re: Alternator cut-out

[FRANK_SCHULZ@xxxxxxxxx]

> Well, no one replied so I'm going to go ahead and try making a switch to
> turn the alternator off and on for the purpose of finding that ever
> elusive HP.  I'll report back with my findings.

ok maybe I can help, (in my poor student days I designed a voltage
regulator replacement for my '80 scirrocco! after 2 factory units died)

be careful that you don't blow up you alternator!

My feelings are that the alternator was designed to see a battery
connected at the output.  I can imagine a slight possibility that the
alternator regulator feedback loop could go unstable if you simply
disconnect the power output of the alternator from the battery.
It all depends on the design of the regulator, it could "overvoltage"
and fry the regulator.

Idealy you would want to cut the current in the rotor "field windings".
This may not be as easy to do, since the alternator probably feeds the
regulator via its own separate diode bridge, once the alternator is powered
up and spinning it could be self sustaining even with the ignition
switch inputs to the regulator disconnected.. depends on the design.

You could wire a switch in series with the rotor brushes, but
you'd want to suppress the inductive voltage glitches when opening
the switch to protect the regulator.

If ya really wanna try this.. here's a science project..!
Here's a possible safe solution, wire up 5 hefty diodes in series with
the alternator output, ~10 or 20 amp rating (or just use diodes from an
old diodepack from an alternator).


battery+-- ~ --|<|---|<|---|<|---|<|---|<|----- alternator output
              |                              |
              |_____________. \._____________|
                    really HEAVY DUTY switch!

normally the switch is closed, the wiring/switch should be rated to handle
the max alternator current.  (plus a fusable link ~ for safety?)

When you open the switch, 3 to 3.5 volts are drpped accross the diodes.
The regulator still senses the alternator o/p and
maintains say max 15.5 volts, simultaneously the current drain
from the alternator drops to almost 0, no more load!
If the regulator goes unstable, the alt output will voltage be safely
clamped to about 3.5 V above the battery voltage.

But.. say this buys you 2 or 4 hp, If you already have around 200hp
can you really feel a 1/2% or 1% or 2% increase in power?

2 hp == 2 x 746 watts = 1492watts,  ==  1492watts/13.5volts = 110amps!
say with 80% efficiency,  80% x 110 = 88 amps
there4 you'd gain ~2 hp if you had a load of 88 amps, and then switched it off.
but normally your alternator load is much less than 88 amps

- -Frank Schulz '92 318is

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