Help with Current Transformer ?

T

The Fuel Tank

Guest
Hello everyone,



I've just started at an electronics design company and they give me some
homework to find out things to help me along the way of learning, i think
this is a good way of learning but this weeks question has got me blocked?



I'm currently designing a 12v to 240v inverter, i got the basic thing
working and now its time to do the little jobs like current limiting for a
start.



I am using a current transformer on the primary side of the transformer with
the start and finish winding through the middle of the CT, i was told to
cross the wires going through the middle of the CT, i asked why do i need to
do that ..? He looked at me and said this was this week's homework, find out
why i am doing this?



I have given him many answers but not the one he wants...

Can anybody help?. I'm stuck for a reason why, he says when i have the
answer why it will stick with me for a longer time...



Please help..



Thanks

Dean....
 
The Fuel Tank (Dean) wrote...
Can anybody help?. I'm stuck for a reason why, he says when
i have the answer why it will stick with me for a longer time...
Click to expand...
But if we simply tell you the answer, it won't stick with you.

Thanks,
- Win

whill_at_picovolt-dot-com
 
I read in sci.electronics.design that The Fuel Tank
<lwbg22108@blueyonder.co.uk> wrote (in <3UFac.809$Id.48@news-
binary.blueyonder.co.uk>) about 'Help with Current Transformer ?', on
Wed, 31 Mar 2004:

I am using a current transformer on the primary side of the transformer with
the start and finish winding through the middle of the CT, i was told to
cross the wires going through the middle of the CT, i asked why do i need to
do that ..? He looked at me and said this was this week's homework, find out
why i am doing this?
Click to expand...
It's a bit difficult to understand what you are doing, but if you are
running both start AND finish wires through the centre of the
transformer, from the same side, you have no net primary current,
because it flows in one direction in one of the wires and in the
opposite direction in the other wire.

Use Courier font:

Wires enter from the same side
______
| |
|______|
--->-------------->-- Start
---<--------------<-- Finish
______
| |
|______|

No net current

Wires enter from opposite sides
______
| |
|______|
---?--------------?-- Start Which way should the arrows go?
---?--------------?-- Finish
______
| |
|______|

What is the *net* current then?



--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
I read in sci.electronics.design that Winfield Hill
<Winfield_member@newsguy.com> wrote (in <c4fajk0uql@drn.newsguy.com>)
about 'Help with Current Transformer ?', on Wed, 31 Mar 2004:
The Fuel Tank (Dean) wrote...

Can anybody help?. I'm stuck for a reason why, he says when
i have the answer why it will stick with me for a longer time...

But if we simply tell you the answer, it won't stick with you.
Click to expand...
But we do give hints, if you ask nicely.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
I will explain better :)

As i am designing a inverter i have on the primary side 2 side by side wound
coils (@17T each wound the same way)
The start of the one and the finish of the other are the centre tap(to
ground) The two wires left are driven by two mosfets giving a 50Hz sq wave
output on the second @ 240V.
|--\/\/\/\/\/\/--|--\/\/\/\/\/\/\/\--|
| T |
| |

primary

Before the two inputs are connected they both go through a current
transformer, but not side by side, the question was that they cross over
each other as they both go through the CT ..?
Why ? why not next to each other ..?

| |
| |
\ /
\ /
\ /
X
/ \
/ \
/ \
| |

The CT is going the round the X part, i'm getting a voltage or Approx 3 volt
after going through a bridge and a 470R across the output.
I just asked the question of why do they have to be crossed in the middle ?
he says they have to be for a reason and i just don't know ..?
i just need at least some help or a hint ..?
has it got anything to do with the phase shift ..? i dont know..
Pleeeeeeeeeeeeeeeeeeeeeeease.......


Thanks.. :)




"John Woodgate" <jmw@jmwa.demon.contraspam.yuk> wrote in message
news:LhKHeNBUezaAFwZi@jmwa.demon.co.uk...
I read in sci.electronics.design that The Fuel Tank
lwbg22108@blueyonder.co.uk> wrote (in <3UFac.809$Id.48@news-
binary.blueyonder.co.uk>) about 'Help with Current Transformer ?', on
Wed, 31 Mar 2004:

I am using a current transformer on the primary side of the transformer
with
the start and finish winding through the middle of the CT, i was told to
cross the wires going through the middle of the CT, i asked why do i need
to
do that ..? He looked at me and said this was this week's homework, find
out
why i am doing this?

It's a bit difficult to understand what you are doing, but if you are
running both start AND finish wires through the centre of the
transformer, from the same side, you have no net primary current,
because it flows in one direction in one of the wires and in the
opposite direction in the other wire.

Use Courier font:

Wires enter from the same side
______
| |
|______|
--->-------------->-- Start
---<--------------<-- Finish
______
| |
|______|

No net current

Wires enter from opposite sides
______
| |
|______|
---?--------------?-- Start Which way should the arrows go?
---?--------------?-- Finish
______
| |
|______|

What is the *net* current then?



--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Click to expand...
 
On Thu, 1 Apr 2004 00:00:03 +0100, "jennifer wiliams"
<jw012e9899@blueyonder.co.uk> wrote:

I will explain better :)

As i am designing a inverter i have on the primary side 2 side by side wound
coils (@17T each wound the same way)
The start of the one and the finish of the other are the centre tap(to
ground) The two wires left are driven by two mosfets giving a 50Hz sq wave
output on the second @ 240V.
|--\/\/\/\/\/\/--|--\/\/\/\/\/\/\/\--|
| T |
| |

primary

Before the two inputs are connected they both go through a current
transformer, but not side by side, the question was that they cross over
each other as they both go through the CT ..?
Why ? why not next to each other ..?

| |
| |
\ /
\ /
\ /
X
/ \
/ \
/ \
| |

The CT is going the round the X part, i'm getting a voltage or Approx 3 volt
after going through a bridge and a 470R across the output.
I just asked the question of why do they have to be crossed in the middle ?
he says they have to be for a reason and i just don't know ..?
i just need at least some help or a hint ..?
has it got anything to do with the phase shift ..? i dont know..
Pleeeeeeeeeeeeeeeeeeeeeeease.......
Click to expand...
1) Instantaneous currents will add or subtract.

2) DC currents will not normally be detectable by a transformer.

3) The detector is a full wave bridge.

Therefore..........


(I'm feeling an attack of misogyny coming on, or maybe it's just
misanthropy.)

RL
 
This is a strange one.. i don't know what to say ..?
I will think on it during the night and look again in the morning ..

Thanks again ..
Keep them coming in ....

Dean...

"legg" <legg@nospam.magma.ca> wrote in message
news:pjjm60tpgmmih4dj1h1kvi3t098ahaomdl@4ax.com...
On Thu, 1 Apr 2004 00:00:03 +0100, "jennifer wiliams"
jw012e9899@blueyonder.co.uk> wrote:

I will explain better :)

As i am designing a inverter i have on the primary side 2 side by side
wound
coils (@17T each wound the same way)
The start of the one and the finish of the other are the centre tap(to
ground) The two wires left are driven by two mosfets giving a 50Hz sq
wave
output on the second @ 240V.
|--\/\/\/\/\/\/--|--\/\/\/\/\/\/\/\--|
| T |
| |

primary

Before the two inputs are connected they both go through a current
transformer, but not side by side, the question was that they cross over
each other as they both go through the CT ..?
Why ? why not next to each other ..?

| |
| |
\ /
\ /
\ /
X
/ \
/ \
/ \
| |

The CT is going the round the X part, i'm getting a voltage or Approx 3
volt
after going through a bridge and a 470R across the output.
I just asked the question of why do they have to be crossed in the middle
?
he says they have to be for a reason and i just don't know ..?
i just need at least some help or a hint ..?
has it got anything to do with the phase shift ..? i dont know..
Pleeeeeeeeeeeeeeeeeeeeeeease.......


1) Instantaneous currents will add or subtract.

2) DC currents will not normally be detectable by a transformer.

3) The detector is a full wave bridge.

Therefore..........


(I'm feeling an attack of misogyny coming on, or maybe it's just
misanthropy.)

RL
Click to expand...
 
On Wed, 31 Mar 2004 21:31:05 +0100, "The Fuel Tank" <lwbg22108@blueyonder.co.uk>
posted this:

I am using a current transformer on the primary side of the transformer with
the start and finish winding through the middle of the CT, i was told to
cross the wires going through the middle of the CT, i asked why do i need to
do that ..? He looked at me and said this was this week's homework, find out
why i am doing this?

I have given him many answers but not the one he wants...
Click to expand...
He probably meant to say that you should "reverse" one of the wires
rather than "cross" them. That would have practical, beneficial, results.

If he did indeed want you to simply cross them, then the proper answer
to "Why?" is "Because you told me to." There is no practical reason supported
by electrical theory that indicates that crossing is any different than
side-by-side.

Tell him I told you the answer and that if he thinks it is wrong that I
will be here to defend my position. Also tell him not to pick on people who
can't fight back.

Jim
 
I read in sci.electronics.design that jennifer wiliams
<jw012e9899@blueyonder.co.uk> wrote (in <X3Iac.1686$Id.1374@news-
binary.blueyonder.co.uk>) about 'Help with Current Transformer ?', on
Thu, 1 Apr 2004:
Before the two inputs are connected they both go through a current
transformer, but not side by side, the question was that they cross over
each other as they both go through the CT ..?
Why ? why not next to each other ..?
Click to expand...
There is no per se reason for the wires to cross. The reason why your
wires need to cross is something to do with what they are connected to,
and you don't tell us that.

| |
| |
\ /
\ /
\ /
X
/ \
/ \
/ \
| |

The CT is going the round the X part, i'm getting a voltage or Approx 3 volt
after going through a bridge and a 470R across the output.
I just asked the question of why do they have to be crossed in the middle ?
he says they have to be for a reason and i just don't know ..?
i just need at least some help or a hint ..?
has it got anything to do with the phase shift ..?
Click to expand...
What phase-shift? Don't clutch at buzz-phrases, think instead.

i dont know..
Click to expand...
I don't understand your diagram, because you haven't labelled where the
ends of your wires go to, and you need to put arrows put on the wires,
showing the directions of the currents. If you do that, you will
probably see the reason yourself, especially if you answer the questions
in my previous post....
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
Thanks people.. i will be back with he's answer :)
Dean...


"James Meyer" <jmeyer@nowhere.com> wrote in message
news:6ppm605cbp5t5d0du1k5t9upu5nou90egc@4ax.com...
On Wed, 31 Mar 2004 21:31:05 +0100, "The Fuel Tank"
lwbg22108@blueyonder.co.uk
posted this:

I am using a current transformer on the primary side of the transformer
with
the start and finish winding through the middle of the CT, i was told to
cross the wires going through the middle of the CT, i asked why do i need
to
do that ..? He looked at me and said this was this week's homework, find
out
why i am doing this?

I have given him many answers but not the one he wants...


He probably meant to say that you should "reverse" one of the wires
rather than "cross" them. That would have practical, beneficial, results.

If he did indeed want you to simply cross them, then the proper answer
to "Why?" is "Because you told me to." There is no practical reason
supported
by electrical theory that indicates that crossing is any different than
side-by-side.

Tell him I told you the answer and that if he thinks it is wrong that I
will be here to defend my position. Also tell him not to pick on people
who
can't fight back.

Jim
Click to expand...
 
Well I've been through it with another designer and found out why the
primary wires had to be crossed over going through the CT..
I can explain better now....
As I'm making a 12 to 240v inverter, I have a step-up transformer being
controlled by two mosfets and a SG3525 ic, I have two positive pulses going
to the two 17T primary on the main step-up transformer, as I switch on the
ic and transformer/fet supply I turned the IC oscillator up slowly and
bingo, 240V 50Hz out the secondary @300w :)
I'm happy now its working, so I turned it back down and put a load on it,
then back up it went and bingo, No problems as yet (only a few oscillator
timing problems but that was sorted with a cap change).
Next stage is to sort the current limit out,
Where the two positive pulses wires are connected to the primary on the
transformer I put them through the middle of the CT, but that's where I was
asked to cross them both over each other as they go through the CT. Yes one
wire goes in side A and out side B while the other wire goes in side B and
out side A, i did not word it very good in the first place. sorry.
As I never realised they have to be crossed over because they are both
positive pulses and that meant that the CT would not be alternating at they
would be both 0v to 12v as it fires up, by crossing them both over each
other would swing the voltage back the other way in the CT giving a -12 to
+12 and a smooth output on the CT controlling the current limit circuit.
I will update you more as I progress with the inverter soon as I'm waiting
for some more mosfets after my current limit needed more caps to dampen the
voltage going to the IC and when I disconnected the load the two mosfets
went bang.. I think it they did because I got to sort the voltage feedback
out next...

To be continued....

Dean...

Thanks again people :)


"Jamie" <jamie_5_not_valid_after_5_Please@charter.net> wrote in message
news:106nh8l68a9te0f@corp.supernews.com...
its to prevent EMF from radiating from out side from where the point of
the pairs end .
inverters normally use high frequency.
on that given note, they also normally twist them a couple
of turns. the twisted effect helps reduce the surface effects.



jennifer wiliams wrote:

Thanks people.. i will be back with he's answer :)
Dean...


"James Meyer" <jmeyer@nowhere.com> wrote in message
news:6ppm605cbp5t5d0du1k5t9upu5nou90egc@4ax.com...

On Wed, 31 Mar 2004 21:31:05 +0100, "The Fuel Tank"

lwbg22108@blueyonder.co.uk

posted this:


I am using a current transformer on the primary side of the transformer

with

the start and finish winding through the middle of the CT, i was told
to
cross the wires going through the middle of the CT, i asked why do i
need

to

do that ..? He looked at me and said this was this week's homework,
find

out

why i am doing this?

I have given him many answers but not the one he wants...


He probably meant to say that you should "reverse" one of the wires
rather than "cross" them. That would have practical, beneficial,
results.

If he did indeed want you to simply cross them, then the proper answer
to "Why?" is "Because you told me to." There is no practical reason

supported

by electrical theory that indicates that crossing is any different than
side-by-side.

Tell him I told you the answer and that if he thinks it is wrong that I
will be here to defend my position. Also tell him not to pick on people

who

can't fight back.

Jim
Click to expand...
 
its to prevent EMF from radiating from out side from where the point of
the pairs end .
inverters normally use high frequency.
on that given note, they also normally twist them a couple
of turns. the twisted effect helps reduce the surface effects.



jennifer wiliams wrote:

Thanks people.. i will be back with he's answer :)
Dean...


"James Meyer" <jmeyer@nowhere.com> wrote in message
news:6ppm605cbp5t5d0du1k5t9upu5nou90egc@4ax.com...

On Wed, 31 Mar 2004 21:31:05 +0100, "The Fuel Tank"

lwbg22108@blueyonder.co.uk

posted this:


I am using a current transformer on the primary side of the transformer

with

the start and finish winding through the middle of the CT, i was told to
cross the wires going through the middle of the CT, i asked why do i need

to

do that ..? He looked at me and said this was this week's homework, find

out

why i am doing this?

I have given him many answers but not the one he wants...


He probably meant to say that you should "reverse" one of the wires
rather than "cross" them. That would have practical, beneficial, results.

If he did indeed want you to simply cross them, then the proper answer
to "Why?" is "Because you told me to." There is no practical reason

supported

by electrical theory that indicates that crossing is any different than
side-by-side.

Tell him I told you the answer and that if he thinks it is wrong that I
will be here to defend my position. Also tell him not to pick on people

who

can't fight back.

Jim
Click to expand...
 
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