Alternators

[edgar t. kremer]

Automotive alternators don't need a "load", per se. If you consider the battery
a "load", this is true.
The reason battery charging alternators destroy themselves is that the sense
wire from the alternator to the
battery sends the battery voltage back to the alternator voltage regulator. If
the battery voltage is low, e.g. 12 volts, the regulator boosts the output of
the alternator until the battery "sends back" a voltage of approx. 14.1 volts.
If there is no battery to "send back" a voltage, the output of the alternator
will continue to rise, eventually destroying itself.

> Automotive alternators need a load present or you can blow output diodes
> because voltage soars to a high value without a load present.
>
>

 

[heinz wittenbecher]

Edgar,

Does the Alternator sense via the wire connected to the battery, i.e.
Alternator output terminal or via the 2 wires plugged into the alternator
case terminals?

I.e. Can you run the alternator with only the 2 wires plugged in or is the
output --> battery connection required also?

Just wondering as I'm going to be messing around in that department this
weekend.

Heinz

> Automotive alternators don't need a "load", per se. If you consider the
battery
> a "load", this is true.
> The reason battery charging alternators destroy themselves is that the
sense
> wire from the alternator to the
> battery sends the battery voltage back to the alternator voltage
regulator. If
> the battery voltage is low, e.g. 12 volts, the regulator boosts the output
of
> the alternator until the battery "sends back" a voltage of approx. 14.1
volts.
> If there is no battery to "send back" a voltage, the output of the
alternator
> will continue to rise, eventually destroying itself.
>

>
> > Automotive alternators need a load present or you can blow output diodes
> > because voltage soars to a high value without a load present.
> >
> >
>
>

 

[edgar t. kremer]

Heinz,

The two small wires plugged into the alternator: one is for the idiot light,
the other is the sense wire.
Without the charging wire connected to the battery, the sense wire will "send
back" the static battery voltage of 12.6 volts telling the alternator to
increase its output, possibly resulting in a "runaway".

> Edgar,
>
> Does the Alternator sense via the wire connected to the battery, i.e.
> Alternator output terminal or via the 2 wires plugged into the alternator
> case terminals?
>
> I.e. Can you run the alternator with only the 2 wires plugged in or is the
> output --> battery connection required also?
>
> Just wondering as I'm going to be messing around in that department this
> weekend.
>
> Heinz

 

[heinz wittenbecher]

Thanks Edgar,

I'll make sure they are all connected.

Heinz

>
> The two small wires plugged into the alternator: one is for the idiot
light,
> the other is the sense wire.
> Without the charging wire connected to the battery, the sense wire will
"send
> back" the static battery voltage of 12.6 volts telling the alternator to
> increase its output, possibly resulting in a "runaway".
>

>
> > Edgar,
> >
> > Does the Alternator sense via the wire connected to the battery, i.e.
> > Alternator output terminal or via the 2 wires plugged into the
alternator
> > case terminals?
> >
> > I.e. Can you run the alternator with only the 2 wires plugged in or is
the
> > output --> battery connection required also?
> >
> > Just wondering as I'm going to be messing around in that department this
> > weekend.
> >
> > Heinz
>
>

 

[edgar t. kremer]

You must have a different wiring diagram. The one in my GM shop manual X-7725
shows the alternator with three wire emanating from it: #10 red (charging), #16
brown (tell-tale), #12 white (sense).

> Seriously, I don't have
> any separate battery sense wire on my coach, and I don't recall seeing one in
> the wiring diagrams.
> Only disadvantage to such a "machine sensed" alternator is that it doesn't
> compensate for voltage loss through the isolator diodes. Since mid-'70s GM
> alternators run higher voltage than just about any other (up to ~15 - 15 1/2
> volts IIRC), this is a moot point.
>
> Rick Staples
> '75 Eleganza
> Louisville, CO
>

 

[edgar t. kremer]

An excellent description of alternator theory is presented in shop manual X-7525,
Page 6Y-18. Pay particular attention to the operation of zener diode D2.