[AT] Tractor wiring

charlie hill chill8 at cox.net
Thu Nov 18 11:54:58 PST 2004


Very good answer Dudley.  While I don't keep sharp on electricity I do know 
a bit about it.  I just wanted someone to make the point that resistance 
plays a part in this equation and it is not quite as simple as A x V = W. 
I would have made the point myself but then I might have had to explain it 
and everyone would have been painfully aware that I had already said more 
than I know.  LOL

Charlie
----- Original Message ----- 
From: "Dudley Rupert" <drupert at premier1.net>
To: "Antique tractor email discussion group" <at at lists.antique-tractor.com>
Sent: Thursday, November 18, 2004 5:30 AM
Subject: RE: [AT] Tractor wiring


> Charlie:
>
> I think you're right that the work (or energy) that the starter must put 
> out
> is the same whether it is 6 or 12 volt and that this work is equal to the
> voltage at the starter times the current flow through the starter.
>
> However, as Dean has stated below, the battery/starter wire does in fact
> have resistance (i.e., it is not a perfect conductor) so there will be an 
> IR
> drop (which is sometimes called voltage drop) across the wire length. 
> This
> means that the voltage at the starter is not actually the battery voltage
> but it is equal to the battery voltage minus this voltage drop.
>
> For a 6-volt starter to get the same electrical energy as a 12-volt 
> starter
> in theory requires it to get twice as much current.  So, if the
> battery/starter wire is the same size in both a 6 and 12 volt system the 
> IR
> drop across the wire to the 6 volt starter is twice that of the 12 volt
> system and thus, in reality, less than 6 volts is available at the 
> starter.
> So, the 6-volt starter actually needs more than twice as much current in
> order for it to get the required electrical energy input.
>
> Another important factor to keep in mind here is the heat loss that occurs
> across the battery/starter wire.  Heat loss is equal to the square of the
> current times the resistance.  As discussed above, the 6-volt starter 
> system
> requires more than twice as much current as does the 12-volt system and 
> thus
> the battery/starter wire in the 6-volt system is required to dissipate 
> more
> than 4 times as much heat.  Since the DC resistance of a copper wire is 
> not
> only inversely proportional to its' cross sectional area and directly
> proportional to its' length, it is also directly proportional to the 
> ambient
> temperature of the wire (at least over quite a wide temperature range). 
> So,
> as the resistance of the battery/starter wire goes up (due to heat) so 
> does
> the IR (voltage) drop across it resulting in even less voltage at the
> starter thus requiring more current at the starter in order for it to get
> the required input energy and ...  You get the point.  In summary I would
> guess that for batter/starter wire sizes in the #1, #2, #4 & #6 gauge 
> range
> one would have to go up at least one and maybe two wire sizes for a 6 volt
> starter system in order to keep the heat loss and voltage drop to values 
> no
> greater than that of a 12 volt starting system.
>
> A long ramble ... hope it adds to the discussion.
>
> Dudley -
> Snohomish, Washington
>
>
> -----Original Message-----
> From: at-bounces at lists.antique-tractor.com
> [mailto:at-bounces at lists.antique-tractor.com]On Behalf Of Dean VP
> Sent: Wednesday, November 17, 2004 8:30 PM
> To: 'Antique tractor email discussion group'
> Subject: RE: [AT] Tractor wiring
>
> Charlie:
>
> It's not wattage that is the primary concern. It's IR drop. When starting
> with only 6V to begin with you want as little voltage drop as possible due
> to the high current.
>
> Dean A. Van Peursem
> Snohomish, WA 98290
>
> I'm a walking storeroom of facts..... I've just lost the key to the
> storeroom door
>
>
> www.deerelegacy.com
>
> http://members.cox.net/classicweb/email.htm
>
>
>
> -----Original Message-----
> From: at-bounces at lists.antique-tractor.com
> [mailto:at-bounces at lists.antique-tractor.com] On Behalf Of charlie hill
> Sent: Wednesday, November 17, 2004 6:52 PM
> To: Antique tractor email discussion group
> Subject: Re: [AT] Tractor wiring
>
> Here is my question on this subject.
>
> Why are battery cables for 6 volt systems heavier than those for 12 volt
> applications?   Assuming both are powering a starter to turn a similar 
> size
> engine (similar load), why would the 6 volt cable need to be bigger.  It
> flows more amps but less voltage and should flow roughly the same wattage 
> in
>
> both applications.
>
> Charlie
>
>
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