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Where to put CTs to measure line current

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#1 scott.mckirdy


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Posted 01 June 2003 - 12:06 PM


I have a simple 3 phase direct on line motor starter with the contactor output feeding the motor and pfc capacitors. The motor leads and the capacitor leads are connected directly to the contactor terminals.
I want to measure the supply line current with CTs, but due to panel layout I could not fit the CTs on the contactor input cables. I can fit them on the output side of the contactor, but since the capacitor leads and the motor leads terminate here, I am confused about whether I should pass the motor leads or both the motor and capacitor leads through the CTs for them to read the proper supply lne current. I realise that the capacitor and motor leads are carrying currents of different phase and amplitude so its not straight foreward. Am I making a big mistake by not fitting the CTs on the input side of the contactor which has only 1 cable per phase? I cannot connect the capacitors at the motor terminals because the motor has sealed in leads for IP68 immersion ability!



#2 marke


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Posted 01 June 2003 - 09:08 PM

Hello Scott

Welcome to the forum.

If you wish to measure the motor current only, then connect the CTs on the output of the contactor with only the motor cables passing through them.
If you wish to measure the line current, then you can connect the CTs on the output side of the contactor with both the motor cable and capacitor cable passing through the CT. i.e. the CT sees the current going to the motor and the current going to the CT. This is exactly the same as connecting on the input side of the contactor.
Note the cable to the motor and the cable to the capacitor must pass through the CT in the same direction from the contactor.

Best regards,

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