A few days ago I was explaining this subject to someone for what seemed like the 500th time, so I took some time to sit down and write it up. I hope you like it?
How the Model T Ford generator, cutout and battery charging system works.
When the engine is stopped and the generator is in a quiescent state the four field winding pole shoes maintain an alternating N-S-N-S magnetic polarity from their last use. The weak magnetic lines of force from the pole shoes are impressed upon the generator armature windings, but no current is produced because the armature is not rotating.
When the engine is started, the generator armature begins to turn producing a small voltage in the armature windings, a portion of this voltage is picked off by the third brush and fed to the field windings increasing the pole shoe magnetism and more armature voltage is produced. This armature to field winding feedback loop continues to increase the generator output voltage to full operating level and is referred to as “building up”. The inability of the generator to “build up” is a common generator failure and has several causes. This armature-third brush –armature feedback loop also controls the output current of the generator when operating normally. To explain another way the position of the third brush controls the field winding pole shoe magnetic strength impressed on the armature windings and hence the output of the armature windings.
The armature output is wired to the generator terminal bolt where the cutout is connected. The cutout has two windings; a voltage and a current winding.
The “build up” described above continues until the voltage reaches 7.2 volts in the cutout voltage winding which operates the spring loaded relay armature closing the cutout contacts connecting the generator output to the battery. At this point the generator charging current is flowing to the battery via the cutout current winding holding the cutout armature contacts closed and charging the battery.
As long as the generator continues produces current to the battery the cutout contacts will remain closed and the ammeter will show the charging rate as set by the third brush in the generator.
As the engine is slowed down or stopped the generator charging rate decreases to a point where current is flowing from the battery to the generator reversing the current flow in the current winding which forces open the cutout contacts disconnecting the battery from the generator. This is why the cutout is called a “reverse current cutout”
This cutout connecting and disconnecting the generator to the battery process goes on continually depending upon the speed of the engine while driving. There is heavy current flow at the cutout contact points and much arcing occurs during connection and disconnection which directly affects the long term reliability of the cutout.
Thank you for taking the time to explain, Ron.
Thank you for putting it into terms I can understand. However, I do believe you will be explaining it a 501st time :-)
Thanks Ron , for explaining it again .!!!
Greetings From Holland
Great explanation and Thank You for taking the time and energy to get it to us.
I must be an oddball as I do not understand what happens when the battery is fully charged and the engine is still running at a high enough rpm to generate 7.2 volts to make the cutout close. As the battery charges, the battery voltage goes up and the charge amperage would go down. When this happens, does this cause the amperage going through the cutout winding to drop enough to open the cutout contacts?
Do not confuse the cutout cut in voltage with battery voltage.
The generator is a constant current source and when connected to the battery via the cutout current winding will continue to push current into the battery regardless of charge level.. Keep an eye on your battery water level!
The reverse current cutout is an off/on switch and does not variably regulate current to the battery. The third brush regulation generator only regulates to the extent of the third brush setting. It is very rudimentary but was the cheapest solution Ford could find.
That's the answer I was afraid of getting. Just doesn't seem right to have a device that will NOT protect either the battery OR generator while operating. I do believe, it's a little too late to change Henry's mind now.
Thanks for the additional explanation.
Excellent write up for electrical dummies like me. Now if I could only figure out how they initially load the smoke into the wires!
Your best hope is using FunProjects Voltage regulator.
Thank You for the suggestion. I was thinking the same thing.
Thanks for taking the time to write that out Ron..I needed that explained to me.
A word of explanation about the FunProjects voltage regulator is in order.
The FP VR modulates the charging output of the third brush regulation generator to the extent the battery actually NEEDS charging. Given the nature of the original Ford charging system this is the bet of all worlds making it easy on the marginal generator, and battery at the same time.
When the battery fully charged it simply spins happily on its bearings without working itself to death and boiling the hell out of the battery. .
Here is a link for the visual thinkers which explains cutout operation in more concise detail. The cutout utilizes a shunt winding and a series winding, cleverly wound, to connect and disconnect the generator output to the battery as the generator output is above or below the battery charging threshold voltage.
Here is a simple system diagram. I probably should have included this with my original post to clarify how the were connected.
The cutout has two coils, one that operates on voltage and one that operates with current flow. The voltage coil will develop a magnetism that pulls the cutout contacts closed. The current operated coil can develop a magnetism that either aides, helps the voltage coil pull the cutout contacts closed, or if the current in that coil is reversed, will push against the voltage coils pull and lets the cutout contact open.
When the Model T engine is first started the cutout contact is open, the generator is not delivering any current so the cutout current coil has no effect. As the generator builds up voltage the cutout voltage coil will pull the cutout contact closed, connecting the generator to the battery and current will flow from the generator to the battery through the cutout current coil. With current flowing in this direction the current coil will develop magnetism that will help the voltage coil hold the cutout contact closed.
When the engine slows down to stops, the cutout contact is still closed, the voltage coil is energized by the battery, through the closed cutout contact. AND now since the generator is not powered by the engine, the battery now tries to run the generator as a motor. This causes current to flow from the battery to the generator, opposite as before. This reverse current flowing in the current coil to develop magnetism that will cancel the voltage coils pull on the cutout contact and the cutout contact will open.
The key to the cutout is that the current coil can either help the voltage coil pull the cutout contact closed (current flowing from the generator to the battery) OR push against the voltage coil which allows the cutout contact to open (current flowing from the battery to the generator).
My 2˘, hope it helps, Mike
When using the Model T for tours about once a month or short trips to town from time to time, it is not so important to prevent over charging the battery. Every time you use a starter or lights you are discharging the battery and the generator will re-charge it. However, if you are going to drive all day long every day, the voltage regulator would be a good accessory so that you don't need to worry about over charging the battery. Back in the day, when people would drive all day long, they would turn on the lights to keep the battery from overcharging. That works too if one remembers to turn them off and if the needle goes to discharge when the lights are on, don't leave them on for long periods.
Ron, thanks for your explanation. It was very thorough. Unfortunately, in a few days this post will go into the archives and the only way to find it will be using Keyword search.
Ron: Is your 10.07 am post a positive ground circuit? jb
Yes it is. I didn’t catch that, but it doesn’t make any difference. With the exception of the ammeter the Model T system is not polarity sensitive. Thanks for pointing it out.