Alternator disconnected from battery while running : Risk ?

[GALAWA]

Oh you electric wizard ,
If the alternator is disconnected from battery while the engine is still is running does the alternator or its voltage regulator risk being damaged ?

According to some old rumors the answer would be yes, internal rectifying diodes were supposed to suffer from over-voltage .

But do you know whether this still applies with modern alternators and regulators ?

[Ostinato]

Absolutely if you consider Balmar alternators modern. It is catastrophic.

[Sailmonkey]

Yes, it still applies. You might get lucky and only damage the diodes and not destroy them, but they will fail at a later date.

[Wotname]

Originally Posted by GALAWA

Oh you electric wizard ,
If the alternator is disconnected from battery while the engine is still is running does the alternator or its voltage regulator risk being damaged ?

According to some old rumors the answer would be yes, internal rectifying diodes were supposed to suffer from over-voltage .

But do you know whether this still applies with modern alternators and regulators ?

More than just a rumour.

The problem occurs because of output current is interrupted thus its EM field collapses cutting the stator coils of the alternator. This sudden collapse of the field through the coils creates the voltage spike that kills the diodes.

The size of the spike is directly proportional to the size of the output current at the time it is interrupted i.e. large current, large spike and vice versa.

Modern alternators have the same coils and the same diode packs so the problem remains the same.

Note, the regulator itself is not affected.

[john61ct]

Absolutely, yes.

What scenario were you contemplating where this may happen?

There are protective devices supposed to absorb the sudden load dump, and special switches to prevent the problem, but best practice is to always ensure there is a place for output to go.

[Jim Cate]

[QUOTE]There are protective devices supposed to absorb the sudden load dump, and special switches to prevent the problem, but best practice is to always ensure there is a place for output to go.
/QUOTE]

Or switch off the field supply first... then is is ok to disconnect the output while running.

Jim

[decktapper]

GALAWA:
My guess is that your concern is when you click your 1 2 all none switch from 1 to all or 2 to all when changing batteries, or may be that when someone is switching from 1 to 2 or 2 to 1, instead of going through all they will go through None, am I correct? I think this cost me an alternator one time.
Thanks.

[transmitterdan]

You can buy zener diodes that "protect" the alternator diodes. Zap Stop is a brand. But you can buy the diodes pretty cheaply from Digi-Key or Mouser in the US. Farnell in EU may have them.

All they do is stop the voltage from rising too high and causing the rectifiers to avalanche. These protective diodes don't have to handle a lot of power.

Disconnecting the field circuit before disconnecting the battery is a good idea. Some battery switches include a small extra switch for this purpose.

[john61ct]

Originally Posted by Jim Cate

Or switch off the field supply first... then is is ok to disconnect the output while running.

That's what I meant, like Blue Sea's AFD feature on its 1/2/B/O switches.

But apparently (per MS) even experienced pro's often fail to wire these properly, same as the Start Isolation feature on their ACRs.

It's not "while engine running" that's the problem, it's "while alt is outputting".

It's also good to be able to shut off field current from the helm, for situations where you need all power for propulsion.

[jamhass]

Originally Posted by john61ct

It's also good to be able to shut off field current from the helm, for situations where you need all power for propulsion.

My older Volvo does not have a fuel shutoff solenoid so the "ignition" switch does not turn off the engine, only the alternator field. Thus, it is easy to safely shutdown the alternator with the engine running via the "ignition" switch.

As implied above, it would be straightforward to add this feature to engines that do disable fuel flow via the "ignition" switch.

[Maine Sail]

Originally Posted by jamhass

My older Volvo does not have a fuel shutoff solenoid so the "ignition" switch does not turn off the engine, only the alternator field. Thus, it is easy to safely shutdown the alternator with the engine running via the "ignition" switch.

As implied above, it would be straightforward to add this feature to engines that do disable fuel flow via the "ignition" switch.

Just be sure it is actually cutting the brush/field wire and not just a 12V excite wire. Once an alt is excited you can remove 12V from the excite wire and most alts will still keep going. Interrupting a 12V excite is not the same as cutting the field and this is often confused..

[jamhass]

Originally Posted by Maine Sail

Just be sure it is actually cutting the brush/field wire and not just a 12V excite wire. Once an alt is excited you can remove 12V from the excite wire and most alts will still keep going. Interrupting a 12V excite is not the same as cutting the field and this is often confused..

MS:

Very good point, thanks.

... but:

How does one tell? In my case, the switch shuts off power to the Balmar external regulator, which then controls the alternator. I have on occasion switched it off and to date with no ill effects. I do know (from my battery monitor) that the alternator is not generating with the switch off.

[transmitterdan]

Jim,

If the VR has no power then the field circuit should be well and truly off. Assuming it is installed per Balmar instructions.

[Maine Sail]

Originally Posted by jamhass

MS:

Very good point, thanks.

... but:

How does one tell? In my case, the switch shuts off power to the Balmar external regulator, which then controls the alternator. I have on occasion switched it off and to date with no ill effects. I do know (from my battery monitor) that the alternator is not generating with the switch off.

Sorry I assumed from the post you had a stock Volvo alternator.. With an external regulator, shutting the regulator off cuts field and thus de-powers the alternator safely...

[jamhass]

TD, MS:
Thanks. Always something to learn.