Re: DC motor - uh oh, in over my head

On Mon, 14 Aug 2006 16:54:46 -0500, Richard J Kinch
<kinch@xxxxxxxxxxx> wrote:

Don Foreman writes:

The sigh is a nice note. Slightly subtler than something like "oh,
the cretins I patiently suffer..." , and certainly more civil than
"wrong, you bloody idiot!"

Well, you're not wrong, you're quibbling in the Socratic sense. You're
correct that if you shrink a capacitor to arbitrarily small value, that it
eventually becomes as if you had no capacitor at all. A physical problem
is turned into a word game involving definitions. This is a hazard of all
casual discussion of complex subjects, and you can't get more casual-but-
complex than Usenet, eh?

Socrates led an existance supported by his patrons but the rest of us
have to buy our capacitors. A vague "use a capacitor" may not be
real helpful to the OP who said he's not an EE and would like a bit
of help. How much is needed is not a quibble if he doesn't know.

The "word game involving definitions" was devoid of definitions of
either "large" or "small". Shame on us. I'll treat this with specific
values and partnumbers rather than vague notions.

The size and number of caps will be determined by ripple current as
well as capacitance. A data point: Xicon LS2222M2D-3550, 200
WVDC, 2200 uF, 5 amps RMS permissible ripple current , $7.25 each from
Mouser. Four of these, $29 worth plus shipping, would pass the
ripple current test, but 4 x 2200 uF still leaves about 70 volts
peak-to-peak ripple with a 2.85 ohm (60 volts/21 amps) load. It also
has an avg value of 125 volts even with halfwave rectification,
considerably higher than 60 volts, as you noted previously -- so
without some voltage control means something will smoke. Unless
the voltage control matter is addressed, that is neither a viable
design nor good design guidance.

Halfwave rectification with a snubber will deliver a safe 52 volts for
4 bux. There will be some torque ripple; whether this is onerous
can only be determined by trying it in the intended applications. The
ubiquitous AC-driven universal motor also has significant torque
ripple, does a great many jobs quite satisfactorily.
.

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