Isolation transformer draws excessive current under no load

[Robert Macy]

On Feb 11, 6:55 am, "Phil Allison" <phi...@tpg.com.au> wrote:

"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 makes a good point about the terms being thrown around here.
Magnetizing current is NOT core inductance current. It is magnetizing
current. The better the core material the less this current will be.


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

You use a pair of windings to get to 1kVA, therefore per winding is
500VA, so at full load each winding is capable of 120Vac at 4.17A into
a RESISTIVE load of 28.8 ohms.

Now, the core winding's inductive reactance is at least 3 times that
usually more than 5 times that or, 144j ohms [the j signifies reactive
impedance of the core and that the current through the inductor will
be out of phase with any load by 90 degrees.] At 60Hz that core's
inductance will be about 382 mH.

When wired in parallel to achieve the full 1kVA capability and NO LOAD
each winding will have approx 0.834A for a total of 1.67A current. Of
course this is reactive current and does not consume significant
power.

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. Industrial consumers with lots of motors often have a
rack [building] full of such capacitors that are switched in and out
depending on the correction they need.

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

A reasonable model predicts lower current than you saw, but 2.5A is
still possible, just seems high.

Further the winding resistance is often less than 1/10 of load, which
implies less than 3 ohms, probably more like 1.5 ohms per winding.
since they're in series to the load.

The magnetizing current is not really the same as the core inductance
current with the core inductance current in parallel with the load,
albeit 90 degrees shifted. Magnetizing current is actually what it
takes to 'turn on' the magnetic material. Really shows up if you try
to measure the core's inductance with a little meter and only put
0.1Vac across a winding, you'll find the inductance you measure is
almost nil. All caused because you haven't supplied the required
'magnetizing current' to overcome the material's coercivity.

Hope this is more clear.

 

[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

 

[Gareth Magennis]

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.


Gareth.

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

 

[mike]

On 2/11/2012 3:49 PM, Gareth Magennis wrote:

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.


Gareth.




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.

 

[spamtrap1888]

On Feb 11, 5:13 pm, mike <spam...@gmail.com> wrote:

On 2/11/2012 3:49 PM, Gareth Magennis wrote:

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.

Gareth.

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.

I don't see how a radar detector is going to help him.

http://www.ukspeedtraps.co.uk/passport8500.htm

 

[Franc Zabkar]

On Fri, 10 Feb 2012 19:22:33 -0000, "Gareth Magennis"
<sound.service@btconnect.com> put finger to keyboard and composed:

Measure the RMS power consumed by the transformer.

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
--
Please remove one 'i' from my address when replying by email.

 

[JW]

On Sun, 12 Feb 2012 01:05:04 +1100 "Phil Allison" <phil_a@tpg.com.au>
wrote in Message id: <9pnaslF48nU1@mid.individual.net>:

"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 ?

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]

On Fri, 10 Feb 2012 12:34:41 -0800 mike <spamme9@gmail.com> wrote in
Message id: <jh3v48$emd$1@dont-email.me>:

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.

 

[Robert Macy]

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.

 

[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"

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 ?

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

** Good.

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.

** 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

 

[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.

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

 

[Robert Macy]

On Feb 13, 4:37 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.

...  Phil

I stand by what I said as technically correct.

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

Quit spewing vitriole, making erroneous statements, *and* misleading.

CONTRIBUTE, EDUCATE, and LEARN!

 

[spamtrap1888]

On Feb 13, 8:04 pm, Robert Macy <robert.a.m...@gmail.com> wrote:

On Feb 13, 4:37 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.

...  Phil

I stand by what I said as technically correct.

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

Quit spewing vitriole, making erroneous statements, *and* misleading.

CONTRIBUTE, EDUCATE, and LEARN!

What are your PFC capacitors doing when the transformer supplies a
load? And if you were to switch them out when there was a load on the
secondary, wouldn't it make more sense just to open the primary when
there was no load? Then you wouldn't get any idle current draw at all.

 

[Phil Allison]

"Robert Macy = Troll "


** Is there any way to this MORON up ??

( Load of totally absurd crap deleted )



.... Phil

 

[Robert Macy]

On Feb 13, 9:15 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

There is a difference between
CANNOT supply reference material,
DID NOT supply reference material, and
WILL NOT supply reference material.

I simply DID NOT for several reasons. Takes a lot of time. Your
attitude appears closed, thus, I did not expect you to take the time
to read what I took the time to research for you; and not sure anyone
else is interested.

However, I AM INTERESTED, and upon searching, found over 15,000
related articles. The artricles either dismiss the small effects/
distortions and then describe linear transformer models, or support
the models I use/modified/developed. However, after one of the most
promising articles also misused the term 'magnetizing current' as I
had; I decided to rederive my transformer model from basics. I already
have distrust for information in printed sources. I'm going to
specifically look into whether the nonlinearity of the magnetizing
current does lessen with load. I suspect that in an isolation
transformer it does NOT, because the full voltage pretty much remains
constant across the core. However, in an audio matching transformer,
going from no load to full load drops the voltage across the core
approx. in half and indeed the magnetizing current would then change,
with less voltage across the core, the magnetizing current would also
lessen.

If the core material of an isolation transformer is extremely high
quality with low coercivity, whether the transformer is loaded, or
unloaded, will make small difference to the inductive current flowing
through the core, BECAUSE the voltage the core must sustain stays
pretty constant, whether unloaded or fully loaded. The inductive
current through the core appears in parralell to the load and is
pretty much constant. Being summed in quadrature just makes it appear
that it is getting smaller.

For example consider an isolation transformer that has 1A core current
(assume linear) with NO LOAD, one would measure 1A of current flowing.
Now load the transformer with a relatively small 5A load, now one
would measure only 5.1A, which makes the core current APPEAR to be
less as load increases, when in actuality, the core current has stayed
the same 1A inductive current.
= sqrt( 5*5 + 1*1 ) = 5.1 Is your comment that the core's current
decreases with load based upon such observation?

Your personal attack is unwarranted. I'm not a "...lying, bullshitting
NUT CASE !!" But this discussion is not about how you perceive my
character, rather this discussion is about explaining whether the OP's
transformer is broken or not, based upon his observation that the
transformer's leakage current is 2.5A. From that question the
discussion has become a positive LEARNING experience! so, STICK to
the SUBJECT.

However, thanks for the pricelss comment:
"Clueless to the core." LOL!
What did I do to deserve such PUNishment?

 

[Robert Macy]

On Feb 14, 9:35 am, spamtrap1888 <spamtrap1...@gmail.com> wrote:

On Feb 13, 8:04 pm, Robert Macy <robert.a.m...@gmail.com> wrote:





On Feb 13, 4:37 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.

...  Phil

I stand by what I said as technically correct.

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

Quit spewing vitriole, making erroneous statements, *and* misleading.

CONTRIBUTE, EDUCATE, and LEARN!

What are your PFC capacitors doing when the transformer supplies a
load? And if you were to switch them out when there was a load on the
secondary, wouldn't it make more sense just to open the primary when
there was no load? Then you wouldn't get any idle current draw at all.

The cap in parallel will continue to 'remove' the inductive current -
as far as the AC mains sees it. The inductive current remains whether
the transformer is loaded or not.

Completely disconnecting everything from the AC mains will always save
power.

As far as I know, no industrial power consumer adds caps just for
transformer core currents, only for large motors that are extremely
inductive loads.

 

[Phil Allison]

"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

 

[Sylvia Else]

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.