According to Grant Erwin :

[ ... ]

Many
people have told me that the wall voltage must be wired to the slider, but it
can't be and isn't.

Grant,

I'm sorry, but in *none* of the followups in this thread have I
seen anyone saying that the wall power should be connected to the wiper.

I have seen *many* (and I am among those) who said that the wall
power must *not* be connected to the wiper. Could you go back through
and find the ones which suggest connecting the wall power to the wiper?

There have been some who asked whether the wall power *was*
connected to the wiper, as that is the most logical reason for the
breaker popping at low settings but not at high settings -- at least
before you came back and said that once the power was turned on you
could go through the full range with no subsequent popping of the
breaker.

Good Luck,
DoN.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

DoN. Nichols wrote:
According to Grant Erwin :

[ ... ]


Many
people have told me that the wall voltage must be wired to the slider, but it
can't be and isn't.


Grant,

I'm sorry, but in *none* of the followups in this thread have I
seen anyone saying that the wall power should be connected to the wiper.

I have seen *many* (and I am among those) who said that the wall
power must *not* be connected to the wiper. Could you go back through
and find the ones which suggest connecting the wall power to the wiper?

There have been some who asked whether the wall power *was*
connected to the wiper, as that is the most logical reason for the
breaker popping at low settings but not at high settings -- at least
before you came back and said that once the power was turned on you
could go through the full range with no subsequent popping of the
breaker.

I understand the miscommunication. What I meant to say was that many people
have told me that in their opinion the only logical reason I'd see the
(undesired) behavior I observed was that wall power was connected to
the slider. However, I wrote it in such a way that it was possible to
interpret it the way you did. I never meant to suggest that anyone said
that the correct way to wire it was to wire wall power to the slider.

Tech writing.

Grant

Grant Erwin wrote in
:

I have a 20 amp variac. Yesterday I tried plugging it into
a 20A circuit with its output voltage set to 30% of input
voltage, and it (consistently) popped the breaker. When I
set it to 100% of output voltage, however, it didn't pop
the breaker. Obviously the magnetizing current is higher
with the variac turned down. The question is why? What's
going on here?

I believe the variac and my electrical circuits are both in
good working condition, in other words I don't believe this
is faulty behavior.

Grant Erwin
Kirkland, Washington

Grant,

I had the same experience. One of our physics profs bought 5
120v/2KW variable voltage transformere from MPJA. They were
to be used to control heat tapes wrapped around a vacuum
chamber. He complained that they popped 20A breakers at
turn-on about 90% of the time. Only one transformer per
breaker. I went through all the possible senarios that have
been mentioned. They would trip the breaker loaded, no-load,
wiper in any position form zero to full. Wiring was correct.
Called MPJA and was told that we needed higher current
breakers. Not happy with that blunt answer! Since the
transformers were "Made in China" and labeled "Variac", but
didnt look up to what I come to expect from VARIAC, I
contacted "VARIAC". The person I talked to there said the
same thing...bigger breaker. He just said that high current
autotransformers...even theirs...have a very high initial
current draw.

When I look at the windings, there aren't that many...and of
about 12Ga wire, primary and secondary being the same. I
suppose that the inductive reactence at 60Hz is relatively
low compared to the very low resistance of the windings...not
limiting the inital current at turn-on...and depending where
in the voltage cycle that occured.

Also emailed VARIAC about the name, since the labeling on
these Chinese knock-offs were an obvious trademake violation.

Not an engineer...just a tech...passing on information...I
won't argue the theory.

Ken

Ken Moffett wrote:
Grant Erwin wrote in
:


I have a 20 amp variac. Yesterday I tried plugging it into
a 20A circuit with its output voltage set to 30% of input
voltage, and it (consistently) popped the breaker. When I
set it to 100% of output voltage, however, it didn't pop
the breaker. Obviously the magnetizing current is higher
with the variac turned down. The question is why? What's
going on here?

I believe the variac and my electrical circuits are both in
good working condition, in other words I don't believe this
is faulty behavior.

Grant Erwin
Kirkland, Washington


Grant,

I had the same experience. One of our physics profs bought 5
120v/2KW variable voltage transformere from MPJA. They were
to be used to control heat tapes wrapped around a vacuum
chamber. He complained that they popped 20A breakers at
turn-on about 90% of the time. Only one transformer per
breaker. I went through all the possible senarios that have
been mentioned. They would trip the breaker loaded, no-load,
wiper in any position form zero to full. Wiring was correct.
Called MPJA and was told that we needed higher current
breakers. Not happy with that blunt answer! Since the
transformers were "Made in China" and labeled "Variac", but
didnt look up to what I come to expect from VARIAC, I
contacted "VARIAC". The person I talked to there said the
same thing...bigger breaker. He just said that high current
autotransformers...even theirs...have a very high initial
current draw.

When I look at the windings, there aren't that many...and of
about 12Ga wire, primary and secondary being the same. I
suppose that the inductive reactence at 60Hz is relatively
low compared to the very low resistance of the windings...not
limiting the inital current at turn-on...and depending where
in the voltage cycle that occured.

Also emailed VARIAC about the name, since the labeling on
these Chinese knock-offs were an obvious trademake violation.

Not an engineer...just a tech...passing on information...I
won't argue the theory.

Ken

I'm nearly ready to call it random chance that the breaker tripped
at the low setting but then not when I turned the setting up to 100.
I'm planning to buy one of these:

http://www.ametherm.com/Data%20Sheets/SL32%202R025.pdf

and put it in series with the hot wire. Other guys have had success
with this technique.

GWE

On 14 Oct 2007 03:05:09 GMT, (DoN. Nichols)
wrote:

According to Grant Erwin :

[ ... ]

Many
people have told me that the wall voltage must be wired to the slider, but it
can't be and isn't.

Grant,

I'm sorry, but in *none* of the followups in this thread have I
seen anyone saying that the wall power should be connected to the wiper.

I have seen *many* (and I am among those) who said that the wall
power must *not* be connected to the wiper. Could you go back through
and find the ones which suggest connecting the wall power to the wiper?

There have been some who asked whether the wall power *was*
connected to the wiper, as that is the most logical reason for the
breaker popping at low settings but not at high settings -- at least
before you came back and said that once the power was turned on you
could go through the full range with no subsequent popping of the
breaker.

Good Luck,
DoN.


I think by "must" he means it must be, for it to behave that way = not
that it is the right way.

--
Posted via a free Usenet account from http://www.teranews.com

For a while I couldn't plug the variac in at all without popping the breaker. No
load on the variac, nothing else plugged into the 20A circuit, no difference
caused by the position of the variac lever. After letting the breaker be on for
about 20 minutes, however, now I can plug it in again.

It looks like this unit (which, by the way is a Superior Electric Powerstat)
just barely sneaks past my 20A panel breaker and then only sometimes.

I only have a little more derusting to do, then I'll pull apart the unit
and check all its wiring. At a minimum I'll fuse the hot lead and not the
neutral.

GWE

Superior Electric is a quality unit. Nothing wrong with the design.
The magnetizing force might be just high enough to break over the breaker.

The breaker might be aging as well. It was an odd thought to me when a
HVAC guy stated it. If your house is 10 or more years old it might be
the case.
They might be fast acting and not slow blow or delayed type for inductive loads.

A 20 amp breaker in a shop is normally a pain! Looks like you found one.

Martin

Martin H. Eastburn
@ home at Lions' Lair with our computer lionslair at consolidated dot net
TSRA, Life; NRA LOH & Patron Member, Golden Eagle, Patriot's Medal.
NRA Second Amendment Task Force Charter Founder
IHMSA and NRA Metallic Silhouette maker & member.
http://lufkinced.com/


Grant Erwin wrote:
For a while I couldn't plug the variac in at all without popping the
breaker. No load on the variac, nothing else plugged into the 20A
circuit, no difference caused by the position of the variac lever. After
letting the breaker be on for
about 20 minutes, however, now I can plug it in again.

It looks like this unit (which, by the way is a Superior Electric
Powerstat)
just barely sneaks past my 20A panel breaker and then only sometimes.

I only have a little more derusting to do, then I'll pull apart the unit
and check all its wiring. At a minimum I'll fuse the hot lead and not the
neutral.

GWE


----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

Martin H. Eastburn wrote:
...
A 20 amp breaker in a shop is normally a pain! ...

Whatta ya mean?

On Sun, 14 Oct 2007 14:20:46 -0700, Grant Erwin
wrote:

For a while I couldn't plug the variac in at all without popping the breaker. No
load on the variac, nothing else plugged into the 20A circuit, no difference
caused by the position of the variac lever. After letting the breaker be on for
about 20 minutes, however, now I can plug it in again.

It looks like this unit (which, by the way is a Superior Electric Powerstat)
just barely sneaks past my 20A panel breaker and then only sometimes.

I only have a little more derusting to do, then I'll pull apart the unit
and check all its wiring. At a minimum I'll fuse the hot lead and not the
neutral.

GWE

Somebody suggested putting a light bulb in series with the
power input to the Variac. They had them setup that way as a
way of protecting against a bad device being hooked up to
the output. If you set yours up that way and added a
bypassing switch across the light bulb it would most likely
stop popping the breaker. ie open the bypass switch, turn
on/plug in the Variac, apply bypass switch. A bit
round-a-bout, but simple enough and a lot better than
beating up your breaker. Even the Square D line of breakers
go bad if you pop them enough.

--
Leon Fisk
Grand Rapids MI/Zone 5b
Remove no.spam for email

On Sun, 14 Oct 2007 14:20:46 -0700, Grant Erwin
wrote:

For a while I couldn't plug the variac in at all without popping the breaker. No
load on the variac, nothing else plugged into the 20A circuit, no difference
caused by the position of the variac lever. After letting the breaker be on for
about 20 minutes, however, now I can plug it in again.

It looks like this unit (which, by the way is a Superior Electric Powerstat)
just barely sneaks past my 20A panel breaker and then only sometimes.

I only have a little more derusting to do, then I'll pull apart the unit
and check all its wiring. At a minimum I'll fuse the hot lead and not the
neutral.

GWE

Variacs are usually designed to run at pretty high flux
density because core temperature rise is not normally a problem.
This means that the magnetising (i.e.no-load) current can be a
lot higher than in comparable fixed ratio transformers.

The toroidal core used is a completely closed iron
circuit with no residual air gap. The the reluctance of this iron
circuit is so low that, when power is switched off, it remains
magnetised at almost the full value of the flux density that
existed at the instant of disconnection of the supply.

Steady state, this is no problem but it can result
in possible large current peaks for the first few cycles after
power is re-applied.

If power is re-applied at the same phase instant as
the previous disconnection phase instant there is no transient
disturbance and it simply resumes normal no-load current.

If power is removed at the peak positive flux
density instant in the supply wave and power later re-applied at
the instant that requires peak negative flux density the core
saturates because it cannot support the doubled flux swing. Very
large peak currents then occur that decay in the first few cycles
of the supply waveform.

This means that the switch on current surge appears
to be random because the actual value depends on the precise
instants when power is removed and when it is re-applied.

Jim

wrote:

On Sun, 14 Oct 2007 14:20:46 -0700, Grant Erwin
wrote:


For a while I couldn't plug the variac in at all without popping the breaker. No
load on the variac, nothing else plugged into the 20A circuit, no difference
caused by the position of the variac lever. After letting the breaker be on for
about 20 minutes, however, now I can plug it in again.

It looks like this unit (which, by the way is a Superior Electric Powerstat)
just barely sneaks past my 20A panel breaker and then only sometimes.

I only have a little more derusting to do, then I'll pull apart the unit
and check all its wiring. At a minimum I'll fuse the hot lead and not the
neutral.

GWE


Variacs are usually designed to run at pretty high flux
density because core temperature rise is not normally a problem.
This means that the magnetising (i.e.no-load) current can be a
lot higher than in comparable fixed ratio transformers.

The toroidal core used is a completely closed iron
circuit with no residual air gap. The the reluctance of this iron
circuit is so low that, when power is switched off, it remains
magnetised at almost the full value of the flux density that
existed at the instant of disconnection of the supply.

Steady state, this is no problem but it can result
in possible large current peaks for the first few cycles after
power is re-applied.

If power is re-applied at the same phase instant as
the previous disconnection phase instant there is no transient
disturbance and it simply resumes normal no-load current.

If power is removed at the peak positive flux
density instant in the supply wave and power later re-applied at
the instant that requires peak negative flux density the core
saturates because it cannot support the doubled flux swing. Very
large peak currents then occur that decay in the first few cycles
of the supply waveform.

This means that the switch on current surge appears
to be random because the actual value depends on the precise
instants when power is removed and when it is re-applied.

Jim



Well, that is certainly the information I was looking for! Thank you!

Grant Erwin

"Grant Erwin" wrote in message
...
For a while I couldn't plug the variac in at all without popping the
breaker. No load on the variac, nothing else plugged into the 20A circuit,
no difference caused by the position of the variac lever. After letting
the breaker be on for
about 20 minutes, however, now I can plug it in again.

It looks like this unit (which, by the way is a Superior Electric
Powerstat)
just barely sneaks past my 20A panel breaker and then only sometimes.

I only have a little more derusting to do, then I'll pull apart the unit
and check all its wiring. At a minimum I'll fuse the hot lead and not the
neutral.

GWE

Also be absolutely certain that the neutral connects to the zero output end
of the main winding. If reversed, both output leads are 120 volts to ground
when the variac is set for zero out. Very hazardous.

Don Young

According to :

[ ... ]

Variacs are usually designed to run at pretty high flux
density because core temperature rise is not normally a problem.
This means that the magnetising (i.e.no-load) current can be a
lot higher than in comparable fixed ratio transformers.

The toroidal core used is a completely closed iron
circuit with no residual air gap. The the reluctance of this iron
circuit is so low that, when power is switched off, it remains
magnetised at almost the full value of the flux density that
existed at the instant of disconnection of the supply.

Interesting.

Steady state, this is no problem but it can result
in possible large current peaks for the first few cycles after
power is re-applied.

If power is re-applied at the same phase instant as
the previous disconnection phase instant there is no transient
disturbance and it simply resumes normal no-load current.

OlK.

If power is removed at the peak positive flux
density instant in the supply wave and power later re-applied at
the instant that requires peak negative flux density the core
saturates because it cannot support the doubled flux swing. Very
large peak currents then occur that decay in the first few cycles
of the supply waveform.

This means that the switch on current surge appears
to be random because the actual value depends on the precise
instants when power is removed and when it is re-applied.

O.K. Then this suggests to me that the best way to control a
large one like his (the rating of his breaker) is to replace the switch
with a solid-state relay, which maintains the "on" state until both the
control voltage is off, *and* the current through the load drops to
zero. This should minimize the spikes when switching on to half of
worst-case. (Even better would be if the turn-on control could be
zero-crossing as well.

Glad to see an explanation of the phenomenon which makes sense,
and this at least shows that the 30% setting of the wiper when it
tripped the breaker was just a coincidence.

Enjoy,
DoN.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

DoN. Nichols wrote:

According to :

[ ... ]


Variacs are usually designed to run at pretty high flux
density because core temperature rise is not normally a problem.
This means that the magnetising (i.e.no-load) current can be a
lot higher than in comparable fixed ratio transformers.

The toroidal core used is a completely closed iron
circuit with no residual air gap. The the reluctance of this iron
circuit is so low that, when power is switched off, it remains
magnetised at almost the full value of the flux density that
existed at the instant of disconnection of the supply.


Interesting.


Steady state, this is no problem but it can result
in possible large current peaks for the first few cycles after
power is re-applied.

If power is re-applied at the same phase instant as
the previous disconnection phase instant there is no transient
disturbance and it simply resumes normal no-load current.


OlK.


If power is removed at the peak positive flux
density instant in the supply wave and power later re-applied at
the instant that requires peak negative flux density the core
saturates because it cannot support the doubled flux swing. Very
large peak currents then occur that decay in the first few cycles
of the supply waveform.

This means that the switch on current surge appears
to be random because the actual value depends on the precise
instants when power is removed and when it is re-applied.


O.K. Then this suggests to me that the best way to control a
large one like his (the rating of his breaker) is to replace the switch
with a solid-state relay, which maintains the "on" state until both the
control voltage is off, *and* the current through the load drops to
zero. This should minimize the spikes when switching on to half of
worst-case. (Even better would be if the turn-on control could be
zero-crossing as well.

Glad to see an explanation of the phenomenon which makes sense,
and this at least shows that the 30% setting of the wiper when it
tripped the breaker was just a coincidence.

Enjoy,
DoN.

Somehow I cant imagine that the material in the "iron core"
has that large a hysteresis in the B H curve to retain
that much field.
But it would account for the effect.
...lew...

On Oct 16, 3:30 am, (DoN. Nichols) wrote:

O.K. Then this suggests to me that the best way to control a
large one like his (the rating of his breaker) is to replace the switch
with a solid-state relay, which maintains the "on" state until both the
control voltage is off, *and* the current through the load drops to
zero. This should minimize the spikes when switching on to half of
worst-case. (Even better would be if the turn-on control could be
zero-crossing as well.

Think about it a bit more. The flux is at a maximum at the zero
crossing of the current. So switching at the current zero crossing
would insure that the core is magnetized as much as possible. Turning
on at the voltage zero crossing would insure that the breaker would
trip half of the time.


Dan

On Oct 16, 1:45 pm, Lew Hartswick wrote:


Somehow I cant imagine that the material in the "iron core"
has that large a hysteresis in the B H curve to retain
that much field.
But it would account for the effect.
...lew..

I have seen the same kind of thing with aorospace transformers
designed for minimum weight.. The secondary was feeding a full wave
rectifier and charging a capacitor. Sometimes this would cause the
capacitor to charge to a much higher voltage than was intended.

Dan