Re: 6 volt starter with 12 volt system

Posted by Bob M on January 17, 2005 at 11:38:38 from (151.190.254.108):

In Reply to: Re: 6 volt starter with 12 volt system posted by lee on January 17, 2005 at 10:19:49:

Lee � your analysis is good as far as it goes. However it neglects one key element, a phenomenon in DC motors known as �back EMF�.

Very simply back EMF is a voltage generated by the armature spinning in the magnetic field and which opposes the system voltage. Back EMF will vary from 0 volts (armature not turning) to very near supply voltage (armature spinning freely with no load).

This means that your equation is correct for predicting the initial inrush current, i.e. when you first hit the starter switch and the armature just starts to turn. However once the armature gets spinning on 12 volts, the back EMF for a 6 volt starter may be as high 6 or 7 volts. Therefore once it gets the motor tipping over, your 6 volt starter may actually draw LESS current on 12 volts than it did on 6 (yeah I know it sounds counterintuitive....)

Granted the higher inrush current on 12 volts is harder on brushes, commutator, etc And mechanically the higher initial torque surge is harder on the Bendix drive (that �bang� you may hear when you hit the button). But this is somewhat offset by the reduced current once the starter is spinning, also by the fact the engine often starts more quickly so starter isn�t required to grind on like it did on 6 volts.

Bottom line � so long as the starter spins the motor, it won�t burn out any faster on 12 volts than it will on 6, and may in fact last longer. Case in point: The first 12 volt alternator conversion I ever did (a �56 S-180 farm truck) is still going strong and still has same 6 volt starter on it before I did the conversion � nearly 30 years ago...

Follow Ups:

• Re: 6 volt starter with 12 volt system Doug in OR 11:45:40 01/17/05 (3)
  • Re: 6 volt starter with 12 volt system Bob M 12:12:25 01/17/05 (2)
    • Re: 6 volt starter with 12 volt system Doug in OR 13:55:19 01/17/05 (1)
      • Re: 6 volt starter with 12 volt system lee 14:05:22 01/17/05 (0)

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