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Brian Whatcott

On Sun, 28 Sep 2008 18:18:46 +0200, wrote:

I was a a tradeshow where I saw one of the tiny Fischer Panda DC generators
with variable rpm. Nice and (much) too expensive for me, but I am thinking
of putting something like it together myself so I was quite interested in
their implementation.

I spoke with the guy doing the demo and he told me they vary the RPM from
2200 to 2800 only. Seems to me that doesn't make a lot of sense, because I
don't think the extra fuel consumption between idle @ 2200 and idle @ 2800
would be worth the effort of putting in the control circuitry?

I could also hear the little generator increase its rpm substantially above
its minimum when producing only 10A @ 24v, so I wonder if, in practice, you
would get ANY benefit from the variable RPM, because I think most would
shut the generator down when battery charging was down to only 10A. I also
wonder why they feel the need to increase RPM at all at that load since
just controlling the alternator excitation would surely get you 10A at 2200
rpm.

So does anybody understand their choices? Vibration, noise perhaps? Or is
this whole variable rpm implementation of Fischer Panda nothing but a
marketing ploy???

Greetings,

Frank

I know nothing of this gen set. But I can understand the design
trades in question.
Start with this design approach:
low idle speed at no [electrical] load.
Go to max revs at max [electrical] load.

This strategy offers economical loiter (but with the possibility of
coking up a small engine) and a LONG latency to rated load supply.

Instead, idle at relatively high speed. This gives increased idle
consumption, but
1) reduced latency time to supplying max load
2) reduced output dip because there is more flywheel energy available
for the initial surge.

Brian W Altus OK

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