Variac saturation question

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Magnetics is not my strength, so please bear with me.

My basic question is what the ramifications are of running a 120V (30A max
rated load) variac at 240V input.

It seems that there are two cases to consider:

1 - No load saturation possibility
2 - With load saturation

Even at 240V, no-load saturation seems unlikely, right?

What about with a load? It seems that the flux density should only be a
function of load current, but certainly with 240V input there is more
potential maximum output power, so something has to give.

Any help would be appreciated.

Bob
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Re: Variac saturation question

"BobWanker"
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** Impossible -  the AC fuse or circuit breaker with blow instantly due to
massive saturation of the core.




....  Phil






Re: Variac saturation question
On Sat, 15 Jan 2011 16:30:34 -0800, the renowned "BobW"

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It will burn out if the fuse doesn't go.  

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The flux in the core is about constant from no load to full load.

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You can run a 240V variac on 120V, but not the other way around.

You could use a 120V variac across half of a split-phase supply and
get adjustment from, say 120VAC to 240VAC provided you don't exceed
the maximum current off the wiper.



Best regards,
Spehro Pefhany
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Re: Variac saturation question

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Yeah, running the variac on half of the line is the other option. The
120V->240V net output should be enough range for what I need.

Thanks.

Bob
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Re: Variac saturation question
On Sat, 15 Jan 2011 16:57:23 -0800, the renowned "BobW"

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Sure. You could also stick a DPDT switch in there and get 0-240VAC out
in two (high/low ranges), with twice the settability of a 240V variac.

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Best regards,
Spehro Pefhany
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Re: Variac saturation question

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That's a good idea, but not appropriate for what I'm trying to do.

I'm using an arc welder transformer as a power source for a TIG welding
setup. It's working well but I want a throttle (like in a real TIG machine).
I don't think I need the full 0-240V to control the flame heat.

The thing that I really don't understand is this -

In an unloaded variac, the current magnitude (regardless of its phase) is
very small, right? If this is true, how can it be anywhere near saturation
even if run at twice its rated voltage. Certainly, the current will go up
with load, so won't the flux density go up with load?

I just don't get this part, but I believe what people are saying so I'm not
planning on trying it.


Bob
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Re: Variac saturation question
On Sat, 15 Jan 2011 17:25:40 -0800, the renowned "BobW"

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Yes, it's a small fraction of full-load current.

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The primary current and secondary current change together, but work in
opposite directions (Lenz's law) and thus cancel out, leaving the flux
unchanged.

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Best regards,
Spehro Pefhany
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Re: Variac saturation question

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Thank you, Spehro. This makes perfect sense, now.

Looking at how a convential transformer works, I can see that the net flux,
due to the load-related primary and secondary currents, is zero.

It's interesting to analyze the variac (autotransformer) in a similar way.
The primary and secondary currents aren't as obvious, but they still result
in zero net additional flux due to the load.

I guess the old adage is wrong - you CAN teach an old dog new tricks.

Bob
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Re: Variac saturation question

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Bob-

I understand that saturation occurs when voltage peaks exceed a critical
value.  It might be interesting to drive a transformer through a current
limiting resistor, and look at an oscilloscope waveform across the
resistor as you increase voltage.  At saturation, I would expect to see
current spikes that correspond to voltage peaks.

You could use your Variac as a source, and drive a lower voltage winding
of another transformer, to see the effects of increasing voltage beyond
the normal value for that winding.

Be careful.  The results could be shocking!

Fred

Re: Variac saturation question

"Fred McKenzie"
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**  When a transformer core is approaching saturation, you will see pulses
of current a twice the AC frequency -  but they are NOT simultaneous with
the AC voltage peaks. Strange but true, each current pulse reaches its
maximum value at the same time the applied AC voltage goes through zero.

In the situation alluded to by the OP  -  the saturation of the core would
be so great that for most of each half cycle current flow would be limited
only by the resistance of the winding on the Variac.  In the case of a 3kVA
unit -  that resistance would be a fraction of 1 ohm.



....  Phil






Re: Variac saturation question

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We purchased several variable autotransformers from MPJA:
 http://www.mpja.com/prodinfo.asp?number15%163+TR
The cores must saturate easily, because with no load, they
would trip a 30A panel breaker almost every time you turned
them on.  :(  Sent them all back.

Ken

Re: Variac saturation question

"Ken Moffett"
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** Not a single thing wrong with those units.

You need to get circuit breakers that can stand a decent inrush surge  -  as
is 100% normal for large variacs.

The first half cycle will pull up to 700 amps peak with a 2kVA unit.

So called "motor start" ones might be the go.



....  Phil




Re: Variac saturation question

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Phil-

Are you sure about that?  I would have thought the inductive reactance
would immediately limit current.  If you applied DC to the input, I
would expect a time delay before the core saturated and current jumped
to the DC resistance value (voltage leads current).

Of course your claim does explain why the 30 Amp breaker popped, but the
Variac's 20 Amp fuse did not blow.  On the other hand, I once had a
similar problem that turned out to be a defective breaker!

Fred

Re: Variac saturation question

"Fred McKenzie"
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** Absolutely.

A variac is constructed similar to a toroidal transformer with a wound strip
iron core - means that saturation comes on very suddenly and is drastic once
the rated AC voltage is exceeded.


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**  Not for long -  then it saturates.


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** The core and winding are sized to be JUST saturation free on 50 or 60 Hz
power at the rated voltage.At switch on, particularly if the switching point
coincides with zero voltage, the core will go into hard saturation for a
short time.

Switching on at the peak of the AC voltage minimises inrush surge and
operating the unit at half the rated AC voltage virtually eliminates it.


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** Fuses are normally in the output circuit of a variac - since the max
current rating applies at any voltage.



.....   Phil





Re: Variac saturation question

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Are you using GFCI breakers?


Jamie


Re: Variac saturation question

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The units are "rated" at 15A...on 30A breakers There was no
load on the outputs of the transformers.  They were the only
device on the 30A circuit.  Several different
circuits/breakers were tried. These are standard
institutional distribution panels for a Physics lab.

This is mostly just a heads-up to anyone thinking of buying
one and using it on a 15A circuit.

Ken

Re: Variac saturation question

"Ken Moffett"
"Phil Allison"
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** There is a very simple way to reduce inrush surges so the breaker never
nuisance trips.

 It's called an NTC thermistor.

 http://www.ametherm.com /

This one looks about right:

http://www.ametherm.com/datasheets/sl321r030.html

The 1 ohm cold resistance will limit the peak surge ( half cycle) current to
about 100 amps.

Any decent 15 amp breaker should survive that.



....  Phil






Re: Variac saturation question
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You are in for a nice explosion, if the fuse will not act first.
Most transformers I know are ~5-30 percent below saturation.
Doubling the voltage, will turn the transformer into a dead short.

Re: Variac saturation question
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   Not enough Xl to properly limit current at 240V; the variac will burn
out, period.

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