# Selection of External Current Transformer simulate this circuit – Schematic created using CircuitLab

I am designing a product for motor protection circuit. where I want to use customized external CT for application which gives me better accuracy in current reading

Below list are power ranges of motor and their measuring current range. Based on current range I would like to choose my external CT

My questions are, as per CT image attached

Calculation of burden resistor

1. How is burden VA calculated?

2. How to calculate burden resistor if I want to interface with PIC16F886?

3. How CT size can be reduced? What parameter do I need to change to make as per size 2 to meet accuracy requirements?

4. If I select this CT, what will be max current I can supply above without it going to saturation?

5. What is effect on burden VA if the current exceeds 100A?

6. How to calculate r1, r2, r3 for this circuit?

Considering AC1030 data sheet. AC1030 datasheet Ip=30A I max=75A turns ratio: 1000:1 supply voltage = 220VAC min power continuous CT can sense = 220X30 = 6.6 kW Max power sense for short period=220*75=16.5 kW Irms=1.4*30A=42A

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?

• Shopping questions are off topic on this site. Suggest you Google suppliers like Digi-Key, Mouser, Newark, Allied, Sampson CT's, etc. – Sparky256 Feb 8 at 4:36
• I've read your post twice but I don't understand what your actual question is. Please edit to clarify. – Transistor Feb 8 at 7:29

## 2 Answers

Note that, according your circuit, for 100A in the transformer primary, you are having 5A in the secondary. This is a very dangerous design for a person, even more for a PIC and for an Arduino board.

I suggest you to look for an Arduino 100A Current Sensor. Besides, if Arduino is your flavour, I also suggest you to give a try to the Arduino Stack Exchange.

• After doing some Search.I came to know i cant use 100/5A Ct here, i need to use 1000/1A,2000/1A , Now few more doubt need to get clarify – AMPS Feb 11 at 3:47
1. How is burden VA calculated?

From $$\ P = VI = \frac {V^2}{R} = I^2R \$$

Since you have Pmax (3.75 VA on CT label) and Imax (5 A) use $$\ P = I^2 R \$$ for your calculations. $$\ R_{max} = \frac {P}{I^2_{max}} = \frac {3.75}{5^2} = 0.15 \ \Omega \ \text {max}\$$ .

1. How to calculate burden resistor if I want to interface with PIC16F886?

Ideally your burden would generate 5 Vp-p at 5 A. $$\ 5 \ \text V_{p-p} = \frac {5}{2 \sqrt 2}\ \text V_{rms} = 1.77 \ \text V_{rms} \$$.

The problem is that the maximum voltage you can generate with the maximum burden is $$\ 5 \times 0.15 = 0.75 \ \text V_{rms} \$$. This means you will be using 42% of the resolution of the CT. This might be a good thing to prevent overvoltage during spikes.

1. How CT size can be reduced? What parameter do I need to change to make as per size 2 to meet accuracy requirements?

I don't know.

1. If I select this CT, what will be max current I can supply above without it going to saturation?

Unclear. If you select which CT?

1. What is effect on burden VA if the current exceeds 100A?

The transformer may saturate so you want to avoid that situation.

1. How to calculate r1, r2, r3 for this circuit?

We have calculated R1 above.

The R2, R3, C1 network needs to have a low enough impedance to hold the common point steady despite any capacitive coupling to the AC mains. Lower value resistors would help but at the cost of current consumption and time taken to stabilise after power-up.