Question #1.
Failure mechanism:
When an alternator is running and delivering current, the current through
the stator windings creates a magnetic field proportional to the amount of
current flowing. The current varies with the resistance of the load, and if
there was no voltage regulator control of the rotor current (and therefore
rotor magnetism), the current and stator voltage would vary also with the speed
of the rotor. In other words, the voltage induced in a wire by a changing
magnetic field depends on the strength of the field and on the rate of the
change. When the stator current in an alternator is suddenly interrupted, the
magnetic field that the current created collapses suddenly too. The lines of
force in the collapsing magnetic field move through the stator windings and
induce a voltage opposite the that which had produced the current in the first
place. The collapsing field dumps a fixed lump of energy into the stator
windings that wants to go through the reverse biased rectifier diodes. Since
the diodes present a high resistance in that direction, the voltage builds to
above the diodes peak inverse voltage rating, at which point the diode starts
conducting heavily, and the energy is dissipated through and in the diodes
themselves. If there is enough energy stored in the stator windings, the diode
is destroyed. Note that this is more likely to happen the larger the current
flowing (i.e. the greater the load on the alternator) when the switch was
opened.
The above description fits what happens when a switch in the output circuit
of an alternator is opened.
The failure you describe is less common I think, because I have never heard
of it, but it would have to depend on a similar mechanism due to the fast
collapse of the current through the rotor. I have gotten away with doing that,
shutting off the "ignition", on several occasions, but I would never do it
deliberately. My first thought is that the risk relative to size of the
alternator load might be reversed. If there were sufficient load on the
alternator then the induced emf (electro motive force) might dissipate through
the load before the voltage built up enough to kill the rectifier diodes. Seems
to me that there ought to be a greater risk to the voltage regulator in
shutting off the "ignition" than to the output rectifier diodes.
Steve Thomas
C&C27 MKIII
Port Stanley, ON
----- Original Message -----
From: Gary Russell via CnC-List
To: C&C List
Cc: Gary Russell
Sent: Tuesday, June 21, 2016 07:36
Subject: Stus-List Alternator Failure
Well, It's happened again. The "ignition" key on my diesel got shut off
while the engine was running (before stopping the engine with the stop lever)
and it fried the diodes in the alternator.
Question #1. Can anyone explain the failure mechanism to me?
Question #2. On my last boat I was able to buy a protection device (I think
it was called a "Zap-Stop") that protected the alternator in the event someone
shut off the key with the engine running. The dealer I bought it from is now
out of business and I can't seem to find it anywhere. Does anyone know how I
can protect the alternator from this happening again?
Thanks,
Gary
S/V Kaylarah
'90 C&C 37+
East Greenwich, RI, USA
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what we do, please help us pay for our costs by donating. All Contributions are
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