Isolation transformer draws excessive current under no load

On Feb 10, 4:42 pm, Ian Jackson
<ianREMOVETHISjack...@g3ohx.demon.co.uk> wrote:
In message
41177fb9-3a45-4b99-b150-285f289a5...@z31g2000vbt.googlegroups.com>,
Robert Macy <robert.a.m...@gmail.com> writes









On Feb 10, 1:36 pm, Robert Macy <robert.a.m...@gmail.com> wrote:
On Feb 10, 9:40 am, nesesu <neil_sutcli...@telus.net> wrote:

On Feb 10, 8:17 am, JW <n...@dev.null> wrote:

Hi all,

Is there any reason that a 120VAC to 120VAC  isolation transformer would
draw 2.54A on its primary when there is no load present on it's
secondary?

Part

reference:http://www.temcoindustrialpower.com/products/Transformers/FT2036.html

datasheet:http://attachments.temcoindustrialpower.com/product_info/Federal_FB_1...

I finally got around to wiring this transformer, and I noticed something
that doesn't seem quite right to me. I have it wired for 120VAC (H1
connected to H3 and H2 connected to H4) and it seems to be working as I
get about 120VAC on the secondary, ( wired X1 to X3 and X2 to X4)
but with
no load on the secondary, the transformer is drawing 2.54 Amps.

Looking for a sanity check I guess. I'm beginning to think the thing may
be defective... Email to the Temco has produced no response as of yet.

As a quick test, disconnect all the windings from each other and then
power up any ONE winding and see what the current draw is.
I see that it is a 15kVA rated, so it is not unreasonable that the
magnetizing curent is that high, but the actual power dissipation
would be much less than that current suggests. It is probably working
correctly, but a bit of an overkill for the average workbench.

Neil S.

Sounds VERY reasonable for a 15kVA transformer. 2.5 A suggests
coupling ratio on the order of 0.982, not bad for an AC mains
transformer that weighs that much.

If it bothers you, you can add  a high quality AC cap in parallel
around 55 uF. That should 'resonate' out the reactive current assuming
120Vac, 60Hz yields around 127 mH.

Or, ignore it and let your house wiring dissipate a bit of power less
than 1W ?

ARRRGGG!  That'll teach me NOT to do my own research.  Now, I have to
reply to my OWN posting!
ok your transformer is the  1kVA version of that series, the smallest
transformer.

current of max load is 1kVA/120 or 8.3A.
reactive current is 2.5 A that implies the core reactive impedance
from its inductance is around 3.3 to 1  Seems a little low, but in
range for a super cheap transformer that will get hot while running. I
would have expected more like around 1A, or less.

Here are some 'good' rules of thumb:
The core inductance reactance is probably 5 to 10 times the load
impedance, I've seen as low as 3 times.
The winding resistance is usually split half in pri and half in sec.
actually more like 45% in primary and 55% in sec.and the total is less
than 1/10 of load impedance.

So the transformer is in 'range' but that current does seem pretty
high for what should be a high quality transfomer.  Could be a short
somewhere. Depending on how the transformer is wound, you may or may
not learn much from measuring the DC resistance of each winding.  You
could power each winding with 120Vac and measure the current of each
winding, but then again.

If you don't need much power through this thing, like less than 500W,
wire it for 240/240 and that'll lower the core current for you.

The symptoms described sound very similar to using a 60Hz transformer on
a 50Hz supply (as might happen if you are using American equipment in
Europe), and the transformer hasn't got enough iron in it - so it's
saturating. However, that's not what you are doing.

If saturation IS the problem, you can usually confirm it (under no-load
conditions) by winding the supply voltage up on a variac, and measuring
the current the transformer draws. It will rise suddenly when the core
starts to saturate. Although the problem is much more likely to be
shorted turns, a quick test for saturation might be interesting.
--
Ian
Click to expand...
When I buy transformers I always buy ones rated for 50 Hz, that way
on 60 Hz they run cooler. That does not apply to ferroresonant
transformers of course, but I have never bought one so it doesn't
matter.
 
On Feb 13, 10:15 pm, "Phil Allison" <phi...@tpg.com.au> wrote:
"Robert Macy"

** Is there any way to shut fools like you up ??
   ----------------------------------------------------









Adding caps will indeed have NO effect on the tranny, but WILL improve
the PF as seen by your AC mains,

** Absolute BULLSHIT !!!!!!!!!

which means the amount of power you
drop in your wiring [and pay for] will be less.

** Absolute IDIOCY !!!!!!!

Adding caps is a STANDARD way to adjust power factor to 1.

** But never used with an off load transformer - because that idea is 100%
STUPID.

YOU are an obsessed IDIOT with a one track mind.

Piss off.

I stand by what I said as technically correct.

** LOL -  that only makes you a BIGGER fucking idiot.

Many, many technical journals, text books, and supporting calculations
based upon terms of definition confirm what I said.

** Shame you cannot supply one that backs up your idiotic comments about
unloaded transformers.

Listen PAL  !!!!!!!!

YOU  are nothing but a lying, bullshitting  NUT CASE !!

Clueless to the core.

FOAD.

...  Phil
Click to expand...
Phil is mentally disturbed, probably autistic, although he is
technically knowledgeable. He'd be okay but for his illness.
 
"William Sommerwanker TROLL"


** Big shame narcissistic fucking idiots like you still roam the earth.

FOAD now !!
 
"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:9qdfqcFditU1@mid.individual.net...

"William Sommerwanker TROLL"
** Big shame narcissistic fucking idiots like you still roam the earth.
FOAD now !!
Click to expand...
ROAR!
I am a tyrannosaur! I will rip off your limbs with my silly two-fingered
arms, and greedily scarf them down.
If you are still alive after all your limbs have been consumed, I will bite
through your skull and savor the crunching sound.
ROAR!
 
It should have read...

ROAR!

I am a tyrannosaur! I will rip off your limbs with my silly two-fingered
arms, and greedily scarf them down.
If you remain alive after all your limbs have been consumed, I will
bite through your skull and savor your agonized screams of fear
and excruciating pain, before you pass forever from this world.

ROAR!
 
"William Sommerwanker TROLL"


** Big shame narcissistic fucking idiots like you still roam the earth.

FOAD now !!
 
On Sat, 18 Feb 2012 21:31:17 -0800 (PST), spamtrap1888
<spamtrap1888@gmail.com> wrote:

On Feb 18, 4:22 pm, "Phil Allison" <phi...@tpg.com.au> wrote:
"Cydrome Leader"

I'd say your transformer is bad or somehow connected wrong.

** No way.

The OP's data shows it is operating normally and correctly.

10 degrees temp rise and a PF of 0.15 is A-OK.

....  Phil

Why doesn't the transformer data sheet include idle current, power
factor, etc.? Or is it embedded in some transformer standard I have to
spend 2000 swiss francs to get?
Click to expand...
If it is covered under a IEEE standard it would be about uS$100 to $200.
I have several of them already, but maybe not the one for your
transformer. Nor is the IEEE standard a mandatory one, check local
regulation for what is mandatory.

Speaking of, i need to go buy some more standards right now.

?-)
 
On Feb 17, 6:25 pm, "Phil Allison" <phi...@tpg.com.au> wrote:
"Robert Macy" =  one stubborn fucker

A cap does improve PF...

** But only with theoretical transformers -  NOT real ones.

First, use a Linear Model to represent the isolation transformer.

** Waste of fucking time and effort  -  as it only repeats the same FUCKWIT
error you have been sprouting here all along.

The primaries of  REAL  commercially made E-core transformers are  NOT
linear inductors !!!!!!!!!!!

The off load primary current at rated voltage is *dominated by the third
harmonic * of the AC supply frequency.

The laminated iron core is then saturating, quite heavily.

JW's 1kVA iso tranny is a very typical example of this fact.

You will  NOT  find this information on webs sites that merely discuss
transformer basics.

You WILL  find this if you test a cross section of commercial E-core
transformers with the aid of a variac, RMS current meter and a scope
monitoring the current waveform.

This has NOTHING do with badly or well made transformers -  all makers do it
to save weight and cost.

BTW:

I happen to own a 1kVA transformer very similar to that described by the OP.

Tested as above, these are the figures:

VAC   A rms    I peak

30       0.08       0.11
50       0.11       0.14
70       0.20       0.35

90       0.45       1.0
110     1.0         2.0
120     1.4         2.9
130     2.2         4.6

Up to 70 volts AC, the tranny is approximately linear with an effective
inductance of about 1.1 H.

At and above 90 volts AC it suddenly changes -  current starts to increase
exponentially and the wave becomes very peaky with a 1:2 ratio between rms
and peak values.

At 130 VAC input, effective primary inductance ( based on simplistic
calculations)  has dropped to less than 0.2H due to core saturation.

I must have tested hundreds of E-core trannys this way in the last 20 years
or so and ALL do much the same thing.

Toroidal and C- core types are different.

....  Phil
Click to expand...
For various reasons, it took MUCH longer than I anticipated to post
back here!

After more accurately modeling a REAL transformer based upon PA's
data, I am convinced that adding a cap in parallel will NOT improve
the PF.

The best intuitive way to explain is to simply say: in order to reduce
PF, a resonating cap is added in parallel to cancel the effect of the
inductance. The effect relies upon the inductance to be a CONSTANT
value, the core of a typical isolation transfomer during its operation
is NOT constant {reducing dramatically as the peak current flows. With
this changing value of inductance all the 'goodness' of adding the cap
completely disappears. Using the following simple model, I could not
even change the cap to some 'optimum' value. PF just stayed bad, did
not get worse, just stayed bad, and the cost of adding any cap was
wasted. .

Small discussion about the dataset. The slight increase in apparent
inductance going from 30 to 50 can be explained as being due to the
coercivity of the core material. At low currents, the BH curve loop
being followed is more horizontal than the BH curve being followed as
the voltage increases and current increases, the inductance then
starts dropping due to the saturation of the core. If you plot the
data set as Apk/(sqrt(2)*Arms) vs Vac, you'll see a strange shape to
the curve. plot as 20*log10(Apk/Apkcalc) vs Vac and it is VERY
interesting to notice a 'step' and then constant value. see the
undershoot, overshoot, and ringing of the data as Vac increases. Note:
The following model does NOT display this type of performance. I
wonder if it was caused by a 'two-step' saturation? In other words,
material saturated leaving another material that saturates at a higher
current value, like regions in the core?. I'm going to go back and
try 3 inductors in series, to see if I can get a 'better' fit - air
core inductor, inductor 1 saturating first, inductor 2 saturating 2nd.
Only mentioned as interesting, do not think a finer model will result
in a different conclusion, though.]

LTspice has a simple nonlinear model, called "Behavioural Model", for
an inductor. The inductor's flux is: Flux = tanh(x), where x is the
current through the inductor.

The model is supposed to follow the saturation curve fairly well, but
assumes ZERO coercivity, in other words, zero hysteresis. The model is
like following the 'centerline' of the hysteresis curve.

Several observations, I could NEVER get the model to fit the data
provided by Phil Allison, [which translates to still don't have a good
model]. However, after some 'adjustments'. the characteristics of the
model did fit the characteristics of the data [No time to EXACTLY
create/present the tables of comparison, will do later]

Suffice to say, as the input voltage increased; the rms current went
up faster than if the transformer's core were a constant inductor, the
peak current for the rms current went up at about the same ratio as
PA's data. At higher voltage, the peak current noticeably distorted
the current waveform into appearing to have severe 3rd harmonic
distortion. Actually, instead of sinusoidal, looks triangular.

Using this model, I calculated an appropriate cap, added it to the
circuit, and found NO EFFECT on PF !!!

I then started changing the cap's value, looking for some optimum, and
found none.

CONCLUSION: For "real" isolation transformers, it is NOT POSSIBLE to
add a cap to 'adjust' the PF for the load.

I defer to PA's experience with a multitude of manufactured
transformers, in defense of my comments, my experience was limited to
custom transformers [whose performance was always better than
commercially available] and my own transformers, which perform a bit
better. Example, 100 turns to get 1 Henry. No DC current is allowed,
but you do measure 1H inductance with only 100 turns. Coercivity is
about 1/100th silicon steel, which my understanding is usually used in
commercial transformers. So what I'm used to working with does
approach 'ideal' inductance.


The simple model is here for any interested:
name the file something ending with .cir LTspice will run the
simulation. You're on your own for changing values.
Note the Rcore value was added to represent the unloaded 40+W
dissipation, you will find its absence/presence does not affect the
conclusions.

TEST_ModelBehaviour - nonlinear inductor using behavioural Model
* for use on LTspice
*
..tran .1 20 19.95 .1m
..param k0=sqrt(2)
..param k1=120
Vac 1 0 AC={k1} SIN(0 {k0*k1} 60)
Racmains 1 IN 0.01
*Cc IN 0 30uF
Rpri IN 3 0.26
..param kk0=1
..param kk1=1.2/{kk0}
..param kk2=0.412
Lcore 3 0 Flux={kk2*kk1}*tanh(x/{kk2})
Rcore 3 0 350
..end
 
On Feb 21, 10:18 am, Robert Macy <robert.a.m...@gmail.com> wrote:
On Feb 17, 6:25 pm, "Phil Allison" <phi...@tpg.com.au> wrote:





"Robert Macy" =  one stubborn fucker

A cap does improve PF...

** But only with theoretical transformers -  NOT real ones.

First, use a Linear Model to represent the isolation transformer.

** Waste of fucking time and effort  -  as it only repeats the same FUCKWIT
error you have been sprouting here all along.

The primaries of  REAL  commercially made E-core transformers are  NOT
linear inductors !!!!!!!!!!!

The off load primary current at rated voltage is *dominated by the third
harmonic * of the AC supply frequency.

The laminated iron core is then saturating, quite heavily.

JW's 1kVA iso tranny is a very typical example of this fact.

You will  NOT  find this information on webs sites that merely discuss
transformer basics.

You WILL  find this if you test a cross section of commercial E-core
transformers with the aid of a variac, RMS current meter and a scope
monitoring the current waveform.

This has NOTHING do with badly or well made transformers -  all makers do it
to save weight and cost.

BTW:

I happen to own a 1kVA transformer very similar to that described by the OP.

Tested as above, these are the figures:

VAC   A rms    I peak

30       0.08       0.11
50       0.11       0.14
70       0.20       0.35

90       0.45       1.0
110     1.0         2.0
120     1.4         2.9
130     2.2         4.6

Up to 70 volts AC, the tranny is approximately linear with an effective
inductance of about 1.1 H.

At and above 90 volts AC it suddenly changes -  current starts to increase
exponentially and the wave becomes very peaky with a 1:2 ratio between rms
and peak values.

At 130 VAC input, effective primary inductance ( based on simplistic
calculations)  has dropped to less than 0.2H due to core saturation.

I must have tested hundreds of E-core trannys this way in the last 20 years
or so and ALL do much the same thing.

Toroidal and C- core types are different.

....  Phil

For various reasons, it took MUCH longer than I anticipated to post
back here!

After more accurately modeling a REAL transformer based upon PA's
data, I am convinced that adding a cap in parallel will NOT improve
the PF.

,,,snip...!!!
Click to expand...
What a WRONG STATEMENT! Adding a cap in parallel to the isolation
transformer 'should' improve PF, but not as much as one would like.,
but does improve it, without makinganything worse, except cost.

The error was caused by monitoring the current through Rpri, not
through Racmains, so OF COURSE PF never changed!!!

Didn't discover the error until went to a three inductor model which
matches the data fairly closely.

Note: the behavioural model should be fairly accurate, because the
transformer has a constant voltage across it, so it is possible to
approximate the hysteresis loss with a fixed resistor. The transformer
has enough voltage that each cycle the same amount of power is lost.
I would NOT use this model tor represent an audio transformer, where
the drive can vary a great deal.

The 'three' inductor model starts very closely matching the data,
results and data are at the end of this posting. Same conclusion,
adding a cap helps PF.


For the single inductor model, I added a residual amount of AIR
inductance [about 30mH] and got the following:

The model very closely models observation, at low voltages current is
fairly sinusoidal, near 70+ Vac, the thrid harmonic starts to dominate
the wave shape, DRAMATICALLY. We're talking major 'pointy' waveforms.
I can't post the plot, but you can copy the model below and see for
yourself.

Checking the model's match to the data:
LTspice PA's Data
Vac Arms Apk Arms Apk
30 0.068 0.098 0.08 0.11
50 0.118 0.172 0.11 0.14
70 0.180 0.270 0.20 0.35
90 0.275 0.447 0.45 1.0
110 0.744 1.68 1.00 2.0
120 1.38 2.96 1.4 2.9
130 2.08 4.22 2.2 4.6

simulating the circuit with the model:
NO LOAD [add Rcore = 350]
120Vac Arms Vinrms Pwr(Rcore) PF
no cap 1.408 119.9 41.54 0.25
28uF 0.762 120 41.57 0.45
56uF 1.542 120 41.61 0.22

FULL LOAD 14.4 ohm [add Rcore = 350]
120Vac Arms Vinrms Pwr(Rcore) PF
no cap 8.594 119.9 1,020 0.99
29uF 8.533 119.9 1,020 1.00

CONCLUSION: adding a cap in parallel to an isolation transformer,
whether loaded or unloaded, is expected to improve PF

copy and name something ending in .cir:
TEST_ModelBehaviour - nonlinear inductor using behavioural Model
* Voltage is V(in), current is I(Racmains)
* Real power is average of V(in)*I(Racmains)
* apparent power is V(in)rms times I(Racmains)rms
* Plot these three: I(Racmains), V(in), V(in)*I(Racmains)
..tran .1 10 9.95 .1m
..param k0=sqrt(2)
..param k1=120
Vac 1 0 AC={k1} SIN(0 {k0*k1} 60)
Racmains 1 IN 0.01
*Cpf IN 0 28uF
Rpri IN 3 0.26
*
..param kAIR=.03 ;set to 'residual' inductance in Henries
..param kk0=1.2 ;set to 'initial' inductance very low current
..param A0=0.274 ;set to current where var. ind. drops to half
..param kk=1.13
..param kk1={A0*kk} ;set to current for half saturation
..param kk2={kk0-kAIR}
*
Lcore 3 0 Flux={ {kk2*kk1}*tanh(x/{kk1})+{kAIR}*x }
*
*Rcore 3 0 350
*Rload 3 0 14.4
..end



= = = The three inductor model:

LTspice PA's Data
Vac Arms Apk Arms Apk
30 0.071 0.114 0.08 0.11
50 0.130 0.210 0.11 0.14
70 0.231 0.383 0.20 0.35
90 0.464 0.830 0.45 1.0
110 0.922 1.74 1.00 2.0
120 1.40 2.83 1.4 2.9
130 2.21 4.58 2.2 4.6

NO LOAD
120Vac Arms Vinrms Pwr(Rcore) PF
no cap 1.441 119.9 41.30 0.24
29uF 0.642 120 41.31 0.54
64uF 1.728 120 41.32 0.20

FULL LOAD 14.4 ohm
120Vac Arms Vinrms Pwr(Rcore) PF
no cap 8.708 1,032 119.9 0.99
29uF 8.622 1,032 119.9 1.00


TEST_MB three - nonlinear inductors using behavioural Model
* for use on LTspice
*
..tran .1 20 19.95 .1m
..param k0=sqrt(2)
..param k1=120
Vac 1 0 AC={k1} SIN(0 {k0*k1} 60)
Racmains 1 IN 0.01
Cpf IN 0 29uF
*
Rpri IN 3 0.1
*
* first inductor
..param kk0=1.2
..param kk1={kk0-mm0}
..param kk2=.2
Lcore01 3 4 Flux={kk2*kk1}*tanh(x/{kk2})
* second inductor
..param mm0=0.185
..param mm1={mm0-nn0}
..param mm2=1.17
Lcore02 4 5 Flux={mm2*mm1}*tanh(x/{mm2})
* third inductor == air core
..param nn0=.02
Lcore03 5 0 {nn0}
*
Rcore 3 0 350
*Rload 3 0 14.4
..end
 
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