Guest

Posted: Fri Jan 15, 2010 9:06 pm Post subject: math

From: jessejenks(at)hotmail.com

To: aeroelectric-list(at)matronics.com

Subject: math

Date: Fri C 15 Jan 2010 15:05:38 -0800

     .ExternalClass .ecxhmmessage P {padding:0px;} .ExternalClass body.ecxhmmessage {font-size:10pt;font-family:Verdana;}    

OK C I wanted to at least try to understand the math for my starter fat wire. Here's what I came up with using Bob's example of wiring a 110V 1500w heater from page 8-7 in the AEC as a model.

On the schematic from Skytec I got 200A (at) 12V which is 2400w. At 10v (battery voltage) that will be 240A. I calculated starter resistance to be .042 Ohms. The wire resistance will be .0023 Ohms for 15' of #2 wire (assuming 0 resistance from airframe ground). This gives a total circuit resistance of .0443 Ohms and a circuit current of 227.27 amps. Voltage at the starter will be 9.55V C and power at the starter will be 2 C170.43 Watts C for a power loss of 9.6% and a Voltage drop of .45 Volts. I did the same calculations for #1 wire C and the difference seems minor: .02V and 9.52W C or .6% power.

So I have two questions; Did I do that right? If so C is #2 wire acceptable C and what standard are we using for power loss in the starter wire?

That's actually 3 questions I guess.

Also C I realize this only addresses the power loss problem. Wire insulation heating is the other problem C but I assume power loss will be the more restrictive of the two.

Thanks.

Jesse

Thanks Bob C I already see that I made some poor assumptions and omissions in my math attempt. I forgot to include the resistance of the battery C and I guess calculating the starter's resistance is not as simple as dividing Volts by Amps as in the case of the heater. I thought I had it figured out C but now it seems complicated again. Thanks for "individuals willing to share a career's worth of experience".

 It seems that sometimes people size the wire simply based on the Volts C current draw of the device C and the length of the wire. You give some lengthy example calculations that go deeper to include resistance of various devices in the circuit. How do i know when it's ok to "grab any wire chart and hook things up accordingly"?

 I hadn't noticed Z-32 before. Is that the standard for a rear battery? I hadn't considered using a relay. If the bulk of the ebus load is resistive C shouldn't a switch be able to handle the 10 amps that the #14 wire can cary? Wouldn't a relay just be another possible point of failure? I'll go that route if it's necessary C but I would rather keep it simple at this point.

 Ok C I'll use #2 wire to the starter C but why #4 at the battery? That seems strange. Won't that negate the benefit of the longer piece of #2? Let me guess at your answer: Is it because you're worried about fatiguing the battery terminals and the voltage drop of those short pieces of #4 is not very significant to the overall circuit C and the temp rise is not really an issue? Is fatigue as much of an issue with more flexible welding cable?

 The picture I have in my mind though is those pieces of #4 acting like a traffic-jam to electron flow. is that incorrect?

 Jesse

  	  Quote: 	
		  [b]At 02:16 PM 1/14/2010 C you wrote:I'm trying to get to the point where I can actually run some wires so I feel like I'm getting something done while I continue to learn about and plan my electrical system. I read the section in AEC on wire sizing C but it leaves me feeling like I won't be able to figure this out on my own. I understand that care must be taken here C but in real life do I need to do a whole page of algebra for each wire? Is there a quick reference rule-of-thumb chart somewhere?    Yes C it's in the chapter on Wire in the    'Connection . . . it's also found at:http://www.aeroelectric.com/articles/wiresize.pdfCan someone give me a best guess on wire size for a 12' e-bus alternate feed wire from my rear mounted battery considering a 6 amp max load (a guess in itself at this point)?    Run a 14 AWG for this wire . . . yes C oversized    but will accommodate future changes without    having to pull new wire. Keeps voltage drop low    too. See figure Z-32.While you're at it C how about for the battery contactor wire? Is 22awg still ok for a 12' run?    yesAlso C I deduced from the chart on Skytec's website that to be safe I would use 1 gauge cable for the battery-starter connection C which appears to be about 15'. Any thoughts on this? Obviously I would rather go a little smaller if possible. I plan to use welding cable.    #2 is fine for all runs except those that    connect to battery. Drop to #4 for these    short jumpers.The last question is; can I run these 3 wires together in the same holes(fat battery C contactor C and e-bus)?    don't know why not . . . ...Bob[/b]http://www.matronics.com/Navigator?AeroElectric-List 	
 		 	   		  Your E-mail and More On-the-Go. Get Windows Live Hotmail Free. Sign up now.

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

Display posts from previous:

	Matronics Email Lists Forum Index -> AeroElectric-List	All times are GMT - 8 Hours
Page 1 of 1

Jump to:

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum