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amps drawn by 6volt starter on 12v conversion

E

e-manhunt

Member
Anyone have any idea how many amps my 6 volt starter draws on my 1940 M? I believe the starter is original, but tractor was converted to 12v. thanks.
 
If it is 50 degrees out about 375 to 400. If zero, near 600. A cheap 18$ meter that is placed against the side of the bat cable will give instant readings well enough to diagnose issues. The 6v starters last as long on 12v as they do on 6. They draw more amps for far less time. JimN
 
Assuming the cranking load remains the same for both six volts or twelve volts, the amperage draw on twelve volts should be about one half of the draw for six volts.
 
With respect, the 12v starter has more resistance to electricity than the 6V (about twice the Ohms (small though they are in number)But the 12v bat on a 6v starter will force more current through it and spin the enging substantially faster. This "faster" comes from amps, as they are series wound motors and go as fast as they can for the current driven through them. JimN
 
Theoretically a 6 volt starter's current draw will double when supplied 12 volts.

However actual 12 volt current will be considerably less on account of the increased starter RPM on 12 volts. (The faster a DC motor spins the LESS current it draws.)
 
True except it is spinning the engine faster and that is work and work is ampsX volts. If the six volt starter was driven to turn the engine over the same speed as the 12 volt battery spins it it would need that exact same 12 volts to do it. Thus the 12 volts puts considerably more current through the windings. Not twice, but much more. JimN
 

Ah Ohm's Law, I remember it well. It describes the relationship between Volts, Amps, and resistance. V = IR for you college types :)

In this case the resistance is the starter motor, which is R in the equation.

Most people will tell you that the Amps are cut by 1/2 when you go to 12V. That is incorrect. In reality the Amps DOUBLE.

It's not "the same power." It's actually FOUR times as much power. That's why the engine spins over faster.
 
A strict application of Ohm's Law only works on a resistive load like a light bulb. In a starter which is an inductive load you must deal with counter electromotive force or impedance. The starter is really both a starter and a generator at the same time. The starter pulls maximmum current when stalled and less when it spins because of the counter voltage being generated.
 
I was just thinking that you have many 'theories' here (that make your head hurt), but no one has quoted actual meter readings. Spend the $18. :roll:
 
Dear JMOR and others,
Many years ago I checked the amp draw of a 12 volt system on an engine mounted on an engine stand with a Sun VAT-40. The draw was 185 amps. Then I substituted a 6 volt battery and the draw was 320 amps, with the engine turning more slowly. The same phenomenon will occur with a weak but not too weak battery. As the starter approaches stall, current flow rises as voltage drops up to a point. This apparent violation of OHM'S law is due to the fact that the starter is both a starter and a generator and when spinning rapidly produces a counter voltage reducing current flow. However, if we were to conduct this experiment with a locked engine, we would have a resistive rather than inductive load and OHM'S Law would be obeyed.
 
(quoted from post at 15:19:33 12/08/08) Dear JMOR and others,
Many years ago I checked the amp draw of a 12 volt system on an engine mounted on an engine stand with a Sun VAT-40. The draw was 185 amps. Then I substituted a 6 volt battery and the draw was 320 amps, with the engine turning more slowly. The same phenomenon will occur with a weak but not too weak battery. As the starter approaches stall, current flow rises as voltage drops up to a point. This apparent violation of OHM'S law is due to the fact that the starter is both a starter and a generator and when spinning rapidly produces a counter voltage reducing current flow. However, if we were to conduct this experiment with a locked engine, we would have a resistive rather than inductive load and OHM'S Law would be obeyed.
Click to expand...
Makes perfectly good sense to me.
Thanks Tom43.
 
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