fusible link questions

[KD6JDJ at aol.com]

  There are some several statements made here that are not exactly true, and
(I think) can be misleading, especially if you are actually intending to
really build (or design) something. If you guys are just exchanging thoughts ,
I opologize for intruding.
  I have tried to make an automotive alternator using rare earth for the
rotor. It was VERY tough for me to get the results that I was trying for. NOTE
-- The curent in the stator produces a magnetic field that rotor has to
contend with. No problem at low output current, but , 100 amps thru the stator
produces a magnetic field that should be considered since the rotor is
influenced by it.
  I suggest that power disipated in the rotor will be around 40 to 60 watts
with 12 VDC applied to it. The power is a little difficult to remove from it
because of its location.
  I think that a good way to look at the analysis of the diode power
disipation is to consider that each diode WILL drop ~ 0.7 volts forward
voltege drop. And there will always be 2 diodes in series with any current
path from the generator to the load. So a 100 amp load DC curent may be
considered to have been to delivered equally thru 3 paths of 2 diodes each.
How about   -- 33 amps thru 0.7 v equals about 23 watts EACH DIODE for a total
of about 140 watts.
  [[ I am curious about how a professor has determined that automotive
alternator diodes will have a forward voltage drop of 2 volts.]] 
  There is no standard that restricts the automotive alternator to being delta
connected. there are MANY Y connected stators. In fact, I thought that most
alternators were Y connected. I assume it is clear that a given stator will
produce a lot more current when it is DELTA connected compared with when it is
Y connected. The RPM for max output will differ greatly of course.eilds or
stator, or both.

     Jerry


>> 
>> I hadn't even considered the power dissapated by the field coil.  It
>> should be small compared to the power dissapated by the rest of the
>> system.  If not, it can be replaced with a ceramic or rare earth
>> permenent magnet.
>> 
>Which would produce a non-regulatable device. You would then require an
>active regulator to dissipate the extra power produced externally.

That's the point of the whold discussion.  I would run the alternater at
a higher voltage and use an efficient switching regulater to step it
down.  Much like a transformer, a switching regulater will step up the
current when it steps down the voltage.

A switching regulater does not regulate by dissapating the extra power.


>Most good alternator diodes are rated at 300PIV. 
>If the diodes have a .6 volt forward drop, at 100 amps they are
>dissipating 60 watts each. In 3 Phase, they share the load, so likely
>see closer to 35-40 amp peaks, and 33 amp average for 20 watts per
>diode, or 200 watts? total diode dissipation at full load. This does not
>change with voltage, but does with load. Even if the load is split
>between the two diodes of each pair, that would be 100 watts total diode
>dissipation - a good reason to have fins and a fan. This dissipation is
>an order of magnitude higher than the field or stator losses.

I discussed this very thing with one of my electronics professors.  Yes,
silicon diodes drop 0.6 or 0.7 volts normally.  The diodes in an
automotive alternater, however, will drop more like 2V under heavy load.

An automotive alternater is wired in delta, and uses six diodes to
rectify the output.  The current from any coil will go through two diodes
in its path through the load.  That gives us a total voltage drop of 4V. 
Since we're supplying around 14V, the diodes dissapate about 28% of the
total output.  This means that if the alternater is providing 1000 watts
to the load, it is dissapating an additional 280 watts in the diodes. 
The stator windings are dissapating some power, too.