What is the instrument transformer? Its construction, working, advantages, disadvantages & Applications

An instrument transformer is a type of transformer that is used in electrical power systems to measure or monitor high voltage and high current circuits safely and accurately. It is designed to step down the voltage or current of a high voltage or high current circuit to a lower level that can be easily measured by standard instruments such as ammeters, voltmeters, and wattmeters.

Construction of the Instrument Transformer:

Instrument transformers are generally of two types, current transformers (CT) and voltage transformers (VT). A current transformer consists of a primary winding and a secondary winding. The primary winding is connected in series with the high-current line that needs to be measured, while the secondary winding is connected to the measuring instrument. A voltage transformer also consists of primary and secondary winding. The primary winding is connected to the high voltage line, and the secondary winding is connected to the measuring instrument. Both types of transformers have a magnetic core made of laminated steel to enhance the magnetic coupling between the primary and secondary windings.

Working of the instrument transformer:

Instrument transformers work on the principle of electromagnetic induction. When a high current or voltage flows through the primary winding, it creates a magnetic field around the transformer core. This magnetic field induces a proportional voltage or current in the secondary winding, which is then used to power the measuring instrument. The voltage or current ratio between the primary and secondary winding is determined by the turn ratio of the transformer.

Advantages of Instrument Transformers:

  1. They provide isolation between the high voltage or current circuit and the measuring instrument, ensuring safety for the operator.
  2. They enable the use of standard measuring instruments for measuring high voltage or current circuits.
  3. They help in accurate measurement of high voltage or current circuits as they have a high degree of precision.

Disadvantages of Instrument Transformer:

  1. They can be expensive, especially for high voltage and high current applications.
  2. They introduce some amount of error in the measurement due to the transformation process.
  3. They require regular maintenance to ensure their accuracy.


  1. Power generation and distribution systems
  2. Electrical substations
  3. Industrial applications such as motor control and instrumentation
  4. Metering and monitoring systems for electrical energy consumption.

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