Question Regarding Why 12v Alternator Won't Self Excite.

MisterBill2

Joined Jan 23, 2018
13,798
Maybe I missed it mentioned, but it will most likely take more than a drill to spin the alternator especially once the battery gets enough of charge that it starts to charge faster. I can' t tecal the numbers off hand and at work at the moment. I think you will need at least four horsepower to make it work.
Post #4 discusses that watts out is less than mechanical watts in.. At 100% efficiency the alternator in the original post will take over two HP at full output. More than most drill motors can supply.
 
Post #4 discusses that watts out is less than mechanical watts in.. At 100% efficiency the alternator in the original post will take over two HP at full output. More than most drill motors can supply.
Thanks. It's hard to read it all sometimes on a phone. I happened to glance at a battery charger and remembered how it seems to charge on a curve and what the curve was. I couldn't quite divide out four large batteries in my head to what a typical car battery would be.
 
"The tire won't hold air." OK, let's engineer a device that will force enough air into the tire so it will self seal. No, just put a patch on the leak you morons. (not aimed at anyone here) This is what I've seen so many times.
As someone who now and again gets to change semi trailer tires on the side of the road I am more than grateful for this idea!! Luckily the replacements are already mounted on rims, and as long as we don't have to beat the rim loose it will only take about 15 minutes versus close to an hour or more. On the shoulder of the interstate that could be life or death. It will still leak air, but it won't go flat.
 

shortbus

Joined Sep 30, 2009
9,762
I have looked into replacing the rotating field coil in the rotor with a serious circular magnet like the ones used in powwerful loudspeakers.
The loudspeaker magnet thing is done by 2 different groups of people for quite a while now. The first is the DIY wind power people to generate electricity. The second is the people who turn an alternator into a electric motor, mostly for powering a bicycle. Googling either application will get a lot of information on what and how.
 
geekoftheweek, I was just making a comparison to how sometimes we over-engineer something that doesn't need to be.
I'll agree on the over-engineer statement. There are a lot of things though that may look overdone, but have a useful purpose. I feel the same about back up cameras and a few other "safety" devices. They're helpful, but they make people lazy because there is a sensor or something to look for instead of them actually paying attention.
 
I'll bet if you hook it up to a 12v battery, it will work even at 200 RPM.
Highly unlikely. Ever notice your headlights get dim if your engine sputters for some reason or idles really low? Most times the engine has to be over 800 RPM itself to get the alternator spinning fast enough. Common thing in pre fuel injection days.
 

MisterBill2

Joined Jan 23, 2018
13,798
geekoftheweek, I was just making a comparison to how sometimes we over-engineer something that doesn't need to be.
The army already has that air-delivery system on some vehicles. I don't know how well it works for tires with bullet holes, and I know that it does not work when the vehicle gets the wheel blown off by a mine. But it probably works for some situations. BUT for the complexity and cost of that setup, per wheel, you could drive a long way on the flat , buy a new tire and wheel, and still get by a lot cheaper.
The rest of us just need a few cans of that "Instant Flat Fix" stuff that inflates and seals the tire just like that. Often, though.it needs to have the wheel jacked up anyway.
 
@MisterBill2 The army can drive on a flat in the field, but the DOT will park you on the side of the road if they see it. It obviously won't save a blow out, but it has it's uses. Drive axles are a whole different story than a trailer axle. I can only imagine how complex the army would make it on top of what it would take anyone else to do it.

Kind of the same with cameras and such. They do have uses for certain situations.

I usually have a can of instant repair in the house garage in some form. A dried up plug kit somewhere too.

Edit...

I'll look into the army setup later. I'm probably comparing things that aren't remotely similar other than they air tires.
 
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Tonyr1084

Joined Sep 24, 2015
7,199
The army already has that air-delivery system on some vehicles.
We're getting off topic. I wasn't intending to redirect the conversation to tires or anything else. I was just making the point that sometimes engineers look for a complex solution to a simple problem.

So far what I've read in this thread is that a self exciting alternator won't self excite unless it's spun fast enough and that it may do so from residual magnetism, which I'm a little skeptical about that. A small magnet in the armature - again, I'm not the expert here - but a small magnet in the armature might be the solution to creating a self exciting alternator. Again, I don't know that to be the case. But I would think so.

I have a lawn mower. In fact, I've owned quite a few lawn mowers with self exciting ignition systems. They don't need to spin real fast to generate a spark. I've also owned a few snow blowers with self powered headlights. It uses the same magneto magnet to excite a current in a second coil, one designed to deliver probably (I don't know this for a fact) voltages anywhere from 9V to 15V, depending on the RPM of the motor. A lawn tractor I briefly owned had one that could recharge a 12 volt battery. Crudely, but it did the job.

Energy can not be created. It comes in one form and we can convert it to another. In the case of a gasoline engine, energy comes from the fuel. We turn it into heat and mechanical energy. Some of that mechanical energy is converted into electrical energy. Alternators I've grown up with required an excitation current in order to get it up and generating a voltage. I've always wondered what would happen if I slapped a magnet to the side of an alternator. Would it self generate? Would it OVER generate? Standard alternators control the output voltage by varying the input current to the armature. When spun faster it would generate a higher voltage in the stator. Too high a voltage and the armature current would be reduced to balance the output. That's as much as I know. Or think I know. Yes, I know that alternators generate three phase AC. Diode packs rectify it to DC. A regulator controls how much current is going to the armature (the spinning part). There are two slip rings (not commutators) in a standard alternator. There, you now know how much I know about alternators.

If we're going to have a conversation about flat tires perhaps someone should start a fresh thread.
 

MisterBill2

Joined Jan 23, 2018
13,798
Permanent magnet devices are indeed different. Usually they do not produce as much power, and second they are not feedback regulated at all.
Most cheap bicycle "generators" are in fact alternators, possibly rectified but not always.And usually not regulated. With the very wide voltage variation they are mostly of marginal value. The more expensive packages may include regulation and even a battery, and be much more reliable.
 

Tonyr1084

Joined Sep 24, 2015
7,199
Geez MisterBill2 I remember those AC generators you clicked against your tire on the bike. What a drag. Literally.

After a very long ride, close to if not over 100 miles (no exaggeration) it got dark. My friend had a battery powered headlamp on his bike but I was the cool kid who had the generator. It got dark and I flipped on the generator. Within 100 yards my legs were spent. So we swapped bikes and went another 100 yards. After that we were dead in the water. Had to call my dad with the truck to come pick us up.

The next day I pulled that stupid thing off my bike. It sat around for some period of time till one day I took a 12 volt transformer and connected it directly to the output of the generator. It began to vibrate at 60 cycles. So I gave it a spin in one direction. It began running like a motor spinning at (probably) 60 RPM. Stopped it and retried the experiment spinning it in the other direction and it again spun like a motor at 60 RPM.

Yes, they're largely useless, and take tremendous amounts of leg energy. Not so much a problem for fresh legs, but legs that spent the entire day cruising up and down the Pacific Coast Highway and side streets as well as the beaches, the load was just too much.

The trip was from approximately Whittier CA to Seal Beach, to Laguna Beach and back. We didn't have any kind of odometer, but the same equivalent drive via CA Highways was about 50 miles one way. And that was the more direct way. We didn't make it all the way home but were close. So around 100 miles is a pretty fair estimate of how far we rode. Dumb kids. What did we know of limitations? Nothing stopped us. Not until I switched on that generator.
 

MisterBill2

Joined Jan 23, 2018
13,798
bike generators were not that efficient, and probably to avoid bulb burn-out the better cheap ones used a heavier-duty bulb that drew a lot more current. That was possibly before Zener diodes were ever used to limit voltage, as in some of the Limey Motorcycles.
 

bwilliams60

Joined Nov 18, 2012
1,437
First of all, alternators do not have armatures, starters and generators do. They have rotors and they do in fact retain residual magnetism. I have a video on my YT channel which shows this. In a self exciting alternator,. A diode trio is used to bleed off a small amount of current to feed the voltage regulator and get it started. Remember, a magnetic field cutting across a stationary conductor will induce a voltage/current into that conductor. That is enough to turn on the regulator, which is of special design I might add, and provide enough current to the rotor to get it charging. Basic rules of magnetism. Slapping a magnet on the side of an alternator will do nothing. The magnet has to cut across the coils to induce voltage. Hope this helps.
 
We're getting off topic. I wasn't intending to redirect the conversation to tires or anything else. I was just making the point that sometimes engineers look for a complex solution to a simple problem.
My apologies for that. It's just sometimes the complex solution actually has real value. It's much nicer to have a driver pull up to the shop at 3 am and say my tire is leaking because it was able to make it back to the yard rather than to load up everything and hit the road and go change a tire who knows where.

I'm referring to the Meritor system I deal with... the military who knows

Back to the alternator...
 

DNA Robotics

Joined Jun 13, 2014
635
I put new diodes in a spare alternator for my 1971 Datsun 240Z.
No internal regulator.
The 12 volt draw to the rotor is 2.5 amps.
Ammeter readings, spinning on my lathe:
With no power to the rotor there was no output.
-2.5 amps (rotor) at 700 RPM
0 volts (breaking even) at 800 RPM
5 amps at 1000 RPM.
The alternator spins 1.73 times faster than the engine.
Idling at 600 RPM, the alternator spins at 1038 RPM.
If you have headlights and heater on, they are discharging the battery BUT the alternator is making 5 amps at idle.
 
If you have headlights and heater on, they are discharging the battery BUT the alternator is making 5 amps at idle.
I never thought of it that way. Thanks for taking the time to work this out.

The only thing is it will for sure take a decent amount of time to charge a battery enough to make it work at those speeds. Probably longer than a drill will want to hold up to.
 
Hi! I have a question about one of my recent projects including a car alternator. Please take note I am not trained in electrical engineering but I am aware of the dangerous current that an alternator can output. I am taking the right safety precautions, gloves, outside, etc. with a friend.

I recently bought a self exciting car alternator, 12v 105 amps, and expect to use it the same way a car uses it. I am trying to generate 13-14v and amperage in order to charge a 12v battery that has been taken out of the car.
Here is the alternator that I bought on eBay:
Alternator for Chevy High Output One Wire 105 Amp 21-7127-SE105; 400-12340 | eBay

I thought at first I could just spin the alternator shaft with a drill at 1800 rpm and get 14v on my meter, but I now realize that is not the case. I understand how a car engine spins the shaft of an alternator at high rpm and as it is connected to the battery, the battery begins to charge. The alternator that I bought claims to be 1 wire, or self exciting which I have learned that it does not need input current to create a magnetic field.

So I have a self exciting 105 amp car alternator, a 12 volt car battery, and all the wire in the world. I also have meters, and a way to spin the alternator shaft at high rpm.
My question is: how do I generate electricity with the alternator to charge the 12v battery using a drill to spin it. I have no clue on the wiring!!!

NOTE: I am getting approximately 500 millivolts when I spin the alternator at high rpm.

Thanks again,
Benjamin
As far as I know all automotive alternator circuits depend on a small current flow through the warning light to initiate self excitation then the half wave diodes of the rectifier pack take over when the warning light should go off.
 
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