Author Topic: Playing with alternators.....PART 1  (Read 35698 times)

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Offline oztules

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Playing with alternators.....PART 1
« on: May 03, 2012, 07:30:58 am »
============================

It has been very busy lately, and have not had time to write anything useful.... but it's time to start.....


Alternators from cars are everywhere, and are a great resource to play with and generate some power for your battery bank.

Bolted to a small petrol motor, they can be a very useful adjunct for your batteries where ever they are ... in cars, tractors, dozers  battery banks.. etc.

It pays to understand what you have and why it works as it does.... so in remote places you can get it going if it fails.

To this end, this will be a treatise on building your own generator from any spare petrol/diesel motor with a small car alternator, and understanding it so that no matter what goes wrong, you can fix it. This will apply to your car as well.

We'll start this with a picture of what we want to achieve:

1035-0

This is a cobbled together generator set that cost almost nothing. The motor is from the dump, the alternator is from the dump, and the electronics for control is from the dump.... old computer boards etc. It will cost no money if you scrounge. We will rebuild the alternator from scratch, so as to make irrelevant if it was external regulation or internal regulation. It will do what we want, and be fully controllable for batteries from 6 v to 12 v.... or more if we wish.... all from the same motor and alternator setup. It will max out at about 30-35 amps dependent on the rotor winding.

Background:

The humble car alternator is a simple device (regardless what the auto elecs tell you). It consists of a rotor for providing the magnetising field, and a stator for generating the EMF. It has in it's native form either an internal regulator or an external regulator.

Lets look at a victim first.

1036-1


When we pull it apart, we find a rotor, and we will discuss this first.

Typical rotor looks like this:
1037-2

When we rip it apart it looks like this:

1038-3

It has two claws oppose to make for a group of magnetic " teeth."  It has a coil of wire to act as the electro magnet to provide the magnetising field, and some slip rings to allow some brushes to energise the rotor winding (coil).... and thats it. ( see the two wires coming from the slip rings that would go to the rotor coil)

When we provide current to the coil via the brushes and slip rings, we make an electromagnet with a flux field proportional to the current flowing in the rotor. ( it's and ampere turns thing... remember this part for later on). The number of "teeth" will dictate the pole count  or number of magnetic poles.

The winding makes two poles N and S and makes one rotor plate all N or all S, and they inter leave to give us N-S-N-S etc

If we provide more current to the rotor it will have a higher flux,......... less current, then less flux..... so we can control the flux if we chose.

This rotor assembly rotates within the stator..........


This is a typical stator:

1039-4

In the above stator, there are three"skein" windings, wound in such a way as to provide effectively three sets of 12 legs.... equating to 6 full coils per phase... or 12 half coils? if there is such a thing. It is the legs that are important. They are the things that couple the flux simultaneously and add their EMF to each other in the phase to get the final EMF per phase group of "coils"

The three windings are 120 electrical degrees apart per pole, and so give us three phase.  It is wound as three phase star, with the three phases joined together at a single point to make a star point. The three "loose "ends are the output points.

The star point can be important for some alternator  indicator purposes for external regulator systems.... or for 8 diodes if fitted...but not so important for us at this stage except to say it is star wound.

Then we have the diode block. This is simply a three phase rectifier consisting of 6 diodes.... or sometimes 8 as in the one below

1040-5

Here we can see the 8 diodes (normally 6). The first 6 make up the normal rectifier bridge, the last two are connected to the star point, and provide a star delta network to provide extra current, and to drop the ampere turns for reasons we will see later on.... but mostly you will see only 6 diodes connected as a normal three phase rectifier.

Next we have the brushes that provide the power to the rotor field via the slip rings. It is self explanatory.... except for a few tricks you need to know.

1041-6

This particular brush set has an inbuilt regulator in the black box on the back..... and we don't care really, as it will be a nuisance in this configuration. Others with an external regulator (like the one pictured in the first photo) will be much harder to see, as they are deep inside the alternator, whereas the internal regulator types usually make it easy to replace bodgy regulators. The ones with external regulators, are generally not accessible without opening the alt up, as they dont have regulators to fail and replace. They only need brush replacement after very very long intervals.... and so seem to be difficult to get at. They do have a tiny hole to use a piece of wire to keep the brushes in place while you press the rotor back in the shell, but that is as good as it gets for maintenance. The internal ones are easy, and are accesable from the back with two screws generally.

The other generalisation is that those with external regulators will have one side of the rotor brush set earthed, and pulse the positive side, and the internal ones will have the positive side of the rotor "hard wired", and pulse the earth side.... this is important to recognise.... ( one of the "tricks" you need to know)

The regulator we will build here will pulse the negative side of the rotor field, and so if an external regulator was fitted, we will need to isolate the grounding of one of the brush terminals so that both of them float above ground. We will then hard wire one to b+ and pulse the b- side.

Another picture of a brush set with an internal regulator.... but spliced into it is a home brew regulator... of the type we will make next.
1042-7


Here is a close up of an alternator (in this service) of the type with an external regulator, the brush housing has been modifies inside so that one side is not earthed as it was for external regulation originally.

We can now see the positive hard wired to one side of the brush input (N) (red wire) and the other side of the brush input (F)( by fiddling with the internal brush earthing and floating it) going to the regulator (green/yellow wire)... hope that makes sense...

1043-8

We have run out of pic room so time to stop. We need to do some simple schematics (pen on paper works for me) so as to explain some of this better.... next post
Flinders Island...... Australia

Offline oztules

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Re: Playing with alternators.....PART 1
« Reply #1 on: May 03, 2012, 06:04:55 pm »
Lets have a look at some simple (poorly drawn) schematics of what we are going to play with.

First here is a simple diagram of the business end of the alternator. It shows the stator coils in star form, with three outputs U V and W and the neutral point as SP (star point?)

It is a simple matter of a stator with a rectifier, either 6 or 8 diodes in the diode blocks. The rectifying power diodes are either pressed in or soldered into the heat sink. There are two of them, as one heat sink will be the batt- output and grounded to the casing, and one heatsink will be floating above the casing (electrically) and becomes the take off point for the big B+ battery connection.

By necessity, the diodes are of two types. One group on one heatsink is cathode to casing of the diode (for the B+) and one group is anode to the diode casing and so this becomes the B- heatsink (grounded)......... So if you have to replace a diode or two, you need to know which heat sink it is from to get either a cathode to case or anode to case replacement from another alternator.

1044-0

The diodes press out ok, but you need to unsolder the stator from the diode blocks to get them out where you can terrorise them  properly. Support the aluminium heat sink as close to the diode casing as possible, so you can drift or press it out without deforming the aluminium.

In the case for the soldered type (my favourite), then a good heat gun will heat the heat sink up easliy to solder temp, and it falls out/off, and then simply sit the replacement in and add more solder.

So thats the diode block. The six diode ones are the same setup as the ones we use for rectifying the output of a three phase alternator wind generator.

The 8 diode ones are essentially the same, but they have 2 extra diodes in the same orientation as the six units, but their leads go to the star point, instead of one of the phase outputs as do the other 6 diodes.

This is for a star/delts switch. ....... If the voltage drop in the stator over a two phase group (to make up a star phase) gets excessive, then the star point starts to conduct, and this turns the stator into a delta setup. It does two things, firstly it lowers the resistance of the stator winding in half (resistance ...... but also due to phase addition, two legs provide extra current to help the third leg... we can get twice the current), but more importantly, it lowers the AMP TURNS in the stator. This allows the magnetic field of the rotor  to better concentrate the flux into the coils of the stator...... short explanation:

These alternators are the same as the air coil alternators in their schematic..... but they follow entirely different rules for their max power.
For the axial we usually build, we are concerned with the stator resistance. The emf in the coils less the battery voltage x the current in the coils will dictate the output. There is very little armature reactance to worry about.

With iron cored alts like these, it is the same thing for low power, but as we increase the current in the stator, we set up a very powerful back MMF (magnetomotive force). This is in opposition to the magnetising field of the rotor.... so for a set rotor current (magnetising force), the back MMF will fight it more and more as the current in the stator increases. (the current in the stator produces it's own field, amplified by the core of the stator, and reflecting at the rotor teeth.... we have no concentrator in the axials, and large air gaps, so it has little effect)

This is easily seen if when we build a motor genset like we are going to....... if we make it as simple as possible, and just wire the brushes directly to earth and b+ but with a 5w globe in series as the brushes, we will have a set current in the rotor. We may see 5 amps at 1500rpm into a flat battery load...... if we increase the rpm we expect the current to rise..... but it wont. We can rev up to 12000 rpm, and there will be barely any change at all. This is because we expected to increase the EMF with revs, but instead of this happening, the back MMF cancelled any further current in the stator, so the magnetising force was unable to further influence the stator..... we flat line.

We get over this to some extent by using a regulator instead of a constant current source like the globe. Now as we want more power past the 5A, we can simply pour more voltage into the rotor. It will draw more current which in turn gives us more MMF to aim at the stator.... so we have effectively driven the constant current point higher up the amp scale.

When the rotor has the full 12v across it, we are at max current  from I=E/R... so if rotor resistance is say 6 ohms, and voltage is 12v then current max will be 12/6=2amps. If we have a lower resistance rotor the same formula applies. In a small auto alt, 5A seems to be around the maximum current draw, and so our upper current limit seems to be about 35-55 amps. If we drove the rotor at a higher voltage, say 24v, then this may  increase a bit, but the rotor may suffer overheat in this instance. This can be fixed with either a fixed upper pulse rate in the regulator (volume control thing)  to keep to 5A, or a resistor in the line to the rotor to keep the current in the design region...... or even another rotor coil outside the alternator. As was seen in the previous post, the rotor presses apart, and so if using it for 24v operation only, then possibly the best thing is to simply rewind the rotor  with more turns of thinner wire.

The "pot assisted" regulator will alleviate most of the 24v rotor problem.

They design these things to run at or near saturation. So if 5A is saturated, then driving it harder will do little if any good. In this case, we use the pot to drive the  regulator to drive the alternator at 24v to near saturation and current limit, and no more. The rotor current should still be around design. A normal 24v reg will overdrive in a large bank and stationery situation, as it will turn on fully into a flat bank, and it won't know the extra rotor current is doing no good.

So EMF is not the problem, we can get heaps of voltage out of these things, but current will be limited at some point.

It is unwise to run the alternator unloaded, as if you rev it up a bit, the high emf may destroy the main diodes. This is probably a cause of most diode faults in car alts.... unless you decide to replace the battery back to front.... that will be a destructive excercise as well.

Here is what we will see in an internal regulated alternator. There are a further three diodes of low amperage that connect to the U V and W outputs like the main diode block.....3-6 amp diodes.

They are hard wired to the regulators positive input. They provide the excitation current for the rotor.

1045-1

To get it started at low rpm, the small indicator light on the dashboard of the car will light up when the key is turned. This is because the light is connected to the B+ of the key switch, and the regulator tries to ground the rotor negative slipring to ground. The current going through the light to the rotor positive slip ring. The light sees this as low impedance load (short?)..... and so it thinks it is between b+ and ground.... it lights up.
It is a very small current, but enough to make some field in the rotor.

As the car engine starts up, the potential in the stator starts to rise under the influence of the revs and the small field in the rotor. It is below "cut in for the batteries" and so as the field builds, the small diodes direct all the emf into the rotor and it quickly causes the stator to increase output.

As this voltage in the stator rises to battery potential, we can see that the point the light is connected to in the alternator is now at around the same potential as the b+ from the battery, and as such no more current flows in this circuit.... the light extinguishes and the system is running normally. From this point on, the three diodes provide the excitation current for the rotor, and it is self sustaining.

We wont avail ourselves of this, and the three diodes are not useful to us at all.. but for the sake of completeness I described it's operation.

Note: if when testing a car alternator,  if you short this point to b+, and the revs are above idle, these little diodes will try to charge the battery as well..... and they will die.. Thats why we won't use them here, they would be effectively wired to b+, and try to charge the battery, rather than just run the rotor field requirements. They need a highish impedance to b+  (small globe on the dash) so they cant try and play with the battery directly.

We will connect the regulator + directly to B+ AND the brush+ for the rotor when we hook the battery to the charger, and that will provide the necessary drive for the rotor b+. Then we need only pulse the b- rotor brush to ground, and we can control the output...... and we can do it with this little circuit.

1046-2

This circuit as is, will replace any internal regulator that has failed.... it is that simple.
To take control of the voltage points, we need only put a potentiometer in series with the voltage divider formed with the 1k and 1.5k divider network on the input of the zener.

If we place a 5k pot in series with the 1.5k resistor to ground, we effectively give the 1k resistor a bigger say in the divided voltage that the zener sees. If we trim the pot to zero ohms, the circuit will regulate to 14.4v or so, if we turn the pot to higher resistance, the divided point will increase voltage, even though battery voltage remains constant, so we fool it into thinking the battery is more highly charged than it really is. With 5k in series, we can get the voltage down enough to charge a 6v battery , at fully off, it will charge a 12v battery.

The big problem with these motor generators is starting the charge on a flat battery. The regulator will naturally turn on fully to charge the flat battery up as fast as possible. This usually loads the motor up very severly.

If we have a "volume" control on the front, we simply turn it to zero, start the motor and load it as we please until it gets charged up a bit, and then turn it to full for unattended regulation to 14v or so.

I have to end this at this point... things to do....


Comments and corrections welcome



...............oztules


Flinders Island...... Australia

Offline ChrisOlson

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Re: Playing with alternators.....PART 1
« Reply #2 on: May 03, 2012, 07:14:58 pm »
That alternator looks like a 12 pole.  The GM ones are 14 pole.  Good writeup on it - looking forward to the rest of the story.  Be interested in seeing how you do the voltage regulation for a 24 volt system using a 12 volt alternator.  I did that once, but I had to replace the rotor in it with a 24 volt one.  Although I used the stock 12 volt stator and even left the 12 volt regulator in it with the regulator grounded so it would feed full field power to the rotor winding.

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Offline Watt

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Re: Playing with alternators.....PART 1
« Reply #3 on: May 03, 2012, 09:09:28 pm »
That alternator looks like a 12 pole.  The GM ones are 14 pole.  Good writeup on it - looking forward to the rest of the story.  Be interested in seeing how you do the voltage regulation for a 24 volt system using a 12 volt alternator.  I did that once, but I had to replace the rotor in it with a 24 volt one.  Although I used the stock 12 volt stator and even left the 12 volt regulator in it with the regulator grounded so it would feed full field power to the rotor winding.

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We have a Case W20B loader which is 24v.  I had to get us out of pinch once when it had a bad alternator, smoked to be exact.  It had the gm style single wire alternator.  I took an off the shelf 12v model and stuck a 24v single wire regulator in it.  Works fairly well the only draw back being no charge at idle.  So long as the guys are using it, it charges well and keeps them from letting it idle all day too.  So, in that regard, it works great and saves diesel. 
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Offline philb

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Re: Playing with alternators.....PART 1
« Reply #4 on: May 03, 2012, 09:54:16 pm »
Excellent writeup Oztules. I especially like your explanations. I'm looking forward to hearing how you can reliably charge a 24 volt bank (like mine) with a 12 volt alt. The 12 volt alts are plentiful around here while 24 volt ones are rare without a lot of cash out of pocket.

Offline oztules

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Re: Playing with alternators.....PART 1
« Reply #5 on: May 03, 2012, 09:58:50 pm »
Yes,Watt, those are my experiences as well. The 12v rotor will survive 24v automotive duty as it is fully driven on an ad hoc basis, and the cooling can keep up it easily. With a stationary version, driving a flat battery bank, it will spend more time driven hard, but due to the excessive rotor current, a regulator will still probably hold it off from cooking.... but I don't know in that application for sure.

Chris drove it to beyond what even the regulator would do, and I have no doubt for extended runs. That may be the difference.

In that little regulator circuit above, if you replace the 1k5 of the divider,with a pot rather than seriesing the resistor with a pot, it will be a 6-12-24 and beyond volt generator. My guess is it may be ok for extended periods at 18v or even 24.... not so sure about any higher for extended runs..... the 1k resistor won't be happy at higher voltages, but that can be changed  up, and use a higher value pot too. the 337 may need upgrading too.

Because you can derate (turn it down as far as you want/need) you can turn it down  from full bore. But I don't know for sure.

Only going to burn up the rotor and regulator .... so no real damage done I guess to find out. (both easy to fix)

 
.............oztules
Flinders Island...... Australia

Offline oztules

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Re: Playing with alternators.....PART 1
« Reply #6 on: May 03, 2012, 10:15:21 pm »
Phil,
As mentioned in the post above, and the main article. I suspect that 24v bank charging will be fine, provided you use this style of regulator.
Chris drove the rotor very hard. I'm guessing 10A into the rotor (if I=E/r, then for 12v rotor @5a, we find R @ about 2.7R at 28v then 28/2.8= about 10A

Now, the discussion above regarding reactance, tells us that if the rotor iron was already at design saturation at 5A, then 10 amps would do no more than heat the rotor and some small improvement.

So with the volume control on the front, if we drive the current just up to flatline and no more, we will probably still be driving only 5A through the rotor for essentially the same performance. If we used a 24v regulator, then it may drive the rotor too hard, and that would be cause for problems.

So on balance, I think the pot controlled regulator will lend itself to that without problem.... only a free dumpster alt to lose.... but I think it will be fine.

Cut in revs will be double, but everyone uses far too big a pulley on these anyway.



..............oztules
Flinders Island...... Australia

Offline philb

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Re: Playing with alternators.....PART 1
« Reply #7 on: May 03, 2012, 10:58:00 pm »
Thanks for the further explanation Oztules.

It seems only the upper half of your first and second posts were showing when I made my previous comments.  I got out of the post, hit the "show unread posts since last visit" and the rest of both posts appeared.  It makes more sense now.
 
I can't quite make out what the npn numbers are in your little circuit. The zener value is 7.5 volts, I think. Could you help me out? 

Offline oztules

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Re: Playing with alternators.....PART 1
« Reply #8 on: May 04, 2012, 12:30:57 am »
I can't read my writing most of the time, so I guess it was a long shot for any one else.

The bc337 and the bd647 are the two trannies. When the divider has more than 7.5volts across it, the zener turns on the bc337 shutting the bd647 down and opening the rotor to ground path and current stops flowing in the rotor, when the voltage drops across the divider and the zener no longer conducts, the 337 turns off  (470R) and the bd647 turns back on  (1K) and provides an earth to the rotor again.

It's easy to see now how increasing the 1k5 will make the zener turn on sooner, and so kid the regulator to turn off earlier. If we increase the 1k, then the opposite will happen, and the battery voltage will have to be much higher to turn it off.


oztules
Flinders Island...... Australia

Offline Bryan1

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Re: Playing with alternators.....PART 1
« Reply #9 on: May 04, 2012, 03:14:51 am »
G'day Oz,
                Top write up mate and now you got me wondering???

                 I scored a new 24 volt 45 amp truck alt and here I was thinking I could just use 2 old 5 hp B&S motors to make one then couple it up for my shed charger. With the season changed over here and many projects in my shed still waiting if my batteries aint got the grunt then no work will be done using my machineshop.

                So with this 24 volt alt will it need the Oztules treatment or will it be good to go.?????

Cheers Bryan

P.S. this project was in the back of my mind and Oz goes and does this tutorial to muddle my mind AGAIN

Offline oztules

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Re: Playing with alternators.....PART 1
« Reply #10 on: May 04, 2012, 06:36:40 am »
Bryan,
 I think all of the generator units would benefit from a control circuit to limit the current into the batteries when they are low.
Once they are near reasonable charge then it is not so critical. It is the first stage of charge into a low bank that is difficult for a straight alternator regulator to handle with a small motor.

This conversion will only make it much nicer to operate.

I would do it as a matter of course. You can live without it, but it will make it much more pleasurable to operate with it.

I used the same circuit modified for 24v to drive the field coils in the generator ( not alternator... she is getting on in age ..the old 10 ton girl) of my caterpillar grader some years back ( have to modify the earth points of the field).... no problems since.

It's friday.... so we have  imbibed too much of the golden stuff to comment further at this stage, and the boss is in party mode, so I'm signing off for now.

Cheers ... and I know you will understand.



......... oztules
Who's mind is muddled up enough for both of us at present  :P  ... and currently listening to guns and roses at high power.... built that too.
Flinders Island...... Australia

Offline niall

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Re: Playing with alternators.....PART 1
« Reply #11 on: May 04, 2012, 07:20:36 pm »
interesting.....

i like the diagrams drawn with a pen and paper as well.. :)

but i,m curious how you got one  "claw" of the shaft ?

i tried beating a couple into submission with no luck ....they just would,nt come off , in the end i put a hacksaw cut through the bit closest to the shaft and it "sprang" open and fell off....but that made it kind off unusable ... :o

after a botched rebuild with micro wave ferrite mags it seemed using a stainless shaft had less leakage which lead to a stronger field on the claws ...kind off... :P

its interesting to...the coil was was also green enemal

Offline Watt

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Re: Playing with alternators.....PART 1
« Reply #12 on: May 04, 2012, 08:47:54 pm »
niall, a press works very well for the task of separating a rotor. 
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Offline oztules

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Re: Playing with alternators.....PART 1
« Reply #13 on: May 04, 2012, 09:23:43 pm »
Yes, it's surprising how easily they come apart if you slip them into thick walled tube........
 and use a dinky little press like this.
1048-0


It's what we in the business commonly refer to as overkill.... or completely over the top

1049-1

A shop press will do as good a job.



...............oztules
Flinders Island...... Australia

Offline Watt

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Re: Playing with alternators.....PART 1
« Reply #14 on: May 04, 2012, 09:35:45 pm »
Wow, nice brake for sure.   I like overkill :P
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