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

#### Benjamin0904

Joined Oct 3, 2020
32
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

#### schmitt trigger

Joined Jul 12, 2010
498
Do you have a battery connected when you perform your experiments?

For a generator to produce electrical power, the armature must rotate in a magnetic field.
No way around this.
Unless your generator has permanent magnets, the field must be externally excited.

Which means that a battery must be externally connected to allow the field regulator to draw current and then excite the field.

Now, there are some alternators with a small exciter generator rotating with the main generator, that will pick up the small magnetic remanence in the steel lamination and produce just enough current to excite the main generator’s field. Whose armature will now provide the current to bootstrap the field and allowing the generator to produce the actual load current.

But as far as I know, in an automotive generator there is no auxiliary field exciter, and the initial excitation must be drawn from the battery.

Joined Jul 18, 2013
25,973
Alternators generally will not self excite, i.e.absence of PM field, as opposed to the DC variety, which readily self-excite.

#### LowQCab

Joined Nov 6, 2012
2,499
A GM "One-Wire" Alternator WILL self-excite,
here's the trick ........
If You will notice, the Crank-Pulley on the average large Engine is very
close to ~8-inches in diameter,
but the Alternator-Pulley is only in the range of ~2-inches in diameter,
this creates a ~4-to-1 RPM increase compared to the Crankshaft RPM.
So, at an Idle-Speed of ~800-RPM the Alternator is spinning ~3200-RPM.

The Alternator "may-not" "Self-Excite" until it reaches around ~10,000-Rpm.
This only has to occur 1 time,
as long as the Alternator RPM is maintained above approximately ~2000-RPM,
below that point,
it may "drop-out" if it has a moderate Load on it.

Some GM Alternators will self-excite at as low as ~5000-RPM,
depending upon the model.

~1800-RPM is no where near enough RPM for self-excitation to occur.

Maximum Current-Output "usually" occurs at around ~8-to-~10,000-RPM.
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#### MisterBill2

Joined Jan 23, 2018
13,088
In addition to the fact that there is not enough residua magnetism to produce much voltage, there is also the issue of POWER. Tor ever watt of electrical power out there must be a watt of mechanical power into the alternator. ( I assumed 100% efficiency to make the math simpler) So to get 105 amps at 12 volts out you will need at least 12X 105 mechanical watts in, or 12X105/760 horsepower That is close to TWO horsepower spinning the shaft. You will not get that from most drills.
One more thing is that most alternators spin at twice to 4X the engine speed. So at 1800 engine RPM you have at least 3600alternator RPM
A cheap trick to get an alternator charging without needing a battery is to use a permanent magnet brush type heater blower motor to generate a bit of DCvoltage to excite the alternator field. Once that is done the alternator can power it's own field and keep producing current. So the blower motor can be driven through a manual clutch, such as a rubber wheel agaunat the back side of the drive belt A very simple mechanical trick.

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Joined Jul 18, 2013
25,973
What I meant was an alternator does not self excite right off the bat, at least the ones I have attempted would not, not like a DC wound field generator will.
I remember going out to service some DC versions that all of a sudden would stop self generating, and having to flash the field with my car battery to restore the residual.

#### Tonyr1084

Joined Sep 24, 2015
7,083
OK, the obvious has been stated, insufficient RPM's. My initial thought was 'what direction are you spinning it?'. I don't know if that will make any differences. Perhaps you've spun it in the wrong direction. Hey! Here - I'm guessing. But I do understand the concept of self excitation (not speaking from personal experience for those with perverted thoughts).

Years back I met an old fella who claimed he worked for the railroad. There was a diesel engine that was running but had no electrical power. I asked how they started it. "ADS" or Air Driven Starter. OK, that can spin the diesel engine; and it doesn't need electrical power to spark the fuel. The fuel self ignites under compression. The problem was that the lights had been left on and the batteries were drained. With no electrical power to excite the alternator(s) (I don't know if they have a single or multiple alternators) the electrical power would not come back on.

Everyone was talking about changing the batteries, but in the field that's a rather cumbersome job. When he arrived and surveyed the issue he simply took an old coat hanger and two batteries out of his hand held flashlight and tapped the hanger to the post on the alternator and the other end of the batteries to chassis ground and that was enough voltage to fire up the alternator. I immediately understood the brilliance of that simple solution. So often I see engineers trying to engineer a solution to a simple problem. "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.

One company I worked for spent millions of dollars on a project that failed. Engineers sat for weeks trying to figure out the problem. Me? I was working as a micro section technician. I asked to have one of the suspect chips to dissect. Upon opening the metal chip I found 0.003" clearance between the leads and the top of the metal lid. Under normal operation pressures exerted on the chip would deflect the lid as much as 0.005". Engineers had ordered the chip with 0.015" clearance. The manufacturer certified they had 0.03" clearance. Nope. Everyone decided there was something going on that was unaccounted for and decided to scrap the project. Then I opened the chip and found what I found. When I told them the results they all decided that the certificate of conformance was in error by a factor of 10. Yes, someone just moved the decimal place one spot to the left and thought they had a good product. The owner of the company that manufactured the chips put his hands in his pocket and in a defeated voice he said "We'll remanufacture the chips." Sometimes you need to listen to the little guy.

So, since I don't know this for a fact, I'm just poking ideas out there. Maybe you have to spin the alternator in the right direction. Maybe I'm wrong. I don't claim to be an expert here, just the little guy who might be on to something.

#### MisterBill2

Joined Jan 23, 2018
13,088
Spin direction does matter because the fan on an alternator is spin sensitive, and alternators need a lot of air. Consider that the 100 amp alternator is wound with #12 wire, only rated for 20 amps in still air.
Brushed generators are a different animal completely, and for them, rotation determines polarity on normal systems.

#### vu2nan

Joined Sep 11, 2014
269
The chosen direction of rotation of the alternator would be the one that augments its cooling fan function in the hot working environment.

It would otherwise have no bearing on the functioning of the alternator.

Nandu.

Joined Jul 18, 2013
25,973
It would otherwise have no bearing on the functioning of the alternator.

Nandu.
Other than phase rotation!

#### MisterBill2

Joined Jan 23, 2018
13,088
An internally rectified alternator is what we are talking about. The ones with the bank of selenium rectifiers went away about 1962 for most applications. That is, in automotive and truck applications.

#### vu2nan

Joined Sep 11, 2014
269
The automotive alternator's design is unique.

It's a 3-phase alternator with a half-wave rectifier for field excitation, a full wave rectifier for DC output and a voltage regulator for field control.

The 3-phase windings are located in the stator and the field winding in the rotor (known as 'claw pole' rotor on account of its construction).

Voltage regulation is achieved through on-off switching of field current between 2 set points (e.g. 13.8 & 14.2 V for a 12V system).

The voltage regulator assembly supplies field current, switched by a transistor, via integral carbon brushes, to the rotor slip rings.

When the ignition switch is turned 'on', the battery furnishes initial field excitation to the alternator, through the 'not charging' indicator lamp.

Once charging is initiated, so is self-excitation and the lamp goes out.

Nandu.

#### MisterBill2

Joined Jan 23, 2018
13,088
The alternators that I am familiar with are of the "Mopar" persuasion and set up differently. The stator has three windings, Delta connected to a six-diode three phase bridge array. The older ones had one rotating field brush grounded to the frame, the newer ones had both field brushes not grounded.. Or maybe I got that backwards. The older ones used a single coil mechanical regulator, the newer ones used a two terminal electronic regulator, one wire to the 12 volt buss and the other to the field. So the field current came from the battery and without a battery it probably would not work. But it did not need a battery good enough to light the dome light to start charging. So with a jump start you could drive all day.
We did that in a 66 Rambler American with a manual trans for a few weeks. Even in the rain, when the engine stalled at a traffic light, we were good. The light turns green, two jump out in the pouring rain and push, the driver popped the clutch , engine started and we were back inside in less than 15 seconds and away we went. And laughed about it for a long while. That was about 1968, I think. The best push-start car ever.

#### shortbus

Joined Sep 30, 2009
9,631
They will self excite, but when first used, out of the box they may need "flashed", to restore the magnetism on the rotor. Just like the old time generators did. This video explains. If the alternator is the one in the link, it also may need flashed if it sits for too long.

#### MisterBill2

Joined Jan 23, 2018
13,088
I have looked into replacing the rotating field coil in the rotor with a serious circular magnet like the ones used in powwerful loudspeakers. That would improve the efficiency because of not needing any field current, but it would also mean that voltage regulation would need to be done with electronics, possibly an SCR phase control system. Suddwnly it is a lot more complex. In addition, there is the challenge of getting that magnet exactly balanced and keeping it exactly balanced. At 5000 RPM balance is important.

#### LowQCab

Joined Nov 6, 2012
2,499
I have looked into replacing the rotating field coil in the rotor with a serious circular magnet like the ones used in powwerful loudspeakers. That would improve the efficiency because of not needing any field current, but it would also mean that voltage regulation would need to be done with electronics, possibly an SCR phase control system. Suddwnly it is a lot more complex. In addition, there is the challenge of getting that magnet exactly balanced and keeping it exactly balanced. At 5000 RPM balance is important.
Microwave Oven Magnets may also work for that purpose.

There are now purpose-built FET Rectifier/Regulators (for Motorcycles) that
handle Regulation with very reasonable efficiency.
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#### bwilliams60

Joined Nov 18, 2012
1,433
A few things about your alternator. It is a GM 10SI "generator" as GM likes to call it. It is indeed a single wire alternator with a case ground. I have build hundreds of these in my lifetime and have used them in heavy equipment, cars, trucks, marine (with screens) and a dozen other applications I have chosen to forget. An alternator does not care which way it spins to work. As mentioned, it only matters when the cooling fans (internal or external) or directional. Otherwise it should work. They turn at a ratio of roughly 2.7-3.2:1 in most applications and require about 1500 RPM to excite when they have some residual magnetism in them. As @shortbus pointed out, it is possible that the magnetism has been lost and since it is from China and probably sat on a shelf and had a long boat ride, that is possible. In order to restore it on this unit, remove the black plug. Take a jumper wire when the alternator is connected and jump from the B+ post to the #1 "R" terminal. That will restore the residual magnetism in that alternator. You may get it to start charging if it has a WYE wound stator in it as they charge at a higher rate at low speeds than a delta wound stator. I have my doubts though and you will probably have to find something faster to drive it or some sort of multiplication on your pulleys. When it starts to charge, it becomes a strong magnet and will require more power to drive it so your drill will probably never serve the purpose. Flash it, add RPM and you should be good. Cheers

As for permanent magnet alternators, they are already a thing and are being used in industrial applications where the output is always known and no further requirements are necessary. They are only as strong as the magnets in them so not very popular.

#### MisterBill2

Joined Jan 23, 2018
13,088
The application that comes to mind for a PM alternator without much regulation is to drive it with a faster 2-stroke engine as a quick charger for boosting car batteries. Delivering 30 tom50 amps for a few minutes would be all that they had to do, and the current would be controlled by adjusting the speed. And able to crank an engine for a few seconds.

#### Tonyr1084

Joined Sep 24, 2015
7,083
Thought I had a photograph of this machine I built several years ago. Built it from a 2 stroke tiller machine and a 60A alternator out of my wife's crashed Celica. There is a DPDT switch with center off. In the down position the inverter is grounded and the battery is being charged. In the center off position the inverter is off but the battery is still charging. In the up position the ignition is grounded, thus shutting the motor off. It will charge a car battery. I don't recall how quickly it will charge, but the frame was built to fit a smaller car battery. I no longer have the battery so I haven't used this thing in years. It will probably still run but it may take some work to get it to start. The bottom yellow wire is connected to a lightbulb. When the battery is charging (I believe) the light goes off. When it's not charging the light is supposed to light up, indicating a charging fault. Toyota. gotta love them. Oh, the motor? It's a Honda.

#### geekoftheweek

Joined Oct 6, 2013
885
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.