Will 6 volt generator work on 12 volt, part 2

Posted by dick on August 27, 2001 at 19:43:59 from (63.120.111.98):

I expect some others besides me noticed the great divergence of opinions in the discussion stimulated by Michael about 2 pages ago (follow the link). I don't have any great stake in this either way, but I do have a tractor which was converted to 12 volt which is still using the original generator, so I was curious. The following is what I was able to glean from a variety of sources, but mainly from the 4th edition of the Electrical Engineers' Handbook by Pender and Del Mar (1949), written back when generators were the primary means of charging automotive batteries and such. Any errors of interpretation are my own, but I think this is interesting and (maybe) important enough that I ought to take a crack at indicating how I think it works and have you other folks tell me where I'm mistaken.

Turns out that 3-brush generators were designed to be constant CURRENT devices over a wide speed range (in some as low as 600 RPM). This was done by pulling the field current and the output (to the battery) current from the same electic field so that as the output current tends to go up with increased speed, the field current (which is producing the electric field) goes down, providing negative feedback, and the system is self-limiting. So much for current, what about voltage?

According to Pender and Del Mar, one of the ways to damage one of these generators is to put a significant resistor in between its output terminal and the battery. The voltage at the output terminal goes up to maintain the generator's designed output current, and the amount of current through the field winding also increases nearly proportionately because of the increased voltage. The increased current in the field increases the electric field, which supports the increased output voltage.

Limits on this are the amount of heat the generator is able to handle (especially in its field windings where the heat produced scales according to the square of the voltage divided by the resistance), how much load the shaft is able to handle (doubling the output voltage would roughly double the shaft work, voltage times current), or something else I haven't identified, which could be any of a number of things like limitations on electic field strength, increased resistance in the field windings due to higher temperature, etc. Bottom line appears to be: Keep the output voltage increase within reasonable bounds and you get away with it, push it too far and something breaks.

The voltage regulator or cut-out prevents the generator from burning out (or blowing up the battery, etc.) by switching the field current on and off so that the average output current is what's needed. Obviously this has to be changed if the system's voltage is changed.

So my present conclusion is that there is no inherent limitation (other then what the field windings and shaft, etc. will tolerate) to doubling the output from these generators. Most of these old 6-volt generators evidently were overdesigned enough that they'll work in a 12-volt system, at least for a long time. Kind of like the starters, where I guess we've all pretty much agreed a 6-volt one is OK in a 12-volt system. Since a generator is kind of like a motor (starter) run in reverse (mechanical work in, electricity out) rather than vice-versa, this makes a lot of sense to me at this point.

I'll be interested in other opinions.

• Original discussion

Follow Ups:

• Re: Will 6 volt generator work on 12 volt, part 2 Burl 03:56:20 08/28/01 (2)
  • Re: Re: Will 6 volt generator work on 12 volt, part 2 Bob M 12:02:00 08/28/01 (0)
  • Re: Re: Will 6 volt generator work on 12 volt, part 2 ralph 08:42:40 08/28/01 (0)
• Re: Will 6 volt generator work on 12 volt, part 2 Good post Dick 01:58:43 08/28/01 (0)
• Re: Will 6 volt generator work on 12 volt, part 2 gene b 20:02:07 08/27/01 (2)
  • Re: Re: Will 6 volt generator work on 12 volt, part 2 John Blanc 04:07:35 08/28/01 (1)
    • Re: Re: Re: Will 6 volt generator work on 12 volt, part 2 gene b 20:47:45 08/28/01 (0)

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