Lew and Others,
I have been following this thread very closely and do agree with all that has
been written... I do not post much and then there is a question that I would
like to pose to you... I had brain cramp and turned off the batt switch before
turning off the engine, I have a Balmar 200 amp alternator, after I realized
what I had done, I quickly shut down the engine... I then proceeded to remove
the alternator and have it bench tested at the local alternator shop... Which
turned out to be nothing wrong with the alt., now how do you see how this was
the case, that no damage came to the alt.???
TIA
Sea Ya
Larry
S/V Two Step-N
N 40* 53.320'
W 073* 47.122'
----- Original Message ----
From: Lew Hodgett <[EMAIL PROTECTED]>
To: [email protected]
Sent: Wednesday, May 7, 2008 12:23:40 AM
Subject: Re: [Liveaboard] "Fied" nectin
" capteric36" wrote:
> The field connection on a battery switch is
> there to protect the alternator diodes in the
> event that the battery switch gets turned to the
> off' position while the alternator is spinning.
Close, but no cigar.
> If
> there is still field current but no load (battery)
> the output voltage of an alternator will very
> quickly rise high enough to destroy the diodes.
True.
> Thus the field connection in the battery switch is
> designed to open before the battery connection,
> shutting down the alternator. DO NOT rely on this,
> as the time it takes the magnetic field in the
> alternator to collapse may often be enough time to
> destroy the diodes.
True.
Basic rule of the boat battery switch.
If the alternator is rotating, DO NOT TOUCH THE BATTERY SWITCH.
WHY?
The generated alternator voltage that is developed without being
connected to a load (Battery) will generate an inverse voltage that
will destroy the silicon diode bridge.
The above is totally independent of the alternator field circuit.
Turn the battery switch "ON" or "OFF" only when the engine/alternator
is at rest.
The use of the oil pressure switch in the field circuit is used to
reduce the starting torque requirement of the cranking motor.
There is enough residual magnetism in the rotator of the alternator to
generate a limited amount of power at the output terminals, thus the
need to be connected to the battery.
An old trick, if the AC output terminals are available, is to connect
a half wave rectifier bridge to the coil of a relay whose contacts are
in the field circuit.
Thus the cranking motor did not hasve to overcome the alternator
starting torque.
As soon as the engine is cranked over and starts, enough voltage is
generated to close the relay thus applying power to the field circuit
which starts the alternator charging.
No AC terminals available, the oil pressure switch works just fine.
Lew
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