MTFCA Forum

Without getting too technical, with rear end ratios-etc, about what is the generator rpm at 20 mph?

From memory, the generator gear is a similar size to the crankshaft gear so its speed will be similar, and 20mph is around 800rpm.

On the KR Wilson test bench, the ratio is 1:1 with the electric motor. So the generator spinning speed would be about 1725 RPM 60 cycle 120 volt.
The engine RPM would be around 800 at 20 MPH (just looked at a chart last night).
Crank gear is 24 tooth, cam gear is 48 tooth, generator gear is 16 tooth. At 800 RMP the cam gear would be turning 400RPM (1/2 engine RPM), if my math is right, the generator gear would be turning 3 times as fast as the cam gear, so about 1200 RPM.

Looks like the generator needs to be well-balanced to run the car at 45-50 MPH... or maybe just leave it off the car.

I’ve had no problem with the generator turning 6000 rpm!

You might want to put a scattershield on that thing... !

Wow Les:

6000 RPM of generator if used with standard Model T rear end gearing of 3.6363... would result in the T going just under 100 MPH. I don't want a test drive to prove that out ha ha. Maybe you are running smaller tires than 3-1/2 x 30. That would be one fast T. I think I would bring spare underwear if I were the test driver. Just kidding.

A T generator turns 1/2 over crank speed.

Mark's math's had it right, Ford specs, For optimum output at 20 MPH the generator is 1200RPM.

Bob, just looking at your test set up, wouldn't the generator voltage just run wild with out having a cutout or regulator on it?

I’ll be darned if I know, I hook up an analog amp meter and set the third brush to 6-8 amps with the generator turning about 1500 rpms But, I’ll be the first to say, I know just enough to be highly dangerous when it comes to such.

You'd be better served by running your test at typical motor speed of 1725 RPM and then running the amperage down to around 4A. That's plenty.

easier on the generator and easier on the battery...

Frank, a cut out has nothing to do with how much a generator puts out, the third brush sets output.

Jeff, true, but the 3rd brush sets the amps not voltage. A 3 brush generator unregulated will just keep pumping out voltage until it has a melt down. That's why they need to be earthed out when running with out a cutout or reg. If that red wire is running to what?? on bob's tester, say to a battery without a cut out then it's trying to motor.

Kerry

cutouts do not regulate anything beyond make/break of a circuit ...they simply close/make-contact at their prescribed voltage is reached, which in the case of a 6V system, is about 7V. If the 3rd brush amperage is set crazy-high, then the voltage input to the battery will be excessive, but when limited/adjusted to 4-12A, the voltage will remain within the constraints of what the battery can handle (though at high amperage will significantly tax the battery and reduce life). When voltage output drops at lower RPM the current from the battery will buck that from the generator and the electromagnet will release it's hold on the points and they will open or "cut-out".

now, what you say about motoring is correct, when the generator output drops to less than 6V, the current will reverse, and the generator will "motor". With a clamp-on ammeter it could be easy to mistake generator Amp output for generator Amp draw at low RPM if one is not alert and careful. If it were my setup, it most certainly would have a cut-out in the circuit (and mine does), and also sufficiently heavy gauge wire on the generator-to-battery connection so as to not influence/bias amperage readings.

Scott, this I know, but it was all in relation to the way Bob's tester is showing for what can be seen in the photo. in order for any voltage or amp control a cut out or reg is needed in the testing, regardless of battery voltage.

it SHOULD be in the circuit, but when running at speed, it is not NEEDED (if the generator is working).

I am with you, in that any lack of output, or otherwise malfunction of the generator will cause troubleshooting to be muddled and difficult to trace to a cause. The fact that this is a "test setup" begs for a cut-out to be in place. Even so, (testing aside) a working generator running at higher RPM as described in previous posts, will put current INTO a battery without a cutout in the circuit. This is why when a cut-out failes or sticks "closed" during a run (like there is no cut-out in the circuit...just a straight connection), it is never detected by the ammeter, nor causes a problem until the engine idles or is shut off and it becomes most apparent that the generator is trying to "motor" and shows a discharge on the ammeter.

I found that,my generator output peaks at about 23 mph.which equates to about 900 engine rpm. Consequently the generator turns about 1350 at this speed. The generator doesn't put out any higher current beyond this point. Scott's recommendation makes the speed setting simple..
I use my drill press to spin the generator to set the third brush. I prefer about 2 to 3 Amp. This is enough to keep,the battery charged. Turn signals and stop lights on mostly country roads. And very limited night driving.
Art Mirtes

Something that I'll add relative to the cut-out discussions:

1. if you are building a test set, it really does need a cut-out in the circuit
2. the set needs a way to electrically close the cut-out independent of the generator running, so that you can "motor" the generator and measure the amperage draw while doing so...too high of a current and the generator has a mechanical bind or electrical fault which will need to be rectified before adjusting for operation in the car.

Scott
The ammeter shows 7 Amp in your picture showing the current draw of the generator.
1. Do you allow the generator to turn during this test or is the generator not allowed to turn.
2. What would the max allowable current be.
3. It seems like a simple switch would work to turn off and on.

Art, the picture was taken years ago with the generator under power, so it is misleading relative to my commentary...sorry about that. It is showing 7A output and 7V output to the battery as you can see.

as for testing the motoring current, to your questions:

1. I allow the generator to turn with no drag when measuring "motoring" current
2. I cannot recall, but will check this AM. It's been a long time since I was rebuilding generators...mostly am doing carbs now
3. you're right...the switch on the lower right in the utility box turns the 1725 motor on/off

for testing motoring current (checks for mechanical binding or electrical faults in the generator), the two switches on the panel are momentary "ON" (normally OFF). Because the meter I purchased is not a +/- meter, the switch under it reverses the leads to the meter and it is pushed and held UP. Then the other switch is pushed UP and that electrically closes the cut-out to allow the battery to back-feed to the generator. The resulting voltage shown is the draw-down on the battery, and the current measured is the current draw from the battery. It works very well and since they are sprung to be in a normal test position unless manually held, there is no way to accidentally screw up the normal output testing of the generator which is what most people would want anyway. The motoring current is just a nice way to see how well the brushes are seated and how freely the armature turns in the case with properly adjusted bearings, etc.

Now, I'll throw some opinion out: there is a point where the generator builds up and will overcome the cut-out and put a charge to the battery...there is also a point (RPM) at which the generator is not going to continue to build up. Where is that point? At what RPM? Well, depending on the condition of your generator, the condition of your battery, the condition of your wiring (do you have the wrong gauge wire going to the battery?? Many do!) the amount of charge regulated by the third brush, every combination of variables will determine "when" the generator has built it's greatest output. Do you want to guess at what RPM that threshold is for your particular set up? Not me, that's for sure. That is why I test everything at 1725 RPM at the armature. That is a convenient motor to hook up and is pretty much the generator speed that an un-modified T is going to run at on a level paved road at a mildly elevated speed. In my mind, there is no reason to set a generator for puttering speed and then unexpectedly have to get on an open road and 'let 'er rip' and have more output or stress on the device than was tested for. I test for maximum stress/speed and end up with a very reliable and functional unit.

Art

I just ran a generator for a few minutes to top up my shop 6V battery...leaving it connected to the electric motor, "motoring" the generator from the battery produced a reading of 3A and that is with the added drag of the electric motor attached. So I'd say that a generator which will motor at <3A by itself (or lower) is fine. This generator is my shop standard which I use to check the test stand prior to putting a new generator on it for adjusting. It has been rebuilt (by me) and probably has 3-4 hours of use on it over the past 6-7 years.

Hopefully I've covered all of your questions.

Scott, Very nice generator test stand. I wish I had one like that, then I could work on developing a voltage regulator replacement for the one that is no longer available. Have the circuit and simulation done but no substitute for bench testing.

Scott
The Model T Ford generator is a "third brush regulation type, but you already knew that.
It was the most inexpensive type generator available at the time and probably why Ford (actually Fred Allison in the Ford electrical engineering department who developed the FA Starting and Lighting System) they specified it. After buildup to 7.2 volts which is the desired cutout cut in voltage. It is a constant current source over a range centered on about 1725 RPM (about 27 MPH) based upon rear axle ration, the "null" point and third brush setting. It has no dynamic current regulation based upon the batteriers actual need for charge. The current output will decrease as speed significantly increases because the interacting relationship between the armature and field windings has a limit. To say this another way the output will decrease at extreme RPMs far beyond usual engine speeds. I have a better explanation of this attribute if you email me.
Here is a photo of my two generator testers now owned by Don Lang and steve Tomasso.

I thank both Scott and Ron for their explanations.
I have wondered if a 1950s generator regulator could be connected to the third brush wire and eliminate the third brush. Seems like it could be made to,work. This is a little more messy then the simple to install regulator that had been available until recently.
Art Mirtes

Art, now that’s interesting? Love to hear from some of the experts, excuse the pun, in this field. Best John

MKossor wrote: ↑

Thu Oct 21, 2021 10:35 pm

Scott, Very nice generator test stand. I wish I had one like that, then I could work on developing a voltage regulator replacement for the one that is no longer available. Have the circuit and simulation done but no substitute for bench testing.

MK_VR.JPG

Question: Why do we need a Voltage Regulator with cut-out sitting on top of the generator? Why not just a cut-out there (solenoid or diode) and a device at the battery to regulate the charging voltage, that is what the concern is about.

(?) Using a GM 3 element 6V regulator on a Model T generator:**** Leave the third brush in place, but disconnect the internally grounded end of the field coils and bring it out to an external insulated terminal on the generator case. Mark it "F". Using a 6 volt GM Delco 3 element regulator, such as one for a 1950 Chevrolet, connect the output terminal of the T generator to the ARM terminal of the GM regulator. (Don't use the T cutout.) Connect the formerly grounded end of the T field windings to the FLD terminal on the GM regulator. Connect the BATT terminal on the GM regulator to the generator wire *that leads to the ammeter *on the T. Attach a ground wire from the T engine to the GM regulator to assure an electrical ground path between them. Set the T third brush to limit the T generator's output to 10-12 amps before connecting the GM regulator. That should limit the output of the T generator to a safe maximum while allowing the GM regulator to regulate voltage and current within that limit and provide a cutout. I haven't tried this, but I think it will work. Try it at your own risk. I have never stayed at a Hampton Inn.

All a cutout can do is prevent current from flowing from the battery to the generator when the generator RPM is too low to allow generator voltage to be higher than battery voltage. It provides no regulation function beyond that.