Let us now investigate the strength of the magnetic field as manifest within a s

Question

Let us now investigate the strength of the magnetic field as manifest within a single phase one turn primary winding Current Transformer identified as a donut CT.

The CT pictured on the right has a primary black cable entering through the opening (the donut hole) and has a current of 350 Amps, 60 Hz flowing through this primary. The output of the CT (the gray and white wires) are feeding a relay monitoring the primary current. The relay has an internal impedance of 0.075 Ohms (resistive). The CT has no secondary resistance of any consequence. What is the primary voltage on the large black wire going through the donut hole CT?   

Explanation / Answer

CT ratio 2000:5

given that current through primary is 350 A

current in secondary is = 350*5/2000

= 0.875 A

secondary connected relay impedance = 0.075 ohm

so voltage across secondary V2 =RI = 0.075*0.875=0.065625

in transformer V1/V2    = I2/I1

V1 = V2I2/I1

= 0.065625*5/2000

= 164.0625 *10-6 V

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