Variac saturation question

"BobWanker"

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

.... Phil

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

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

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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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.

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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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.

Best regards, Spehro Pefhany

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"it's the network..."                          "The Journey is the reward"
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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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Yes, it's a small fraction of full-load current.

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

Best regards, Spehro Pefhany

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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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Not enough Xl to properly limit current at 240V; the variac will burn out, period.

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

"Fred McKenzie"

** 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

"Phil Allison" wrote in news: snipped-for-privacy@mid.individual.net:

We purchased several variable autotransformers from MPJA:

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

"Ken Moffett"

** 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

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

"Fred McKenzie"

** 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.

** Not for long - then it saturates.

** 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.

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

..... Phil

Are you using GFCI breakers?

Jamie

"Phil Allison" wrote in news: snipped-for-privacy@mid.individual.net:

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

"Ken Moffett" "Phil Allison"

** There is a very simple way to reduce inrush surges so the breaker never nuisance trips.

It's called an NTC thermistor.

This one looks about right:

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