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Instrument

Transformer

Electrical

Measuring

Instruments

Instrumentation

Topics

1. Introduction2. Uses

of

instrument

transformer

3. Advantages4. Current

transformer

5. Shell

type

current

transformer

6. Ring

type

current

transformer

7. Burden

of

an

instrument

transformer

8. Phase

diagram

Introduction

These are special type of transformers used for

the measurement of voltage, current, powerand

energy.

As

the

name

suggests,

these

transformers are used in conjunction with therelevant

instruments

such

as

ammeters,

voltmeters, watt meters and energy meters.

Types

of

Instrument

Transformer

Such transformers are of two types :1. Current Transformer (or Series Transformer)2.

Potential Transformer (or Parallel Transformer)

Current transformers are used when the magnitude

of AC currents exceeds the safe value of currentof measuring instruments.

Potential transformers are used where the voltage

of an AC circuit exceeds 750 V as it is notpossible

to

provide

adequate

insulation

on

measuring instruments for voltage more thanthis.

Use

of

Instrument

Transformer

Measurement of current as CTThe primary winding is so connected that the

current to be measured passes through it andthe secondary is connected to the ammeter. The function of CT is to step down the current.

Instrument

Transformer

as

CT

Instrument

Transformer

as

PT

Advantages

of

Instrument

Transformer

1. The measuring instruments can be placed for

away

from

the

high

voltage

side

by

connecting

long

wires

to

the

instrument

transformer.

This

ensures

the

safety

of

instruments as well as the operator.

2. This instrument transformers can be used to

extend the range of measuring instrumentslike ammeters and voltmeters.

Disadvantages

of

Instrument

Transformer

The

only

main

draw

back

is

that

these

instruments can not be used in DC circuits.

Current

Transformers

In order to minimise the exciting ampere turns

required, the core must have a low reluctanceand small iron losses. The following three types of core constructions

are generally employed :

1. Core type2. Shell type3. Ring type

Core

type

Shell

type

•^

In shell type, the windings are placed at thecentral limb, thus it gives better protection tothe windings.

Ring

type

•^

Ring

type

core

is

commonly

used

when

primary

current

is

large.

The

secondary

winding is distributed round the ring and theprimary winding is a single bar.

-^

It is a joint less core and there is very smallleakage reactance.

Ring

type