I Never Thought Of This!!!

It is true on these self-excited generators that the field current is produced by the generator, and the magnetic field strength could be increased by increasing field current. The problem here is that increasing the current flowing in the field windings produces more heat which will break down the windings. Since field strength can't safely be increased (much), this leaves either increasing rpm's or increasing the number of windings.

That's the theorey. In practice, generators (and motors, too) have what's called a "speed droop:" they produce less voltage under load than when under no load. This is due to voltage drop inside the windings themselves due to internal resistance. Say the windings have a resistance of 0.1 ohm. Run this on a circuit with a 0.9 ohm resistance. The 12 volt generator will still put out 12 volts, although you'll "see" only 10.8 volts at the terminals because you lost 1.2 volts inside the windings (12volts/1 ohm total = 12 amps current. 12 amps x 0.1 ohm = 1.2 volts lost inside the gen).

For this reason, the no-load voltage of small generators is higher than stated: my 6-volt gen puts out 8.4 volts no-load with the tractor at idle, for instance. If I put it under load, voltage drops, because of the voltage "lost" inside the gen.

As far as tractors running fine with 6 volt gens on 12-volt systems: I guess you could increase field current to increase the voltage output: if the windings are heavy enough to take the extra current. You could also put a little smaller pulley on to the up the rpm's. Also remember that just as a 6-volt gen puts out more than 6 volts at idle, then at higher engine speeds they will produce a higher voltage yet. So if it breaks the 12-volt mark at higher engine speeds, I see no reason why it won't charge. The max amperage available will be less due to the size of the windings, but with a good battery you may not even notice. Hope that helps. Dave

Stated "If you run a 12 v generator at no load (0 amps or open circuit), it will produce 12 volts, and 12x0=0 watts"
Where does the generator get this voltage from? You don't want to tell me that if you spin a generator buy it's self "without" any battery or (voltage source) in the loop, it will produce voltage, current or wattage?

Stated "Eventually the winding amperage and heat is so great that the windings start to break down. This is what limits generator output" Then why isn't a voltage regulator called a amperage regulater?

To prove the therory you just stated, take a generator and disconect all the leads to the generator, start your prime mover and tell what voltage, amperage or wattage you get at any rpm!

Answer=None!

T_Bone

On the second question, by "limit" I mean design limitation. There is no "topping" governor or overload relay on these generators, like larger (multi KW or MW) ones do. As you reduce the resistance in the output current circuit, more current will flow. Using my example, if you replaced the output wire with ga. 2 copper and grounded it, the total circuit resistance would be only 0.1 ohm (plus a little bit for the ga. 2 copper, which is close to zero, maybe 0.0001, but we'll round off to 0.1 to keep the math simple). The armature produces 12 volts as always, but since the circuit resistance is only 0.1 ohm, that means 120 amps is flowing! 120 amps through the gen windings (since the gen is part of the circuit) will heat them up and melt them. Whatever amperage is the winding "melting" amperage is the limit of the generator's output. Big generators have water cooling in the windings to delay this as much as possible and also improve efficiency (work better cold).

By "open circuit" or "no load" I mean no load applied to the generator: lights off, no battery to charge, etc. The generator is doing no work. Even with this no-load condition, a voltmeter across the terminals shows 8.4 volts for me. This is because there is current flowing in the field windings, which are grounded. Taking ALL the wires off the generator "should" result in nothing, just like you said. I think this is where the misunderstanding is. You're supposed to disconnect the field ground from the generator when running without the battrey for this reason: running an excited generator (one that's producing voltage) for a long time with no load will hurt it. Taking the field wire off disables the field current, and kills (most of) the magnetic field.

I say "most of" and "should" because even with the field circuit open (not grounded) the iron core retains magnetism, and even this little bit produces some voltage in the armature!

I took the resistor in my four-positon switch out, so my "low charge" is now equivalent to a open field circuit. This way, I just leave the switch in the off position, and don't have to worry about re-connecting wires when I do get a battery (and a box to put it in). I'll just flip to "high charge" and go. I gather off this board that that is where this switch was usually kept anyway, unless you're running for a really long time, which I don't do.

It's called a voltage regulator because it regulates output voltage. A simple one like mine accomplishes this by a relay which opens if voltage is too low (cutout). A better one will keep output voltage from going too high by inserting a resistor into the field circuit, reducing the field current when rpms go up.

I think the big confusion was caused by my use of the term "open circuit," which I meant to apply to the output circuit only.

Hope that all makes sense.
Dave

You made me go back and hit the books to refresh my mind and even learned something new about 3 brush generators and series/parrell switch for 24/12v systems.

Thanks,
T_Bone

Still it's a clever idea I never thought of either....

Consider a generator with a "rated" output of 13.6v @ 65amps = 884 watts with a 12v voltage regulator and a 12v battery.

That same generator will have a output of 884 watts at 19.8v@44.5amps with a 18v voltage regulator and a 18v battery.

That same generator will have a output of 884 watts at 26.4v@33.5amps with a 24v voltage regulator and a 24v battery.

That same generator will have a output of 884 watts at 39.6v@22.3amps with a 36v voltage regulator and a 36v battery., etc.

The voltage regulator controls the field voltage to the generator thus regulates the amperage output. But the generator is still only rated at 884 watts no matter what voltage is input.

Thats why it's called a voltage regulator folks!!!
and is a self excited generator.

T_Bone

Consider a generator with a "rated" output of 13.6v @ 65amps = 884 watts with a 12v voltage regulator and a 12v battery.

That same generator will have a output of 884 watts at 7.2v@122.8amps with a 6v voltage regulator and a 6v battery.

As you can see the amperage output is nearley doubled because the voltage is half of the rated input.

Now you can see why generators are rated in watts rather than amperage. If we try and control the amperage for regulation we would have some very heavy duty controls but voltage is only a "potenial" or assumed "force" therefor it's very easy to control output with small voltage controls and low "control" amperage.

T_Bone

My two cents :)
- Dave