# What’s the current transformer working principle?

FAQ | Jul 03,2023

Current Transformer Working Principle:

The current transformer is an electrical device that is widely used to measure alternating current (AC) in high-power systems.
Current transformer working principle is based on the principle of electromagnetic induction. Here’s how it works:

Construction: A current transformer consists of a primary winding and a secondary winding. The primary winding is connected in series with the current-carrying conductor, through which the current needs to be measured. The secondary winding is connected to an ammeter, protective relay, or other measuring devices.

Turns Ratio: The turns ratio of a current transformer is very high, typically ranging from 100:1 to 5000:1. This means that the number of turns in the secondary winding is significantly greater than the number of turns in the primary winding.

Magnetic Flux: When an alternating current flows through the primary winding, it creates an alternating magnetic field around the winding. This magnetic field induces a voltage in the secondary winding according to Faraday’s law of electromagnetic induction.

Step-down Effect: Due to the high turns ratio, the induced voltage in the secondary winding is much lower than the voltage in the primary winding. This step-down effect allows the current to be accurately measured by standard instruments designed for low currents.

Burden: The secondary winding of a current transformer is connected to a load or burden, which is usually an ammeter or protective relay. The burden impedance determines the accuracy and performance of the current transformer. It is important to match the burden impedance with the current transformer’s specifications for accurate measurements.

Current Ratio: The current ratio of a current transformer is defined as the ratio of the primary current to the secondary current. It is determined by the turns ratio and is a fixed value for a given current transformer. For example, a current transformer with a turns ratio of 1000:1 will have a current ratio of 1000:1.

Accuracy Class: Current transformers are manufactured with different accuracy classes, which define their performance characteristics. The accuracy class specifies the maximum permissible error under specified operating conditions. Common accuracy classes for current transformers are 0.1, 0.2, 0.5, 1, and 3, with class 0.1 being the highest accuracy.

Overall, current transformer working principle of a current transformer involves stepping down the high current in a power system to a level suitable for measurement or protection purposes using electromagnetic induction and a high turns ratio between the primary and secondary windings.

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