[MikeK]

Originally Posted by steve s

Mike ,
I feel like we're speaking two different but related languages: you, engineering and me, physics (although perhaps somewhat out of date). Perhaps you could answer a question to see if we have some common ground.
Do we agree that Ohm's Law, V=IR, cannot reliably used for circuits with inductance or capacitance elements?

Steve, Good grief, kind of a loaded question. Even what appears to be a simple purely resistive circuit, like a battery, two wires, and a light bulb will have some, albeit near immeasurable elements of reactance/inductance and capacitance. That said, Ohm’s law is never an absolute.

Anything that functions on the relationship between electricity and magnetism, like any motor, will be highly inductive and, while running will not simply follow Ohm’s law. Case in point: The starter motor in this discussion. If it draws 125A @ 6V while running at cranking speed it WILL NOT draw twice that @12 while running. For most parallel dual field DC series motors that number will be about 1.4X the amperage if you double the voltage. Now, does this mean you will not get 4X the power? Yes and no. While running, no. Stalled, yes. At that point it will draw near 2X the amperage. The motor will transition from 4X “power” (actually ~3.8X if you want to set up a dyno on a test bench for the starter in question) to about 2X as the inductive reactance changes in a non-linear relationship with armature rotational velocity. The near-stall speed 4X (OK, ~3.8X) is what imparts the increased momentum energy in the armature/Bendix that creates the nasty ‘crash’ engagement.

On anything I’ve posted on Model A forums I’ve always tried to keep things in perspective of the audience. Not too many engineers here, so I have rarely delved into application of the work of Henry, Farad, Steinmetz, etc. . . for electrical problems. Even if I did, this forum lacks a math editor/font, and if I started posting anything with calc or functions derived from the work of anyone beyond Volta, Ampere, and Ohm I’d be talking to an audience of one (You!). I spent too many years scribbling that kind of stuff on blackboards in front of wannabe EE’s and ME’s with blank looks.