Scared Me 3 Times Until I Used My Head

[George House]

My generator on the ‘26 roadster quit charging. Nothing showing on the ammeter. Then I tried the ‘ice pick light’ with grounding clip to the frame and at the cutout to generator connection. Nothing. Knowing that I have a Ken Kopsky rebuilt generator (12-15 years ago), I removed the generator dust cover to see the 5 amp fuse. Before I removed it I knew I had to unfasten the hot wire to cutout and wrap it with electrical tape. Now wrapping needle nose pliers with electrical tape so as not to damage the plastic fuse if it was good, I proceeded to remove the fuse. It was shot. After a quick trip to town to secure 2 fuses (to have an extra) I easily installed it. Now to reconnect the hot wire. This is where the cussin starts. THREE TIMES I touched the screwdriver shank to the cutout cover causing a spark and losing that tiny screw and lock washer !! Finally got smart and disconnected the yellow hot wire at the firewall and easily connected to the cutout. Lesson Learned ;o)

[Art M]

George,
Your 5 amp fuse in your generator is interesting to me. Could you describe where it is connected. Does it reduce the chances of the generator burning up if the battery connection to the generator is lost.

Art Mirtes

[John Codman]

A good practice to follow is before performing any electrical work, disconnect a battery cable.

[Moxie26]

A 20 amp fuse, as pictured here, was installed in the generator by some as protection to your system against stuck original style cut out points.

[George House]

Art, I’m sorry but Ken Kopsky quit ‘customizing’ generators with fuses but said they protect the field coils from burning up. I have 2 more generators I’d like to have fuses incorporated. Ken did relay some pictures to me. Lets see if these help:

[Ron Patterson]

The fuse that can sometimes be found inside the Model T Ford generator case as shown in Ken Kopsky's photos IS NOT 25 Amp and it IS NOT to protect the generator from stuck cutout points.
It should be a 4 or 5 Ampere fuse and it is installed to protect the generator in the case of the battery is disconnected from the charging circuitry allowing the generator to destroy itself.
Ron Patterson

[RVA23T]

Something, that has yet to be identified, caused the fuse to blow in the first place, before the pliers got to the new fuses.

[Art M]

Ron Patterson,
Thanks for the explanation. That is exactly what I was looking for. I might add one to my spare generator. My other generator burned up about 5 years ago because of a loose connection

Art Mirtes

[Art M]

George House,
Thanks for the pictures.

Art Mirtes

[Ron Patterson]

Art
When the internal generator fuse blows the third brush regulation field circuit is opened fully preventing the generator from producing armature current.
Ron Patterson

[Moxie26]

Mr. Patterson... The fuse installed is a protective unit for the generator, but what causes that fuse to disconnect? ..,...or "blow" as you said in your posting .

[Ron Patterson]

This a an area of the Model T charging system that is commonly not understood.
To understand this fuse and what it protects you first have to understand how the third brush regulation generator works.
I wrote an article a few years ago describing normal generator operation, how the "building up" process works. See attachement.

MTFCA Charging Article-final.pdf

(267.19 KiB) Downloaded 69 times

If no battery is connected the generator build-up process continues uncontroled causing the generator voltage to rise to over 25 volts and a the 15-20 amps current produced in the armature (looking for a battery) but has no where to go. The armature and field windings get very hot very fast. So hot as to melt the solder retaining the multiple armature winding ends in the commutator (ever seen the solder thrown in a circle inside the generator case?) or burning out (burnt insulation on each field winding) one of the four series wound field windings. Either will cause the generator to quit charging and now requires a complete rebuild to the tune of $400-$500.
The fuse we are discussing is commonly installed in series with the wire from the third brush to the 4th field winding (see attached diagram).
If the battery becomes disconnected while the generator is operating the fuse will almost immediately go open disconnecting the field circuit feeding voltage to the four poles. At that point the generator will cease to produce current therby saving it from disaster indicated above.
Hope this helps

[Ron Patterson]

Why the Model T Ford generator destroys itself when the battery is disconnected.
I described the Model T generator “building up” process in the article entitled “Model T Ford Battery Charging System” in the technical section of the” Model T Ford Fix” website.
When the (Reverse Current Relay type) cutout fails to operate or there is otherwise no battery connected to the battery the armature and third brush” building up” process continues uncontrollably. The armature voltage increases to 30+ volts and there is no load on the armature output to pull off the significant current being produced. A great amount of heat is produced in the armature and field windings and it becomes a race to see which fails first stopping the building up process. The field windings can simply burn out or the solder in the commutator segments where the winding connects melts and is found thrown around the inside of the generator case. In either case the feedback loop fails and the generator ceases to function.
This is one of the biggest downside risks of the third brush regulation generator design.

[Moxie26]

Thank you Ronald.