What limits the current in an unloaded transformer?

[FunFred]

I got to do some experiments recentley and I realise that my
understanding of transformers has some holes.....

First I got hold of a variac and some resistor loads which I attached
to the transformer.

The transformer got hot, but not too hot, not much new there.
When I wound up the input voltage to the primary it got hotter. OK so
far...

Then as I was in a hurry and I wanted to test some other transformers
I connected just the primmary windings.

This is were it went a bit strange...

Just sitting at standard mains voltage, the transform did not do much,
but when I wound the input voltage up, the input current suddenley
climbed, transformer got a lot hotter and before I knew it the primary
coil had burned out.

I repeated this on a couple of scrap transformers and the same thing
happed.

So the question is... If there is virtualy no current on an unloaded
transformer why does the current suddenley climb when you go over it's
rated voltage?

I can only guess the core saturates and the side effect is the primary
starts conducting current and this causes it to get hotter and finaly
burns out.

Nobody I knows seems to be able to confirm this ... it's been bugging
me!
and googling just throws up hundereds of other issues....

Cheers

FunFred

 

[Phil Allison]

"FunFred"

I got to do some experiments recentley and I realise that my
understanding of transformers has some holes.....

First I got hold of a variac and some resistor loads which I attached
to the transformer.

The transformer got hot, but not too hot, not much new there.
When I wound up the input voltage to the primary it got hotter. OK so
far...

Then as I was in a hurry and I wanted to test some other transformers
I connected just the primmary windings.

This is were it went a bit strange...

Just sitting at standard mains voltage, the transform did not do much,
but when I wound the input voltage up, the input current suddenley
climbed, transformer got a lot hotter and before I knew it the primary
coil had burned out.

I repeated this on a couple of scrap transformers and the same thing
happed.

So the question is... If there is virtualy no current on an unloaded
transformer why does the current suddenley climb when you go over it's
rated voltage?

I can only guess the core saturates and the side effect is the primary
starts conducting current and this causes it to get hotter and finaly
burns out.

Nobody I knows seems to be able to confirm this ... it's been bugging
me!
and googling just throws up hundereds of other issues....

** 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

 

[default]

On Fri, 09 Nov 2007 03:21:41 -0800, FunFred
<alanfredperry@googlemail.com> wrote:

I got to do some experiments recentley and I realise that my
understanding of transformers has some holes.....

First I got hold of a variac and some resistor loads which I attached
to the transformer.

The transformer got hot, but not too hot, not much new there.
When I wound up the input voltage to the primary it got hotter. OK so
far...

Then as I was in a hurry and I wanted to test some other transformers
I connected just the primmary windings.

This is were it went a bit strange...

Just sitting at standard mains voltage, the transform did not do much,
but when I wound the input voltage up, the input current suddenley
climbed, transformer got a lot hotter and before I knew it the primary
coil had burned out.

I repeated this on a couple of scrap transformers and the same thing
happed.

So the question is... If there is virtualy no current on an unloaded
transformer why does the current suddenley climb when you go over it's
rated voltage?

I can only guess the core saturates and the side effect is the primary
starts conducting current and this causes it to get hotter and finaly
burns out.

Nobody I knows seems to be able to confirm this ... it's been bugging
me!
and googling just throws up hundereds of other issues....

Cheers

FunFred

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.
--

 

[John Larkin]

On Fri, 09 Nov 2007 03:21:41 -0800, FunFred
<alanfredperry@googlemail.com> wrote:

I got to do some experiments recentley and I realise that my
understanding of transformers has some holes.....

First I got hold of a variac and some resistor loads which I attached
to the transformer.

The transformer got hot, but not too hot, not much new there.
When I wound up the input voltage to the primary it got hotter. OK so
far...

Then as I was in a hurry and I wanted to test some other transformers
I connected just the primmary windings.

This is were it went a bit strange...

Just sitting at standard mains voltage, the transform did not do much,
but when I wound the input voltage up, the input current suddenley
climbed, transformer got a lot hotter and before I knew it the primary
coil had burned out.

I repeated this on a couple of scrap transformers and the same thing
happed.

So the question is... If there is virtualy no current on an unloaded
transformer why does the current suddenley climb when you go over it's
rated voltage?

I can only guess the core saturates and the side effect is the primary
starts conducting current and this causes it to get hotter and finaly
burns out.

Nobody I knows seems to be able to confirm this ... it's been bugging
me!
and googling just throws up hundereds of other issues....

Cheers

FunFred

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

 

[FunFred]

On 9 Nov, 16:31, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:

On Fri, 09 Nov 2007 03:21:41 -0800, FunFred



alanfredpe...@googlemail.com> wrote:
I got to do some experiments recentley and I realise that my
understanding of transformers has some holes.....

First I got hold of a variac and some resistor loads which I attached
to the transformer.

The transformer got hot, but not too hot, not much new there.
When I wound up the input voltage to the primary it got hotter. OK so
far...

Then as I was in a hurry and I wanted to test some other transformers
I connected just the primmary windings.

This is were it went a bit strange...

Just sitting at standard mains voltage, the transform did not do much,
but when I wound the input voltage up, the input current suddenley
climbed, transformer got a lot hotter and before I knew it the primary
coil had burned out.

I repeated this on a couple of scrap transformers and the same thing
happed.

So the question is... If there is virtualy no current on an unloaded
transformer why does the current suddenley climb when you go over it's
rated voltage?

I can only guess the core saturates and the side effect is the primary
starts conducting current and this causes it to get hotter and finaly
burns out.

Nobody I knows seems to be able to confirm this ... it's been bugging
me!
and googling just throws up hundereds of other issues....

Cheers

FunFred

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.

 

[John Larkin]

On Fri, 09 Nov 2007 14:51:32 -0800, FunFred
<alanfredperry@googlemail.com> wrote:

On 9 Nov, 16:31, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Fri, 09 Nov 2007 03:21:41 -0800, FunFred



alanfredpe...@googlemail.com> wrote:
I got to do some experiments recentley and I realise that my
understanding of transformers has some holes.....

First I got hold of a variac and some resistor loads which I attached
to the transformer.

The transformer got hot, but not too hot, not much new there.
When I wound up the input voltage to the primary it got hotter. OK so
far...

Then as I was in a hurry and I wanted to test some other transformers
I connected just the primmary windings.

This is were it went a bit strange...

Just sitting at standard mains voltage, the transform did not do much,
but when I wound the input voltage up, the input current suddenley
climbed, transformer got a lot hotter and before I knew it the primary
coil had burned out.

I repeated this on a couple of scrap transformers and the same thing
happed.

So the question is... If there is virtualy no current on an unloaded
transformer why does the current suddenley climb when you go over it's
rated voltage?

I can only guess the core saturates and the side effect is the primary
starts conducting current and this causes it to get hotter and finaly
burns out.

Nobody I knows seems to be able to confirm this ... it's been bugging
me!
and googling just throws up hundereds of other issues....

Cheers

FunFred

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?

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!

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!!

You're both right.

John

 

[Phil Allison]

"FunFred"

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

** 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.

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

** 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.

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

** Surprises lotsa folks, that one.

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



...... Phil

 

[Phil Allison]

"FunFred"

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!!

** 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