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Posted by D Sherman [72.47.153.24] on Thursday, January 26, 2012 at 12:11:49 :
In Reply to: voltage reduction posted by Dave K. Royal [75.81.249.81] on Wednesday, January 25, 2012 at 21:17:27 :
Sometimes the best way to figure out how to run a 6 volt motor off of 12 volts is to do just what you did -- try various resistor combinations until it seems to work reasonably well. It's hard to calculate just how much current a motor will draw under various load conditions. I don't know about vacuum boosters, but I know windshield wipers vary all over the place depending not just on what's on the windshield but where the wiper is in its cycle. Small motors are usually wound so that they try to maintain constant speed by drawing more current when they're loaded down more. The trouble with using dropping resistors is that when the motor tries to draw more current, the dropping resistors drop more voltage (like a valve in a pipe turned part-way closed) and the motor slows down more.
Whenever you use dropping resistors, motor performance will suffer compared to having a "stiff" voltage source like a 6 volt battery or a true voltage regulator. How big a deal that is depends on the application. For a heater fan, it's no problem at all. For a windshield wiper, in my experience simple dropping resistors render the wiper useless. I suspect a vacuum pump is somewhere in between, but probably just fine with the right value resistor. You can check the voltage across the motor terminals and see how much it varies as it builds up vacuum. The danger is that you could be giving it too much voltage which could eventually burn out the motor. If you don't have a meter, at least feel the motor and the resistors after a long period of operation and make sure they're not very hot.
16 amps is a lot of amps, even at 6 volts. That's about 100 watts, which is plenty enough to get things pretty hot. If the motor really is drawing 16 amps, you've got 100 watts heating up the motor plus another 100 watts heating up the resistors.
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