Hi Steve,
Perfect explanation! That makes sense to me. I never understood why
turning off the key would create the reverse voltage, and perhaps it was
just a coincidence that the alternator was noticed to be dead a week or so
after the key was turned off. I have no disconnect between the alternator
and my house bank, so it's a mystery why my relatively new alternator
died. Thank you!
Gary
S/V Kaylarah
~~~~~~~_/)~~~~~~
On Tue, Jun 21, 2016 at 9:42 AM, S Thomas via CnC-List <
cnc-list@cnc-list.com> wrote:
> 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 <cnc-list@cnc-list.com>
> *To:* C&C List <cnc-list@cnc-list.com>
> *Cc:* Gary Russell <captnga...@gmail.com>
> *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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