Variac for Hotwire

Doing more searching I found this:
Loading Image...

But it also looks as if there might be variacs that don't need the isolation
transformer. Or is it that some just don't use one. What's the scoop?

http://www.public.iastate.edu/~orman/air/cutter/hotwire.html

Post by Grant
Hello,
I'm looking for info on using a Variac for a hotwire. I know nothing
about it but did a google group search and there was some mention at this
group about them being best.
Is there any thing special about them I should be aware of?
I am gunna be cutting fairly wide 2# EPS foam. The hotwire will need to
be mostly about 2' long but up to about 4' long at times.
One guy on this group said you needed a step down transformer before the
variac. What's the scoop?
There seems to be a bunch of variac's on eBay. Any recommendations as to
brand or model?
Thanks,
Grant

Third Stage Hovernaut

2004-11-04 06:08:39 UTC

Post by Grant
http://www.public.iastate.edu/~orman/air/cutter/page9.jpg

Hey,

Since you are at Iowa State look at:
Loading Image...

This shows a 120 dimmer switch in front of a 100 dollar battery
charger. The charger output voltage is reduced by reducing the
incoming 120 voltage from the line. the output Voltage would be 14
voltage but the dimmer is turned down to allow only 1.5 volts at the
charger out.

Also the picture shows a safety strap across the wire to prevent
the dread hot wire tatoo.

Also at the site is pix showing using weights to apply continous
gentle pressure to the bow allowing one to stay away from the fumes
when cutting also use lower voltages to cut with.

http://www.public.iastate.edu/~pfoley/uh/2001GWC/gwc.htm

--Pat

Post by Grant
But it also looks as if there might be variacs that don't need the isolation
transformer. Or is it that some just don't use one. What's the scoop?

Ken Roberts

2004-11-03 18:36:25 UTC

grant,

i use a hotwire directly connected to the ac line.

a variac IS the best way to make a hot wire, because you get precise control.

the key here is to know the limits of your equipment. if your variac is not
suitable for 120vac then you need a step down transformer. you should be able
to handle at least 10 amps, preferrably more. you need not worry about ccs
ratings, icas is good enough. in other words, the intermittent rating on the
variac is ok. maybe the person talking to you was thinking of a dimmer switch,
which doesn't have precise enough control to be useful without a step down
transformer.

you may want to make more than one hotwire frame. make the bow at least a foot
longer than your wire, because you use that extra room up quickly with springs
and such. it's possible to make a spring loaded bow, but it's harder that way.
a spring loaded bow will mean you need less room at the wire side and your bow
can be shorter.

generally, i make a bow out of 1/2" conduit and put in some old spark plugs into
the end. i tape them so that when i change my wire i don't lose the spark
plugs. the wire is tied around one spark plug electrode and also to a spring.
the other end of the spring goes around the second spark plug. current is
applied to each end of the wire, not through the spring. applying current
through the spring will turn your spring into a long wire rather than a spring.

you want enough tension on the spring when cold to account for the wire's
expansion when it heats up. for a 4-foot wire, you will get quite a bit of
expansion, maybe as much as 4". more than that, you need to keep enough tension
on the wire to keep the wire fairly straight as it goes through the foam. the
center will tend to trail behind, so if you are making a curved surface you will
get a hollow on the part which is inside the curve and a hump on the outside.

you need to balance tension required with the wire's tendency to break when it's
hot. you also need to balance the thickness of the wire with strength, because
the thicker the wire the more current it carries before getting hot. more
current means bigger (and more expensive) variac, etc.

my variac is rated for 22a icas, 17a ccs @ 120vac. variacs in general are a
donut of ferrous material, wrapped by a single layer of transformer wire. there
is only a primary circuit, there is no secondary winding. IN OTHER WORDS YOU
BETTER USE A FUSE ON YOUR PLUG! you could potentially get a direct connection
between you and the wall outlet with this thing.

anyway, the top of the donut is cleaned of wire insulation on the primary, and a
sweeper arm connects to the wire at any point. if you're hooked up to 120vac
and there are 120 windings, then your sweeper will give you 1v increments. my
variac gives me about 2.25v increments, which is still good enough but i could
stand smaller increments. if you can find out how many windings are in there
you could shop around a bit to get the best deal, but mostly you would only be
able to tell by plugging it in and measuring the voltage jump.

the wire i use is standard safety wire. it's stainless steel and maybe 1/32 in
diameter, i don't know the gauge.

for multiple bows, if you have enough room and are willing to tolerate the extra
bulk you can use the large bow on the smaller projects. you can move the
electrical wire from the end of the wire to the middle. the best way to hook
the wire to the transformer is alligator clips or mini battery charger clips.
get ones that have insulation around most of the clip and then be careful not to
melt through the insulation. wrap the wire around the bow down to the hot wire,
and have your transformer wires leave all from the same spot. getting a cheapo
extension cord and cutting the end off works fine. a switch on the bow helps
but is not entirely necessary. what helps a lot is to have the wire from the
transformer to the bow be very long. it lets you put the variac on a table and
you can walk to where the project is. a 25 foot extension cord is adequate
usually, though i have 50 feet.

the variac rating being so high isn't because the variac actually puts out that
much current. the variac is an expensive thing, and the important part is that
when a short happens the thing that blows is a fuse or a wire, not a variac.
sooner or later you WILL make a short circuit, so the fuses come in really
handy. i'd put one on the input of the variac and on the output as well.

Post by Grant
Hello,
I'm looking for info on using a Variac for a hotwire. I know nothing about
it but did a google group search and there was some mention at this group
about them being best.
Is there any thing special about them I should be aware of?
I am gunna be cutting fairly wide 2# EPS foam. The hotwire will need to be
mostly about 2' long but up to about 4' long at times.
One guy on this group said you needed a step down transformer before the
variac. What's the scoop?
There seems to be a bunch of variac's on eBay. Any recommendations as to
brand or model?
Thanks,
Grant

Al A.

2004-11-03 23:36:43 UTC

On Wed, 03 Nov 2004 18:36:25 GMT, Ken Roberts
<***@kroberts.in.9ci.com> wrote:

<SNIP lots of great info>

Post by Ken Roberts
is only a primary circuit, there is no secondary winding. IN OTHER WORDS YOU
BETTER USE A FUSE ON YOUR PLUG! you could potentially get a direct connection
between you and the wall outlet with this thing.

This is a very important point. The reason for the isolation
transformer before the variac is generally for just the reason stated
above, so that the variac output is NOT directly connected to the
wall. As Ken says, the fuse is a must, but do be aware that if you
get YOURSELF connected to the line, that (depending on the size of the
fuse, how you got connected and lots of other variables) by the time
you draw enough current to blow the fuse, you may be dead. Being dead
makes it much less likely that you will finish your hovercraft.
Bummer. An Isolation transformer helps prevent those sorts of things.
Another good idea, IMHO, is to ONLY run your hotwire setup on a GFI
protected circuit, particularly if you omit the isolation transformer.
That will trip much faster than a fuse if you should get zapped.

Not trying to be the safety police. Working safe usually dosen't take
much more effort that taking chances. Just my 2 cents.

-Al, who has been badly zapped enough times to decide that no hobby
(or job) is worth dying over.

Grant

2004-11-04 01:24:29 UTC

Sorry I'm not an electrical wizard. You guys are sort of confusing me. Al
says put the Isolation Transformer before the Variac. What is that? Is it
different from the Step Down Transformer that is in the picture on the
website I posted above?

Are you guys saying the picture of the variac and then the stepdown
transformer is not good enough. Sorry but I'm not good at this electrical
stuff like this.

Do you guys have any brand and model numbers for the parts that would work?

Post by Al A.
On Wed, 03 Nov 2004 18:36:25 GMT, Ken Roberts
<SNIP lots of great info>

This is a very important point. The reason for the isolation
transformer before the variac is generally for just the reason stated
above, so that the variac output is NOT directly connected to the
wall. As Ken says, the fuse is a must, but do be aware that if you
get YOURSELF connected to the line, that (depending on the size of the
fuse, how you got connected and lots of other variables) by the time
you draw enough current to blow the fuse, you may be dead. Being dead
makes it much less likely that you will finish your hovercraft.
Bummer. An Isolation transformer helps prevent those sorts of things.
Another good idea, IMHO, is to ONLY run your hotwire setup on a GFI
protected circuit, particularly if you omit the isolation transformer.
That will trip much faster than a fuse if you should get zapped.
Not trying to be the safety police. Working safe usually dosen't take
much more effort that taking chances. Just my 2 cents.
-Al, who has been badly zapped enough times to decide that no hobby
(or job) is worth dying over.

Ken Roberts

2004-11-04 07:19:20 UTC

grant,

sorry for being vague. i'm going to describe this in mechanical terms in hopes
that it will make more sense. i'll leave a lot out of it because it doesn't
pertain to this application. first though i'm going to make a couple
suggestions. sorry but this is a long one and i'm not sure i explained things
very well.

if you go with a transformer to isolate your variator, i would recommend
dropping down to 40 or 50 volts feeding into the variator. it will give you
more control over your wire's temperature no matter what variator you use, and
it will provide all the isolation that the isolation transformer would. don't
go with 12 volts because with the longer wire lengths that becomes a bit low.
i'm sorry but i don't have part numbers or brands. my variator was hacked out
of a piece of antique electrical gear, and it has no brand listed.

if you buy your transformer and variac in a store, be sure to tell the salesman
that you will drive use the transformer to isolate the variac. the reason for
this is that there's a power rating for each thing. the isolation transformer
needs to be a little bit beefier than the variac does because there is some lost
power in each one. the transformer will draw more power than it puts out, and
so will the variac. you need to be sure that the transformer's output matches
the variac's input capacity. the capacity is usually measured in VA (volt-amps)
or kVA (kilovolt amps). 1 kVA = 1000 VA. you get VA by multiplying the voltage
and the amperage.

the explanation, which you can read or ignore as you wish:

a transformer is like an automatic transmission in a car, only there is just one
speed and it's always in gear. you choose the ratio just like you would choose
a gear box ratio. you put in wall current and you get out something else. if
your reduction ratio is 2:1, then you put in 120v and get out 60v. you can't
change the ratio after you buy the transformer, but some transformers have
multiple "taps" on the "secondary" which means you can choose what ratio you
want of the ones that are available.

if you have an isolation transformer, the voltage you get out is the same as the
voltage you put in. it's a 1:1 ratio.

any real transformer (that has a primary and secondary winding) has an effect
that makes it limit the amount of power you can get out of it, based on the size
of the transformer. if you go above this power the transformer saturates and
the power coming out hits a ceiling. this is why the isolation transformer is
being recommended. it's similar in concept to a torque converter in an
automatic transmission, though the analogy isn't perfect. it's similar in that
when the load gets too high it starts to slip a bit, which means if you hang on
to both wires at the same time you won't cook quite as fast. there isn't any
solid electrical connection between the wall outlet and the transformer output.
the equivalent is the torque converter since there's an input shaft with vanes
on it and an output shaft with vanes on it, but the power is transferred through
fluid. the transformer equivalent is an electrical circuit that makes a
magnetic field that drives a second electrical circuit. there's a limit to the
middle part.

a variac doesn't have that limiter. you have a direct electrical connection to
the wall outlet and so you can suck in as much juice as the wall outlet can
provide, and you cook faster.

Post by Grant
Sorry I'm not an electrical wizard. You guys are sort of confusing me. Al
says put the Isolation Transformer before the Variac. What is that? Is it
different from the Step Down Transformer that is in the picture on the
website I posted above?
Are you guys saying the picture of the variac and then the stepdown
transformer is not good enough. Sorry but I'm not good at this electrical
stuff like this.
Do you guys have any brand and model numbers for the parts that would work?

Post by Al A.
On Wed, 03 Nov 2004 18:36:25 GMT, Ken Roberts
<SNIP lots of great info>

This is a very important point. The reason for the isolation
transformer before the variac is generally for just the reason stated
above, so that the variac output is NOT directly connected to the
wall. As Ken says, the fuse is a must, but do be aware that if you
get YOURSELF connected to the line, that (depending on the size of the
fuse, how you got connected and lots of other variables) by the time
you draw enough current to blow the fuse, you may be dead. Being dead
makes it much less likely that you will finish your hovercraft.
Bummer. An Isolation transformer helps prevent those sorts of things.
Another good idea, IMHO, is to ONLY run your hotwire setup on a GFI
protected circuit, particularly if you omit the isolation transformer.
That will trip much faster than a fuse if you should get zapped.
Not trying to be the safety police. Working safe usually dosen't take
much more effort that taking chances. Just my 2 cents.
-Al, who has been badly zapped enough times to decide that no hobby
(or job) is worth dying over.

Al A.

2004-11-04 23:57:28 UTC

On Thu, 04 Nov 2004 07:19:20 GMT, Ken Roberts

Post by Ken Roberts
grant,
sorry for being vague. i'm going to describe this in mechanical terms in hopes
that it will make more sense. i'll leave a lot out of it because it doesn't
pertain to this application. first though i'm going to make a couple
suggestions. sorry but this is a long one and i'm not sure i explained things
very well.

Grant,
Ken gives a pretty good analogy with the "one speed automatic
transmission" description. To extend it just a bit more, one could
think of the variac as being like one of those contiuously variable
belt transmissions like you find on a snowmobile. You can pick any
ratio, but lacking a fluid coupling, if you jam up the output, it
won't slip. Not a perfect analogy, but close enough for our purposes.

i apologize also for being vague. I do this sort of stuff for a
living (electrical/power electronics) and have a nasty tendancy to
assume others are familiar with such things. I guess that would make
me a lousy teacher.

What Ken said is correct. The chief advantage of an "isolation"
transformer is that there is no DIRECT (ie. copper wire-to-copper
wire) connection between the output (secondary) and the input
(primary). The electrical energy is transferred from primary to
secondar via a magnetic coulping. The great thing about that is that
while you can still get shocked by touching both secondary wires;
a) in this case the secondary voltage is quite low
b)(as ken said) the current is limited by the carrying rating of the
transformer (but could still be dangerous)
c) touching any one of the secondary wire and GROUND will not
complete a circuit through your body.

"C" is the important one. With just a variac, whick lacks the magnetic
isolation, touching one wire and ground gives you a 50-50 chance of
being tied directly between the power mains. Ouch.

In practice, it really dosen't matter if the isolation transformer
goes before or after the variac, the effect is the same. That assumes
correct ratings, etc. but the details aren't too important, as this is
the "esoteric theory" department.

-AL

Grant

2004-11-05 10:57:33 UTC

Al,
But what I get your saying is that you need the isolation transformer before
the variac. That is just the opposite of the website I posted here:
http://www.public.iastate.edu/~orman/air/cutter/page9.jpg which places the
isolation transformer after the variac.

Neither of you guys are saying that this drawing is correct?
Thanks,
Grant

Ken Roberts

2004-11-05 17:13:25 UTC

grant,

it's really close to irrelevant which thing comes first, unless the maximum of
one of the transformers is exceeded by being either first or second.

the only thing i can think mattering is that with the isolation transformer
second the variac is still subject to line voltage. if a short circuit involves
the variac, it still will pull the entire power available from the wall.

there may be something that makes it better to put the variac first and the
isolation transformer second, but i don't know what it would be. my own
inclination would be to put the isolation transformer first and get its benefit
for the entire device.

a couple things about your picture:
- your variac shows 12-30v output. most standard variacs go from 0 to 100% of
the total input voltage. the next most common thing is to increase the
voltage, i've seen as much as 120% increase. if your variac puts out less
voltage you need to be sure your isolation transformer doesn't lower your
total voltage too much.
- your variac circuit shows two windings, a primary and a secondary. this is
not how most variacs work--there's only one winding. be sure what you have,
because if you have a second winding you don't need the isolation
transformer at all. the variac will perform both parts in that case.
- if your drawing is an indicator of true life, you plan on having an ac
outlet and plug on each device. in that case you can switch from one way to
the other just by changing the order of the plugs.
- you still want a fuse on both ends of the total device to keep yourself and
your equipment from cooking anything.

Post by Grant
Al,
But what I get your saying is that you need the isolation transformer before
http://www.public.iastate.edu/~orman/air/cutter/page9.jpg which places the
isolation transformer after the variac.
Neither of you guys are saying that this drawing is correct?
Thanks,
Grant

Matt&Chris Starbuck

2004-11-07 02:20:15 UTC

Guys, you DON'T need a transformer with a variac.
A variac IS a transformer. The secondary is not connected to the primary.
It is only magnetically or electromagetically coupled, just like any OTHER
transformer.
A variac is a transformer where the primary and secondary are wrapped around
a hollow core, and you change the reluctance of the core by moving a slug in
and out of or around in it. Changing the reluctance changes the amount of
magnetic coupling of the two different sets of windings.

The transformer deal is for use when you use a dimmer switch with 120VAC to
run your wire. Without the transformer, it will work but will be very
touchy on adjustment. Putting the dimmer between the wall outlet and the
transformer (120VAC-to-24VAC) will put you down in a range that is more
usable, and increase the resolution of knob adjustments on the dimmer.
The dimmer switch puts out a chopped or clipped waveform, which transformers
don't really like, but it works FINE. It just makes the transformer hum a
little louder, and may make your hotwire vibrate at certain settings. It
works better with a ceiling fan(motor)dimmer switch. Costs about three
times as much a bottom-of-the-barrel light dimmer. It's not needed, but it
IS more compatible with a transformer.
So..... 120V wall supply goes to dimmer. Dimmer feeds primary(120V) winding
of transformer available at Radio Shack. Secondary of transformer feeds
your hotwire. In this case the primary will vary zero to 120v depending on
what you do with the dimmer knob. The secondary will vary zero to 24vac in
proportion with the primary (around 20% of primary's voltage setting).

I posted a .jpg diagram of this setup years ago on Hoverlovers, and maybe
even here if alt.rec was ever allowing attachments. I can send it to anyone
who requests it.

Matt

Post by Ken Roberts
grant,
it's really close to irrelevant which thing comes first, unless the maximum of
one of the transformers is exceeded by being either first or second.
the only thing i can think mattering is that with the isolation transformer
second the variac is still subject to line voltage. if a short circuit involves
the variac, it still will pull the entire power available from the wall.
there may be something that makes it better to put the variac first and the
isolation transformer second, but i don't know what it would be. my own
inclination would be to put the isolation transformer first and get its benefit
for the entire device.
- your variac shows 12-30v output. most standard variacs go from 0 to 100% of
the total input voltage. the next most common thing is to increase the
voltage, i've seen as much as 120% increase. if your variac puts out less
voltage you need to be sure your isolation transformer doesn't lower your
total voltage too much.
- your variac circuit shows two windings, a primary and a secondary.

this is

Post by Ken Roberts
not how most variacs work--there's only one winding. be sure what you have,
because if you have a second winding you don't need the isolation
transformer at all. the variac will perform both parts in that case.
- if your drawing is an indicator of true life, you plan on having an ac
outlet and plug on each device. in that case you can switch from one way to
the other just by changing the order of the plugs.
- you still want a fuse on both ends of the total device to keep yourself and
your equipment from cooking anything.

Matt&Chris Starbuck

2004-11-07 01:57:54 UTC

Al A.

2004-11-06 04:02:19 UTC

On Sat, 6 Nov 2004 17:57:54 -0800, "Matt&Chris Starbuck"

Please do not misunderstand me. I am not trying to be argumentitive
here. A Variac is NOT isolation transformer. It does not have a
primary and a secondary. It is version of a class of transformers
called "autotransformers" that partly share the same windings for
both primary and secondary. This makes for a unit that is physically
much smaller than an equivelent isolation unit (how much smaller
depends upon the input to output voltage ratio) by trading off the
electrical isolation. In a Variac, the voltage is not varied by
changing the reluctance of the core, it is varied by a sliding contact
(think of a "tap" in the coil) that moves around the edge of the
winding, which is is left uninsulated. The geneirc name for a Variac
is a "variable autotransformer", Variac being a brand name originally
belonging to the GENRAD Instrument company.
Check this out of more info. Note that there is only one winding:

http://instrumentation-central.com/Staco/PDFCutSheets/VT%20Schematic.pdf

The device you are describing that varies the coupling between the
primary and secondary windings is most commonly found on welding
equipment. If this is what you have, then you are correct, no seperate
isolation transformer would be necessary. It is refered to as a
"loosely coupled" or "variable coupling" transformer, but is not a
"Variac."

regards,
-AL

Matt&Chris Starbuck

2004-11-07 20:28:24 UTC

No problem Al and Ken.
MY old variac that I got from Prestolite where I used to work was a real
transformer. You had to crack a cam lock loose and crank a handle to move
the slug up and down- similar to my Napa arc welder that I have now.

Nevertheless, I stand corrected. Thanks. Don't want to endanger anybody
with bad info.

Regards,
Matt

Post by Al A.
On Sat, 6 Nov 2004 17:57:54 -0800, "Matt&Chris Starbuck"

Please do not misunderstand me. I am not trying to be argumentitive
here. A Variac is NOT isolation transformer. It does not have a
primary and a secondary. It is version of a class of transformers
called "autotransformers" that partly share the same windings for
both primary and secondary. This makes for a unit that is physically
much smaller than an equivelent isolation unit (how much smaller
depends upon the input to output voltage ratio) by trading off the
electrical isolation. In a Variac, the voltage is not varied by
changing the reluctance of the core, it is varied by a sliding contact
(think of a "tap" in the coil) that moves around the edge of the
winding, which is is left uninsulated. The geneirc name for a Variac
is a "variable autotransformer", Variac being a brand name originally
belonging to the GENRAD Instrument company.
http://instrumentation-central.com/Staco/PDFCutSheets/VT%20Schematic.pdf
The device you are describing that varies the coupling between the
primary and secondary windings is most commonly found on welding
equipment. If this is what you have, then you are correct, no seperate
isolation transformer would be necessary. It is refered to as a
"loosely coupled" or "variable coupling" transformer, but is not a
"Variac."
regards,
-AL

Ken Roberts

2004-11-07 01:00:09 UTC

matt,

your arrangement might actually be safer than the variac. since it has a
primary and a secondary, it will certainly not need an isolation transformer.

Post by Matt&Chris Starbuck
No problem Al and Ken.
MY old variac that I got from Prestolite where I used to work was a real
transformer. You had to crack a cam lock loose and crank a handle to move
the slug up and down- similar to my Napa arc welder that I have now.
Nevertheless, I stand corrected. Thanks. Don't want to endanger anybody
with bad info.
Regards,
Matt

Post by Al A.
On Sat, 6 Nov 2004 17:57:54 -0800, "Matt&Chris Starbuck"

Post by Matt&Chris Starbuck
Guys, you DON'T need a transformer with a variac.
A variac IS a transformer. The secondary is not connected to the

primary.

Post by Matt&Chris Starbuck
It is only magnetically or electromagetically coupled, just like any

OTHER

Post by Matt&Chris Starbuck
transformer.
A variac is a transformer where the primary and secondary are wrapped

around

Post by Matt&Chris Starbuck
a hollow core, and you change the reluctance of the core by moving a slug

Post by Matt&Chris Starbuck
and out of or around in it. Changing the reluctance changes the amount

Post by Matt&Chris Starbuck
magnetic coupling of the two different sets of windings.

Please do not misunderstand me. I am not trying to be argumentitive
here. A Variac is NOT isolation transformer. It does not have a
primary and a secondary. It is version of a class of transformers
called "autotransformers" that partly share the same windings for
both primary and secondary. This makes for a unit that is physically
much smaller than an equivelent isolation unit (how much smaller
depends upon the input to output voltage ratio) by trading off the
electrical isolation. In a Variac, the voltage is not varied by
changing the reluctance of the core, it is varied by a sliding contact
(think of a "tap" in the coil) that moves around the edge of the
winding, which is is left uninsulated. The geneirc name for a Variac
is a "variable autotransformer", Variac being a brand name originally
belonging to the GENRAD Instrument company.
http://instrumentation-central.com/Staco/PDFCutSheets/VT%20Schematic.pdf
The device you are describing that varies the coupling between the
primary and secondary windings is most commonly found on welding
equipment. If this is what you have, then you are correct, no seperate
isolation transformer would be necessary. It is refered to as a
"loosely coupled" or "variable coupling" transformer, but is not a
"Variac."
regards,
-AL

Matt&Chris Starbuck

2004-11-08 05:50:34 UTC

Yer preachin' to the choir Ken

Post by Ken Roberts
matt,
your arrangement might actually be safer than the variac. since it has a
primary and a secondary, it will certainly not need an isolation transformer.

Post by Al A.
On Sat, 6 Nov 2004 17:57:54 -0800, "Matt&Chris Starbuck"

Post by Matt&Chris Starbuck
Guys, you DON'T need a transformer with a variac.
A variac IS a transformer. The secondary is not connected to the

primary.

Post by Matt&Chris Starbuck
It is only magnetically or electromagetically coupled, just like any

OTHER

Post by Matt&Chris Starbuck
transformer.
A variac is a transformer where the primary and secondary are wrapped

around

Post by Matt&Chris Starbuck
a hollow core, and you change the reluctance of the core by moving a slug

Post by Matt&Chris Starbuck
and out of or around in it. Changing the reluctance changes the amount

Post by Matt&Chris Starbuck
magnetic coupling of the two different sets of windings.

http://instrumentation-central.com/Staco/PDFCutSheets/VT%20Schematic.pdf

Post by Matt&Chris Starbuck

Post by Al A.
The device you are describing that varies the coupling between the
primary and secondary windings is most commonly found on welding
equipment. If this is what you have, then you are correct, no seperate
isolation transformer would be necessary. It is refered to as a
"loosely coupled" or "variable coupling" transformer, but is not a
"Variac."
regards,
-AL

Barry Palmer

2004-11-10 16:53:01 UTC

I should mention, there being newbies about, that the foam Sevtec sells should
not be hot wire cut, as Cyanide gas would be liberated in such a process. the
foam as Sevtec uses can be easily cut with an ordinary kitchen knife or safety
razor blade, anyhow. There is simply no need for a hot wire cutter.

While I am at it I might dispel another myth, that it would "improve"
construction if the builder could get 4 foot by 8 foot sheets, instead of the 2
by 4 foot sheets Sevtec gets shipped via UPS. I do have 4 X 8 sheets shipped,
and they will work fine, but shipping costs are higher. The builder should
have a flat surface nearly as large as the largest hull panel to work on so
that flat skinned sheets can be made, regardless of initial foam size. Joints
in individual flat panel foam do not weaken the structure, they strengthen it,
at the cost of a minute amount of weight.

The current craft I am operating is made up of 1 1/2 inch squares of foam,
which are mounted on a cheesecloth like scrim to facilitate handling. I am
using this type of construction as the craft was layed up in a mold, and this
eases holding the squares against the skin in the mold during bonding so that
vacuum bagging is unnecessary.

Vacuum bagging is simply not necessary in any Sevtec design built by a
homebuilder, as the fiberglass skins are layed up on top of the foam and
gravity, with help from a paint brush and/or a ribbed roller will make for an
excellent bond to the foam, and produce a lighter weight product than when
layed up in a mold.

Subject: Re: Variac for Hotwire
Date: 11/7/2004 9:50 PM Pacific Standard Time
Yer preachin' to the choir Ken

Post by Ken Roberts
matt,
your arrangement might actually be safer than the variac. since it has a
primary and a secondary, it will certainly not need an isolation

transformer.

Post by Matt&Chris Starbuck
No problem Al and Ken.
MY old variac that I got from Prestolite where I used to work was a real
transformer. You had to crack a cam lock loose and crank a handle to

move

Post by Matt&Chris Starbuck
the slug up and down- similar to my Napa arc welder that I have now.
Nevertheless, I stand corrected. Thanks. Don't want to endanger

anybody

Post by Matt&Chris Starbuck
with bad info.
Regards,
Matt

Post by Al A.
On Sat, 6 Nov 2004 17:57:54 -0800, "Matt&Chris Starbuck"

Post by Matt&Chris Starbuck
Guys, you DON'T need a transformer with a variac.
A variac IS a transformer. The secondary is not connected to the

primary.

Post by Matt&Chris Starbuck
It is only magnetically or electromagetically coupled, just like any

OTHER

Post by Matt&Chris Starbuck
transformer.
A variac is a transformer where the primary and secondary are wrapped

around

Post by Matt&Chris Starbuck
a hollow core, and you change the reluctance of the core by moving a

slug

Post by Matt&Chris Starbuck
in

Post by Matt&Chris Starbuck
and out of or around in it. Changing the reluctance changes the

amount

Post by Matt&Chris Starbuck
of

Post by Matt&Chris Starbuck
magnetic coupling of the two different sets of windings.

http://instrumentation-central.com/Staco/PDFCutSheets/VT%20Schematic.pdf

Post by Matt&Chris Starbuck

Barry Palmer, for <A
HREF="http://members.aol.com/sevtec/sev/skmr.html">Sevtec</A>

Ken Roberts

2004-11-06 06:49:57 UTC

matt,

i have a variac i can show you that does not have a secondary and it's visibly
provable. it's quite possible that on some variacs there is a primary, in which
case you would not need the transformer. i've had ... 3 of them, and they were
all the way i described. still have two, but one is blown from being too small
for the wire.

my variac is a toroidal core, there is no slug. the variable part is a contact
arm that sweeps across the wires of the transformer. the thing you're talking
about is not what i'm talking about. what i have goes from zero volts to full
wall voltage, and draws whatever current is necessary to maintain the load.

your variable reluctance transformer will be even more like an automatic
transmission's torque converter because under no load it will probably read
fairly high voltage but as the load increases the voltage will drop as the
current rises. i suspect what you're looking at is a variable CURRENT supply,
which might work just as well for this. the variable reluctance transformer
will probably not only adjust the amount of coupling but also the saturation
level of the transformer.

it's a lot of years since i was looking at this sort of thing, but if you want a
picture matt i'll send you one of my variac with no cover. the mechanism is
clearly visible.

Post by Matt&Chris Starbuck
Guys, you DON'T need a transformer with a variac.
A variac IS a transformer. The secondary is not connected to the primary.
It is only magnetically or electromagetically coupled, just like any OTHER
transformer.
A variac is a transformer where the primary and secondary are wrapped around
a hollow core, and you change the reluctance of the core by moving a slug in
and out of or around in it. Changing the reluctance changes the amount of
magnetic coupling of the two different sets of windings.
The transformer deal is for use when you use a dimmer switch with 120VAC to
run your wire. Without the transformer, it will work but will be very
touchy on adjustment. Putting the dimmer between the wall outlet and the
transformer (120VAC-to-24VAC) will put you down in a range that is more
usable, and increase the resolution of knob adjustments on the dimmer.
The dimmer switch puts out a chopped or clipped waveform, which transformers
don't really like, but it works FINE. It just makes the transformer hum a
little louder, and may make your hotwire vibrate at certain settings. It
works better with a ceiling fan(motor)dimmer switch. Costs about three
times as much a bottom-of-the-barrel light dimmer. It's not needed, but it
IS more compatible with a transformer.
So..... 120V wall supply goes to dimmer. Dimmer feeds primary(120V) winding
of transformer available at Radio Shack. Secondary of transformer feeds
your hotwire. In this case the primary will vary zero to 120v depending on
what you do with the dimmer knob. The secondary will vary zero to 24vac in
proportion with the primary (around 20% of primary's voltage setting).
I posted a .jpg diagram of this setup years ago on Hoverlovers, and maybe
even here if alt.rec was ever allowing attachments. I can send it to anyone
who requests it.
Matt

Post by Grant
Al,
But what I get your saying is that you need the isolation transformer

before

Post by Grant
http://www.public.iastate.edu/~orman/air/cutter/page9.jpg which places the
isolation transformer after the variac.
Neither of you guys are saying that this drawing is correct?
Thanks,
Grant

Rick

2004-11-06 17:04:30 UTC

Post by Ken Roberts
matt,
i have a variac i can show you that does not have a secondary and it's visibly
provable. it's quite possible that on some variacs there is a primary, in which
case you would not need the transformer. i've had ... 3 of them, and they were
all the way i described. still have two, but one is blown from being too small
for the wire.
my variac is a toroidal core, there is no slug. the variable part is a contact
arm that sweeps across the wires of the transformer. the thing you're talking
about is not what i'm talking about. what i have goes from zero volts to full
wall voltage, and draws whatever current is necessary to maintain the load.

You sure what you're describing is not a variable resistor?

Post by Ken Roberts
your variable reluctance transformer will be even more like an automatic
transmission's torque converter because under no load it will probably read
fairly high voltage but as the load increases the voltage will drop as the
current rises. i suspect what you're looking at is a variable CURRENT supply,
which might work just as well for this. the variable reluctance transformer
will probably not only adjust the amount of coupling but also the saturation
level of the transformer.
it's a lot of years since i was looking at this sort of thing, but if you want a
picture matt i'll send you one of my variac with no cover. the mechanism is
clearly visible.

Post by Grant
Al,
But what I get your saying is that you need the isolation transformer

before

Ken Roberts

2004-11-07 00:58:58 UTC

Post by Rick

You sure what you're describing is not a variable resistor?

yes, i'm positive. it says "variac" on it stamped in the metal. the wire is
heavy gauge copper and it reads far less than one ohm across the primary wires.

a variac is an ac version of a potentiometer. mine weighs about 25 lbs or so
and it generates almost no heat when in use. none that you can notice, but i
know it must generate some because of eddy currents in the core. there are no
features to dissipate heat.

here:

http://instrumentation-central.com/staco/EngineeringDrawings/2510.pdf

this describes a variac that is similar to mine. it includes mechanical data
as well as an electrical schematic and specifications.

Grant

2004-11-06 22:28:44 UTC

Ken,
Look on this page here : http://www.action-electronics.com/variac.htm#Vary
does the one that says isolated have an isolation transformer inside? Not
that I'm gunna buy it...too expensive.

Now I've also read on the web about a lot of people using a battery charger
controlled by a dimmer like this:
http://www.oldf.homestead.com/foamcutting.html . Would this be similar to a
variac controlling a transformer. Is not a battery charger just a
transformer from AC to DC?

Does anyone here know what would be the difference of using AC or DC through
the hotwire?

Also could not a variac control the battery charger...which I do by chance
have laying around.

Grant

Ken Roberts

2004-11-07 01:21:18 UTC

grant,

i would contact them and ask. they don't have any diagrams there so it's hard
to say for sure.

for that matter, unless you have a burning need for a shiny new variator i
wouldn't buy one from there anyway. go to ebay and get one for $50.

right now there are two pages of variacs on ebay. some are brand new even with
digital readouts. if you have time be patient and decide in advance that you
don't want to pay more than x amount, including shipping. you'll probably lose
a few but you'll get one eventually.

Post by Grant
Ken,
Look on this page here : http://www.action-electronics.com/variac.htm#Vary
does the one that says isolated have an isolation transformer inside? Not
that I'm gunna buy it...too expensive.
Now I've also read on the web about a lot of people using a battery charger
http://www.oldf.homestead.com/foamcutting.html . Would this be similar to a
variac controlling a transformer. Is not a battery charger just a
transformer from AC to DC?
Does anyone here know what would be the difference of using AC or DC through
the hotwire?
Also could not a variac control the battery charger...which I do by chance
have laying around.
Grant

Matt&Chris Starbuck

2004-11-07 02:26:18 UTC

I just looked at that link after posting my other post.
They seem to show options with or without the variac.

The one without the variac shows an isolation transformer feeding your
"business" transformer. The isolation transformer is not needed in that
case. The secondary is only magnetically coupled to the wall, not
electrically.

The isolation transformer is also not needed with the variac either.

Matt

Al A.

2004-11-06 04:30:14 UTC