Re: AC motor speed control question (really my question was about phase control)

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "michael curtiss" <lucknau-at-yahoo-dot-com> 


Hi there,

Thanks, you're right, I meant phase control.

So, what the phase control circuit does is not change
the motor speed continuously, but rather it bumps the
position of the spinning electrode(s), to match the
sine wave of the coil.

And if I did want to vary the speed of a synchronous
motor (I don't, though, I now realize) I'd then have
to adjust the frequency of the electricity.

So I guess my next question is about the variac.

I've read that I probably wouldn't want to use a light
dimmer on a circuit like this, because the dimmer
controls voltage basically by just lopping off chunks
of the sine wave, which in turn can result in voltage
surges generated by the inductive load (the inudction
motor).  And these voltage surges can fry the dimmer,
and possibly the operator?

Is this true, and if so, does a variac face similar
issues?

I guess I'd prefer the light dimmer route because its
cheaper and will hold me over till I find a variac.

BTW, I wrote a long question, asking about how the
phase controller functions, but just found a previous
post (Re: RSG sweet-spot.. and probably others) that
helped clarify.

Michael




 > Terminology mistake? Your stated goal of building a
 > speed control for a synchronous gap is a paradox.
 >
 > The SRSG depends on the speed NOT varying any--not
 > even a little. Otherwise, it wouldn't be
 > synchronous.
 > However, it is possible to build a phase shifter
 > using
 > a variac and an appropriately sized capacitor. This
 > allows the operator to remotely sync the gap with
 > the
 > AC sine wave for best performance. Is this what you
 > had in mind?
 >
 > The ARSG on the other hand, does require an external
 > speed control. If you base the async rotary on a
 > so-called "universal" motor (angle grinder, electric
 > drill, electric weed-eater, vacuum cleaner blower,
 > etc.), you may vary the speed using a variac or even
 > a
 > cheap lamp dimmer. With such an arrangement, there's
 > no need to worry about surges, spikes, or other
 > weirdness. Just plug it in and adjust the speed to
 > your liking.
 >
 > Best Regards,
 >
 > --- Tesla list <tesla-at-pupman-dot-com> wrote:
 >
 >  > Original poster: "michael curtiss"
 >  > <lucknau-at-yahoo-dot-com>
 >  >
 >  > Hi all,
 >  >
 >  > I accidently sent a blank email as my first post
 >  > ever
 >  > to this group.  My apologies.
 >  >
 >  > I'm building an SRSG, and want to make a speed
 >  > controller circuit for my AC motor.
 >  >
 >  > What I've been reading is that there's a danger
 > of
 >  > kickback spikes when trying to vary AC current
 > with
 >  > something like a variac.  There're a lot of
 >  > references
 >  > out there to a motor speed control circuit on
 > John
 >  > Freau's site, but the link no longer exists.  I'm
 >  > wondering if anyone knows of a good way (and/or
 >  > schematic) to safely vary the speed of an AC
 > motor.
 >  >
 >  > thanks,
 >  > Michael Curtiss
 >  >
 >  >
 >  >
 >
 >
 > =====
 > Gregory R. Hunter
 >
 > http://hot-streamer-dot-com/greg
 >
 >
 >
 >



		
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