On Wed, 16 Jul 2003 03:03:48 -0700, "Ed Price"
wrote:
"Glenn Ashmore" wrote in message
...
First of all, teh Zap Stop is not intended to protect anything from
external spikes. There are very few situations on a boat that will
cause a damaging spike back into the alternator other than a lightning
strike and in that case all bets are off.
The Zap Stop is only intended to protect the alternator from self
destructing from a load dump. It does nothing else. It does this by
clamping the voltage lower than the damaging point for the rectifier.
In a load dump the voltage spike is what fries the rectifier diodes not
the amperage.
And how do you "clamp" a voltage when a voltage source is trying to drive it
higher? As the Zap-stop does, by presenting a conductive path that has a
very low resistance. The current is limited only by the source's internal
resistance. I have no idea what the internal resistance of a large
alternator is, but I would guess maybe 0.01 ohms. The important point is
that you have to be able to sink a lot of current during the alternator's
field winding voltage excursion. Putting the protective device on the load
side of the alternator diodes is a solution that requires a heavy current
draw through the alternator diodes during a protective event.
For illustration, lets say we have a big honkin' 250 amp Balmar cranking
out full power 15 volts into the house bank (3,750 watts) when somebody
turns the master switch and the output voltage starts to rise. The
regulator is still supplying the same current to the field and the RPM
has not changed. As no additional energy is being supplied to the
alternator the total power output remains the same.
Absolutely bogus assumption about power output remaining constant.
I am assuming that your alternator has an electronic, not a mechanical,
regulation scheme. Are you claiming that the regulation can track load
variations during normal operation, but, if the load is suddenly shed, it
takes hundreds of milliseconds to react?!
Power is volts *
amps so as the voltage rises the amperage has to drop. For that 200-300
milliseconds that it takes for the regulator to adjust the field current
the zenier has to absorb that 3,750 watts of excess power. Diodes that
can handle this amount of power for that length of time are not hard to
find. Digikey sells them for about a buck apiece.
I checked my Digikey catalog, but can't find any "zenier" diodes. Perhaps
you could fix your spull chucker; your consistent use of the wrong spelling
is beginning to bug me, as real experts know the names of their tools. You
know, you pound nails with a hammer, and discussing nail technology is
disconcerting when the other guy keeps talking about his hummer.
But regardless of how much diodes cost, where you buy them, and whether they
will fail trying to carry x kiloamps for y milliseconds, the initial point I
asked was why it was good practice to put the protection on the load side of
the alternator diodes. Telling me that you can get away with it is not a
good answer.
Ed
Ed - there is a simple answer.
VOLTAGE is the killer, not current. What determines the maximum
output of an alternator? Unlike generators that had a current
regulator, alternators are regulated by the internal resistance of the
stator circuit. The diodes in an average 100 amp alternator are good
for close to 200 amps (continuous) of total output, but only for about
150 volts.
Now 150 volts is MORE than adequate for normal circumstances, but when
you experience a "load dump" the voltage can very quickly exceed even
300 or 600 volts for a short time (minimum of number of stator poles
of cycles) as the collapsing magnetic field in the stator collapses,
inducing voltage in the field, which can also contribute to the
voltage spike). Without a load of some sort to keep that voltage tamed
down, the voltage rating of the diodes can be grossly exceded. The
current flow, being extremely short, does not get the diodes hot
enough to fail from overcurrent, as the current is more or less self
limitting.
The Zap stopper shunts this current to ground, keeping the voltage
well controlled for that very short time - and again, the current is
not high long enough to heat the slow-blow protection fuse enough to
pop it.
The trans/zorb or zap stopper can also protect against the same kind
of spikes should the field circuit open under load - and the
collapsing field current induces a (negative?) spike in the stator
which passes through the diodes and is clamped by the zap stopper.
Now, if you attempted to install the zap-stopper on the stator side of
the rectifying diodes, you could have a serious problem - as there is
AC on the stator, not DC. You would need 2 zap stoppers per phase - or
one per diode - to clamp the voltage internally. That is SIX on an
average 3 phase alternator. And ONE on the rectified side does the
job, just as well.
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