The Ford Barn

Pills · 02-22-2020, 12:42 PM

Contrary to popular belief, the Model A 3 brush generator develops constant current; not constant voltage. The primary control of voltage is the resistance of the entire charging circuit from the generator output, through the battery, to the frame. If you have high voltage at the cutout output terminal, above 8+ volts at 8 amps, check the voltage drop from there to the battery negative post. It should be 0.5 volts or less. If more, it is probably your wiring, so fix that.

Also check the voltage drop from your positive post to the car frame. It should be infinitesimal.

If it is not those, it is your battery resistance. You can have high battery resistance from a defective battery or an overcharged battery. Fix that and you are home free.

Now, since voltage varies directly with the charging circuit resistance, it follows that when there is a break in the charging circuit, the resistance becomes infinite and the voltage skyrockets. When that happens, the voltage breaks down the generator winding insulation and away we go.

Conversely, we are told to ground the generator output wire to the cutout if the charging circuit has broken (a scary thought). In this case, the resistance is zero therefore the voltage is zero and, without voltage, there can be no current flow. No current flow, no burned windings.

Joe K · 02-22-2020, 04:56 PM

Without a battery to hold it back, a Ford generator can put out up to 32 volts, or even a bit more.

Thus an original generator CAN be used with a 12v system - and will charge just fine.

And 12v works just fine on the starter motor. Wind-up is a bit fast and may cause problems with the bendix - but the starter will be fine.

Not critiquing "finer tuning" of a change to 12V. Many do it as it opens the door to greater capacity, better regulation, more reliability in circuits.

But - it can be done - not that big a deal.

Joe K

Purdy Swoft · 02-22-2020, 05:20 PM

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The battery controls the voltage of the system . As long as it remains positive ground no wires will need to be changed . All that will be necessary is to first install a 12 volt battery and connect it positive ground . It will be necessary to either install a resistor at the coil connection if a six volt 1.5 OHM coil is used . I prefer to use the more powerful 12 volt three OHM coil with a built in resistor for a cleaner setup that still appears original . If negative ground is needed it will first be necessary to connect the battery negative ground . The coil wires will need to be reversed and so will the wires at the ammeter . I almost forgot but if the system will be 12 volts , the bulbs will need to be changed and a resistor may be needed at the horn and electric windshield wiper motor if used .

02-22-2020, 12:42 PM	#1
Pills Junior Member Join Date: Sep 2014 Location: Rhode Island Posts: 9	On Model A Generator Voltage Contrary to popular belief, the Model A 3 brush generator develops constant current; not constant voltage. The primary control of voltage is the resistance of the entire charging circuit from the generator output, through the battery, to the frame. If you have high voltage at the cutout output terminal, above 8+ volts at 8 amps, check the voltage drop from there to the battery negative post. It should be 0.5 volts or less. If more, it is probably your wiring, so fix that. Also check the voltage drop from your positive post to the car frame. It should be infinitesimal. If it is not those, it is your battery resistance. You can have high battery resistance from a defective battery or an overcharged battery. Fix that and you are home free. Now, since voltage varies directly with the charging circuit resistance, it follows that when there is a break in the charging circuit, the resistance becomes infinite and the voltage skyrockets. When that happens, the voltage breaks down the generator winding insulation and away we go. Conversely, we are told to ground the generator output wire to the cutout if the charging circuit has broken (a scary thought). In this case, the resistance is zero therefore the voltage is zero and, without voltage, there can be no current flow. No current flow, no burned windings.

02-22-2020, 04:56 PM	#2
Joe K Senior Member Join Date: Jul 2010 Location: Cow Hampshire Posts: 4,188	Re: On Model A Generator Voltage Without a battery to hold it back, a Ford generator can put out up to 32 volts, or even a bit more. Thus an original generator CAN be used with a 12v system - and will charge just fine. And 12v works just fine on the starter motor. Wind-up is a bit fast and may cause problems with the bendix - but the starter will be fine. Not critiquing "finer tuning" of a change to 12V. Many do it as it opens the door to greater capacity, better regulation, more reliability in circuits. But - it can be done - not that big a deal. Joe K __________________ Shudda kept the horse.

02-22-2020, 05:20 PM	#3
Purdy Swoft Senior Member Join Date: May 2010 Location: Alabama Posts: 8,099	Re: On Model A Generator Voltage Sponsored Links (Register now to hide all advertisements) The battery controls the voltage of the system . As long as it remains positive ground no wires will need to be changed . All that will be necessary is to first install a 12 volt battery and connect it positive ground . It will be necessary to either install a resistor at the coil connection if a six volt 1.5 OHM coil is used . I prefer to use the more powerful 12 volt three OHM coil with a built in resistor for a cleaner setup that still appears original . If negative ground is needed it will first be necessary to connect the battery negative ground . The coil wires will need to be reversed and so will the wires at the ammeter . I almost forgot but if the system will be 12 volts , the bulbs will need to be changed and a resistor may be needed at the horn and electric windshield wiper motor if used .