Phase Control
Date : Mon, 18 Oct 2004 11:08:30 -0600
Subject : RE: AC motor speed control question
(really my question was about phase control)
Original poster: "Lau, Gary"
<gary.lau@hp.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.com/tesla/sync_gap.htm working right.
Regards, Gary Lau
MA, USA
> -----Original Message-----
> From: Tesla list [mailto:tesla@pupman.com]
> Sent: Monday, October 18, 2004 10:00 AM
> To: tesla@pupman.com
> Subject: Re: AC motor speed control question (really my question was
about phase
> control)
>
> Original poster: "michael curtiss"
<lucknau@yahoo.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@pupman.com> wrote:
> >
> > > Original poster: "michael curtiss"
> > >
<lucknau@yahoo.com>
> > >
> > > Hi all, I accidentally 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.com/greg
finis