K12GT alternator output

[twist]

I was looking at the specifications of various BMW bikes, and noticed something that may or may not be a typo.  So my GS has a 600W alternator, and GS Adventure has 720W.  Those sound about right.  The new K1200GT has a 945W alternator (if it's not a typo).  For comparison other K1200's (k12s, k12r and k12r sport) have 580W.  If 945W is correct, then holy crap that's a lot of watts!  3 extra sets of lights wouldn't even strain it!

[HankB]

The alternator on my K75RT was 700W and I think the one on my K12RS is 800, so 945 doesn't sound like it is outside the realm of possibility. BMW basically puts an automotive alternator in their bigger bikes.

[Mr Sunshine]

BMW basically puts an automotive alternator in their bigger bikes.

Except it runs like a normal stator as other sport bikes.  They just make sure it has more than enough juice for whatever you want to run.  One more reason why I am pretty sure my next street bike is going to be a BMW.

[throttlemeister]

Except it runs like a normal stator as other sport bikes.  They just make sure it has more than enough juice for whatever you want to run.  One more reason why I am pretty sure my next street bike is going to be a BMW.

Nog on their boxers they don't. Just your ordinary car alternator bolted on top, belt driven from the crank. There's a reason I always called BMW agricultural. Of course now that I have one, that regularly gets thrown in my face but I don't care.

[HankB]

Except it runs like a normal stator as other sport bikes.  

I'm not sure what you mean by that. I'm familiar with two kinds of charging systems used on modern bikes. On my wife's Suzuki GZ250, there's a stator inside the crank case that is excited by permanent magnets on a rotating piece. The stator has three windings that actually generates three phase power which is rectified through diodes. Since permanent magnets are used, the output only depends on RPM. To control the voltage, excess energy is dissipated in a voltage regulator that starts to conduct over a specified voltage. The excess energy is dissipated as heat. This is why you see finned voltage regulators on bikes like this mounted in the air flow where they can stay cool at higher RPMS when cruising. A down side to these is that they can't be designed with too much excess capacity otherwise the regulator would have to dissipate too much heat when full output is not needed.

With automotive style alternators, the field is generated with another winding on a rotor instead of permanent magnets. The current through the field can be varied to match the output to the load. There's more parts, hence more weight and they take more space. But they can be designed with a lot of excess capacity which is only drawn on when needed.

There's more about alternators at Wikipedia. http://en.wikipedia.org/wiki/Alternator

That's my story and I'm sticking with it. But if anyone has better info, feel free to correct me.

-hank

[Mr Sunshine]

Nog on their boxers they don't. Just your ordinary car alternator bolted on top, belt driven from the crank. There's a reason I always called BMW agricultural. Of course now that I have one, that regularly gets thrown in my face but I don't care.

We aren't talking about the Boxer now are we?  

I had one and I might end up with another again.

[Mr Sunshine]

I'm not sure what you mean by that.

In BMW motorcycle land the K bikes are inline-4 motors that run a stator.  The R bikes are boxer twins and the run a belt driven alternator.  As you stated they are two different types of charging systems.  You originally said alternator and I was correcting you about the K1200GT's charging system being a stator and not a alternator.

[twist]

To control the voltage, excess energy is dissipated in a voltage regulator that starts to conduct over a specified voltage. The excess energy is dissipated as heat. This is why you see finned voltage regulators on bikes like this mounted in the air flow where they can stay cool at higher RPMS when cruising. A down side to these is that they can't be designed with too much excess capacity otherwise the regulator would have to dissipate too much heat when full output is not needed.

So then how can the new K-GT generate 945W?  I imagine that the bike needs less than 400 to run, even with everything turned one (heated grips+seat+high beam).  Are you saying that it's constantly dissipating 545W as heat through the voltage regulator?  That'll cook the voltage regulator pretty quickly.  From what I understand, if the voltage goes above 14.5, then VR simply starts disconnecting from the coils and reconnecting back at such intervals that it's ouput is still 14.5V.  So if the alternator output is 29V, then the VR only connects to the coils for 50% of the time (it connects and disconnects quickly enough that its output is almost DC).  This means that the excess energy is then not dissipated through the VR, but through the coils.

[Mr Sunshine]

So then how can the new K-GT generate 945W?  I imagine that the bike needs less than 400 to run, even with everything turned one (heated grips+seat+high beam).  Are you saying that it's constantly dissipating 545W as heat through the voltage regulator?  That'll cook the voltage regulator pretty quickly.  From what I understand, if the voltage goes above 14.5, then VR simply starts disconnecting from the coils and reconnecting back at such intervals that it's ouput is still 14.5V.  So if the alternator output is 29V, then the VR only connects to the coils for 50% of the time (it connects and disconnects quickly enough that its output is almost DC).  This means that the excess energy is then not dissipated through the VR, but through the coils.

Who knows but I'm sure BMW has it covered.  This is another reason to get the GT over the FJR.  You can light the world and not worry about it.

[twist]

This means that the excess energy is then not dissipated through the VR, but through the coils.

I should correct myself here.  The excess energy is not dissipated through the coils.  It's never generated.  When the VR draws current from the coils, it makes it harder for the engine to turn because it also has to generate the energy that is being drawn.  If you have coils that don't have any current drawn from them, the engine doesn't have to work as hard to turn the magnets.  So in reality, the extra energy you are not drawing from the generator should stay in your gas tank  

[HankB]

In BMW motorcycle land the K bikes are inline-4 motors that run a stator.  The R bikes are boxer twins and the run a belt driven alternator.  As you stated they are two different types of charging systems.  You originally said alternator and I was correcting you about the K1200GT's charging system being a stator and not a alternator.

I originally said alternator since that's what they have used in all k-bikes since they started making k-bikes. Here's the alternator for a K75RT:
http://realoem.com/bmw/showparts.do?model=0573&mospid=47906&btnr=12_0754&hg=12&fg=10
And here's the one on my K1200RS:
http://realoem.com/bmw/showparts.do?model=0554&mospid=48866&btnr=12_0771&hg=12&fg=10
(Don't be fooled because they call it a generator. The picture clearly shows an alternator. Likewise for the K1200R:
http://realoem.com/bmw/showparts.do?model=0594&mospid=48551&btnr=12_1357&hg=12&fg=10
Listed as 50A, that's 700W.

I thought maybe the airheads used stators, but going back to 76, they clearly show an alternator:
http://realoem.com/bmw/showparts.do?model=0311&mospid=47870&btnr=12_0737&hg=12&fg=10
But it looks like my wife's F650CS has a stator:
http://realoem.com/bmw/showparts.do?model=0184&mospid=47820&btnr=12_1069&hg=12&fg=10
(Described in translation as a magneto.)

Sorry twist, no listing yet for the new GT. But suffice to say it's got a true alternator and electrical output will be enough for anything short of trying to light up a runway.


Oh, yeah... the k-bikes also included an inline 3 (K75*) that also had an alternator.

HTH,
hank

[Mr Sunshine]

Well damn I could have sworn it was internal.  Interesting that it looks like it is still belt driven.

[HankB]

Well damn I could have sworn it was internal.  Interesting that it looks like it is still belt driven.

The oilheads use a belt. The K-bikes have a plate with three vanes on something gear driven from the flywheel (or something in the engine) and a cup with three vanes on the end of the alternator. Between the vanes are rubber pieces variously described as goobers or monkey nuts. I kid you not! Described as "RUBBER MOUNTING" (part 9) in the first link.

-hank

[ksann]

Damn, 80amps!

Nowadays many cars have more, but not that long ago, that was an upgrade for a car.




ken