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Old 11-07-2014, 09:38   #106
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Re: High Rated Alternator with Alternator to Battery Charger

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Originally Posted by transmitterdan View Post
That's how the "belt saver" function works. It decreases the field voltage thus decreasing the output current.
Since we have Balmar products on board, I decided to actually read the manuals for them.

Not terribly helpful in explaining just how the belt load manager achieves its goal.

They describe it three different ways:
1. The field is reduced. That could mean just about anything,
2. The field potential is reduced. That implies voltage like you say, but doesn't make sense to me how that could physically matter.
3. The field pulse bandwidth is widened. I understand the pulse control of the field, but do not understand their use of the term bandwidth here. I suspect they actually mean the pulse width or duty cycle, and not bandwidth.

My thinking is that the voltage on the field wire is used as a gate - voltage on equals current flow, voltage off equals no flow. It doesn't matter what that exact voltage is, only that it is nominal and how long it stays "on" vs. "off". The current will always be enough to cause full field when "on" regardless of the specific voltage. This would be PWM, if that is what they are using.

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Old 11-07-2014, 09:55   #107
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Re: High Rated Alternator with Alternator to Battery Charger

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Originally Posted by Ex-Calif View Post
Can one overdirve an 80 amp alternator (ignoring durability issues) and get 100 amps out of it.
I disagree with Dan - it is not possible to "overdrive" an alternator to produce more than its rating. Maybe a tiny amount because of how ratings are calculated (ie, 85 amps out of that 80).

But if the alternator is at a certain speed and excitation at full field, it can produce only what its windings allow it to. Once their magnetic field is saturated, there is no more "there" there - you have reached a physical endpoint and limit.

To go higher would require physically different windings.

BTW, I made the assumption you meant "overdrive" and not "overdirve". If you meant the latter, I'm not touching that with a 10-foot pole! http://www.urbandictionary.com/define.php?term=dirve

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Old 11-07-2014, 10:07   #108
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Re: High Rated Alternator with Alternator to Battery Charger

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Originally Posted by colemj View Post
I disagree with Dan - it is not possible to "overdrive" an alternator to produce more than its rating. Maybe a tiny amount because of how ratings are calculated (ie, 85 amps out of that 80).

But if the alternator is at a certain speed and excitation at full field, it can produce only what its windings allow it to. Once their magnetic field is saturated, there is no more "there" there - you have reached a physical endpoint and limit.

To go higher would require physically different windings.

BTW, I made the assumption you meant "overdrive" and not "overdirve". If you meant the latter, I'm not touching that with a 10-foot pole! Urban Dictionary: dirve

Mark
Alternator:"I don't care if you're offering me overdirve. You're only gettin' 80 amps!"

BTW - That was my unenlightened understanding, that you can't get what isn't there.

And I learned a new word!
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Old 11-07-2014, 12:06   #109
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Re: High Rated Alternator with Alternator to Battery Charger

There is no saturation in an alternator so far as I know. It is a linear device. If it were not so regulators would not work.

In a typical setup the field voltage is limited to the battery voltage. But if you provide a higher supply voltage to the field the alternator will try to make more current (and lots more heat).
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Old 11-07-2014, 12:48   #110
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Re: High Rated Alternator with Alternator to Battery Charger

The maximum output is a fully saturated winding. There is no more to be gained no matter how much current you provide to the excitation field. It is Ampere's Law: for a fixed number of windings around a fixed magnetic path length, current increases linearly to a saturation point where no more current is produced.

At least this is how my creaky brain remembers earlier physics and electronics classes. I am open to be shown otherwise.

I don't agree with the voltage magnitude on the field wire as being so important. Once the current saturates the field coil, it is at maximum and no more current or voltage has any effect.

If voltage was important in the way you say it is, then an alternator could either not reach full output with anything less than a full battery SOC, or the regulator would have a dickens of a time trying to control a voltage that was in constant flux depending on how much it was being controlled by the regulator - a constant chasing of its own tail. Neither of these happen.

Regulators work by switching the field current on and off at high speed with a duty cycle commensurate with the alternator output desired. The old mechanical ones actually used points clacking away to do this, and all new ones use transistors. Yes, the resultant average field current may be lower than full output, but that consists of packets of full field followed by no field. The alternator output would look the same if it behaved perfectly with no hysteresis, no battery connected to it, etc.

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Old 11-07-2014, 13:16   #111
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Re: High Rated Alternator with Alternator to Battery Charger

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Originally Posted by colemj View Post
The maximum output is a fully saturated winding. There is no more to be gained no matter how much current you provide to the excitation field. It is Ampere's Law: for a fixed number of windings around a fixed magnetic path length, current increases linearly to a saturation point where no more current is produced.

At least this is how my creaky brain remembers earlier physics and electronics classes. I am open to be shown otherwise.

That's not what Ampere's law states. The flux is linearly proportional to amps times turns. The iron in the stator might saturate at some high magnetic field strength but that would be highly inefficient. And it would be very difficult to control the current/voltage because saturation is highly nonlinear. I think you are remembering some kind of magnetic amplifier or a constant voltage transformer. They rely on saturable magnetic material but an alternator doesn't work that way.

A PWM buck converter controls the field current by switching the battery voltage on and off hundreds or thousands of times per second. The pulse width (duty cycle actually) determines the field excitation. Field current is proportional to the duty cycle. At 100% duty (on all the time) is maximum excitation. But the magnetics are not saturated. A buck converter just can't make voltage higher than the battery. If you had a higher field voltage available it would drive the alternator current up. That is up until the thing fries due to overheating. There is no free lunch.
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Old 11-07-2014, 15:15   #112
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Re: High Rated Alternator with Alternator to Battery Charger

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Originally Posted by transmitterdan View Post
If you had a higher field voltage available it would drive the alternator current up. That is up until the thing fries due to overheating. There is no free lunch.
I'm having a difficult time with this. Other than overcoming the resistance of the coil circuit, voltage has no effect on magnetic field strength. The resistance of an excitation field circuit in an alternator is low and easily overcome with a nominal amount of voltage (a nominal 12V, in this example). Any higher voltage is meaningless because the maximum amount of current that the coil can use to fully drive the alternator is readily reached at this nominal voltage.

I don't understand how one can drive an alternator output higher simply by increasing the voltage on the field coil. It is only current that can do that.

I understand the relationship between current and voltage - but a battery at 12V can easily supply more current than the field coil can use without needing to resort to a higher voltage.

If you wanted to get 100A out of that 80A alternator, and it was possible using your suggested way, then you would need that 100A output to be at (say) 50V and not 12V. If this was possible, you would destroy the battery well before the alternator had problems.

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Old 11-07-2014, 18:14   #113
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Re: High Rated Alternator with Alternator to Battery Charger

Mark,

We're speaking different languages it seems and way OT. If you want to discuss by PM let me know.
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Old 18-07-2014, 06:29   #114
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Re: High Rated Alternator with Alternator to Battery Charger

I now have a migraine after reading all this.
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