In a message dated 10/18/2006 6:53:21 PM Central Standard Time,
tr3driver@comcast.net writes:
> The OP said he measured the output at 17 volts. That says it was
> generating,
> which means the regulator was supplying rotor current. Alternator
> regulators
> are switch-mode devices, meaning it supplies either full battery voltage or
> none
> at all to the rotor. With full battery voltage applied, the rotor stores a
> magnetic field, which takes time to decay. Hence there will be spikes,
> every
> time the regulator switches off.
Are you sure about that switchmode of operation. I initially assumed it was
switchmode because all my years of designing regulators for large AC
generators were switchmode (or phase controlled rectifier based) designs. But
the
cursory schematics do provide me with a high degree of confidence that it isn't
a
linear regulator.
But, nonetheless, the alternator also contains several spike suppression
devices on both the field supply and the alternator output to protect against
spikes.
>
> Now maybe those spikes won't be big enough to cause a problem, depends on
> how
> quickly the regulator switches and how much inductance there is in the
> rotor.
> But at the very least, they will be applying more stress to the regulator
> than
> usual, and it's the only thing between you and disaster.
I wouldn't worry about the regulator. Any switchmode regulator will have,
and realistically any linear regulator should have, a freewheeling diode to
carry the rotor current when the transistor switches off (unless you are doing
negative forcing but that's getting a bit esoteric). The rotor is a large
inductor and the current will be a function of the average voltage applied.
That is
why switch mode works so well in these applications. When the transistor
switches off the rotor current must go somewhere and the diode provides that
path.
I would be more concerned about the main rectifying diodes. But these are
beefy diodes and can tolerate quite a bit of avalanche as long as the rise time
of the spike is sufficiently slow to allow adequate current spreading.
When you take your alternator into Auto Zone for testing do they connect it
to a battery during the test?
I would be a highly irresponsible design of an alternator that could not be
run open circuit like that.
But we are talking about Lucas.
Cheers
Dave
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