What limits the current in an unloaded transformer?

"FunFred"

** Your intuitive an analysis and experimental results are 100% spot on !!!!

Just connect an AC amp meter ( true rms type if possible ) in series with the primary to see your first contention confirmed.

Apply the famous " I squared R " formula to work out the HEAT in watts being dissipated in that poor primary winding.

Note that as the primary heats up, the " R " value goes up by possibly 2 or

3 times.

Good on you, Fred.

I like your no nonsense style of enquiry.

....... Phil

The inductance only causes reactive current (high impedance) until it saturates - after that happens it is just the low resistance of the wire to limit current - and that doesn't limit any real current, what resistance is there in the wire, heats up.

Ordinarily transformers are designed so they work fine with no load so why are you destroying them?

Some variacs can be made to go to 20%-40% over nominal line voltage and that would destroy a transformer. Many variacs have a tap inside that allows you to select for 100% or 120-140% over voltage. Put it on 100% unless you need to correct for low line or are testing something over nominal line voltage - and fuse the variac; they tend to be more expensive than other transformers, and are a bitch to repair.

At higher voltages (or lower frequencies!) the core will hit magnetic saturation. When that happens, the inductance drops radically and nothing limits the promary current but the copper resistance.

Most transformers are designed to run pretty close to core saturation at rated line voltage and frequency, so a bit more pushes them into their saturation region.

The primary current waveform is interesting. It has the usual quadrature sine wave caused by primary inductance, and usually some distortion caused by the core. As you crank the primary voltage up, the distortion increases, becoming huge spikes as the core saturates.

Note that saturation is caused by the primary voltage, not by a load.

John

The test was to see if the 110 VAC tansformers could have withstood

230 VAC. This was more of a "what's the worst sombody could do". The fault was to be detected and a warning given.

Having the loads connected protected the transformer to a degree. But why?

Does this mean the loads absorbed energy via the magnetic coupling thus reducing the magneticfield imposed on the core which meant the transformer is not so saturated and lasts long? ( OK but not for long )

The result was so non-intuative to me,

The thought was "How could an unload transformer be worse than a fully load one?" thus I was a little supprised!!!

but then I do not normaly go and drive things this hard or have worked with transformers at this level.

This sort of behaviour is mentioned in some texts, but not stated, sort of implied and you have to work it out for your self or do the Test.

A friend has since suggested that if the frequency was doubled the voltage could be doubled. I then pointed out that 100 Hz mains was a little uncommon!! :-)

At least it proves the point "Don't assume, Test!"

Thanks for the comments guys.

Loading the secondary increases the primary current, so causes a little voltage drop in the primary resistance. That reduces the primary magnetization, so the transformer is somewhat less likely to saturate.

You can think of the primary as being a perfect, zero-resistance winding in series with an external resistance equal to the actual primary resistance. So loading the transformer increases the voltage drop in that "external" resistor, so the primary sees less net voltage drive.

The thing that saturates the iron in the primary drive is applied volt-seconds, namely how much voltage is applied for how long. So higher primary voltage, or lower frequency, both push the core towards saturation.

Of course, if you load the secondary too hard, both the primary and the secondary wire heats up, so again the transformer fries. But it doesn't saturate!

You're both right.

John

"FunFred"

** They won't.

But except for very small examples, primary current rises to such a high value it blows the AC fuse immediately.

With larger transformers ( over 300VA) the current is enough to trip the breaker in the AC supply.

** Due solely to primary voltage drop, the load causes the peak voltage applied to the primary to be less by 5 or 10%.

Imagine the resistance of the copper wire disappeared and you added a resistor of the same value external to the transformer. There would be a voltage across that resistor not seen by the primary.

** Surprises lotsa folks, that one.

Hence the persistent myth that an overloaded transformer is "saturating".

..... Phil

"FunFred"

** If the frequency doubles, the power throughput of the transformer is doubled too.

Transformers for use with 400Hz AC power systems on board aircraft are tiny.

....... Phil