alternator damage

[PabloPicasso]

Running the engine with power off can damage the alternator. the recitifier can blow and the aternator can output 100volts or more without this working,

This happened on my boat a couple of years back and literally boiled the battery. I couldn't work out what the smell was!

Eventually I felt the heat coming from the battery and noticed it had adopted a somewhat bowed shape as the plastic sides started to melt!

Keep all the engine switches in the on position when running the engine!!

So 'Oscarprop' I suggest modifying your engine key position so it cannot be knocked off from the helm position by accident. Put a cover on it or move it!!

 

[Talulah]

Running the engine with power off can damage the alternator. the recitifier can blow and the aternator can output 100volts or more without this working,

This happened on my boat a couple of years back and literally boiled the battery. I couldn't work out what the smell was!

Eventually I felt the heat coming from the battery and noticed it had adopted a somewhat bowed shape as the plastic sides started to melt!

Keep all the engine switches in the on position when running the engine!!

So 'Oscarprop' I suggest modifying your engine key position so it cannot be knocked off from the helm position by accident. Put a cover on it or move it!!

My immediate reaction to this is that the alternator was wired up incorrectly.
How was the alternator sensing voltage wired?
I suspect with the key off the alternator sensed a flattened battery and was trying to pump max output in to a battery that wasn't flat.
It could be that whoever wired the alternator up connected the battery sense wires and the exciter wires the wrong way around.

 

[VicS]

My immediate reaction to this is that the alternator was wired up incorrectly.
How was the alternator sensing voltage wired?
I suspect with the key off the alternator sensed a flattened battery and was trying to pump max output in to a battery that wasn't flat.
It could be that whoever wired the alternator up connected the battery sense wires and the exciter wires the wrong way around.

It sounds as though the alternator was battery sensed but that the sensing was disconnected due to incorrect wiring and or incorrect switch operation.

Perhaps the sensing was connected to a battery which was switched out of service. The alternator then seriously overcharged the battery which was in service.

It can easily happen if the sensing wire is connected to the battery side of an isolating switch.

 

[pampas]

or main diode ?

 

[mjf107]

Running the engine with power off can damage the alternator. the recitifier can blow and the aternator can output 100volts or more without this working,

This happened on my boat a couple of years back and literally boiled the battery. I couldn't work out what the smell was!

Eventually I felt the heat coming from the battery and noticed it had adopted a somewhat bowed shape as the plastic sides started to melt!

Keep all the engine switches in the on position when running the engine!!

So 'Oscarprop' I suggest modifying your engine key position so it cannot be knocked off from the helm position by accident. Put a cover on it or move it!!

Assuming by rectifier your referring to the main diodes within the alternator, if these were to blow surely the alternator output would fall to zero?
If the battery were to become detached from the alternator under heavy charge the voltage could indeed rise to over 100 volts this in turn would then blow the diodes!!! I also believe an internal short within the battery could send the alternator into full charge, then when over heated the battery could fail open circuit blowing the diodes. I'm not disputing the fact that you experienced a serious fault with the charging circuit, just that you might have been looking at the resulting damage rather than the route cause. Others have pointed to the alternator being incorrectly wired, was this ever looked into at the time?

Michael

 

[lw395]

There are several failure mechanisms.
1) The residual magnetism in the rotor may be enough to generate a fairly high voltage, which blows up the regulator or the aux diodes which supply it.
2) the regulator goes into oscillation
3) The inductance of the stator generates a spike on switch off, which takes out the regulator
Once the regulator is faulty the alternator can easily drive itself to a high enough voltage to damage diodes. If one of the auxillary diodes which supply the regulator goes, it will sense some fraction of the voltage and try to correct for that, producing rather more than 14V.

Good design of the regulator might make any of the above less likely, but if it's not in the design spec, it may not even be a design goal. Alternators are mostly vehicle designs and vehicles rarely have battery isolators, so regulator designers have no interest in the problem.