Lights Fade with reduced RPM

[48Dodger]

Ok..so I googled it. I found this on Stoveblot's site...I think it answers my question.

"In 1957, all American car manufacturers standardized their electrical systems at 12 volt, negative ground to accommodate aftermarket manufacturers, and to take advantage of the new solid-state electronics, which did not work well in positive-ground systems. The alternator came on the scene soon after, and the old DC generator began a slow decline into obscurity."

http://www.stovebolt.com/techtips/6v_charge_system/index.htm

48D

[Merle Coggins]

OK, you were asking if there were vehicles that used a 12v generator and you found your answer.

I was mearly pointing out that both devices generate AC voltage and output DC voltage. They just go about it in different ways, the alternator's way being more efficient. So, saying that a generator is DC and an alternator is AC is just incorrect.

The generator spins coils of wire within a magnetic field. When a conductor (wire) passes through a magnetic field a current flow is induced into the conductor. The strength of the current or voltage is dependant on the strength of the magnetic field and the speed at which the conductor passes through it. The generator's armature has windings that allow multiple sections of the same conductor to pass through the magnetic field together. This also strengthens the induced current. As the winding rotates it will pass through the magnetic field in both directions. First from N-S, then from S-N. This will cause the current flow to alternate directions as the armature spins. (AC Voltage) However, as the armature spings the winding ends, which are connected to the commutator, with alternate which brush they come in contact with. So even though the current alternates within the winding it exits the generator as a direct current. The voltage regulator is connected to the field coils and when it calls for voltage the field coils are energized to make the magnetic field stronger. When sufficient voltage is obtained the field coils are de-energized and the magnetic field goes away, or minimizes. And as Tod mentioned, these have their limits and aren't as efficent.

An alternator has a rotor with an electromagnetic inside it. The rotor spins inside the stator, which has the windings like a generator's armature. The design of the rotor creates multiple N-S and S-N magnetic fields and as they spin inside the stator windings alternating current is induced into the windings, which needs to be rectified, with diodes, into DC voltage. The voltage regulator controls the output of the alternator by changing the strength of the magnetic field within the rotor. This design makes them much more efficent and they can produce higher currents in a smaller package, which is why they became more popular.

[DollyDodge]

I checked my voltage last night it is 6.25 at the battery and 4.98 at the headlights. Also, my new tail lights do not fade, but I have installed all new wiring for them. I guess I need to start saving for new wiring for the headlights.

[TodFitch]

I checked my voltage last night it is 6.25 at the battery and 4.98 at the headlights. Also, my new tail lights do not fade, but I have installed all new wiring for them. I guess I need to start saving for new wiring for the headlights.

I assume you are using 6006 headlight bulbs rated at 50w at 6.1v (high beam) or 40w at 6.2v (low beam). I also assume you measured when the low beams are on.

The nominal resistance of the low beam filament is about 6.5 amps (6.5 amps * 6.2 volts = 40 watts). (It will vary with filament temperature which varies with voltage, you numbers will vary.) You have two headlights, so that would be about 13 amps for the two. You have a voltage drop of 6.25-4.98=1.27v. To get a 1.3v drop at 13 amps requires only 0.1 ohms resistance. If you were running high beam then that voltage drop would require a bit less than 0.1 ohms to be present.

This is not something you can easily chase down using a cheap multimeter as they don't register resistance that low very well. Basically you are down to just being really suspicious of all connections, switches and wires. If there is any doubt about them, assume they need fixing.

Actually, if you only have 0.1 ohms resistance in the headlight circuit then you are probably doing pretty well. When the generator is really putting out, the voltage at the regulator should be 7.5v if I read the 1946-54 service manual correctly. You will have some loss on the way to the ammeter before the power gets to the lighting circuit but not much, so your voltage at the headlights will be about 7.5v-1.3v = 6.2v which is just about what the bulbs are rated at.

Your tail lights are not dimming because their total current draw is probably 1.5 amps (0.75 amps per bulb) so the voltage drop is much less even with the longer distance and lighter gauge wire. It is also why those filaments are rated at 7v.

Hope I have all that arithmetic correct.

[Frank Elder]

Dave, a smaller pulley won't burn up the genny? Not by any means a expert , but I would think the genny was engineered with a specific pulley size in mind...what about stress on the VR?

Is there an answer floating around somewhere or am I way off base here?

[48Dodger]

Is there an answer floating around somewhere or am I way off base here?

"Conducting heavy current through brushes creates a certain amount of arcing, so generators are limited in their capacity to develop heavy charge current. Also, they don't develop much current flow at low speed. If the pulley on a generator is small enough to spin it fast at idle speed, it will throw the windings out of the armature at highway speed due to centrifugal force. The DC generator has some definite limitations, but it served well in the days of low-speed high-torque engines and simple electrical systems."

That's what I found on Stovebolts site.

48D

[Frank Elder]

"Conducting heavy current through brushes creates a certain amount of arcing, so generators are limited in their capacity to develop heavy charge current. Also, they don't develop much current flow at low speed. If the pulley on a generator is small enough to spin it fast at idle speed, it will throw the windings out of the armature at highway speed due to centrifugal force. The DC generator has some definite limitations, but it served well in the days of low-speed high-torque engines and simple electrical systems."

That's what I found on Stovebolts site.

48D

Cool beans, that is a definitive answer.

[DollyDodge]

Tod: the voltagage I gave was on the high beams. The low beam voltage was about the same. My multimeter is a real cheap one, but I assume it get fairly close voltage readings. The electircal world has always baffled me, but I am learning. I will eventually replace all the wiring. Some of it is new and some is old at this point. For my birthday I got new RI wiring for the Dash to dimmer switch, dimmer switch to headlight junction, generator harness, and a couple others. When I get it all installed I would bet much of my issues will disapear. Some of the old wires are in bad shape. I have taped all the bare spots, but they are old. I replaced the real bad ones, but I am sure the new RI harnesses will make a huge difference. I need to save up for the headlight crossover wiring and new headlight bucket wires. thanks for all the help it really is an education and is starting to make sense to me.

[Dave72dt]

Is there an answer floating around somewhere or am I way off base here?

He may only need to go a little bit smaller, like a 1/4 inch, to get enough to keep them bright at idle and there may not be anything available for that wide belt. Yes, you can damage the gen by spinning it to fast with a heavy amp load applied for a long time. After thirty plus years in the business, I've only seen one come apart. It was a well worn armature that had been turned down several times, undercut very deeply and the solder on the commutator bars finally let go. The heat fom the heavy amp load melted the solder and the bars lifted, destroying brushes, brush holders. Armature was junk but shouldn't have been used in the condition it was in anyway.

Anyone know how fast it has to turn before it grenades? A little algebra and some pulley diameters should be able to get a rpm on the gen.

[DollyDodge]

I think my old truck might be possessed. I was working on the lights on

Friday and noticed the ammeter was no longer working, it was working when I first started to get the old girl running again. Anyway I went to town on Sat bought a SW ammeter for temporary use, hooked it up, and guess what? My lights no longer fade at low RPM!!! Does that make sense? All my lights now work really well. I ordered wires from RI wire to rebuild my old turn signals and once I get the turn signals installed I will be back in shape electrical wise.

[Dave72dt]

Probably some corrosion on the terminals. Disturbing them may have given you cleaner connections and less resistance. Headlights draw the most so would be the most likely to show dimming if not enough current.