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Insight - How Current Transformer Works

Written By: 

Abhimanyu Mathur


Transformer is an electrical device which works on the principle of Faraday’s law of electromagnetism used to step up or step down the input voltage.Transformers are of different types for example: Power transformers, Current Transformers, Potential Transformers, Pulse Transformer, RF Transformer and Audio transformers. The most common application of Current transformer is to reduce the current and provide galvanic isolation in current sensing devices. Some typical examples include SMPS, motor control and electronic lightning ballasts.

Some of you would know how a current transformer works. Windings, Core, connections and other things which many of you have surely seen in the books or  in the diagrams.But how many of you have got the chance to see how actually it looks from inside. A small instrument but which works greater than its size. In today’s insight you will know how current transformers are made to work.

Image Showing A Typical Current Transformer

Fig. 1: 

Image Showing A Typical Current Transformer

Above picture displays a Current transformer. The marking on the device helps us to know the number of turns in secondary coil. Different manufacturers have different format of marking for example this device has marking in the format of 54XXXC in which the X’s are replaced by a three digit.The number represents the turns in secondary coil for instance if it is written 54050C it means that the secondary coil has 50 turns.


Closer View of Current Transformer Packaging

Fig. 2: Closer View of Current Transformer Packaging

This transformer has a polymer housing made from a material known as UL94 V-0 which complies with the RoHS standards. UL94 a code given to the test for flammability of plastic materials used in electronic devices.

The last two alphanumeric represents their flaming rating. In thisV-0 means that the flames will be vanished in maximum 10 seconds with no burning material can fall.


Windings & Pin Structure


Picture Showing Windings Inside A Current Transformer

Fig. 3: Picture Showing Windings Inside A Current Transformer

Bottom view of the transformer is shown in the picture above. Copper wires aregenerally used as current carrying conductor. There are six terminals out of which two are connected with the secondary winding and four with primary winding. Primary winding has area of cross section bigger because resistance of a wire is inversely proportional to its cross sectional area. Input current for the transformer has to be more than secondary. To minimize the magnetic flux density secondary coils have lower cross sectional area.

There are two primary coils in this device in the ratio 1:1 which improve the safe current capacity of the transformer.

Pin Structure:

Pin Structure of A Current Transformer

Fig. 4: Pic Structure of A Current Transformer

It is a 6 pin device which serves as input and output terminals for it. Circular groove at the bottom left corner helps to identify the pin structure. Primarywinding are connected to the pins 1, 6& 2, 5 and 3, 4 to the secondary winding. Turn ratio of the device is 1:1:200, where 200 is the secondary coil turn ratio.

Internal Structure

Inner structure:

Internal Structure of Current Transformer

Fig. 5: Internal Structure of Current Transformer

After cutting down the polymer we saw how the winding overlaps each other. It looks like a ring of copper wire as shown in the image above. The windings are placed concentrically to reduce the flux leakage in the transformer.

Primary Winding and Primary Winding Insulation In A Current Transformer

Fig. 6: Primary Winding and Primary Winding Insulation In A Current Transformer

Core and Coils

There are two primary winding of single turn and it overlaps the secondary. Insulators made from varnish or enamel is used to separate primary and secondary winding.Insulation in transformer prevents conductors shorting.

Current Transformer Core and Coils

Fig. 7: Current Transformer Core and Coils

After shredding some copper wires we saw the heart of the transformer “The Core”.This type of construction of core is known as Toroidal core in which the conductor tightly surrounds the core in a manner that there is no air gap. Ferrite Toroidal core is used for high frequency working to reduce losses as well as its physical size.

Ferrite Toroidal Core Inside A Current Transformer

Fig. 8: Ferrite Toroidal Core Inside A Current Transformer



Some more eleboration is required on a given topic.

Extra discription required.

@ Samir.dod


Will post clear content on Current Transformers

Instrument Transformers are classified into two types: Current Transformers(CT) and Voltage Transformers(PT). These Transformers are mainly used for Metering Purposes and for Protection Purpose in Electrical Power Systems. The Relays are used for protecting electrical  power system. These are made to operate at low voltages & currents. So Current & Voltages are stpped down by means of CT & PT and fed to protection system. In Current Transformers(CT) the primary is connected in series with power line. So current through its primary is nothing but the current flows through that power line. The primary current of the CT, hence does not depend upon whether the load or burden is connected to the secondary or not or what is the impedance value of burden. Generally CT has very few turns in primary where as secondary turns is large in number. Say Np is number of turns in CT primary and Ip is the current through primary. Hence the primary AT is equal to NpIp AT. If number of turns in secondary and secondary current in that current transformer are Ns and Is respectively then Secondary AT is equal to NsIs AT. In an ideal CT the primary AT is exactly is equal in magnitude to secondary AT.

Current Transformer Vector Diagram:


Is - Secondary Current

Es - Secondary induced emf

Ip - primary Current

Ep - primary induced emf

KT - turns ratio = numbers of secondary turns/number of primary turns

Io - Excitation Current

Im - magnetizing component of Io

Iw - core loss component of Io

?m - main flux.

Let us take flux as reference. EMF Es and Ep lags behind the flux by 90o. The magnitude of the passers Es and Ep are proportional to secondary and primary turns. The excitation current Io which is made up of two components Im and Iw.
The secondary current Io lags behind the secondary induced emf Es by an angle ? s. The secondary current is now transferred to the primary side by reversing Is and multiplied by the turns ratio KT. The total current flows through the primary Ip is then vector sum of KT Is and Io.

Current Error or Ratio Error in CT:

From above passer diagram it is clear that primary current Ip is not exactly equal to the secondary current multiplied by turns ratio, i.e. KTIs. This difference is due to the primary current is contributed by the core excitation current. The error in current transformer introduced due to this difference is called current error of CT or some times Ratio Error in Current Transformer.


Current Error = (Ip-KTIs)/Ip