Posted by Bob M on December 09, 2008 at 08:34:53 from (151.190.254.108):
In Reply to: Yo Jim Bob M posted by John T on December 09, 2008 at 07:23:09:
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...)
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