INSTRUMENT TRANSFORMERSPresented by:-M.Aamir Sohail khanSession: 2004-EE-60University college of engineering & technology bahaudian zakariya university Multan, Pakistan

What is Instrument Transformer ?

A transformer that is used in conjunction with a measuring instrument.It utilizes the current-transformation and voltage transformation properties to measure high ac current and voltage.

Importance of Instrument transformersIn dc circuits for current and voltage measurement we use ammeters and voltmeters.For measurement of high current ,it is usual to use low range ammeter with suitable shunt.For measurement of high voltage, low range voltmeter are used with high resistance connected in series.But for measurement of high A.C. current and voltage we cannot use these methods.We use specially constructed instrument transformers.

Types of instrument transformers

These instrument transformers are of two types:-

Current transformersPotential transformers

Current TransformersCurrent transformer normally known as c.t. is a step up transformer.

These are used with low range ammeter to measure current in high voltage alternating circuits where it is not practical to connect instrument and meters directly to lines.

This is step up transformer because when we step up the, voltage increases and current decreases.

The current is step down in a known ratio called current ratio.

Construction of C.T.C.T. has a primary coil of one or more turns of thick wire connected in series with the line whose current is to be measured.

The secondary consist of large number of turns of fine wire, is connected across the ammeter terminals.

Working If a current transformer has primary to secondary current ratio of 100:5 then it step up the voltage 20 times and step down the current 1/20 times of its actual value.If we know the current ratio I1/I2 and the reading of a.c. ammeter, the current can be calculated. Current = ratio ammeter reading

Importance of short ckt.Ammeter resistance is very low ,the current transformer normally works short circuited.

If for any reason the ammeter is taken out of secondary winding then the secondary winding must be short ckted with the help of short ckt switch s.

If this is not done, then due to high m.m.f. will set up high flux in the core and it will produces excessive core loss which produce heat and high voltage across the secondary terminals Hence the secondary of current transformer is never left open.

Potential transformerA PT is a step down transformer having many primary turns but few secondary turns.

In step down the voltage decreases and current increases, thus voltage can be easily measured by using low range voltmeter.

The voltage is stepped down in known ratio called voltage ratio.

Construction and working of P.T.ConstructionA potential transformer has many primary windings but few number of secondary windings that makes it step down transformer.Voltmeter is connected to secondary winding usually voltmeter of 150 v is suitable.WorkingPrimary terminals are connected across the line to which the voltage is to be measured.The voltmeter gives the transformed value of voltage at secondary.The deflection of voltmeter when divided by transformed ratio gives the actual voltage at primary.Line voltage = deflection / trasf. RatioWhere transformation ratio = V2/V1

Precaution for P.T.

Since the secondary of p.t. is connected to relays, their ratings are usually 40 to 100 Watts.

For safety purpose the secondary should be completely insulated from the high voltage primary and should be in addition grounded.

Types of P.T. Some types of p.t. are

Shell type Dry type Oil type

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