current transformer problem

[bjeservice]

Dear friends
i tried to built an electronic control circuit for earth fault protection, so i built a current transformer with the information listed below ( as i know some informations of how to built them ). the current transformer load is 200 ohm resistive, the voltage that i need to deal with is 1.00 volt as the fault current of the transformer is 3.00 Amps in primary so what i need is 5.00 mA in secondary coil.
inner iron diameter of toroidal ct = 318 mm
outer iron diameter=338 mm
width of iron sheet=25 mm
the iron sheet is grain oriented silicon steel and it is heat treated
the coil is 600 turn coil, of 0.25 mm bare wire diameter and wound uniform around the iron core, its resistance is 21.0 ohm.
The problem is :i could not get the 1.00 volt from the 200 ohm load , only a 0.70 volt i get and any winding correction get no use.
could any one inform me why i could not get the one volt from the 200 ohm .

 

[fsmyth]

You can do one of 2 things:
1) raise the winding total to over 900, or
2) reduce the burden (higher load resistance).
<als>
Actually, there is a (3) - you could
loop the C.T. primary. Not too practical if
you are dealing with 4/0 conductors

 

[CarlPugh]

The core you are using is probably too large in diameter and the exciting current is not being measured.
The exciting current is similiar to the no load current of a regular voltage type transformer.

The operation could be improved by:
Using a smaller diameter core.
Using a core with higher permability.

Or it may be possible to use this core with two turn on the primary.
Or can you use a load with a resistance higher than 200 ohm?

Some things that should be checked:
Transformer operates at 50 or 60 hertz?
Are there gaps in the core?
Are any of the turns shorted? (Secondary should be wound on tape or some type of insulating material)Insulated and not bare wire should be used.
The bracket holding the transformer core does not short the core laminations.
The transformer is not tied down with bare wire through the center of the core.

 

[bjeservice]

dear friends:
1- i need that inner core diameter of ct because i am simulating the case of three phase cable that i really need in my practical circuit .

2- so i could not use multi turn primary winding , and therefore could not double secondary turns.

3- i need that burden ( 200 ohm ) for the electronic relay

4- the strange thing is when i increase the turns ( by 100 turns ) i get no-increase in voltage but it decrease a little.

5- when i decrease the turns of secondary ( in order to increase current transformer ratio ) also i saw that the voltage of the burden stop increasing ( i reduce it till 300 turn secondary winding )

6- the frequency is 50 Hz

7- yes when i use another smaller diameter core the transformer works well but as i said i need that core diameter.

8- there is no gaps in transformer

9- i use another lamination core and another wire ( 1.3 mm dia. wire ) and check effect of internal resistance ( it has fallen to 1.5 ohm ) , but the transformer could no give me the response that i need

10- finaly the stange thing i saw is :when i take reading for the excitaion current , my knee point volage is more than 40 volt and here is my test data:
Vs(Volt) Is(mA)
-------- --------
5 9
10 14.4
15 18.50
20 22.70
25 26.70
30 32.00
35 38.30
40 47.70
45 66.40
50 120
54.2 307

i am still asking , why is that ct does not give me the 1.00 volt that i need although it seems so good in design
thank you

 

[itsmoked]

bjeservice; Have you looked at your output secondary with an O-Scope? This would show if any saturation is occurring which would then point at a material or core cross section problem.

 

[bjeservice]

Dear itsmoked
when i calculate my variables, i choose
B=0.0332 Tesla
Ie=3.4 mA
so it must be un saturated ( thats the problem ) as the lamination saturates at 1.9 Tesla,
and as excitation test shows Knee voltage about 40 volt
what is strange why this ct does not give 1.0 volt

 

[fsmyth]

The data you posted implies that the coil resistance
is too high. Try the same number of turns using wire
a couple of sizes larger.
Although, that IS a lot of core material, and you seem
to be trying to get a fairly linear response at a very
low magnetizing current.
Have you tried talking to the folks that actually make
these things? I have found that most magnetics engineers
are very helpful, especially when confronted with an
interesting challenge (as am I, but this is a bigger
item than I have experience with). If you do get vendor
assistance, please post the results. Interesting.
<als>

 

[scottf]

bjeservice-

Based on the dimensions you listed, I calculated the performance of the CT, based on using the type of silicone-steel we use for protection CTs. I calculate the current error to be roughly -25%, which seems to correspond with the output voltage voltage you're getting.

The problem is that the ampere-turns of this design is WAY too low. You're really not going to be able to develop of a CT with a 3A primary and 5mA secondary with the size diameters that you're trying. I'm not even sure you can do it with small diameters either.

If you want to pursue it further, I did calculate the error using Nickel-type core material, and the current error was around 3% if you take the core height (lamination width) up to 75 mm and increase the wire diameter to 0.5mm. Of course this core material is about 10 times more expensive than Si.

 

[bjeservice]

Dear fsmyth
as i mentioned in point 9 upper, i tried the same core with another wire size ( 1.3 mm dia. ) in order to reduce the internal resistance ( and so it reduced from 21 ohm to 1.5 ohm ) the voltage get higher alittle but it does not reach the 1.0 volt

 

[bjeservice]

Dear scottf
*i solved my problem by incraesing the iron area by 60%.
*i am sking what is the starting ampere-turn suitable for the ct core diameter, as you mentioned that it is too low.
* if the exciting current Ie that i calculate is 3.4 mA
isn't the current error = 3.4/5.00 = 0.68

for all friends:

if there are any electronic references that could help , or any expert person that will be helpful to understand this problem please try to inform me on my e-mail that is : bjeservice@yahoo.com

thank you all

 

[CarlPugh]

Are you actually using an electronic relay as you posted on July 31?
If an electronic relay is used, obtain one with a lower burden.

 

[bjeservice]

dear CarlPugh
yes i am using a simple electronic circuit that has an input impedance of 200 ohm , it is designed and built to start cut off of a simple relay at threshold voltage of 1.00 volt.

as i mentioned above i solve the problem by using larger core area, but the question of why that ct does not work is what i am asking.

 

[CarlPugh]

It is the exciting current of the current transformer.
Measure the secondary current while applying 1 volt to the secondary. (with the 1 turn primary open circuit) The current you measure will be the error you are measuring.
(It is probable that the current measured is equal to the error. Have not preformed the calculations to prove it)