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

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

Original poster: "Lau, Gary" <gary.lau-at-hp-dot-com> 

Hi Michael:

For the application of sync RSG phase control, the variac is NOT being
used as a variable power supply.  It is being used as a variable
inductor, and this, in conjunction with the specified motor run
capacitor, creates a variable phase shift between the mains and the
motor.  For this reason, a light dimmer cannot be substituted; a variac
must be used.

Don't worry about spikes.  This is a pretty benign application, and
variacs are pretty robust.  Using a variac as a phase control element
doesn't create any problems unless your motor connection goes open and
your variac starts resonating with the motor run cap, which is why a
fuse is recommended.

It sounds like this may be your first coil?  A sync RSG is a lot to bite
off for a first-time coiler.  I'd recommend starting with a static gap.
It took me years and several attempts before I got my sync RSG
(http://www.laushaus-dot-com/tesla/sync_gap.htm)  working right.

Regards, Gary Lau
MA, USA

 > -----Original Message-----
 > From: Tesla list [mailto:tesla-at-pupman-dot-com]
 > Sent: Monday, October 18, 2004 10:00 AM
 > To: tesla-at-pupman-dot-com
 > Subject: Re: AC motor speed control question (really my question was
about phase
 > control)
 >
 > 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
 >  >
 >  >
 >  >
 >  >
 >