Proper Selection of a Current Transformer, Multirange Ammeter Design

[imranahmed]

Do you know about Current Transformer?

I need to measure current C.T from my meter. The issue is that the C.T`s are available for different current ratings.
for example:
1) 30/5 means if 30amps current pass through C.T it will induced 5 amps current in its secondary and
if 15amps current pass through C.T it will induced 2.5 amps current and so on.But according to formula
Is=(Np x Ip)/Ns==> Is=(1 x 15)/6=2.5A for 30Amps C.T.

2) 60/5 means if 60amps current pass through C.T it will induced 5 amps current in its secondary and
if 15amps current pass through C.T it will induced 2.5 amps current and so on.But according to formula
Is=(Np x Ip)/Ns==> Is=(1 x 15)/12=1.25A for 60Amps C.T.

3) How to calibrate my voltmeter for both C.T`s?

 

[KlausST]

Hi Imran,

here again.

Your software has to know what current transformer you use.
This can be made with a switch (30A/60A) or by keypress... or any other manual input.

* digital with IIC:
If you have a connector for both CTs and you have some pins free on this connector then you can mount a simple IIC EEPROM into the plug of the CT.
In the EEPROM you can stoer the information whether 30A or 60A. You can also store a complete calibration value: like (29.87A full scale). Then you are most flexible and it is easy to use.

* digital with port pin:
just use one or two port input pins and use a pullup in the one CT connector and a pulldown in the other CT connector.

* analog ADC solution:
If you don´t like the EEPROM stuff, then a simple resistor can work also. Lets say a 6kOhms resistor for 60A and a 3kOhms resistor for 30A. Read it it with the ADC and decide what CT is connected.

* analog solution without calibration:
Another possibility is to apply the shunt to the CT. So maybe a 120mOhms to the 60/5 A CT to get 10mV per Ampere
and a 60 mOhms shunt to the 30/5 A CT and get 10mV per Ampere also --> no calibration needed.

One question: why not allways use the 60/5 A CT? the benifit of the 30/5 A CT is not that much.

(Or why not use simply a shunt for 60A and a chopper stabilized OPAMP to amplify the signal. For Ptot < 0.5W @ 60A --> 120uOhms. And 7.2mV....OK forget about this. With a good layout and well chosen amplifiers it is possible, but i think in your application it makes more problems than benifit....)

So i hope there is at least one good solution for you...

Hope this helps
Klaus

 

[imranahmed]

Dear KlausST,

Actually I need to build ammeter for multi-range CT`s.

for example:

If my ammeter able to read all range 0/5 to 1000/5 amps so it good but all CT`s have
its own ratio.If I calibrate my ammeter for 30/5A CT it will not work with 60/5 or others.
What can I do for build multi-range CT operated ammeter?

 

[KlausST]

Hi Imran,

Can you calibrate for 5A and show 0 ... 100%?
Then it's true for all x/5A CTs.

Klaus

 

[imranahmed]

Can you calibrate for 5A and show 0 ... 100%?

Dear KlausST,

This is I need you told but how? :thinker:

- - - Updated - - -

Dear KlausST,

Please visit another new thread "Digital Multimeter Design".

 

[KlausST]

Hi,

DC: (active compensation CTs) take an average of a number of conversions and multiply it so, that if you apply 5A on the input the display shows "100.0"

AC: true RMS, or what? If so, then calculste the value and calibrate it the same way as with DC.

Or is the problem elsewhere?

Klaus

 

[imranahmed]

Dear KlausST,

I do not need to display in percentage I need to display actual value.

For True RMS AC.


If I use 40m-ohms burden resistor for 5A CT.

V=I x R
V=5A x 40m-ohms
V=200mV.

V=I x R
V=2.5A x 40m-ohms
V=100mV.


But it is true if I use only 30/5 or x/5 one CT meter but all CT`s compensation is the problem.

 

[KlausST]

Hi,

Another possibility:
Use a rotary switch that switches different shunt values according to your CT values.

Or use a rotary switch (maybe with binary coding) and connect to port pins to tell the ucontroller which CT you use.


Klaus

 

[imranahmed]

Dear KlausST,

Please visit this thread

"7-segment decimal point management"