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low mod 12v conversion

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karl f

02-24-2004 20:21:14

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i've heard of just changing to a 12v regulator if the rest of the charging system is intact. (and doing bulb swaps and adding the coil resistor)
do you have to maintain the pos ground polarity in this case? or is that what polarizing is for?
thanks
karl

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Gordon in IN

02-25-2004 18:48:57

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Re: low mod 12v conversion in reply to karl f, 02-24-2004 20:21:14

Just a few words (or more) on my personal experience with converting 6 volt tractor ignition systems to 12 volts. I have converted two (Allis Charmers WD's) to 12 volt systems using the (old style) Delco/GM alternators with external regulators. I kept the original 6 volt starter AND 6 VOLT COIL. I measured the voltage across the coil with the 6 volt ignition and duplicated this voltage by sizing the (coil) running resistor with the 12 volt system. I replaced the capacitor(condensor)in the distributor with a NEW 12 volt one. (I did not replace the points.) I kept the original (narrow) pulley on the alternators. (The crank pulley on the AC WD's will accept and function with a "narrow" belt.) I chose an "extra service" or "severe service" belt (like the ones used on riding lawn mowers) of the proper width to fit the alternator pulley. I replaced the bulbs in the lights with 12 volt bulbs. I did not need to adjust the regulators as they both provided the desired voltage when checked in service. One of these tractors has been in service since the early 1970's with no problems. The other has been in service since the early 1980's with only one replacement belt required (because the belt adjustment and tension was not properly maintained). I have one AC WC that was converted using the original generator. The generator was "checked out" and a new set of brushes installed. A variable resistor was installed in the circuit to the field coil and adjusted to provide the desired charging voltage output. Once this resistance was determined, the variable resistor was replaced with a resistor of like value. A running (coil) resistor was utilized as above. A new 12 volt capacitor was installed in the distributor. Replacement of bulbs was also done with the exception of the (sealed beam) head lights which were wired in "series" ot utilize the 6 volt "sealed beam" bulbs and spare 6 volt sealed beam bulbs "on hand". This conversion on this tractor has been in service since the early 1970's with only one replacement of the "original size" belt. (The belt was not replaced at the time of the 12 volt conversion.) I plan on converting another AC WC the same way this spring. Hope this information is of some help to someone, Gordon in IN

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Bob M

02-25-2004 05:31:33

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Re: low mod 12v conversion in reply to karl f, 02-24-2004 20:21:14

Karl - What you describe can be done but it's got a couple drawbacks: 1 - The generator won't charge unless the engine RPM's are run up fairly high. Means if you spend a lot of time running slow or idling the battery won't charge. 2 - Heat! When you double the system voltage, the heat generated in generator field windings heat is quadrupled. Result is hot-running generator and shorter life.

You CAN however keep a positive ground system with this approach - the generator doesn't care about ground polarity. (Most of the early 12 volt generator systems were positive ground anyway�.)

Now if your tractor is for work (not a show tractor), I recommend replacing the old generator with an alternator. A rebuilt alternator will cost about the same as a 12 volt generator regulator. And an alternator will be more reliable, and provides more charging current - especially at idle - than a generator & regulator system.

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buickanddeere

02-25-2004 06:07:20

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Re: Re: low mod 12v conversion in reply to Bob M, 02-25-2004 05:31:33

Wrong on the generator winding heating effect. heat is "power" which is I2R or voltage drop across the windings x amps. That's why we transmit power cross country at 500,000V instead of 4160V. Also why electrical equipment is better wired in at 240/480/600 instead of 120 or 208. The identical generator windings will heat the same at 20 amp load no matter if the output voltage is 7.2,14.4 or 28.8V . Most 6V generator cores are identical to the 12V units. A wee bit more rpm is all that�s required. The main problems with generators not charging are the following. #1 A cheap owner who shuts the machine off every time for the 1 minute off the seat to hook a wagon or open a gate. #2 idling around and never revving up the engine. #3 A worn pulley will not drive the generator up to speed even if the owner splurges on a new belt. #4 Most machines have never had the regulator set up with a voltmeter, just a guess by looking at the ammeter to see if it's charging "much". Modern batteries need 7.2V,14.4 or 28.8V measured at the posts at full rated rpm and all the lights etc turned on.

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Bob M

02-25-2004 08:23:57

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Re: Re: Re: low mod 12v conversion in reply to buickanddeere , 02-25-2004 06:07:20

Hey B&D - I gotta disagree with your analysis! It is correct ONLY when resistance a circuit is increased by the same percentage as the voltage.

However in this situation the field winding resistance remains unchanged. By Ohm's law E = I*R (rearranged I = E/R), when voltage is doubled and resistance stays constant, current is doubled. Therefore since heating is given by W = I^2*R, when you double the current you QUADRUPLE the heating.

Or calculate it another way: Heating is also given by W = I * E. By feeding the fields 12 volts you have doubled BOTH voltage and current. Consequently you have quadrupled the heating.

Real world experience: A couple years ago a buddy did a quick 12 volt conversion on an old Minne Mo by simply mounting a 12 volt regulator and changing the battery. It worked, but he observed the generator case became unusually hot after running a while. The system worked for several months, then the generator just quit. When we opened it up afterward we found the field windings' insulation (lacquer and tape) charred and burned - obviously from prolonged overheating.

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Bob

02-25-2004 09:09:26

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Re: Re: Re: Re: low mod 12v conversion in reply to Bob M, 02-25-2004 08:23:57

Bob M,

I have to agree with you on the field coil heating. B & D shot from the hip on this one!

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buickanddeere

02-25-2004 12:07:55

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Re: Re: Re: Re: Re: low mod 12v conversion in reply to Bob, 02-25-2004 09:09:26

Not a simple situation and we are working from here say evidence indirectly from the non electrically trained person who needs something to blame other than his own handiwork. The main winding heating will be unaffected at the current has not changed nor has the resistance of the main windings. The same I2R heating will occur no matter if the output is 6,12 or 24V. As for the field winding heating that involves several factors. The situation was an old generator that was how close to smoking anyways? How full of dirt and dust was it? And it wasn't speeded up as I suggested. This electrical system wasn't working properly when on 6V either. It's possible the field windings were shorting then and not developing enough magnetic flux to excite for 6V operation either. Voltage output is directly proportional to rpms. The field contacts were likely closed 100% duty cycle which they are not designed to operate even on 6V. More rpm would have prevented that. It's possible the "new" regulation was not properly adjusted by the home tinker who didn't use a voltmeter. It's possible the contacts became welded closed due to being under rated with the 2x field current at 12V. Or the spring was too tight and they couldn't open. Heating of the field windings was increased by a factor of 4 when the regulator was calling for more voltage. Had the installation been engineered, new condition matched components and some rpm. It would still be in service. Reminds me of the stories how some people blame young Billy Bob for blowing up Grandpa's tractor/truck/ etc engine that had sat in the shed for 30 years.Nobody seems to remember it was parked because it was on it's last legs or had quit.

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Bob

02-25-2004 20:40:14

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Re: Re: Re: Re: Re: Re: low mod 12v conversion in reply to buickanddeere, 02-25-2004 12:07:55

Why not just admit trying to get 12 Volts out of a 6 Volt generator is gonna stress the field windings? And who are you calling non electrically trained???

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Bob M

02-25-2004 19:05:20

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Re: Re: Re: Re: Re: Re: low mod 12v conversion in reply to buickanddeere, 02-25-2004 12:07:55

B&D - All of the scenarios you lay out above are valid, and any one could indeed account for a generator failure. However your statement here summarizes the whole sitation very nicely: "Had the installation been engineered, new condition matched components and some rpm. It would still be in service."

Therefore expecting a generator to work well and live long while controlled by a non-matched regulator to deliver double it's engineered voltage is an uncertain situation at best.

(Incidentally when we opened the toasted generator on my buddy's MM we found it's insides to be in excellent shape - even the brushes and commutator. Aside from the burned field winding insulation it looked nearly new...)

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buickanddeere

02-26-2004 04:35:37

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Re: Re: Re: Re: Re: Re: Re: low mod 12v conversion in reply to Bob M, 02-25-2004 19:05:20

Something I remembered to mention 2 seconds after posting is that 12V on a 6V field that wasn't already burned wouldn't particularly stress it. The strength of the magnetic field develped would also be doubled with the 12V instead of 6V. This would also increase the generator output voltage. This would reduce the duty cycle time and heating concerns. The soft iron cores are pretty large and the field windings modest. The excitation wasn't being driven into saturation. So if the 6V generator was running at more than just the break even rpm to make 14.4V and the field wasn't enegergized 100%. The smoke shouldn't have got out of the windings. The person doing the origional 6V to 12V conversion wasn't a pro. There was a 6V system that wasn't working and they threw a 12V battery and regulator into the mix and have come up with the conclusion that the conversion ruined the generator. Sorry, it was already toast before the tinkering started. Install a new field, a regulator that doesn't weld closed and enough rpm the field is energized only 30-70% of the time and it will be fine.

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