Voltmeters and Ammeters are used to measure the voltage and current in the circuits. Using instrument transformers, ranges of these devices to measure can be increased. A current transformer is basically a step-down transformer; hence it steps down the current. When used in conjunction with the low range ammeter, a current transformer (CT) increases the range of the ammeter. Thus a 0-5 A ammeter can be used to measure several hundred or thousand amperes of current.
Similarly, a potential transformer (PT), which is basically a step-down transformer can increase the range of the low voltage voltmeter. Thus a voltmeter designed to measure voltage up to 110V can measure much higher voltages (several thousand volts) when used in conjunction with a suitable potential transformer.
Instrument transformers have many advantages. Some of the advantages are given below:
Advantages of Instrument Transformers
- Single range ammeters and voltmeters can measure a wide range of currents and voltages if used in conjunction with suitable Current Transformers (CTs) and Potential Transformers (PTs).
- The measuring instruments like ammeter, voltmeter and wattmeters etc are incorporated in the secondary circuit and hence they are totally segregated from the high voltage, thereby ensuring safety for the operator and observer.
- The meter need not be insulated for high voltages which would be the case if they are directly included in a high voltage circuit.
- Using a current transformer with suitable split and hinged core, it easy to measure heavy currents in the busbar without having to break the conductor carrying current. The core of the Current Transformer (CT) is opened at the hinge, the current carrying conductor is introduced in the center of the core through an opening made and the core is tightly closed again. The conductor itself acts as a single turn primary winding of the current transformer.