I don’t know how ignition switch would disconnect alternator output form a battery? You need to check that. Perhaps ignition switch shuts off power to the regulator, but event at that alternator should not be outputting current that has nowhere to go. It does sound more like someone moved the house select switch to off while engine was running.
Petar Horvatic Sundowner 76 C&C 38MkII Newport, RI From: CnC-List [mailto:cnc-list-boun...@cnc-list.com] On Behalf Of Gary Russell via CnC-List Sent: Tuesday, June 21, 2016 10:25 AM To: C&C List Cc: Gary Russell Subject: Re: Stus-List Alternator Failure 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 <mailto:cnc-list@cnc-list.com> To: C <mailto:cnc-list@cnc-list.com> &C List Cc: Gary Russell <mailto: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 ~~~~~~~_/)~~~~~~ _____ _______________________________________________ This list is supported by the generous donations of our members. If you like what we do, please help us pay for our costs by donating. All Contributions are greatly appreciated! _______________________________________________ This list is supported by the generous donations of our members. If you like what we do, please help us pay for our costs by donating. All Contributions are greatly appreciated!
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