It's not just the inductance of the wiring and the 3 phase windings on the
alternator though. Mechanism is like a DC-DC with a very slow feedback
loop. This is the rotor inductance causing the problem, during the high load
current time the control current in the rotor is increased to maximum by the
regulator and this current takes a long time to decay when the regulator
senses an over voltage. During this time there can be little load on the
electrical system and the alternator is actively pumping out as much current
as it can! For this reason Alternators use "avalanche rectifiers" that are
basically zeners to control the voltage.
https://www.jstor.org/stable/44472894?seq=1
"Yes, a DC/DC is different; but the same principles apply. An alternator has
no output filter caps, but has lots of inductance. When you open-circuit
it, you get a tremendous voltage spike."
Yes, you can make it whatever you want it to be by changing the time
you choose. A typical load dump pulse is up to 400ms and can be 120V in real
life. The peak current in this time is the maximum current the alternator
can put out, for a car about 150A.
" Suppose something momentarily shorts the DC/DC output, and its capacitors
deliver a peak current spike of 1000 amps; which almost immediately burns
out the short. The wire between the DC/DC and that short has a mere 1
microhenry of inductance. V = L x di/dt. 1 uH x 1000a / 1usec produces a
1000 volt spike.".
-----Original Message-----
From: EV On Behalf Of Lee Hart via EV
Sent: Thursday, 22 April 2021 6:46 am
To: George Tyler via EV <ev@lists.evdl.org>
Cc: Lee Hart <leeah...@earthlink.net>
Subject: Re: [EVDL] From my nissan leaf .com: Why the Leaf 12v system
undercharges the 12v battery.
George Tyler via EV wrote:
> An alternator is very different to a Dc-Dc converter in this respect.
> Modern high frequency DC-DC conv. Have very small inductors storing
> very little energy. An Alternator has a HUGE inductor storing a lot of
> energy, and this is not on the output but in the control side! So has
> much more effect! You won't get much overshoot from the DC-Dc converter.
Yes, a DC/DC is different; but the same principles apply. An alternator has
no output filter caps, but has lots of inductance. When you open-circuit
it, you get a tremendous voltage spike.
A DC/DC has negligible inductance, but huge output filter capacitors.
When you short it, you get a huge current spike. Since there is always
wiring inductance, this high current "charges" the inductance. When the
short suddenly goes away, you still get a huge voltage spike.
"Suppose something momentarily shorts the DC/DC output, and its capacitors
deliver a peak current spike of 1000 amps; which almost immediately burns
out the short. The wire between the DC/DC and that short has a mere 1
microhenry of inductance. V = L x di/dt. 1 uH x 1000a / 1usec produces a
1000 volt spike."
Yes, and you will also get a 1000 Amp current with the same short on
a battery. Car electronics is designed for load dump and from experience it
is quite hard to achieve. This is 0.5J energy, load dump could be 6,000J,
far more of a problem.
I have been thinking of doing this with supercaps with a added small
DC-DC to keep the caps charged from the main battery but finding a place to
take the main battery supply off is not trivial. The Solar panel should be
able to keep the caps charged, except if there is no sun.
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