What is Current Transformer? Where is It Used?

Current transformer is a kind of measurement transformer. These transformers are used to measure the current flowing in a circuit. Current transformers may look like a small box when we look from the outside. Inside this square-shaped form, there are three basic parts. an iron core, that is, the center or the frame, which forms the main structure of the current transformer. The core is the first of the three. It can be thought of as a solid, layered framework. Assuming that this frame is a square, the 1 – primary and 2 – secondary windings wound on two opposite sides they form the other sections. In other words, the current transformer simply consists of an iron core with primary and secondary windings. The secondary winding part of this structure is also important as grounding is done here and this must be done carefully for safety. Also in this transformer there is magnetic flux, and parts such as the feet and the yoke. 

Current Transformer

How Does a Current Transformer Work?

Current transformers are connected in series to  the electrical circuit and the current passing through the primary circuit is reduced by a magnetic coupling system. This is transferred to the secondary, that is, the secondary circuit and the electrical devices connected to this circuit. Thus, a cyclic current is created in the secondary circuit thanks to the current transformer. As a result, the measuring instrument connected in parallel to the secondary winding can measure the current passing through the first circuit, by the transformer.

Current transformers reduce the electric current in the circuits they are connected to as the desired values ​​and thus, by providing the necessary current and isolation, they make the connected measuring device in the secondary circuit functional. The current transformer appears as a measurement transformer that helps to regulate the current, provide the required current and create isolation. The current range required for the instruments to measure is generally determined as 1A and 5A. The current transformer or transformer also sets this range.

How It Is Made

Current transformers are indispensable elements for measuring in circuits. Current transformers connected in series to the circuits have a phase difference between the primary current and the secondary current under normal operating conditions, this phase difference should be approximately zero.

Current transformers are widely used in medium and high voltage circuits. The current in the primary windings is reduced by a magnetic coupling and thus a smaller current is obtained in the secondary winding. The current passing through the system prevents the machines operating or measuring instruments connected to the secondary side of the device from being adversely affected by possible high voltages.

Current Transformer

Current Transformers According to Isolation Types

Considering the structure and working principles of a current transformer, they are very important for operations. In the structure of a current transformer; there are primary and secondary windings, they are on a magnetic core, there are insulators and a boiler filled with oil if the cooling type is oil cooled type.

Like the basic working principles of a normal transformer, in this device, the alternating current on the primary winding creates voltage and induction, that is, magnetic flux. The magnetic is transmitted with the help of the core and the flux on the primary winding is transmitted to the secondary. Variable alternating flux also induces voltage in the secondary one too. This voltage is the cause of the current flow in the secondary circuit of the transformer.

Other Current Transformer Types and Parts

Current transformers have different types in terms of operating principles. For example, there are current transformers with a detachable core. This design can be preferred because of the convenience it provides while mounting; they can be used for constantly running systems. These can often be integrated or mounted in the system without separating the bar or conductor. In addition to these types, there are different transformers according to the voltage used.

In a current transformer, the current passing through the primary is magnetically transmitted to the secondary depending on the conversion ratio of the transformer. There is high voltage in the primary windings and therefore these windings are thick and thy have few turns. On the other hand, since the windings of the secondary part are exposed to a smaller voltage, the windings in the secondary are both thin and are multi-turned.

Current Transformer

Working Principles of CT

When talking about the working mode of the current transformer, the most important feature is related to the secondary circuit. The secondary circuit in transformer actually operates as a short circuit. If the secondary windings in the transformer are left open, there is a risk of creating high voltage. Because the voltage created by the secondary windings increases with the disappearance of the reducing magnetic flux effect of the primary windings. This rising voltage can accumulate at the ends of the secondary windings that are left open and can reach the heating of the main core. More and more voltage is induced at the secondary ends, the magnetism effect increases. This can reach to a risky level, at this stage the devices or measuring instruments may even be damaged. For this reason, attention should be paid to every detail in the structure of the transformer to avoid risks. The secondary ends must always be short-circuited.

To Connect a Current Transformer

For instruments such as instrument transformers, the characteristics of the current to which these instruments will be exposed are important. If the current to be measured is too high for the transformer, this process cannot be done by the current transformer. Or, to measure a very high voltage current, a suitable operation must be arranged or suitable instruments are required, which is costly. In such cases the current has to be reduced to standards that can be measured by a current transformer. In addition to the inability to directly measure the currents that are too high for the current transformer or the high costs of the process, these operations can also be dangerous. It is preferable to have a current reduced to the values ​​required for the current transformer, as measuring high currents may be inappropriate for safety standards. In other words, it may be necessary to reduce the current passing through the circuit to measurable values ​​in order to use the current transformer.


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