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Re: SRSG Phase Control Question
Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <FutureT-at-aol-dot-com>
In a message dated 8/27/01 12:50:13 AM Eastern Daylight Time,
> Original poster: "Dave Kyle by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> I am in the process of completing my SRSG and plan to use John Freau's cool
> SRSG phase control circuit
> http://members.aol-dot-com/_ht_a/futuret/images/phshift4.jpg in place of
> physically rotating the motor. It has been some time since I worked with
> Variacs and I want to check my understanding of the wiring. I believe the
> Variac circuit shown is different from my pre-wired encased Variac; in
> words I will need to rewire the unit such that it will no longer be usable
> as a voltage controller. This is not a problem as it will be dedicated to
> Tesla work anyway but I want to be sure this was the case prior to opening
> it up. I assume a 5 amp Variac will be adequate for this use my 25 Watt,
> 1/30 HP synchronous motor.
Yes, it should be adequate. You'll probably need to use a smaller
value phase shift cap than the value I showed to keep the voltage
across the motor low enough across the variac's range, because
you're using a smaller motor. There are
two types of enclosed variacs; one with a line cord and plug and
an ac recepticle built in, and there's also the type with all the
terminals brought out to screw terminals. Either type can be
used without any modification. With the variacs using screw
terminals, it's just a matter of selecting the two proper terminals.
For the type with the built-in line cord and outlet, you can make
it work by using only the output recepticle terminals, since
these form a series loop through the variac. One side of the
output outlet is connected to one end of the variac winding,
and the other side connects to the wiper, so the terminals
you need are accessible. This necessitates using a male
ac prong as a hot lead for the external wiring, so it won't meet
any type of wiring
code. It's also possible to access the same terminals by
using one of the ac plug prongs (neutral), and one output
recepticle slot (hot lead). This method still probably can't
meet any code, but either method could be used if the
unit is sealed into a cabinet or the connections taped up, etc.
for safety. Some prongs and slots will be unused of course.
You can of course open the housing, and connect
directly to the desired terminals as needed as you suggested.
I just wanted to mention that you can use the variac "as is"
for phase control work, using suitable external wiring connections.
This way, the variac can remain intact, for future use in the
traditional way. (I've never done it, but it has to work)