You can disconnect the battery while the engine is running and cause
no damage to the alternator, provided the alternator is not putting
out much current. (battery fully charged, no accessories drawing
current)

If the alternator is supplying much current then you will probably zap
the diodes in it.

The voltage regulator monitors the alternators output voltage and
changes the field voltage on the alternator to control the output.

The problem is, being that a magnetic field is involved it does not
change instantly. There is a time lag for the field to collapse and
reduce the alternators output.

When the load is suddenly disconnected from the alternator, when there
is a substantial load on it, there is a large voltage generated in the
alternators windings from the collapsing winding magnetic field. (the
magnetic field collapses because there is no more load on it) That is
connected to the diodes.

The voltage regulator sees the large voltage spike and tells the field
winding to shut down the field current but it is too late as there is
no way to stop the magnetic field in the alternator windings from
collapsing. The regulator can stop the field from building up before
the alternator output current goes up but once it is there, the
magnetic field has to first collapse to stop further output.

When the alternator is putting out a large current the alternators
windings have a high magnetic field in them. Disconnecting the
alternators load causes the magnetic fields to suddenly collapse and
generate a large voltage with nothing to limit the voltage. (no
battery to absorb it)

Note: A changing magnetic field is what generates voltage in the
windings. This happened normally when the alternator is supplying
power as wanted. The uncontrolled change is what causes the damage.

Regards
Gary


On Sun, 13 Jul 2003 02:17:15 GMT, "johnh" wrote:

If he's "running the engine while the battery is disconnected," but he
didn't disconnect while it was running, I'm wondering how he got it started.

"Glenn Ashmore" wrote in message
...


SAIL LOCO wrote:
What does fry diodes is breaking the
output circuit WHILE the alternator is producing current

I think that's what the poster said:

I didn't read it that way.

HE said "running the engine while the battery is disconnected."

I am talking about disconecting the battery while the engine is running.

BIG difference.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
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