What are Instrument Transformers

These are transformers that are used in conjunction with measuring instruments and are related to the field of instrumentation. These are mainly used for measurement of current, voltage, energy, power etc. in high power A.C. lines. Here the values of these quantities are so high that they can’t be measured directly by ammeters, voltmeters etc because of the range limitations of these instruments.

 

The two most prevalent instrument transformers today are:-

Current transformer or simply known as C.T. Voltage or Potential transformer or simply known as P.T.

In a current transformer, the number of turns of a secondary is far higher than the primary winding. The primary winding is connected in series with the load in the power line. Due to the difference in the number of turns, the current that is induced in the secondary winding is far less than that flowing through the primary. This current in the secondary is then easily measured by using an ordinary ammeter. This ammeter reading is then multiplied by the conversion factor of the transformer to obtain the actual magnitude of the current that is flowing in the power line.

In a potential transformer, the number of turns of a secondary is far fewer than the primary winding. The primary winding here is connected in parallel with the load of the power line. Again the magic lies in the difference in the number of turns that down steps the voltage induced in the secondary winding by a great amount. This down stepped voltage can then easily be measured by using a regular voltmeter. Again taking in to account the conversion factor of the transformer, the actual voltage across the load in the power line can be obtained using the voltmeter reading.

Another big advantage of instrument transformers is the electrical isolation they provide between the measuring instrument and the power line. This safeguards these expensive instruments from any damage due to undesired fluctuations in voltage and current. In absence of such isolation, these fluctuations at such high voltage and current levels would prove a huge hazard that could cause permanent damage to these sensitive instruments.

There are a few errors though that are encountered in the instrument transformers on account of factors like flux leakage, core saturation, hysteresis etc. These errors should be compensated for in the final reading to obtain more accurate results.

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