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Old 10-07-2014, 11:06   #91
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Re: High Rated Alternator with Alternator to Battery Charger

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So in saying they are accurate then you are saying Calder and Sterling are inaccurate!
I think he is saying that you have not correctly comprehended what you read. I won't speak for him, but that is the point I have been trying to make - neither of them are inaccurate AND what I and others have been telling you is true. You are discounting that possibility.

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Old 10-07-2014, 13:23   #92
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Re: High Rated Alternator with Alternator to Battery Charger

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No, one of the points of this thread was that the A2B can also provide this without an external regulator or changing its internal regulator voltage set point. So far, you are the only one who has disagreed with that...Mark
I haven't ever disagreed with that! I actually said:

"yes the A2B does the job - with the cautions that I and Maine Sail have pointed out"
.

The simple fact is the Sterling A2B is an EXTERNAL regulator. It just achieves the higher charging voltage in a different and I have argued - a more inefficient way.

I think a lot of your posts have just been confusing readers who want to get to the bottom of this - you keep mis-quoting people - which spoils the thread!
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Old 10-07-2014, 14:57   #93
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Re: High Rated Alternator with Alternator to Battery Charger

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I haven't ever disagreed with that! I actually said:

"yes the A2B does the job - with the cautions that I and Maine Sail have pointed out"
.

The simple fact is the Sterling A2B is an EXTERNAL regulator. It just achieves the higher charging voltage in a different and I have argued - a more inefficient way.

I think a lot of your posts have just been confusing readers who want to get to the bottom of this - you keep mis-quoting people - which spoils the thread!
Yes but your cautions make little sense and are based on an incorrect understanding of the process involved.

As to inefficiency , without any real info, we, you, I are all guessing , but I have laid out that efficiencies are less then 100% alternator output are somewhat meaningless.

Nor is Sterlings A2B a regulator in the understood sense. its primarily a DC to Dc convertor.

Quote:
I think a lot of your posts have just been confusing readers who want to get to the bottom of this
Get to the bottom of what precisely ?
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Old 10-07-2014, 15:28   #94
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Re: High Rated Alternator with Alternator to Battery Charger

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I think most regulators, once started, get their field voltage/current from their own internal diode pack.

Besides, even a very low SOC battery bank has mucho capacity to power a field coil.

Mark
I'm not saying the field can't produce any power at reduced voltage. But it can't produce maximum power. An alternator is not a voltage source. The stator is more like a current source with some internal resistance. The field coil is just a piece of wire wrapped so as to create a magnetic field that rotates. That rotating magnetic field is what induces the stator windings to produce a proportional current (which also depends on RPMs). It does not produce a proportional voltage (that's why removing the battery damages the diodes; they can't take the high voltage).

The field wire has some resistance R. The current through that wire is Vfield/R. So the maximum current the field coil can have is directly proportional to the maximum available field voltage. Since the maximum alternator current is also proportional to that field voltage a lower output voltage must mean a lower maximum output current (for the same RPM).

So an external regulator that drags down the alternator output voltage must also reduce the maximum output current (1V reduction is about 7.5% less current). If someone wants to do an external DC-DC converter that maximizes charging current it would be better to have an MPPT function like solar regulators and probably have to be able to buck and boost.

Running the alternator at higher voltage and lower current will improve the heating and maximize power delivered. Hey, this sounds like a product the LiFePo guys could really use...
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Old 10-07-2014, 15:40   #95
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Re: High Rated Alternator with Alternator to Battery Charger

I'm not sure what you are getting at. Full field on an alternator is typically about 7 amps. It doesn't take much to provide this, and even an almost dead battery at 12V can provide the 1A it takes to excite the alternator enough to allow it to start producing enough power to self-excite and continue higher - where full field can be supplied easily.

The specific voltage has little to do with it as long as it is high enough to supply a relatively small amount of current. 12V or 14.8V on the field wire is of no consequence to the alternator output.

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Old 10-07-2014, 16:38   #96
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Re: High Rated Alternator with Alternator to Battery Charger

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12V or 14.8V on the field wire is of no consequence to the alternator output.
Mark,

I think that statement is incorrect. I think the current output from the alternator is directly proportional to this voltage. So going from 14.8V down to 12V means the maximum current output drops almost 20%. That's how the "belt saver" function works. It decreases the field voltage thus decreasing the output current.
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Old 10-07-2014, 16:56   #97
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Re: High Rated Alternator with Alternator to Battery Charger

Perhaps, but are you sure it doesn't just change the maximum field current, as well as refiguring the control linearity between the minimum and new maximum values?

If the field voltage mattered greatly, then no alternator would ever be able to give full output until the batteries were fully charged. That hasn't been my experience.

I have some experience with inductive magnetic fields and none were voltage controlled.

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Old 10-07-2014, 18:03   #98
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Re: High Rated Alternator with Alternator to Battery Charger

The excitation is current controlled, alternators are current controlled devices.

PS oceansailor. The reason diodes blow on disconnection is purely due to inductive di/dt A massive rate of change in current due to the disconnect causes an equally massive rate of change in voltage , the diodes which have ground on their anodes typically blow open circuit.

I have yet to see the so called " warnings " about A2B units technically explained. I mean is this a "gedanken" for you or do you have specific experience of them.

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Old 10-07-2014, 18:24   #99
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Re: High Rated Alternator with Alternator to Battery Charger

Mark,

I'm pretty sure the field winding is just a resistance that obeys Ohm's law.

Consider that an external DC-DC converter could get the most power out of the alternator by forcing the field voltage (and thus current) as high as possible. Then it could have maximum available power for all phases of the battery charge cycle. As soon as it didn't need all that power it could reduce the excitation accordingly. The more I think about it the more I see a product improvement for the DC-DC converter idea. It would be an MPPT controller for alternators. The LiFePo folks could really use something like this.

The Sterling idea is not "bad" but I think it could be improved by being buck/boost and take control of the field current like a regulator. It would be the best of both.
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Old 10-07-2014, 18:36   #100
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High Rated Alternator with Alternator to Battery Charger

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Mark,

I'm pretty sure the field winding is just a resistance that obeys Ohm's law.

Consider that an external DC-DC converter could get the most power out of the alternator by forcing the field voltage (and thus current) as high as possible. Then it could have maximum available power for all phases of the battery charge cycle. As soon as it didn't need all that power it could reduce the excitation accordingly. The more I think about it the more I see a product improvement for the DC-DC converter idea. It would be an MPPT controller for alternators. The LiFePo folks could really use something like this.

The Sterling idea is not "bad" but I think it could be improved by being buck/boost and take control of the field current like a regulator. It would be the best of both.

Jeepers , there a lot of confusion here.

Firstly the alternator s regulated , it will hold its output voltage at its rated set point upto the maximum allowable current the power generation ability of the alternator is capable of. Exceed that and the voltage falls in a fairly linear fashion. Since there is no slippage between engine and alternator and engine sufficient power the curve is very linear.

Hence there is no mppt point to track. ( unlike wind or water driven units were mppt is useful,)

As to an external regulation forcing field current high. , once the alternator has reached its power capacity , no regulator can extract more. Nor can any charging device unless very high power overcome the battery terminal voltage by more then about 0 .2 V

As soon as the DC DC convertor doesn't need power the internal regulator reduces field current appropriately

Having a buck convertor is fairly useless unless you wish to charge systems that are using significantly lower voltages then the standard regulator is set at. That might be useful for lithium ferrous where modern alternators are set a bit high. But that's a tiny market.

For LA there's no point to buck conversion , high powered buck boost has many design issues especially at the equal input output voltage point.

In reality you really never need less output voltage then the stock regulator

Nor is anything gained by controlling the field current. The stock internal regulator does that just fine. If the A2B controlled the field it wouldn't need DC conversion and would have all the issues of accessing the field wire and disabling the internal regulator , issues that the A2B was designed to specifically avoid. Hmmmm

Further note that the A2B does not need to " force the field voltage as high as possible. The DC DC process will load the alternator according , driving the alternator harder and harder and ultimately out of regulation , at this point the alternator is at max capacity without touching the field control system. I don't think people under stand switched mode power conversion all that well.

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Old 10-07-2014, 19:49   #101
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Re: High Rated Alternator with Alternator to Battery Charger

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Jeepers , there a lot of confusion here.
We agree that the alternator outputs a constant current set by the field current. The alternator output voltage is determined by the load resistance multiplied by that constant current. A discharged battery is a non-linear load so it acts to limit the voltage. But if the load was a pure resistor then increasing the resistance would increase the voltage (for a given field current). So the power output would go up but the current would be the same. A DC-DC buck converter can easily make that happen. It could let the alternator voltage go up to 16-18V and it would magically get more power out without increasing the I squared R losses. But to be useful the DC-DC converter would have to tightly control the field winding because as you rightly pointed out a normal regulator would limit the voltage. When the buck converter needed to decrease it's output power it would have to decrease field current to prevent the alternator increasing voltage until something breaks.

My point is that an external DC-DC buck converter would be able to improve the power output from an alternator by 10-20% just by operating at a higher voltage than normal. And now that I've written this I realize there is no point in a boost mode in this idea. So I was a little confused before but now I think this is an even easier idea to implement. I would appreciate if you could try punching holes in this idea.
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Old 10-07-2014, 20:13   #102
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High Rated Alternator with Alternator to Battery Charger

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We agree that the alternator outputs a constant current set by the field current. The alternator output voltage is determined by the load resistance multiplied by that constant current. A discharged battery is a non-linear load so it acts to limit the voltage. But if the load was a pure resistor then increasing the resistance would increase the voltage (for a given field current). So the power output would go up but the current would be the same. A DC-DC buck converter can easily make that happen. It could let the alternator voltage go up to 16-18V and it would magically get more power out without increasing the I squared R losses. But to be useful the DC-DC converter would have to tightly control the field winding because as you rightly pointed out a normal regulator would limit the voltage. When the buck converter needed to decrease it's output power it would have to decrease field current to prevent the alternator increasing voltage until something breaks.



My point is that an external DC-DC buck converter would be able to improve the power output from an alternator by 10-20% just by operating at a higher voltage than normal. And now that I've written this I realize there is no point in a boost mode in this idea. So I was a little confused before but now I think this is an even easier idea to implement. I would appreciate if you could try punching holes in this idea.

Ahem, hold a minute there cowboy

A regulated alternator , is , while in regulation an imperfect voltage source, that is current will vary according to load impedance , but output voltage remains fixed. Changing the load impedance causes a change in current , but not a change in voltage.

Out of regulation , a regulated alternator is an imperfect current source, voltage varies but current remains constant for a given load impedance. To drive the alternator out of regulation the load impedance must be low enough to force the voltage below the regulation set point at full alternator power. Changes in load impedance cause the voltage to change but not the current ( once below the regulator set point )

In both cases where the load requires max power the regulator will drive the field current to maximum , within permitted power dissipation. Nothing more can be realistically extracted.

Let's review the purpose of the sterling A2B.

The primary purpose is to improve the battery charging profile of a stock alternator without requiring modifications to access the field wire and /or disabling the internal regulator. This bring a process only suitable for geeks or requiring the purchase of a speciality alternator. It also preserves the warranty of the stock alternator.

The secondary purpose is to improve the charge profile over a stock regulator. Mainly by raising the absorption set point ( and hence regulation voltage ) of the output to the battery. This ensures a faster charge to absorption point, and a more aggressive absorption charging cycle ( a point sterling harps on about).

Hence the purpose of the boost only DC DC convertor.


Now, as to your point of raising the voltage and then buck conversion. You cannot extract more power from an alternator by raising the voltage. The alternator has a certain amount of magnetic coupling, hence magnetic power transfer. Raising the voltage will cause a corresponding drop in current.

Furthermore the alternator is rated for certain power as a function of cooling, materials etc. It cannot extract more without exceeding that point.

The fact is a 500 watt alternator is just that. You can have it anyway you like 12V x 41.6A , 13.5Vx. 37 A or 18v x 27.7A. The point about I squared losses is valid , but in these circumstances, gains would be tiny as R is so low. and current gain increase is effectively non existent.

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Old 10-07-2014, 22:02   #103
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Re: High Rated Alternator with Alternator to Battery Charger

Dave,

If you are on GMT you're either up early or late<g>.

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Now, as to your point of raising the voltage and then buck conversion. You cannot extract more power from an alternator by raising the voltage. The alternator has a certain amount of magnetic coupling, hence magnetic power transfer. Raising the voltage will cause a corresponding drop in current.
I'm pretty sure that is not correct. The rotating magnetic excitation field induces a current in the stator proportional to the excitation current. The stator output voltage will go to whatever it needs to deliver that current according to Ohm's law. That's why disconnecting the battery from an excited alternator busts the diodes. The stator voltage goes up because R becomes nearly infinite until the diodes avalanche and fail.

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Furthermore the alternator is rated for certain power as a function of cooling, materials etc. It cannot extract more without exceeding that point.
I think that isn't right. Heating is caused by I squared R losses in the windings plus mechanical bearing losses. More voltage does not increase the I squared R losses if current stays the same. That's why alternators are nameplate rated by current. There is some increased load on the bearings because the alternator is requiring more torque to turn the rotor.
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Old 10-07-2014, 22:19   #104
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Re: High Rated Alternator with Alternator to Battery Charger

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It isn't too difficult - Maxwell described it and Tesla implemented it a long time ago. If you graphed amperage through a field coil vs. magnetic flux or dipole moment, you get a sigmoidal curve.

However, the majority of that curve, which is the part used for work, is linear - so for all intents and purposes, the relationship is linear. Put more current in, get more current out.

The slope of the field input/final output curve will depend on the alternator and winding design - but it will be linear in the range it is used.

Mark
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A regulated alternator , is , while in regulation an imperfect voltage source, that is current will vary according to load impedance , but output voltage remains fixed. Changing the load impedance causes a change in current , but not a change in voltage.

Out of regulation , a regulated alternator is an imperfect current source, voltage varies but current remains constant for a given load impedance. To drive the alternator out of regulation the load impedance must be low enough to force the voltage below the regulation set point at full alternator power. Changes in load impedance cause the voltage to change but not the current ( once below the regulator set point )

In both cases where the load requires max power the regulator will drive the field current to maximum , within permitted power dissipation. Nothing more can be realistically extracted.
Marginally hanging on to the discussion but very interested to learn more here...

As a visual kinda guy what I was hoping for was someone (alternator manuf?) had taken a real (stock) P/N XYZ alternator and graphed field current against outputs under load.

I guess that is no longer germane - the question I would like to see settled now is -

Can one overdirve an 80 amp alternator (ignoring durability issues) and get 100 amps out of it.

And I suspect no simply because all kinds of people would already be doing it.
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Old 10-07-2014, 22:39   #105
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Re: High Rated Alternator with Alternator to Battery Charger

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I guess that is no longer germane - the question I would like to see settled now is -

Can one overdirve an 80 amp alternator (ignoring durability issues) and get 100 amps out of it.

And I suspect no simply because all kinds of people would already be doing it.
Yes you can but the windings and/or diodes will probably melt. 100A vs. 80A would create approximately a 60% increase in temperature rise. So the alternator would likely not last long.

Perversely going the other way doesn't gain the same. A 100A alternator operated at 80A has only a 35% decrease in temperature rise. That darn current squared thing is a b#$%h.
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