Yo Jim Bob M

Yo, Jimbo n the Bobster, I got late on the post regarding the starters current draw if 12 verus 6 volts was applied, so have some questions for yall.........

1) Fer as I know the maximum curretn draw is at locked rotor I think yall agree with that....???

2) If the load was pure resisitve and there werent any inductive and magnetic field considerations at 12 volts there would be exactly twice the current flow.. Ohm Law DUHHHHH agree ??

3) Hoewever, due to inductance and magnetic field effects, at 12 volts it wont draw quite twice the current (of course the load affects all this) Yall agree??????????????

SOOOOOOOOOOOOO the big theoretical question is WHY NOTTTTTTTTTTTTTT

Isnt the reason the current decreases as RPM increases due to the magnetic field generated which creates the so called BACK EMF and in affect that sort of a self generating process helps the motor action along because the otherwise necessary external battery current to create the electromagnetic field is reduced due to the fact the spinning of the armature inside the field is creating some of its own magnetic field (its also acting as a DC generator besides just a DC motor) ....... Sooooo since the 12 volts makes the starter crank faster theres even more of that self help action (faster the spin the more magnetic field strength created) which is the reason a 12 volt FASTER SPINNING starter wont really draw quite twice as much battery current as the slower spinning 6 volt starter.....

WHATCHA THINK????????????????

Now if you get into WORK and Energy (volts x amps x time) and instantaneous power (volts x amps) I have to put on my thinkin cap on unless one of you gents want to give us a refresher course......

Gotta work on the RV more today, gettin ready to head south for the winter, there will be a test on the above so I insisit that you both exercise due diligence and all due dispatch in responding to my above inquiry lol

John T Tricity man

Hey John T - I agree 100% with your analysis!!

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Comments to your questions:

#1 (max current occurs under locked rotor conditions). Yes!

#2 (pure resistive load, current drawn on12 volts vs 6). Yes again. Supplying 12 volts to a 6 volt starter results in exactly double the current. But only UNDER LOCKED ROTOR CONDITIONS. (read on...)

#3 (on 12 volts it wont draw quite twice the current). Only at the instant the starter switch is closed is current doubled. Reason: As the armature accelerates it generates increasing "back EMF" in opposition to the supply voltage. The faster it spins the greater the back EMF and therefore the LOWER the current draw. And since the starter spins MUCH faster on 12 volts (thus creating much higher back EMF) the current drawn while spinning will be much less than doubled.

Also consider this: A 12 volt battery has approximately twice the internal resistance of an equivalent capacity 6 volt battery. This furthers limit the maximum current drawn by a 6 volt starter to less than the theoretical 2x.

Bob M (mechanical engineer moonlighting as an EE...)

Thanks Bob, I think now my analysis isnt quite right BUTTTTTTTTTTTTTT it yields the same end affect i.e. curent draw decreases as RPM increases due to that back EMF butttttttt what you say (and I believe is more correct then mine) isnt quite how I had it pictured ........

The back emf is in opposition to that from the battery so the starter sees less voltage therefore draws less E/R current yayyyyyyyyyyyy

I will be glad to hear Jimbos analysis

John T Electrical Engineer acting like I was a mechanical lol You mechanical gents are handy to have around now n then. Be sure n bring your "first wife" to the New York Expo next summer so we can have a little get together maybe let them buy us dinner??????????

I believe the magnetic field comes from the voltage rather than the current although it takes both. To prove my idea use a small 12 volt lawn mower battery that has less cranking amps than a 6 volt and when you hit the starter it will turn faster but won't turn as long.

John T - You can demonstrate the effect of current draw vs starter RPM on a slow-cranking tractor like a JD A or G. Simply turn on the lights then hit the starter. You will see the lights alternately brighten and dim in time with the starter speed variations. (Or with the tractor cranking engage the clutch with the tractor in gear. The lights will nearly go out when the starter stops turning....at least until smoke escapes from the starter...)

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I'm planning to attend the 2009 Expo. Only kicker is my last unmarried daughter is getting married the following weekend. So it's uncertain whether the "Divine Mrs M" will be joining me at Canandaigua for this show.

Checking in on the issue!!!
John as usual you are on the road of reality.
Another factor (way of "seeing"): The locked rotor gives the entire motor time to be saturated with current and magnetism. A running motor goes from .5rpm to several thousand. At several thousand, it is substantially not saturated, and this increases the magnetic reluctance in the windings, laminations, and wires. It is for sure running on AC inside, just ask the wires. Thus serious increase in aparent resistance. Jim