Alternator running unconnected.

[webcraft]

My suspicion that no-one understands electricity at any meaningful level is further strengthened by this thread . . .

I sometimes support High School students in physics, and find the electrics part of the course a bit of a nightmare as I can only cope with stuff that I can understand the basics of. Electricity is all analogies and statements of observed fact, with no real theoretical bedrock . . . at least, at the level that most of us deal with it.

As such, it would be more useful if people could simply relate their own experiences - i.e. have you a) run an alternator with no battery connected and b) connected/disconnected a battery with the alternator running, and what was the result. I would personally be interested in this (statistical) information. I have certainly switched a marine diesel off at least once without (as far as I could tell) damaging the alternator, but am always paranoid about doing so as I have been led to believe that this is 99% certain to kill the alternator, or at least the diodes.

I have to say that if this is really the case, then the fact that this is an easy mistake to make on most of the yachts I have sailed makes this a moronic piece of engineering . . . why can't some kind of breaker be fitted in alternators as standard to prevent this happening? Or fitted as a simple aftermarket mod perhaps?

As I said, I know nothing (Barcelona school of electronics), so no sarcastic replies please)

- Nick

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[MainlySteam]

Brief reappearance - very brief.

"This is getting ridiculous, at first you posted that you dont know much about the subject and now you get paid for knowing this"

People should read other peoples posts, I actually said I knew little about alternators (although it would seem that I actually know more than many), I did not say anything of the sort about batteries.

People can take whoevers view they like, but the fact is that I have a degree in electronic physics and the professionals pay me well for advice on boats. So sorry if I seem a little testy, it is clear that many do not understand what is being said and I am happy to leave at that.

I do trust though, that people will desist, as has happened several times on this thread, from misquoting me - a cynic would suspect that they may do that to try and support their own unsupportable arguments.

As I say, all have fun, I have some peoples new boats to look after today (nice big expensive ones).

John

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[ongolo]

webcraft,

now I am lost - who said switching a marine diesel off will blow the alternator?

What nonese is this? This here is about disconnecting the alternator whilst running or starting it without wires connected, but not about switching diesels off.

Have you noticed that a robot has landed on mars? that would not be possible without an exact undertanding of electronics.

Lord Kelvin had good one about knowledge and being able to express something in numbers. However, all this is no longer informative to anyone and rather weird.

ongolo

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[webcraft]

Well . . . I meant disconnecting the batteries. However, I have been led to believe that this is in effect what happens if the 'ignition' is switched off and the engine left running . . .

In other words, if the engine is not stopped using the fuel cut-off but by using the key, alternator damage may result.

Maybe I have been misinformed for may years . . . and maybe English is not your native language . . . however, I think you should be a little more sparing with the use of words like 'nonsense' and refrain from the rather heavy handed sarcasm.


- N

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[claudio]

Nick,

As long as the alternator output remains connected to the batteries the alternator diodes will be ok. Switching off the ignition usually switches off the field winding only, so there shouldn't be any problems leaving the engine running with the ignition switch off.



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[mickshep]

Hi Webcraft. Just read your last post with interest, My boat is fitted with a Vetus M3.10. Unfortunately the previous owner put the engine control panel low down in the cockpit below the bridge deck. on a number of occasions careless feet have caught the engine kill switch, knocking out the electrical rev counter, would this have the same effect you describe of stopping the engine's electrical side whilst allowing the engine to continue running? and if so am I risking damage to the charging system? Mike

<hr width=100% size=1>"Just because I'm paranoid doesn't mean they're not out to get me."

 

[claudio]

If you switch off the ignition while the engine is running, the alternator may stop delivering output (field coil will be disconnected). This shouldn't harm the alternator, although if you don't notice, you may end up with flat batteries. Doesn't your boat have a buzzer to indicate this condition ?



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[mickshep]

No. Buzzer sounds once the heater plugs have been used to preheat but prior to starting. Once engine fires and charge light goes off, buzzer stops. Only noticed anything was wrong because tacho went off. No key type switch fitted at all though whole system is isolated via main 1,off,2,1&2 type battery switch. Cheers, Mike.

<hr width=100% size=1>"Just because I'm paranoid doesn't mean they're not out to get me."

 

[jfkal]

Correct for this one. Since it needs Battery voltage to "Kick Start" the field coil. Once it puts out voltage and you diconnect the voltage will rise until failure.


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