Why Instrument Transformers Used and Advantages

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 bascially 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 0-5A ammeter can be used to measure several hundreds 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 upto 110V can measure a much higher voltages (several thousnd volts) when used in conjnction with suitable potential transformer

Instrument transformers have many advantages. Some of the advantages are given below:

Advantages:

• 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 current transformer with suitable split and hinged core, it easy to measure heavy currents in the busbarwithout 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 a opening made and the core is tightly closed again. The conductor itself acts as a single turn primary winding of the current transformer.

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Moving Coil (MC) Instruments Advantages Disadvantages

The operation of moving coil meters depends on the electromagnetic effect of the current ( a force is excerted on the current carrying conductor when placed in a magentic field )

Moving coil instruments are of two types:

• Permanent magnet type
• Dynamometer type

Permanent magnet type moving coil instruments can be used only in measuring dc circuits whereas Dynamometer type moving coil meters can be ued to measure both ac and dc circuits

Some of the advantages and disadvantages of moving coil instruments are given below:

Merits or Advantages:

• Moving coil instruments consume quite less power. This is becasue the resistance of moving coil is small
• In Moving coil meters the ratio of torque to weight is quite high and this reduces the errors due to friction
• The scale of moving coil instrument is uniform
• A low range ammeter can be used as both ammeter and voltmeter to measure high currents and voltages
• PMMC meters are free from errors which would be caused by hysteresis and stray magnetic fields
• Damping is very simple and effective in moving coil meters

Demerits or Disadvantages:

• Due to friction at the bearings and temperature variations, errors are likely to be presentand the observed readings are may not be accurate and fresh calibration of the meters may become necessary
• With long usage permanent magnet gets weakened and this likely may introduce errors in readings

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Difference between Current Transformer (CT) and Potential Transformer (PT)

Current Transformers (CT) and Potential Transformers (PT) are used to measure the current and voltage in a circuit of the order of hundreds of amperes and volts respectively. 

A CT has large number of turns on its secondary winding, but very few turns on its primary winding. The primary winding is connected in series with the load so that it carries full load current. A low voltage range ammeter (0-5A) is connected across the secondary winding terminals. Secondary of the CT is practically short circuited since the ammeter resistance is very low. It should be remembered that secondary of the CT should not be made open as it draws heavy current and damages the primary winding of the CT.

A PT has large number of turns in the primary and fewer turns in the secondary and hence it steps down the voltage. The primary winding is connected across the supply voltage and low range voltmeter (0-110V) is connected across the secondary winding terminals

Some of the main difference between current transformers (CT) and potential transformers (PT) are given below:

• The secondary of the CT is almost short circuit, whereas the secondary of the PT is practically a open circuit
• The primary winding of the CT is connected in series with the load so that it carries the full line current, but there is only a small voltage across it. However the primary winding of the PT has the full supply voltage applied across it
• In CT the excitation current I₀ and flux density vary over a wide range whereas in PT, they vary over a limited range only.

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