Phase Control
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Original poster: "michael curtiss"
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
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"
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
> Sent: Monday, October 18, 2004 10:00 AM Subject: Re: AC motor speed control question (really my question was
about phase control)
>
> Original poster: "michael curtiss"
>
> 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
wrote:
> >
> > =====
> > Gregory R. Hunter,
finis