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CT coil question

11/21/2009 2:16 PM

If you pass a 1mm single core cable through a ct coil and get a reading of 5A , what would the reading be if you looped the same cable back through 2 times 3 times 4 times and so on ? Will the reading still be a constant 5A or would it decrease/increase with each loop ?

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#1

Re: CT coil question

11/21/2009 3:17 PM

Each loop contributes 5A worth of flux to the CT. Just like the number of turns in the primary of a transformer. So your reading should be 5A x # of turns.

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#2

Re: CT coil question

11/21/2009 4:27 PM

That trick is done some times when the amperage is small so you can get a good reading on you meter. Just don't forget to divide the reading by the number of turns.

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#3

Re: CT coil question

11/21/2009 5:23 PM

Hi - & welcome to CR4.

Not meaning to be offensive, but is this a homework question? Are you a student? Just curious?

It is, honestly, a pretty trivial question for anyone who has any idea how a transformer works.

Don't take me wrong - I just want to know where you're coming from.

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#4

Re: CT coil question

11/21/2009 6:15 PM

If the meter has been scaled to the same ratio as the CT, the extra turns through the CT may peg the meter, or even burn it out. In other words, if the real range of the meter is 5A (which is common), the multiple currents of 10A, 15A, etc., will be out of range.

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#5

Re: CT coil question

11/21/2009 9:15 PM

Thank you all :)

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#6

Re: CT coil question

11/22/2009 10:48 AM

Of course you would read the multiplications of 5A in each more loop.

We all do the same for low current Clamp metering purposes. In the cases the metering current is too low for correct reading, we create generally ten loops in the clamps and we read the current ten times more. And later we divide what we read to ten. Because divider ten is easy for quick calculation, when you don't have a calculator nearby.

However coming to Current Transformer, this succestion is not correct. The matter is totally different.

Current transformers are special transformers. They built to ensure their magnetic cores to easily saturate just after 5 Amp. This is deliberately made in order to protect the needle pointers of the ampermeters suddenly to rise to hit to the endless and therefore to bent out.

This means that you can multiply the readings by making additional loops inside the CT. before you reach to 5 Amp reading. But you're limited when you reach 5 Amp. Because the core is magnetically saturated at this point.

Current Transformers are built under certain standards. Quick saturated ones are used for metering to protect the measuring instruments.

Semi quick saturated ones are built for overload breaking purposes. They release the breakers, and they permit some temporary overloads, such as transformer or motor demarraged starts.

To give an idea, Quick saturated CT's are saturating around 6-7 amps, since semi quick CT's saturate around 15-30 Amps.

However in general, the CT's are built to saturate, after the rating, indicated on their label. Check the label on the CT if for quick saturation or more.

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#7
In reply to #6

Re: CT coil question

11/22/2009 1:27 PM

CTs for VS drive feedback are generally designed to not saturate for at least 250% with rated CT load impedance attached.

Most CTs with a very low connected load impedance don't saturate easily. It is the VA supported by the CT. As the load impedance increases the VA requirements increase, so the CT becomes more bulky.

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#8
In reply to #7

Re: CT coil question

11/22/2009 2:17 PM

Exactly. You are right.

You can always securely short the output of any CT at anytime without any harm. It continues to supply the correct output value even if the voltage near to zero, due the shorted output.

But in the case the output impedance rises, the output voltage even rises proportionally to higher voltages to maintain the same correct current to flow. (VA equalisation)

Output terminals of a CT cannot be left open (Even under lower loads)

Impedance at the output terminals tries to go endless, Therefore Voltage at the output also tries to reach endless. The result is simple. The dangerous high voltage damages the insulation in the CT.

Current at the output of CT has limited by saturating the core. But the voltage is not limited.

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