Isolation transformer draws excessive current under no load condition

[JW]

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.

 

[N_Cook]

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_120x
240Pri.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.

If there are shorted turns then there would be a difference in DC ohms of
each coil, assuming they should be the same for 120/120

 

[mike]

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.

It's not unusual for a transformer to draw some amps when unloaded.
That's what inductors do. The current should be out of phase with the
voltage.
What you care about is POWER. Measure the RMS power consumed by the
transformer. That's what counts.

If it's actually dissipating 300W, it will get very warm.

 

[William Sommerwerck]

2.5 amps seems like an awful lot of current, in-phase or not.

It sounds as if the unit is "defective" in some undefined. Or it might be
that the transformer's inductance is too low.

 

[mike]

How might you do that?
(I don't have a power meter kicking around)

Sure you do. It's on the outside of your house and goes
round and round...They send you a bill every month.
My base load is 200W, so an additional 300W would be
easy to see.
The newer ones simulate round and round
with an lcd display and have a flashing IR led that can
be used to measure actual power quite accurately.
If you have an old Palm III vintage PDA, I can send you
a program that lets you point the IR window at your power
meter and graph consumption...but a stopwatch counting
the display works as well.

It's best to turn off everything you can to reduce the base
load and improve the precision of your measurement with and
without the transformer connected.

If you wanna spend a few bucks, the P3 Kill A Watt meters
are very handy in this application.

As a go/no-go test, you can put an incandescent light bulb
in series with the transformer and see how bright it gets.

But at 300W, you shouldn't have any trouble sensing transformer
temperature rise with your hand.
OR
Wrap the transformer in insulation
and plot the temperature vs time...weigh the transformer to guess
at heat capacity and calculate the power from that.

So, several ways to "do that".

 

[mike]

2.5 amps seems like an awful lot of current, in-phase or not.

First guess, I'd have to agree. My Old Stancor 250W transformer
draws 9W 18VA .15A RMS unloaded.

It sounds as if the unit is "defective" in some undefined. Or it might be
that the transformer's inductance is too low.

One more thing to try.
Make sure you're not paralleling a primary and secondary winding.

Hook up one winding to the AC. Measure all 4 voltages very accurately.
How you connect the windings can make a difference. Small changes
in coupling due to the arrangement of the windings can produce
small differences in the output. When you start paralleling them,
you can get one winding fighting another. If that's the case,
energizing only one of the 4 windings with the others disconnected should
make the problem go away.
It's not impossible that a manufacturing defect screwed up the turns
count on one of the windings.

In a transformer this big, if you have a shorted turn, you should be
able to smell the result.

Depending on how close the transformer is to the box, the metal
box represents a shorted turn. All the flux is SUPPOSED to be inside
the core....supposed to be. I don't expect taking it out of the box
would make any significant difference, but I'd try it before scrapping
the device.

But the real diagnostic is to measure the real power consumed.
You can chase your tail trying to solve a problem that doesn't exist.
Try the series light bulb.

 

[mike]

Sounds VERY reasonable for a 15kVA transformer.

Huh???
Federal Pacific SE120N1F FT2036 - Isolation Transformer 120 x 240
Primary 120/240V Secondary - 1 kVA 60Hz


2.5 A suggests

 

[David Lesher]

How might you do that?

(I don't have a power meter kicking around)

Buy a Kill-o-watt; a useful tool to have around.

 

[Ian Jackson]

In message

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.

 

[mike]

Er, not everyone lives in North America. I happen to live in the UK. Sorry for your loss.

Tell me again how you might measure the power consumption of a transformer.

Not sure how repeating myself helps, but here goes.
I'd plug it into a Kill A Watt meter. That's how I got the other
experimental
results I mentioned in this thread.

I also disclosed several other techniques.

If you would read the parts you snipped, I also gave you several other
ways to estimate the power consumption of a transformer.

If you don't have a readable utility meter or a stopwatch or a
light bulb or a voltmeter or a scale or a thermometer or a hand, you
probably
should look elsewhere for advice. I'm all out.

Wait.....
I think I figured out how to do it with a used tea bag, the chime
from Big Ben and a plate of fish and chips.
Nope, you're still gonna need the hand.

 

[Phil Allison]

"Robert Macy"

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.


** You are a total ignoramus about transformers.

FYI:

Magnetising current is not inductive - it is highly non linear with peaks
at each zero crossing.



..... Phil

 

[Phil Allison]

"JW"

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?

** Is that 2.54 amps RMS or not ?

In any case, is it getting rather hot and making a humming noise ?

If not, then all is OK.

The magnetising current will drop significantly when a full load is
applied.

BTW:

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





..... Phil

 

[Phil Allison]

"Robert Macy"

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.

** You are a total ignoramus about transformers.

FYI:

Magnetising current is not inductive - it is highly non linear with peaks
at each zero crossing.

Phil makes a good point about the terms being thrown around here.

** The one doing the chucking about is YOU - pal.


Magnetizing current is NOT core inductance current.

** But includes it.

Back to the core' inductance...The core's inductance is usually 5 to
10 times the impedance of full load.


** Wrong - it is way more than that.

The NON LINEAR magnetising current increases with applied voltage and rise
sharply as the max rating is approached.


Now, the core winding's inductive reactance .....

** Is so high the resulting current flow barely matters.

At 60Hz that core's inductance will be about 382 mH.


** It is far more likely to be many Henries.


To get rid of such inductive reactive currents, which cause a
'lagging' power factor, it is possible to add a parallel capacitor
essentially in resonance taking it to near zero, thus 'correcting' the
power factor.

** FFS - give up on this crap.

You cannot correct the PF of an unloaded AC supply tranny with a parallel
cap !!!


It is of note that the current is ALWAYS there, even at full load, in
parallel with your load. If the transformer is made properly, the
waveform will be fairly linear, if the core is starting to saturate,
the current at the peaks will increase due to that saturation


** In reality, the ONLY significant current flow occurs around each
voltage zero crossing.

BTW:

Toroidal AC supply transformers are different.

Inductance is way high and I mag is almost non existent up to the rated
input voltage at the rated frequency.


..... Phil

 

[mike]

I'll bet you don't have a Passport.

I'm intrigued. How do you use a Passport to measure
transformer losses? If you don't have a hand, you can't
even pick it up.

 

[Franc Zabkar]

How might you do that?

(I don't have a power meter kicking around)

My local Aldi has these for AU$13:
http://aldi.com.au/au/html/offers/2827_20656.htm?WT.mc_id=2012-01-30-08-10

The meter also reports the power factor.

Same model number (GT-PM-04), probably made by Globaltronics:
http://www.teknihall.nl/index.php?id=1129&L=nl

- Franc Zabkar

 

[JW]

"JW"


** Is that 2.54 amps RMS or not ?

Yes. I've measured it with the same result using both a Watts Up meter and
a Fluke 87.

In any case, is it getting rather hot and making a humming noise ?

Making a humming noise, yes. It's not getting hot, the external
temperature rise of the case is only about 10c.

If not, then all is OK.

The magnetising current will drop significantly when a full load is
applied.

Nail. Head.

My test setup and how this all started:

120V in is being fed to a variac, then to the isolation transformer, then
the load. On the secondary of the transformer I added a toggle switch that
would connect the secondary in series or parallel depending whether I
wanted 120V or 240V out. When I first wired up everything as I stated in
my OP, being on the cautious side I placed a 1.5A fuse into the variac so
if I mis-wired something there'd be no damage. Well, on first power up I
slowly raised the voltage on the primary and at a little over 110V the
fuse blew.

The actual wattage when the primary is at 120V is only 42W as measured by
the Watt meter. Here's why - the power factor is really crappy at just
about .15. This would explain everything. I wonder if this is typical of a
1KVA transformer? I never thought to check the wattage being drawn...

BTW:

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

Noted, and thanks.

Also, thanks to everyone else for their input - some very good thoughts.
It would appear that there's no problem after-all.

 

[JW]

Depending on how close the transformer is to the box, the metal
box represents a shorted turn. All the flux is SUPPOSED to be inside
the core....supposed to be. I don't expect taking it out of the box
would make any significant difference, but I'd try it before scrapping
the device.

It's potted inside the box, so that's not possible. In any case it would
seem that there's no problem (see my other post), thanks.

 

[William Sommerwerck]

Out of curiosity, I took out my Tenma 72-545 1.5A isolation transformer.

On my Kill A Watt, it draws 60 mA unloaded, 3W / 7VA, with a power factor of
0.48.

I assume the current without a load varies with the primary inductance, not
the rated capacity. So the 2.45A drawn by the OP's transformer seems to be
unnaturally high.

 

[Phil Allison]

"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.



.... Phil

 

[Phil Allison]

"JW"
"Phil Allison"

Yes. I've measured it with the same result using both a Watts Up meter and
a Fluke 87.

** Good.

Making a humming noise, yes. It's not getting hot, the external
temperature rise of the case is only about 10c.

** Sounds about normal for a large e-core tranny

The actual wattage when the primary is at 120V is only 42W as measured by
the Watt meter.

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

Here's why - the power factor is really crappy at just
about .15. This would explain everything.

** Not everything.

You have to know that the current waveform is distorted - THIS fact is
causing the poor PF when unloaded.


.... Phil