Re: Re: Will 6 volt generator work on 12 volt, part 2

Posted by Bob M on August 28, 2001 at 12:02:00 from (165.170.128.66):

In Reply to: Re: Will 6 volt generator work on 12 volt, part 2 posted by Burl on August 28, 2001 at 03:56:20:

Dick - Well thought-out and written post! It squares pretty well with my experience with 6 volt 3-brush generators. Here's couple points to add to your comments:

1 - The cut-in RPM (speed where the cutout relay pulls in and the generator begins charging the battery) will be higher when running a 6 volt generator on 12 volts. I found the engine has gotta be running at around half max RPM before it'll cut in and charge. Likewise the cutout RPM as the motor slows back down will be higher. What's this mean? If your tractor is gonna run mostly at idle or low speeds the generator won't ever cut and and won't keep a 12 volt battery up. However if most of the time is spent at or near full RPM, it won't be a problem.

2 - The heat developed in generator windings varies with the square of the system voltage. This means both the fields and armature of a 6 volt are gonna throw off 4 times the design heat when operated at 12 volts. Bear in mind the 6 volt generator is "closed" (there's no cooling air flow thru it), it has no cooling fan, and it's positioned in a hot location high up under the hood on H's and M's. So overheating the generator on 12 volts becomes a possibility. Now the generator is significantly overdesigned mechanically as you observed. Consequently for a show tractor, or a tractor used only infrequently and for short periods, overheating probably will not be a concern. But if the tractor is to be run continuously at high RPM in high ambient temperature, and especially if you run a continuous high current load (i.e. lights) you may see generator life shortened as a result (heat damage to insulation, dried out bearings, etc).

Also……your statement about doubling of the generator shaft work on 12 volts is correct (power = volts x amps; doubling the volts doubles the shaft work). But that's not likely to have any significant effect on generator life and the generator is substantially over-designed anyway. And the maximum power on 12 volts I figure is a still paltry 170 watts or so (less than 1/4 HP) - it's unlikely to have any effect whatsoever on the life of the shaft, bearings, etc.

Bottom line is a 6 volt, 3-brush generator can be made to work on a 12 volt system. Just keep in mind there's performance compromises and limitations that will result. ...Bob M

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