Isolation transformer draws excessive current under no load

[Robert Macy]

On Feb 14, 7:07 pm, "Phil Allison" <phi...@tpg.com.au> wrote:

"Robert Macy"

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

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  -   pun intended.

FOAD.

...  Phil

Although you are wrong, you say with such conviction that people
believe. I would not have bothered replying and supplying references
supporting my statements, except your claims/comments are proving a
disservice to people seeking information. As an example of the effects
of your incorrect statements, see reply posted by spamtrap1888, whose
question appears to be based upon having accepted your erroneous claim
that as a transformer is loaded, the inductive core current decreases.
Not true. As an isolation transformer is loaded, the inductive core
current DOES NOT decrease. rather stays fairly constant.

Stop misleading people. You do NOT contribute, rather distract/
distort. Stop it.

To prevent/undo the damage you cause, I post the following references:

using advanced google search with the following words
"magnetizing current" transformer distortion
yields 38,200 results, starting at the first, they are worthwhile
reading, such as...

this shows how saturation causes a spike in current as the voltage
waveform crosses zero:
<http://www.allaboutcircuits.com/vol_2/chpt_9/1.html>
section "Transformer Principles" describes constant voltage across the
core, thereby constant inductive current, etc.
<http://lehmanengineering.com/quiz/quiz6sol.html>
also, these two pdf files are a decent background reference:
<http://classicaudio.ru/articles/Output_Transformer_a57.pdf>
<http://www.classicaudio.ru/articles/ot_distortion_p2_a57.pdf>

etc, etc, etc.

These references supported my statements.

..
..
..
However, you did not supply any reference refuting my statements.

 

[Phil Allison]

" Robert Macy = fuckwit TROLL "


( snip piles of the idiot's steaming great turds )


Listen here PAL !!!!!!!!

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

Clueless right to the core.

Fuck OFF !!!!!!!!!!!!!!!!



.... Phil

 

[William Sommerwerck]

God knows, we've tried... but you keep posting.

 

[Robert Macy]

On Feb 15, 1:52 am, Sylvia Else <syl...@not.here.invalid> wrote:

On 14/02/2012 12:52 AM, Robert Macy wrote:





On Feb 13, 5:45 am, JW<n...@dev.null>  wrote:
...snip...

Adding capacitance in parallel has no effect on the tranny whatsoever  -
and it will make the PF worse.

Noted, and thanks.
...snip...

Adding caps will indeed have NO effect on the tranny, but WILL improve
the PF as seen by your AC mains, which means the amount of power you
drop in your wiring [and pay for] will be less.

Adding caps is a STANDARD way to adjust power factor to 1. Used by the
industrial power consumers that are heavily penalized for lagging
power factor. It is cheaper for them to add a building full of caps,
just to shift their PF and pay less for power.

If you do the analysis of power consumption throughout a standard AC
mains power distribution, you will find that a lagging power factor
[caused by motors, etc] INCREASES the power required to simply get
billable wattage to you. And, it's surprisingly large.

But, except for the extra power dissipated in the household wiring, is
not measured by the meter, and not included in the billed energy units.

Sylvia.

True!

Today, the only way that utilities companies can bill for this lost
energy is by assigning a 'penalty' for bad PF.

I'll bet with today's smarter metering, they'll figure a way to
measure it real-time and bill for it.

 

[Robert Macy]

On Feb 15, 3:37 pm, "William Sommerwerck" <grizzledgee...@comcast.net>
wrote:

God knows, we've tried... but you keep posting.

I use google access so the thread line is NOT preserved.

William,

Is this email for me?

Is there an error(s) in what I posted?

 

[Sylvia Else]

On 16/02/2012 9:31 AM, Robert Macy wrote:

On Feb 15, 1:52 am, Sylvia Else<syl...@not.here.invalid> wrote:
On 14/02/2012 12:52 AM, Robert Macy wrote:





On Feb 13, 5:45 am, JW<n...@dev.null> wrote:
...snip...

Adding capacitance in parallel has no effect on the tranny whatsoever -
and it will make the PF worse.

Noted, and thanks.
...snip...

Adding caps will indeed have NO effect on the tranny, but WILL improve
the PF as seen by your AC mains, which means the amount of power you
drop in your wiring [and pay for] will be less.

Adding caps is a STANDARD way to adjust power factor to 1. Used by the
industrial power consumers that are heavily penalized for lagging
power factor. It is cheaper for them to add a building full of caps,
just to shift their PF and pay less for power.

If you do the analysis of power consumption throughout a standard AC
mains power distribution, you will find that a lagging power factor
[caused by motors, etc] INCREASES the power required to simply get
billable wattage to you. And, it's surprisingly large.

But, except for the extra power dissipated in the household wiring, is
not measured by the meter, and not included in the billed energy units.

Sylvia.

True!

Today, the only way that utilities companies can bill for this lost
energy is by assigning a 'penalty' for bad PF.

I'll bet with today's smarter metering, they'll figure a way to
measure it real-time and bill for it.

And if we correct our power factor, our bills will be lower than they
are now.

Oh, wait a minute, what was I thinking?

Sylvia.

 

[Robert Macy]

On Feb 13, 4:42 pm, "Phil Allison" <phi...@tpg.com.au> wrote:

...snip...
**  That is pretty high actually -  a typical 1KVA tranny has about 6 to 8 %
power loss at full load.
...snip...

Agree with that range.

Assume for this transformer the power loss is 8%, 80W.
Further assume this is a 'well designed' transformer where the
designer allowed half the dissipation in the windings and half in the
core.
That means 40W in the core and 40W in the windings, at full load.
Four windings with each carrying half the full load current, 4.167A,
implies 0.576 ohms/winding
two in series would be 1.15 ohm and since most pri/sec are 45%/55%
that would mean
each pri winding 0.52 ohm
each sec winding 0.63 ohm

Now unloaded the two pri windings are passing 1.27A each, dissipating
almost 2 W and the core is dissipating around 40W, so JW would have
measured power for the unloaded transformer at around 42W, which he
said he measured.

JW,
Did you ever measure the winding impedance?
It would be interesting to compare prediction to actual values. [of
course, preaching to the choir, for better accuracy put the two pri in
series and the two sec in series]

 

[Robert Macy]

On Feb 13, 5:45 am, JW <n...@dev.null> wrote:

On Sun, 12 Feb 2012 01:05:04 +1100 "Phil Allison" <phi...@tpg.com.au
wrote in Message id: <9pnaslF48...@mid.individual.net>:

...snip...
Adding capacitance in parallel has no effect on the tranny whatsoever  -
and it will make the PF worse.

Noted, and thanks.

A cap does improve PF...

To answer the question whether an external capacitor placed in
parallel with the input of a transformer improves power factor or not,
for both loaded and unloaded conditions; I started the following
analyses and share with others who may be interested.

First, use a Linear Model to represent the isolation transformer.
Simulate the iso transformer and circuitry by using LINEAR SIMULATION,
that is, only using linear components to represent observed data.

Second, simulate using nonlinear components, specifically the chan
model, which simulates BOTH hysteresis (coercivity) and saturation.
TO BE DONE LATER.

(Model of the 1kVA ISO-Transformer wound for 120/120 operation)

LINEAR SIMULATION
The following transformer model is accepted for low frequency
operation. The transformer is approximated by representing the
transformer as primary and secondary winding resistances connected to
a core which has a parallel resistance to represent the core losses.
The transformer's coupling coefficient was arbitrarily set to one,
more likely to be as low as 0.98. But modifying coupling coefficient
does not appreciably change results.

Values for core inductance and parallel resistor were determined from
data provided by JW and winding resistance values were estimated based
upon experience:

Core Inductance:
Lcore = 0.128 H, based upon JW's measurement of current with NO LOAD
Core Losses:
Rcore = 360 ohms, based upon JW's measurement using Watts Up meter
with NO LOAD
Winding Resistances:
Rpri = 0.26 ohms, Rsec = 0.315 ohms estimated from experience

The measurements that one would obtain with this linear model are
Current NO LOAD = 2.54 Arms
Power NO LOAD = 41.7 W
PF = 0.137

All fairly close.

As a sanity check, what happens at FULL load with 14.4 ohms?
Vout = 115V, Power = 922W, PF = 0.960, which is reasonable.

Load regulation is not that good in this model. The output voltage
with NO LOAD is 120V, but when loaded with 14.4 ohms to get the
expected 1kW output, the output voltage drops to 115V. This
manufacturer probably did what everyone else does and not wind the
transformer EXACTLY 1:1, but add a few extra turns on the secondary to
compensate for this expected drop, more like 100:105 or such. JW could
verify the winding ratio by running the transformer 'backwards'. He
would then see he'd probably only get 110 out the pri with 120 into
the sec. Anybody who has used two doorbell transformers, one down and
one back up just to make an isolation transformer has discovered this
'feature'.

For simplicity, I leave the winding ratio as 1:1, which will not
appreciably change results.


In a previous post I said add a parallel capacitor, 55uF, to adjust
the PF closer to 1.

When 55uF cap is added in parallel to the input of the transformer,
the PF does indeed change.
NO LOAD: PF = 0.997
FULL LOAD: PF = 1.00
And, as I claimed adding a cap improved for both NO Load and FULL
Load.


I've included a copy of the simulation model below, watch out for word
wrap, but feel free to try it in FREE LTspice and see for yourself.

CONCLUSION:
Adding a parallel capacitor to the input of a transformer does help
power factor both loaded and unloaded.


Now, this was LINEAR simulation. I will now more accurately model the
non-linear characteristics of a real transformer and compare to see if
my assertion is still true.





LINEAR SIMULATION:

Version 4
SHEET 1 3060 1012
WIRE 1216 752 1184 752
WIRE 1344 752 1296 752
WIRE 1456 752 1344 752
WIRE 1584 752 1456 752
WIRE 1728 752 1664 752
WIRE 1808 752 1728 752
WIRE 1904 752 1808 752
WIRE 2080 752 1984 752
WIRE 2384 752 2160 752
WIRE 2432 752 2384 752
WIRE 1728 768 1728 752
WIRE 1808 768 1808 752
WIRE 1904 768 1904 752
WIRE 1984 768 1984 752
WIRE 2432 768 2432 752
WIRE 1456 784 1456 752
WIRE 1184 800 1184 752
WIRE 1728 880 1728 848
WIRE 1808 880 1808 848
WIRE 1904 880 1904 848
WIRE 1984 880 1984 848
WIRE 2432 880 2432 848
WIRE 1184 896 1184 880
WIRE 1456 896 1456 848
FLAG 1184 896 0
FLAG 1808 880 0
FLAG 1904 880 0
FLAG 1984 880 0
FLAG 2432 880 0
FLAG 2384 752 OUT
FLAG 1344 752 IN
FLAG 1728 880 0
FLAG 1456 896 0
SYMBOL voltage 1184 784 R0
WINDOW 0 48 52 Left 0
WINDOW 3 47 113 Left 0
WINDOW 123 49 82 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName Vac
SYMATTR Value SINE(0 170 60)
SYMATTR Value2 AC 120
SYMBOL res 1200 768 R270
WINDOW 0 32 56 VTop 0
WINDOW 3 0 56 VBottom 0
SYMATTR InstName Racmains
SYMATTR Value 0.01
SYMBOL ind 1792 752 R0
SYMATTR InstName Lcore
SYMATTR Value 0.128
SYMBOL ind2 1888 752 R0
SYMATTR InstName Lpri
SYMATTR Value 10
SYMATTR Type ind
SYMBOL ind2 1968 752 R0
SYMATTR InstName Lsec
SYMATTR Value 10
SYMATTR Type ind
SYMBOL res 1568 768 R270
WINDOW 0 32 56 VTop 0
WINDOW 3 0 56 VBottom 0
SYMATTR InstName Rpri
SYMATTR Value 0.26
SYMBOL res 2064 768 R270
WINDOW 0 32 56 VTop 0
WINDOW 3 0 56 VBottom 0
SYMATTR InstName Rsec
SYMATTR Value 0.315
SYMBOL res 2416 752 R0
SYMATTR InstName Rload
SYMATTR Value 10MEG
SYMBOL res 1712 752 R0
SYMATTR InstName Rcore
SYMATTR Value 360
SYMBOL cap 1440 784 R0
SYMATTR InstName Cc
SYMATTR Value 55ľF
TEXT 1808 936 Left 0 !K1 Lpri Lsec 1
TEXT 1152 624 Left 0 !.ac LIN 201 50 70
TEXT 1152 552 Left 0 ;.ac LIN 201 50 70
TEXT 2672 800 Left 0 ;NO Load Rload = 10MEG\nFULL Load Rload 14.4 ohms
TEXT 1648 624 Left 0 ;PF = RE(V(in)*I(Racmains))/(V(in)*I(Racmains))
TEXT 1400 936 Left 0 ;NO PF Correction Cc = 55pF\nWITH PF
Correction Cc = 55uF

 

[Phil Allison]

"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

 

[Robert Macy]

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

Thank you for the explanation and the data.

I did see that in my non-linear models the third harmonic peaks
started 'popping' up as the voltage increased.

Obviously, the concept of inductance loses something with that much
distortion.

Will try to curve fit that data to the chan model and see if the
simulations match your measurements. It would be great to have a model
one can trust to at least somewhat 'look' like reality.

 

[Phil Allison]

"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

 

[Cydrome Leader]

Sylvia Else <sylvia@not.here.invalid> wrote:

On 14/02/2012 12:52 AM, Robert Macy wrote:
On Feb 13, 5:45 am, JW<n...@dev.null> wrote:
...snip...

Adding capacitance in parallel has no effect on the tranny whatsoever -
and it will make the PF worse.

Noted, and thanks.
...snip...

Adding caps will indeed have NO effect on the tranny, but WILL improve
the PF as seen by your AC mains, which means the amount of power you
drop in your wiring [and pay for] will be less.

Adding caps is a STANDARD way to adjust power factor to 1. Used by the
industrial power consumers that are heavily penalized for lagging
power factor. It is cheaper for them to add a building full of caps,
just to shift their PF and pay less for power.

If you do the analysis of power consumption throughout a standard AC
mains power distribution, you will find that a lagging power factor
[caused by motors, etc] INCREASES the power required to simply get
billable wattage to you. And, it's surprisingly large.

But, except for the extra power dissipated in the household wiring, is
not measured by the meter, and not included in the billed energy units.

Sylvia.

yes it is. the problem with low power factors is the power loss before the
meter, which isn't measured and can be huge for large customers.

If at home, I toss a giant capacitor or huge inductor across the line, my
meter will pick up all the wiring and heating losses from the capacitors
or inductor.

It will not measure the losses to the power company for me circulating 100
amps to and from the power grid. the power companies losses on that will
be larger than mine, plus it wastes capacity of the grid itself.

For residential customers in the US, it's just assumed we're not idling
huge motors or testing capacitor banks in the dining room, so the
utilities don't pay attention to us.

 

[Cydrome Leader]

JW <none@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_1Ph_120x240Pri.pdf

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.

I just ran a test I have on a similar transformer. Mine is a cutler hammer
1.5kVA unit in the same type of metal box and potted in expoxy and sand -
nothing special.

it's actually a 240/480 to 240/120 unit, but you can switch the H and X
leads with no no big deal.

Anyways, running 125 VAC (line voltage is high around here) across "half"
the secondary (X1 and X2 instead of X1+x3 and X2+x4) resulted in 24 watts
of loss and a current of 0.5A according to the kilowatt meter. The thing
even buzzes somehow.

I'd say your transformer is bad or somehow connected wrong. My transformer
is rated 115C rise, so it's probably no winner when it comes to energy
efficiency. %Z isn't even listed on the nameplate.

 

[Phil Allison]

"Cydrome Lunatic"

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.



that's a pretty horrible transformer if those parameters are A-OK.

** ROTFL - you are one colossal, know nothing bloody fool.



..... Phil

 

[Cydrome Leader]

Phil Allison <phil_a@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

that's a pretty horrible transformer if those parameters are A-OK.

 

[Phil Allison]

"spamtrap1888"
"Phil Allison"

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.

Why doesn't the transformer data sheet include idle current, power
factor, etc.?


** Why should it ?

They are easily measured parameters and of little interest to most users.

If the safety ratings, VA rating, temp rise, dimensions, weight and mounting
details are all described - that is enough.



.... Phil

 

[spamtrap1888]

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?

 

[William Sommerwerck]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:9qb4mgF3fkU1@mid.individual.net...

"Cydrome Lunatic"

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.

That's a pretty horrible transformer if those parameters are A-OK.

** ROTFL -- you are one colossal, know[-]nothing bloody fool.

Phil, do you know the word "empirical"? If not, you should learn it.
Empirical data are knowledge, as assuredly as theoretical considerations.

Last week I measured the no-load drain of a 1.5A isolation transformer. * My
Kill A Watt read 0.06 amperes, which could be anywhere between 0.055 and
0.065 amps. (The display doesn't have enough resolution.) That's 4% of the
rated load. Not bad.

The OP's transformer seems way out of line. It should definitely be placed
off-line. (Ar, ar.)

* I was about to specify 125V, when I realized that, in theory, an isolation
transformer should "work" at any voltage -- short (joke intended) of a
voltage high enough to break down the insulation, etc, etc, etc.

 

[Cydrome Leader]

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?

The only transformers I've seen specs for idle power consumption listed
(but not on the nameplate) are for distribution transformers the power
company would own. They seem to care about how much money it costs to keep
a transformer running for 30 years or until it finally explodes, so every
watt counts.

For loose reference, I recall some 1kVA oil filled poole mount
distribution tranformers with the real step-lap core idling at 14 watts,
or something close to that. They really make 1kVA units for stuff like
streetlights and running switchgear. At the 10kVA oil filled ones with the
typical step-lap cores, something like 45 watts is normal for some brands.

These monsters aren't really rated the same as smaller EI core potted unit
which are only meant to be cheap to produce in mexico or china or wherever
they come from these days.

 

[Ian Jackson]

In message
<03332c05-5f3e-4f02-bf32-4883630b0ef1@s13g2000yqe.googlegroups.com>,
rrusston@hotmail.com writes

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

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.

FWIW (and from what I remember), ferro-resonant (constant voltage)
transformers usually DO get pretty hot on no load. Regardless of the
load current, the input power is fairly constant. What doesn't come out
heats up the transformer.
--
Ian