CT Burden VA calculation

I am designing a product for motor protection ckt. where i wanted to use customized external CT for application.Now i would like to get idea on selecting proper CT for my application.

These are Motor ranges and their Current measurement range. i would like to know

  • Calculating right burden VA for my application

Turns ratio of CT. CT should be as shown picture.
class of CT to get better accuracy
Burden resistor calculation
External CT measurement Circuit for Arduino UNo

My apllication is motor protection Relay. weather i need to go with protection CT 5P10/5p20 or nomal CT can be used

Motor Range Current Range Protection Class
1-7.5HP 0.4A-12.5A 5P10/5P20
3HP-15HP 0.8A-25A 5P10/5P20
20HP-40HP 2-62.5A 5P10/5P20
50HP-75HP 4-125A 5P10/5P20


Why didn't you continue in your original thread http://forum.arduino.cc/index.php?topic=586359.15 ?

Anyway, the device you linked to seems a bit excessive since the main focus of that device appears to be making third party CT's act as a Lecroy current probe.

As previously stated startup current may be as high as 20 times running current so of running current is 125A you will probably have to be able to measure currents reaching a peak value of close to 2.5kA.

If we translate that to the 10 bit A/D-conversion maximum resolution will something like 2.5A

As for calculating burden resistor it's basically just Ohms law.

[edit] I would suggest getting a cheap CT and hook it up to a burden resistor and an oscilloscope to get a better feel of the kind of signal you can expect. I used a schottky rectifier to get a DC signal to my arduino since my project did not require very high accuracy or resolution. With calibration it can still get quite decent.

I wanted to goto basic to understand. When i purchased external CT. They mentioned

turn ratio 1000:1 30/5A 50/5A 100/5A 150/5A

along with 3.75VA burden or 5VA burden.

I would like to know relation equation between them.

When i explore it in google. I didnt get equation to find VA burden .

few people explaining if 1000:1 30/5 if showing error of 5% if same CT primary current applied with 35A then degree error exceeds 10% after that it goes to saturation.

I would like to know this equation by selecting CT

if 1000:1 30/5A 3.75va burden is used .

Max primary current i can pass??
How can i calculate Burden resistor??
what if my short circuit current exceeds 50A-100A what CT will do?

If it explain with one example it will be more helpful

I have attached image for reference. How can i Interface with arduino Uno

How Burden VA is calculated??

how to calculate Burden resistor if i wanted to interface with PIC16F886

How CT size can be reduce? what parameter i need to change to make as per size 2 meet accuracy

if i select this CT , what will be Max current i can supply above which it goes to saturation

what is effect on burden VA , if current exceeds 100A current.

CT image.png

Ciruit image.png

Also , of course , if you are trying to protect a motor in this way ( as is previously said, not a great idea IMO) , you have to be able to measure and act on small over loads and also much larger fault currents ( as implied by other posters ) - so its a complex task and you need to speak to a CT manfacturer. You also need to look at the “ competition” - trips that attach to motor contractors that are co-ordinated with fuses ( and their overload characteristic - fuses, overload devices, contactors and the motor, and it’s duty cycle need to be co-ordinated together ) to provide good protection at a low price.

Considering AC1030 data sheet .

Ip=30A I max=75A
turns ratio: 1000:1
supply voltage =220vAC
min KW continuous CT can sense =220X30=6.6kw
Max Kw sense for short period=22075=16.5kw

burden resistor= 2.5/(42/1000)=59.5ohm
R1=R2=100kohm Resistor.

Is it correct , what are other care i need to consider while taking hardware design.

In this type application we need to use Normal CT or protection class CT 5p10/5p20 Series??
How Burden VA calculated ??
How can i judge Max voltage limit giving to Arduino Board?

The basic laws of resistance and power apply.

V = I R
P = V^2 / R
P = I^2 * R

all in rms units for ac.

The secondary current is N times less than the current being measured for a CT with a turns-ratio of N

For example if a 1000:1 CT is measuring 50Arms with a burden resitor of 200 ohms,
the secondary current = 50/1000 = 0.05Arms.
power in burden resistor is I^2 * R = 0.05^2 * 200 = 0.5W (yes, watts, not VA, its a resistor so true power = apparent

And of course the voltage across the resistor is 0.05 * 200 = 10Vrms

Do you understand about rms, peak and peak-to-peak? That's vital for understanding
the limitations of sampling the CT output with an ADC.

Now i got it. but And of course the voltage across the resistor is 0.05 * 200 = 10Vrms

Vrms =0.7071* vpeak

Hence Positive side max peak voltage is 14.14V and neg side -14.14 V .

Microcontroller wont read negative signal so we lifting the signal using pull up resistor . Thats reason we used divider ckt in application

But Controller will detect 5V where we need to drop signal from 14.14V-->5v to 2.5V and -14.14v to 2.5V-0V

Is it correct or wrong.

my question is Vrms what calculated is actual voltage coming across Burden resistor? or by changing burden

resistor value i can drop to 5v

if i use burden resistor 100ohm ; 0.05*100= 5vrms

if i use burden resistor 56ohm ;0.05*56=2.8Vrms

Yup, that's it.

if i use burden resistor 56ohm ;0.05*56=2.8Vrms

Which is about 7.9volt peak/peak, so still too much for a 5volt Arduino pin.

Then use a voltage divider (with much larger impedance than the burden resistor) to bring it down.

Then use a voltage divider (with much larger impedance than the burden resistor) to bring it down, or a smaller
burden resistor (better anyway as greater overload handling)