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Associate

Join Date: Oct 2013
Posts: 41

Polarity

01/26/2017 9:49 AM

Dear all

why are there additive and subtractive polarities for current transformers (other than current flow in the windings)? What are the benefits one over the other?

thanks for all

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Guru
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#1

Re: polarity

01/26/2017 9:59 AM

Because megnetism and current flow both have direction...

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Guru

Join Date: Apr 2010
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#2

Re: Polarity

01/26/2017 11:51 AM

Current transformers are marked with the polarity so that multiple measurements can be combined. For example, with a balanced load, the sum of the three phase currents in a 3-phase system should sum close to zero. If one transformer is reversed, you might be reading "Ia + Ib - Ic" instead of "Ia + Ib + Ic".

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Associate

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#6
In reply to #2

Re: Polarity

01/28/2017 8:56 AM

Dear sir

good day

thank you for your reply

but do I understand that If terminal X1 is diagonally across the transformer from H1, the current flow will be in a certain direction & When H1 and X1 are adjacent, the current flow It will be in the opposite direction

best regard

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Guru

Join Date: Dec 2010
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#7
In reply to #6

Re: Polarity

01/29/2017 5:13 AM

If primary P1 is positive to P2, then secondary S1 is positive to S2. If mark on one winding is positive to its unmarked: then mark on other winding is postive to its unmarked.

Test is to apply battery P1 to P2, P2 positive - then S1 becomes + to S2

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Guru

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#8
In reply to #7

Re: Polarity

01/29/2017 9:39 AM

Argghh ...........Apologies for error in post #7 !!!!!!!!!!!!!!!!!!!!!!!!

Test is to apply battery to primary P1 to P2, P1 positive - then secondary S1 becomes + to S2.

Letters on primary & secondary may be different but numbers should apply.

For the avoidance of doubt about markings, use test battery - size of battery [and rating/resistance of wires to it] should provide appropriate short-circuit current for CT primary current. Usually, a single cell, alkaline size D/LR20 upwards to vehicle battery is adequate.

Secondary Moving Coil DC multimeter is better than digital for this job. Needle will flick forwards when lead "dabbed" to battery in primary circuit.

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#3

Re: Polarity

01/27/2017 6:58 AM

Many wattmeters & Wh meters give outputs for forward and reverse watts. But if polarity of CT output relative to that in conductor through it is not marked then installation is guesswork.

Polarity is checked by the simple, but definitive, test of applying a battery to the primary conductor and checking direction of current pulse in CT secondary.

You will find drawings where primary has P1, P2 and secondary S1,S2 marks or • at one end of winding - when mark on one winding is + , mark on other winding is +.

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Guru
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#4

Re: Polarity

01/27/2017 3:07 PM

A great place for a stupid joke:-

What is the difference between a battery and a woman?

The battery has a plus!!!

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Active Contributor

Join Date: Jan 2017
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#5

Re: Polarity

01/27/2017 6:49 PM

It is nothing more than a convention of identifying of polarity in a single-phase power transformer that has two HV and two LV terminals. Voltage and current transformers do NOT have such a convention, . . . they have marked terminals, typically H1 & X1 will be of same polarity, . . . while drawings will show (*) on the H & X terminals to show same polarity.

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