480V 3phase amp clamp readings

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sb1233

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Last night at work my partner and I were replacing the bearings on a 3ph 480Volt motor. After we got done we decided to test it. Out of curiosity we decided to put an amp clamp to measure the current. Another guy come along and asked "what would it read it you put T1 and T2 in the clamp?". I told him that it would read 0 because they would cancel each other out being that they are 180 degrees out of phase. So I did it to show him and suprising to me the meter said 3.62A.(on T1 and T2).......When I measured T1 by itself it was 3.59A. When I measured T2 it was 3.6A and when I measured T3 by itself it was 3.62A. SO.....I went back and clamped T2 and
T3 (together) It read 3.59A.. then I clamped T1 and T3 it read 3.6. ..So I come the assumption that when measuring 2 phases together it will give you the 3rd phase current....Is this true? If so I think its because the 2 phases are canceling each other out and your reading the current on the 3rd phase comming back through the motor windings. Does this make sense to anyone?my theory true? Please let me know because this is somehing that was never covered in college.
 
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nakulak

Senior Member
how would a 3 phase motor have 2 leads 180 out of phase ? 3 phase is 120 out of phase. there is a formula to calculate the imbalance - I don't recall it off the top of my head, but I'm sure someone will post it shortly. however, if you are good at math, you can use vectors to do it graphically or with trig using the vectors (or phasors if that is the correct term).
 
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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
As Nakulak says, use can use vector math to calculate the 'net current' being carried on a set of phases. If you know the current and its phase angle on phase A, and the current and its phase angle on phase B, then you can calculate the current on phase C.

What you discovered with your current clamp is that a current transformer reports the net current on all conductors going through its bore. You can have multiple wires for the same phase (eg. parallel conductors), you can have conductors of different phases, you can even loop the same conductor through several times.

You are also looking at "Kirchhoff's Current Law", which is a basic electrical law which can be stated 'all current flowing into something has to flow out again'. Assuming no faults, all current flowing into a motor on phases A and B _must_ flow out again on phase C.

So when you put your clamp around phases A and B, you were directly measuring the vector sum of the current on A and B, and (assuming no faults) that must be exactly equal to the current flowing in the opposite direction on phase C.

Where this is of practical use: if you put your clamp around all three phases, the net current should read zero. If it isn't zero, that means that there is another current path someplace else, eg. a ground fault.

-Jon
 
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sb1233

Member
Thanks for the help guys. And thank you for correcting me on the 180 degrees. (brain fart). I will check out the link that you gave me and hopefully It will help me understand a little better.
 
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