Is our alternator knackered

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. . . and how do we tell?

One little incident that occured today during our mad motor-sailing beat to Porto Santo was that the 1st mate kicked the engine key and switched it off - she had some feeble excuse like trying to stay on board the vessel during one of our extreme attitude manoevres.

I thought it had died, and was looking forward to twelve hours hove too drifing North in the maelstrom followed by a jolly beat back . . . so naturally was delighted to find it was ony a matter of turning a keyl

But now it occurs to me . . . is the alternator / diode pack likely to be knackered, and how do I tell?
 
no, it should not be damaged. Turning the key, switches off the field and the alternator stops generating. The alternator diodes only get damaged if the alternator output is removed from the battery during operation.

If you remove the generator from the battery, the voltage generated starts climbing rapidly to several hunderd volts. The 6 diodes inside the alternator are designed to rectify high currents, but usually do not survive much higher voltages then 50 Volts. So, if you remove the generator off the battery during operation, the diodes get overloaded and burn thru.

Peter
 
Put a voltmeter across the battery when the engine is charging. It should read 14-15V unless the batteries are knackered. If it is reading 12V or less you are likely to have popped at least one of the 3 diodes used to convert the 3-phase output of the alternator to a rectified single phase charging output. Also, at tick over your red ign light may well be dimly lit if one or more diodes is blown.
 
Kandoma and Searush are right. There should be no problem. I guess you saw a warning light and feared the worst.
 
You should be OK ... as the key generally provides exciter current to the alternator - soon after it starts to turn it will generate its own field excitation - reducing the current via key to zero - that's how the red ign light goes out .....

Easiest indication that all is ok - is if you start engine and red light goes out as normal ... if a faint glow still there then possibly damage has occurred.
As another says - if alternator is pumping charge into a battery - you will meter a voltage over 14 ... probably about 14.4 - 14.8 .....
The real problem is if youy actually disconnect the HD side - the actual charge side while the alternator is charging ... the result is a volt surge on the diodes ... POP.

I carry a cheap Drager Batt / Alt LED tester ... all of £9.99 at Halfords ----- I know you are not near one !! It may be a cheap gizmo - but it certainly told me when my diodes had popped ... It is a quick and good DIY tool for diagnostics ...
(Another use I found for it .... when checking deck connectors etc. - a meter will show 12v etc. - but it may be low current due poor connection etc. - the tester will give good indication by its led's of good or poor connection as it puts a small load on ...)

Anyway a check on the ign light and a meter should answer your worry ...

/forums/images/graemlins/cool.gif
 
[ QUOTE ]
3 diodes used to convert the 3-phase output of the alternator to a rectified single phase charging output.

[/ QUOTE ]
I can point out that there are 6 diodes in the 3 phase bridge rectifier and the output isn't single phase it's DC. There is a supplementary set of 3 rectifiers on many (all?) units to supply the excitation current.
Easiest way to blow an alternator up is to connect the battery the wrong way round so make absolutely sure it's right before you connect. Reckon it's worth putting bold markings and labels on the connections as these things are always in a dark corner where you might not be able to see the cable colours.
 
I think that he was worried that by" switching "the engine off instead of stopping it by the normal method of pulling up the stop lever ( I assume it's a diesel) the alternator would be damaged as it would be if the battery switch had been turned off. At least that's the impression I got from the question. At any rate,no damage should be done by switching off the engine this way.
 
If that had been the case,ie the engine was normally stopped by turning off the key,why then was he worried that any damage might have been done?
 
[ QUOTE ]
There is a supplementary set of 3 rectifiers on many (all?) units to supply the excitation current.

[/ QUOTE ]

Some, possibly most, but not all.

I have a circuit diagram for a SEV Marechal alternator which has a different arrangement. It has the usual 6 diode 3 phase fullwave rectifier pack but the output passes through an additional single diode. The feed for the field is taken from between the main rectifier pack and this extra diode. I dont know what you would call this diode but it is an isolating or blocking diode that prevents the battery back feeding the regulator, bypassing the warning light and "ignition switch". Sorry rather difficult to describe but quite clear on a diagram. Irrelevant to this thread any way.

Well done for spotting the error in the description of the main rectifier. I missed that!
 
That's what baffled me!

I can turn the ignition key to my engine off while it's running, nothing happens until I press the stop button (solenoid operated lever on engine).
 
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