Actually I was talking about alternators with an internal regulator with
the concern being toasting the regulator. Old-style external regulator
alternators are easier to interrupt--you can open the field coil, just
like the regulator does.
Regulators in alternators do not work by modulating the field current but
by switching it. They typically have a fixed switch rate, like 400 cps.
When the system needs more voltage the duty cycle varies--like 10% on and
90% off at idle, changing to 100% on and 0% off at full. This kind of
system has to handle back emf from the field coil 400 times a second.
I couldn't find a schematic for a car regulator, but did find one for a
BMW motorcycle, It's got a thyristor in the output that looks like it's
there for transient protection. When the voltage on the output gets too
high a zener triggers the thyristor which discharges through a resistor.
That's a common over voltage protection scheme.
I've used old-style alternators to power tools with universal motors long
ago--it was an easy conversion to make. What probably killed the little
cottage industry of alternator power converters that used to advertise in
the back of Popular Mechanics was internal regulators. You can't get the
alternator to generate higher voltage by doing anything externally.
Not to get too carried away with all this, but switches are not arc
welders--at least not ones that work for long. They snap open rapidly and
are designed to minimize arcing. The original stated concern was not that
an arc would be drawn by the switch opening, but that the high voltage
transient could arc over the open switch contacts. The alternator takes
time to generate the transient--it's a big coil. The magnetic flux has to
collapse and cut through the wires before the voltage is generated. That
takes a long time in the high-reluctance output coil. Since the switch
opening is the initiating condition, by the time the high voltage
transient occurs the contacts will be some distance apart.
-----Original Message-----
From: Randall Young [mailto:Ryoung@navcomtech.com]
Sent: Wednesday, May 07, 2003 5:10 PM
To: fot@autox.team.net
Subject: RE: Alternators and Master Cutout Switches
> A peak of only 140V is not sufficient for an arc across any switch
> designed for even 12V.
Sure it is, if the contacts are close enough together at the instant.
Open circuit on an arc welder is only 30-40 volts.
> The only electronics that would be affected by a
> properly wired straight disconnect would be those on the alternator
> side of the switch, which should be only the regulator and the
> rectifiers. Certainly there is no problem with having a switch that
> connects a resistor before opening the circuit, but it sounds very
> belt-and-suspenders to me.
No argument there.
> You also might be better off with a large
> capacitor (something in the range of 1uf at 480V) than a resistor,
> since it would be open circuit to steady DC and would be unlikely to
> present a hard-to-diagnose switch or wiring failure. You wouldn't't
> even have to have a make-before-break switch to use it. It might be
> what they use internally to limit transient voltage, though a high
> voltage zener is more likely.
For the alternators I've had apart, there is nothing internally to limit
transient voltages, except for blowing diodes. In fact, most of them will
happily generate 120vdc all day with the regulator disabled. This used to
be a popular modification, before gen sets and battery powered tools
became common, as many 120vac tools and lights will run on 120vdc.
I've even seen articles on using a big truck alternator as a welding
source; although I've also heard it's apt to pop diodes.
Randall
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