6 volt motor on 12 volts

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realolman

Senior Member
this is a little off track but it's all electrical, so I hope the moderators will allow it.

I bought a 1956 tractor that originally had a 6 volt system... somebody converted it to 12 v. He left the 6 v starter ...It seems to work... I'm wondering if it's possible it might work too well.

I wonder if someone would be willing to discuss the motor characteristics when running a 6 volt (series wound? ) motor on 12 v ....rpm, torque, current etc. compared to what they would be on 6 v. would they be doubled or is there more to it .

Also, the possibility of inserting a series resistor of some sort, what kind of resistor it might be, where it could be obtained, and whether or not it would be necessary.... is the motor likely to be damaged?

thanks
 

sii

Senior Member
Location
Nebraska
I wouldn't know where to begin on the logistics of this but I know my father did just that on several tractors over the years. Never had a starter issue to my knowledge and I know he left the 6V starters in.
 

dereckbc

Moderator
Staff member
Location
Plano, TX
I can tell you a little about it. Most of it will depend on how heavy a gauge of wire the motor is wound with. If suitable heavy enough it will work.

Dc series motors are strange beast in that they do not work in a way one would think. RPM is proportional to voltage. so going from 6 to 12 volts would leave you to believe the RPM would double, but it will not. Torque is proportional to current, and maximum torque and current is at 0 RPM a decays pretty much at a linear rate until max RPM is reached. So at max RPM you have 0 torques and virtually no current and 0 HP.

You do not have to worry about over RPM used as a starter unless the solenoid fails to engage. Operating a DC series wound motor the fastest way to destroy one is by not having a load on one as the RPM will quickly surpass the safe rotational speed and the motor can literally flies apart and explodes That should not be a problem used as a starter motor as the engine presents a pretty hefty load.

Only real problem is start up current like if the engine is really cold and cranks very slowly. It is possible the excess current could burn up the windings.

Most Series DC motors can easily take 200 to 500% more current than their rated for for short burst. It is common practice to boost torque and is what the electric golf cart enthusiast like myself do all the time. I have a 600 amp controller on a 4 HP 48 volt motor made for 75 amps @ 4800 RPM. My cart will pop a wheelie and squeal the tires, but top speed is only 30 Mph, but it can pull a pickup truck out of the mud or climb a tree. :grin:
 

gar

Senior Member
101107-1850 EST

Very shortly after contact closure the current will be about double. If the wire size from the battery is smaller than for the 6 V condition this will reduce the voltage at the motor a little. Also the battery internal resistance will be higher on a 12 V battery of the same physical size as the 6 V battery.

On the basis that the motor current is higher, then torque is higher, field flux is higher (both higher field flux density and higher armature current contribute to this higher torque). At higher flux density for a fixed armature voltage the the speed will be slower, but the armature voltage also goes up. I can not tell you what the balance will be. The higher torque means higher mechanical shock. Are the mechanical components good for this? I do not know. Almost certainly motor cranking will be better.

I do not know how much voltage drop occurred on a 6 V system, but it may have been on the order of 3 V, thus about 3 volts at the starter motor.

Check the voltage at the starter motor when the solenoid first closes. Try a smaller wire to the starter and see what that does. I believe 200 A may have been a value for 6 V starter motors. So 6 V at 200 A is 0.03 ohms. Fusing current of copper #6 is almost 700 A. #6 is 0.4 oms per 1000 ft. So you won't put 100 ft in the car to get this drop. Need to go to a Nichrome resistor.

See
http://www.sw-em.com/Vintage_Volvo_6V_to12V_conversion.htm
for limited comment on running a 6 V starter on a 12 V system.

Check your peak starting current and the running current as the starter is starting the engine. From this judge what you might want to do with a resistor. I think trial and error will be the easiest if you do anything.

"Direct Current Machinery" by Hempstead S. Bull, John Wiley, 1929, 1948 gives a very brief discussion on series motors.

.
 

gar

Senior Member
101107-2019 EST

A Sears Diehard 24F size has a cold cranking rating of 700 A. Crank current is the current at 7.2 V across the battery. So the internal resistance of this battery is about (12.6-7.2)/700 = 0.008 ohms.

.
 

CJE

Member
In a former life, I worked in the farm implement business. I have done this many times. I only had one problem child out of all of them. The 12 volt battery would slam the drive forward so hard that it would eventually break the nose of the starter. As I recall, after it did it two times in about 3 years, we had our local starter/generator shop convert the starter to 12 volts. Every other one was never a problem.
 

hillbilly1

Senior Member
Location
Atlanta,Ga
I have a 601 Ford that I converted from a six volt positive ground system to a 12 volt negative ground system 20 years ago, I am still running the original starter with no problems, I did change the starter solenoid to 12 volt, as well adding a dropping resistor to the ignition coil.(the old six volt coil will not last long at all at 12 volts!) I also have a 1951 Case that my grandpa bought new that has been converted. (this is a very common conversion)
 

BJ Conner

Senior Member
Location
97006
Eight volt batteries for cars were pretty common. I had a Chevy pickup that had an 8 volt battery. Lots of theoritical problems but noe of them oever showed up.
I have seen cars ( both were pre 1950s Fords) with two six volt batteries in series. 12 to run the starter ( 50s GM engines) , 6 for every thing else. It seems to me that just convering the who car to 12 volts would have been a better solution.
 

realolman

Senior Member
101107-1850 EST

The higher torque means higher mechanical shock. Are the mechanical components good for this? I do not know.

.
...this is what concerns me most.


Thanks very much everyone... I knew I'd get good stuff here. :)
 

dicklaxt

Senior Member
Those older vehicles/tractors had a higher voltage starting/charging systems with an accessory tap to 6volts if I remember correctly but don't remember any of the pros and cons for the 6 volt accessories unless it was just a precusor to better technology:)

dick
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
What did you do about the polarity swap and the cranking direction?
The rotation stays the same with these motors. You are reversing both the armature and the field at the same time. To change rotation you have to reverse one with respect to the other.
 

broadgage

Senior Member
Location
London, England
Those older vehicles/tractors had a higher voltage starting/charging systems with an accessory tap to 6volts if I remember correctly but don't remember any of the pros and cons for the 6 volt accessories unless it was just a precusor to better technology:)

dick
6 volt lamps were said to be more durable than 12 volt, esp if subject to a lot of vibration as on a truck or tractor.
The 6 volt lamps have much shorter and thicker filaments than 12 volt ones.
 

Open Neutral

Senior Member
I bought a 1956 tractor that originally had a 6 volt system... somebody converted it to 12 v. He left the 6 v starter
It will likely work just fine, as gar and others have said. BTDTGTTS. Worst case, use a smaller cable.

You could find a small integral regulator alternator & install it; a 30-35 amp one is more than sufficient.
 

hurk27

Senior Member
Re-built many an old John Deer double pumper and switching the system from 6 to 12, I always left the 6 volt starter, changed from a generator to a 1973-1976 GM alternator with built in regulator, lights were changed to 12. some had magneto ignition but the newer versions had 6 volt coils so they had to be changed, all in all those 6 volt starters held up for years at 12 volt, one guy did have his rewound for 12 volts, and he said, it cranked so slow, it was hard to start, so he put a 6 volt back on it.

Some had compression release so you could spin it up by hand then apply compression and let the fly wheel do its thing, worked with an almost dead battery also, there are still allot of these old John Deers around here.
 

roger

Moderator
Staff member
Location
Fl
Occupation
Electrician
I have a 601 Ford that I converted from a six volt positive ground system to a 12 volt negative ground system 20 years ago, I am still running the original starter with no problems, I did change the starter solenoid to 12 volt, as well adding a dropping resistor to the ignition coil.(the old six volt coil will not last long at all at 12 volts!) I also have a 1951 Case that my grandpa bought new that has been converted. (this is a very common conversion)
I have a model 640 Ford and did the exact same conversion about 15 years ago with no problems to date. I used a piece of kendorf to fabricate the alternator support

Roger
 

dhalleron

Senior Member
Location
Louisville, KY
My dad did that on a truck more than 40 years ago. He said the engine turned over faster and it started better in the winter. It only runs for a second or two anyway. If something doesn't break from the extra kick, then the current shouldn't overheat much of anything.
 
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