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Ramblings about Generators

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Sparky

05-20-2002 16:53:10

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Most people know that if you move a magnet past a coil of wire, it will generate electricity out the ends of the wire. A generator has 2 main parts, the field and the armature. The field is the magnet, or an electro magnet in this case. The armature in a generator is what moves past the magnet or field and is where the electricity is created. There is really no control on the armature. It is going to put out electricity based on whatever the field or magnet does. It is the field that is controlled by the regulator. A 6 volt battery needs about 7.5 volts to charge it. What happens first is the regulator supplies full power to the field. The output out of the armature rises up to over 7.5 volts. The regulator senses this and shuts off the field. The armature output starts dropping below 7.5 volts. The regulator senses that and supplies full power to the field again. All of this keeps happening many times a second. The end result is a fairly steady output out of the armature of 7.5 volts. Now what's going to happen if you use a 6 volt generator on a 12 volt battery with a 12 volt regulator. The field winding has a certain amount of windings to give it a resistance to 6 volts that will limit current to about 2 amps max. If you supply 12 volts to that same field, the current will now be 4 amps. Can the field winding handle twice the current without smoking? Probably, but this would not be the best situation long term. I think the generator field might go up in smoke for sure if trying to charge a dead battery. It would be better to have the generator field rewound for 12 volts. The shop would use more turns of a smaller wire to give it more resistance.

If I were going to try running a 6 volt generator on 12 volts myself without having it converted, I would put a 3 ohm 25 watt resistor in series with the field to limit current to the original 2 amps. The regulator would have to be changed to a 12 volt model, as the original 6 volt one would still try to regulate to 7.5 volts and would go up in smoke trying.

One more thing while I'm ramblin on, a 12 volt battery of the same physical size as a 6 volt, is only going to give you roughly half the cranking time. Granted the starter is going to turn the engine over much faster making it more likely to start, but the 12 volt will run down quicker if it doesn't start.

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Sparky

05-22-2002 14:54:12

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Re: Ramblings about Generators in reply to Sparky, 05-20-2002 16:53:10

All of this reminds me of a story about Stash and Stosh a couple of farmers around here. Stosh had been trying to use his old rusty and dull plow with a team of 6 donkeys. It was tough going. Sometimes the 6 donkeys couldn't even get the plow moving. Well Stash, having some education, came along and right away he saw the problem. So he said, Stosh, you need more donkey power. You should be pulling that plow with 12 donkeys. Well wouldn't you know it, those 12 donkeys really had that old plow moving. Maybe the ground didn't turn over quite like it should due to the dull plow points and rusty plow shares, but the dirt was really flying. All of a sudden the plow beam breaks and now those 12 donkeys are moving even faster. Moral to this story... doesn't matter whether you call it resistance or inductance, too much donkey power is too much donkey power.

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dick

05-22-2002 04:43:04

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Re: Ramblings about Generators in reply to Sparky, 05-20-2002 16:53:10

Maybe I'm missing something, but since the voltage regulator is switching the generator on/off many times/second, isn't an AC circuit analysis needed? Your DC analysis doesn't appear to tell the whole story since it neglects the effects of the coil inductance, which would seem to me to be quite significant.

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gene b

05-22-2002 04:51:15

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Re: Re: Ramblings about Generators in reply to dick, 05-22-2002 04:43:04

life the man said he was rambling should have kept it to himself enough people are confused as it is there are enough problems with people going from 6 to 12 v and haveing problems hardly a day goes by without someone having a mess of their hands

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Steve

05-21-2002 08:50:18

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Re: Ramblings about Generators in reply to Sparky, 05-20-2002 16:53:10

I don't understand your amperage calculation - ohm's law states that if you double the voltage the amperage is CUT IN HALF!
2 amp at 6 volts will be 1 amp at 12 volts, not 4 amps.
Did somebody rewrite the laws of physics or am I missing something - please correct me but I am reasonably sure that ohm's law applies to all electrical devices.

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Daniel

05-21-2002 10:07:58

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Re: Re: Ramblings about Generators in reply to Steve, 05-21-2002 08:50:18

I think you need to check you text books again. I teach electronics every day and for a constant resistance (the resistance of the generator coil for example) doubling the voltage will double the current. I (AMPS) = E (VOLTS) / R (OHMS). 6V/3 ohms = 2A and 12V/3 ohms = 4A. What your thinking of is power (wattage). The reason stoves and water heaters run on 240V instead of 120V is beacuse it will take half as much current to produce the same amount of heat. Less current allows you to use smaller (read cheaper) wire to run to power hungry appliance. It's the same reason you can get away with 4 gauge battery cables on a 12V tractor but need 2 gauge on a 6V tractor. (SMALLER gauge numbers are LARGER wires) Hope I didn't offend anyone. Good Luck with your wiring. Daniel

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Steve

05-22-2002 07:53:46

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Re: Re: Re: Ramblings about Generators in reply to Daniel, 05-21-2002 10:07:58

Thanks for straightening me out Daniel.
Certainly no offense. I appreciate your taking the time to correct my error!
Take care,
Steve

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BIG JOHN

05-20-2002 18:30:38

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Re: Ramblings about Generators in reply to Sparky, 05-20-2002 16:53:10

Sparky, thanks for the good info, it clears out some of the cob webs.

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TomH

05-20-2002 17:16:34

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Re: Ramblings about Generators in reply to Sparky, 05-20-2002 16:53:10

Thanks for the info, you answered some lingering questions I've had for a while. Here are a couple more

Two amps at twice the voltage is still twice the wattage, it's really only amperage that you need to limit?

An electrician friend says every piece of electrical equipment is manufactured with some smoke in it. He says once you let the smoke out the item is ruined. Ever heard of that? ;^)

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Diesel Don

05-22-2002 10:35:46

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Re: Re: Ramblings about Generators in reply to TomH, 05-20-2002 17:16:34

Absolutely!I work as what they call an instrumentation and electrical technician,and I can tell you with absolutely no doubt in my mind that all electrical equipment contains smoke,and if you let it out,the equipment will not work!All kidding aside,because of the winding inductance and the fact that the field in the generator is also broken by the commutator(correct me if I'm wrong but I believe the field is the rotating element)Ohm's law alone will not determine the current drawn through the field.I could go into a little more depth but I don't think you guys want this turned into an AC fundamentals forum-power or tractors!

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Diesel Don

05-22-2002 11:03:24

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Re: Re: Re: Ramblings about Generators in reply to Diesel Don, 05-22-2002 10:35:46

Now to agravate you further,now that I have thought about it,a generator is also affected by what ia called counter emf.As the generator turns and current is produced,it tries to run like a motor.This motoring current does two things-it produces a load on the engine and acts as a limit on how much current the field will accept.Now,before I get permanently banned from this fine discussion board,I shall graciously be quiet!

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Sparky

05-21-2002 06:23:17

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Re: Re: Ramblings about Generators in reply to TomH, 05-20-2002 17:16:34

In the case of adding a 3 ohm resistor in series with the field to limit the current to 2 amps, you're right the total wattage for both field and resistor together would be double, but the wattage for the field alone would be about the same as before.

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Bus Driver

05-21-2002 17:47:33

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Re: Re: Re: Ramblings about Generators in reply to Sparky, 05-21-2002 06:23:17

Sparky, in windings with changing currents, such as the intermittent flow caused by the regulator (or if AC current), the current is limited by inductance. The unit of inductance is the henry, the effect of inductance in an operating system is measured in ohms. So the ohm resistance of a coil or winding measured as DC ohms does not tell us much about how it will act when in an operating system. I have seen some 6 volt generators with 12 volt regulators and 12 volt batteries. Am told that they do work, but that output amperage is low. Have not verified this by testing such a hookup. Plan to do so someday.

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