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Generators & Electric Motors General Discussion

Automotive DC Generator - Practical Upper Limit on Output Voltage


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  #1  
Old 06-27-2018, 10:50:38 PM
Larry Rusch Larry Rusch is offline
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Default Automotive DC Generator - Practical Upper Limit on Output Voltage

If one were to set up a vintage automobile DC generator, 6 or 12 volt, spin it at some normal speed, say 3000 RPM and use a separate variable DC source to excite the fields....what would the upper limit on armature output voltage be? At what point would problems arise with insulation breakdown or commutator/brush action? Would the fields saturate and limit the output? Just a thought problem I've been wondering about.
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Old 06-27-2018, 11:27:22 PM
Birken Vogt Birken Vogt is offline
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Default Re: Automotive DC generator - practical upper limit on output voltage

I doubt insulation would be any problem for voltages you are likely to encounter. But saturation of the iron would be my guess under certain conditions.

If you were a manufacturer and had a 12V alternator frame and wanted to make it 24V, you would wind the stator with twice as many turns of half size wire.

Many common alternator frames are available in a 24V version, if that is what you want.
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Old 06-27-2018, 11:31:01 PM
Thaumaturge Thaumaturge is offline
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Default Re: Automotive DC generator - practical upper limit on output voltage

Much higher than you might think. Seem to recall devices from 60s for running power tools off car gen.
Doc
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Old 06-27-2018, 11:37:55 PM
Vanman Vanman is offline
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Default Re: Automotive DC generator - practical upper limit on output voltage

I think that the limiting factor is going to be commutation. Too many volts per bar and you will start to get flashover. The dirtier the commutator becomes (ie carbon dust from the brushes), the lower the voltage at which this will occur.

In any event, you want to not excite the field beyond the normal current (voltage) of 6 or 12 volts, and

You want to not have sparking- or at least very minimal sparking- at the brushes.
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Old 06-27-2018, 11:52:25 PM
Railroads Railroads is offline
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Default Re: Automotive DC generator - practical upper limit on output voltage

Auto Generators are good for +35 volts DC at 3000 Engine RPM. http://www.netlink.net/mp/volks/htm/gen.htm
Not sure about devices for running tools from actual car gen? Doc, I would like to know a little more about them?

I have been using such a box for car alternators from the 1970's to run power tools at 100V DC. I did see one of these that was old enough to have screw in fuses. (Wish I had bought the darn thing to disassemble to figure out how it worked.)

There is a book on the web from Lindsay Publications called Alternator Secrets that is worth a read.

Typically the old car DC gen's had to be rewound for other voltages and some of them could do 200 watts AC at 60HZ 110V. I have two such books on the topic that explain how to rewind the armatures. Both of these books are now rare and out of print sadly with the demise of Lindsay Publications a couple years ago.

One book is the Lejay Manual 1945 edition and the other is Autopower - Generator Secrets.

Another way to use a car gen is to rewind for higher voltage AC output which requires some engineering as far as fitting either sliprings or jumpering all the commutator bars to form a slip ring. The other end of the single winding would be grounded through the armature and generator frame. Then a transformer would step up the 30 some odd volts to either 110 or 220. http://www.schematicsforfree.com/arc...0Generator.pdf

It's easier to adopt the method of using wire to bridge the commutator bars into a slip ring and then grounding through the armature and generator frame for the other side of the circuit. Adding a 1uf to 2 uf AC capacitor at the output helps with the noise from the crude slipring.

Using these old generators and alternators is one of my specialties these days. I've done so many of these conversions and even designed a few that are yet to be built up.

Using old DC treadmill motors for 110V DC is another fun project. Figuring out how to mount a V belt pulley and the original fan is a pain though.

Converting induction motors into AC generators is another specialty of mine. This is a simple illustration for those unfamiliar with this setup. http://www.qsl.net/ns8o/Induction_Generator.html Big three phase motors work much better but the only PDF on the web is wrong in the wiring and does not make the best use of the motor conversion. There is a tutorial from England that is pretty good though. http://www.stationary-engine.net/for...tion+Generator

Robert

Last edited by Railroads; 06-28-2018 at 12:46:10 AM.
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Old 06-27-2018, 11:52:51 PM
pegasuspinto pegasuspinto is offline
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Default Re: Automotive DC generator - practical upper limit on output voltage

It's possible that a really good alternator shop can rewind it in favor of a new voltage for you too. I have one near me that could probably do it.
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Old 06-28-2018, 03:52:53 PM
Ken Karrow Ken Karrow is offline
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Default Re: Automotive DC generator - practical upper limit on output voltage

I recently bought the Lejay book on Amazon from a used book dealer. Several were listed so I think probably still could find one.
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Old 06-28-2018, 09:59:44 PM
Ed Stoller Ed Stoller is offline
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Default Re: Automotive DC Generator - Practical Upper Limit on Output Voltage

A long time ago, I belted a car alternator up to a lawn mower engine. By increasing the rpm, I was able to increase the AC out voltage to about 120 V. The frequency was about 400 HZ. I think these units are 3 phase AC so one could convert it to DC for universal motors.
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Old 06-28-2018, 11:24:06 PM
Railroads Railroads is offline
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Default Re: Automotive DC Generator - Practical Upper Limit on Output Voltage

Most likely if not using some kind of transformer you were measuring DC 120V. Each phase is not capable individually of doing 120V. Some big alts like the Leese Neville type's have 6 or more sets of phases and can do 300 volts if reconnected. Some even had AC taps for 120V at 400HZ.

The DC coming out of Alts has a a lot of AC ripple and even some transformers can be run. Some of the older TV sets would work. Some would blow up immediately. I mean literally go BANG! Picture tube and all would explode.

Hmm, Someday I need to create a website with all the technical data I have and my experiences with these conversions. Something well detailed but not too scientific so laymen can understand it. Something like what sparweb has setup for wind turbine projects he's done. http://www.sparweb.ca/3_Gen_MoCo/Conv.html

Robert
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Old 06-29-2018, 08:08:38 PM
cornbinder89 cornbinder89 is offline
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Default Re: Automotive DC Generator - Practical Upper Limit on Output Voltage

Quote:
Originally Posted by Railroads View Post
Each phase is not capable individually of doing 120V. Some big alts like the Leese Neville type's have 6 or more sets of phases and can do 300 volts
Robert
never saw one that had any more than 3 phases, and I have been in a lot of them.
Some are Delta wound and have 6 leads, but still only 3 phases. Even the 375 amp oil cooled Delco's have only 3 phases, there is just no gain in going with more phases.
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