On 1/15/2013 11:35 AM, Klaus wrote:
I recall seeing a picture of a mechanical PWM controller comprising of a
segmented copper disk being spun by smaller electric motor. Two brushes
that could be moved relative to one another sat side-by-side on the disk
making for PWM control. I can only imagine the shower of sparks it created
running an electric motor for the little while it lasted, unless they had
some heroic snubber across it.
This idea has surfaced many times. It gets tried, and fails. Arcing is
*tremendous*, and the contacts soon wear out.
A snubber is vital to get it to work at all (for hours, instead of
minutes). A freewheel diode helps a lot more (days, instead of hours).
These weren't available until the last 20-30 years; before that, every
mechanical chopper was a reliability headache.
There are a lot of very sophisticated design tricks that have to be used
to make this kind of mechanical switch-based chopper last long enough to
be at least marginally useful. The usual backyard inventor types usually
miss this.
One trick is synchronous resonant switching. The circuit is arranged so
the switch opens and closes at zero current and/or voltage. This was
used in the 1950's car radio vibrators, to step 6vdc up to 250vdc for
the vacuum tube radios.
The commutator in a DC motor uses these tricks as well. It's designed so
the voltage is zero as the brush crosses from one commutator bar to the
next.
--
The trouble ain't that there's too many fools, but that lightning ain't
distributed right. -- Mark Twain
--
Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm
_______________________________________________
UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub
http://lists.evdl.org/listinfo.cgi/ev-evdl.org
For EV drag racing discussion, please use NEDRA
(http://groups.yahoo.com/group/NEDRA)
