current transformer question?

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 wrote:
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....
Click to expand...
If you pass 2 wires through the CT, that both carry the same current,
their magnetic fields will either cancel or add, depending on which
way each of those currents pass through the hole. Of course, you
could just pass one of them through and it will always work, but will
produce only half as much output.

--
John Popelish
 
Thats one of the answers i gave him but he says its not the reason why they
are both going through, but why are they crossed going through ..?
i can't see it making much change ?
Thanks again ... :)


"John Popelish" <jpopelish@rica.net> wrote in message
news:406B4FDF.67EEF771@rica.net...
The Fuel Tank wrote:

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....

If you pass 2 wires through the CT, that both carry the same current,
their magnetic fields will either cancel or add, depending on which
way each of those currents pass through the hole. Of course, you
could just pass one of them through and it will always work, but will
produce only half as much output.

--
John Popelish
Click to expand...
 
Well i think i will ask him today and find out his reasons why.. i will put
it on here and let you know.. thank you everyone :)
Dean...
"John Popelish" <jpopelish@rica.net> wrote in message
news:406B6877.C13056D@rica.net...
jennifer wiliams wrote:

Thats one of the answers i gave him but he says its not the reason why
they
are both going through, but why are they crossed going through ..?
i can't see it making much change ?
Thanks again ... :)

Perhaps I have missed what you mean by 'crossed'. I assumed you meant
that one of the leads went through with the transformer toward one
side of the window (in the CT), and the other went through with the
transformer toward the other side of the window. If you are talking
about the lead dress of the wires, instead, then I have no idea what
they are talking about. All that should matter with a current
transformer is the net current passing through the window.

--
John Popelish
Click to expand...
 
Hi,

My guess!

Cancellation of interwinding capacitance?


Cheers - Joe
 
On Wed, 31 Mar 2004 21:31:46 +0100 The Fuel Tank <lwbg22108@blueyonder.co.uk> wrote:

| 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?

This will depend on what you mean by crossed. It could simply mean that
one wire goes in side A and out side B while the other wire goes in side
B and out side A. Or maybe it is meant that the wires have to cross each
other at a 90 degree angle as they both pass through the CT window.

In the latter case, it might be to eliminate any cross coupling between
the conductors, but I don't know why that would be needed in this case.

In the former case, it would be to get the sum of the current in the two
wires. If you follow the wiring you first go into the CT window on side
A, then to the transformer primary, then from the other end of the winding
back to the CT window, and in on side A again. The two currents will add
up this way. The other way they would have canceled in the CT and you
would get nothing unless there was a wiring fault in the transformer.

Perhaps the bigger question is why have both conductors going through the
CT at all. If you are measuring system current, one wire will do it. If
the primary is more complex, such as having split windings and more than
two conductors, you would have to measure more than one thing. But in
such a case I would be more inclined to measure them independently.

Give your mentor a counter puzzle ... ask him which way the wires should
go for a three phase system with the primary transformer wired delta.


| 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...

Realize that in a given AC cycle, the currents of each wire in a two wire
transformer primary (or just about any load) are going to be equal and
opposite (in one, out the other). To measure their sum, you invert one.

--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/ http://ka9wgn.ham.org/ |
-----------------------------------------------------------------------------
 
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 :)
<phil-news-nospam@ipal.net> wrote in message
news:c4hr7j1phl@news3.newsguy.com...
On Wed, 31 Mar 2004 21:31:46 +0100 The Fuel Tank
lwbg22108@blueyonder.co.uk> wrote:

| 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?

This will depend on what you mean by crossed. It could simply mean that
one wire goes in side A and out side B while the other wire goes in side
B and out side A. Or maybe it is meant that the wires have to cross each
other at a 90 degree angle as they both pass through the CT window.

In the latter case, it might be to eliminate any cross coupling between
the conductors, but I don't know why that would be needed in this case.

In the former case, it would be to get the sum of the current in the two
wires. If you follow the wiring you first go into the CT window on side
A, then to the transformer primary, then from the other end of the winding
back to the CT window, and in on side A again. The two currents will add
up this way. The other way they would have canceled in the CT and you
would get nothing unless there was a wiring fault in the transformer.

Perhaps the bigger question is why have both conductors going through the
CT at all. If you are measuring system current, one wire will do it. If
the primary is more complex, such as having split windings and more than
two conductors, you would have to measure more than one thing. But in
such a case I would be more inclined to measure them independently.

Give your mentor a counter puzzle ... ask him which way the wires should
go for a three phase system with the primary transformer wired delta.


| 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...

Realize that in a given AC cycle, the currents of each wire in a two wire
transformer primary (or just about any load) are going to be equal and
opposite (in one, out the other). To measure their sum, you invert one.

--
--------------------------------------------------------------------------
---
| Phil Howard KA9WGN | http://linuxhomepage.com/
http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/
http://ka9wgn.ham.org/ |
--------------------------------------------------------------------------
---
Click to expand...
 
On Fri, 2 Apr 2004 15:58:32 +0100 The Fuel Tank <lwbg22108@blueyonder.co.uk> wrote:
| 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...

When you pulse one wire positive, it still comes back over the other wire
in the reverse direction. You want to sum them, so you do have to reverse
the other wire through the CT. In the other pulse cycle it goes back the
other way. It looks like AC to the transformer, as well as the CT.

Now consider designing a three phase inverter (much larger). The transformer
will have three 12 volt delta windings energizing the 230/400 volt wye/star
secondary. Now consider how you are to pass the three wires connected to
the delta secondary through the CT. Unfortunately, a CT is a one phase
device. Three phase power doesn't let you use the same shortcuts as one
phase power, though it does have some shortcuts of its own. You'll probably
need 3 separate CTs with one wire each, and more complex current measuing.
And if you ever do end up designing a three phase inverter, be sure to go
Google for "triplens".

--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/ http://ka9wgn.ham.org/ |
-----------------------------------------------------------------------------
 
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