Measuring Instruments

ELECTRICAL AND INTSRUMENTATION

TECHNOLOGY

CGE535

MEASURING INSTRUMENTS

DC – AC MEASURING INSTRUMENTS

Principle of operations

Electromagnetic – the magnetic effects of currents

Electrostatics – forces between electrically-charged conductors.

Electrothermic – the heating effect of the material.

Electric measuring instruments are used to measure the

voltage, current, power or energy.


The permanent magnet moving coil (PMMC) is the most

common analogue instrument used to measure dc voltage or

current.

This instrument is an electromechanical concept whereby the

input is an electrical signal while the output is the mechanical

force.



The PMMC instrument consists of a permanent magnet, a

small lightweight coil wounded on a rectangular soft core

iron and an aluminium pointer attached to the rotating coil.

When a current is passed through the coil windings, torque is

developed on the coil by the interaction between magnetic

field and the field set up by the current in the coil. The

aluminium pointer moves around the calibrated scale

indicates the deflection of the coil.


An ammeter is the instrument that measures the current in a

circuit. It is connected in series with the components

carrying the current.

An ammeter should have low resistance so that the it will not

alter the load current.

For high range ammeters, a low resistance made up of

manganin (low temperature coefficeient of resistance) is

connected in parallel to the moving coil.


This connection is called shunts. Thus a multirange ammeter

can be constructed by employing several values of shunt

resistances, with a rotary switch to select the desired range.


A DC voltmeter is constructed by connecting a resistor in

series with a PMMC instrument.

A voltmeter should have a very high resistance and it is

normally connected in parallel with the circuit where the

voltage is to be measured.



A moving iron instrument is a type of electromagnetic

instrument.

This instrument measures the AC voltage and current.

The movable system consist of one or more pieces of

specially-shaped soft iron that are pivoted to be acted upon

by the magnetic field produced by the current in the coil.


Construction of moving-iron instrument Moving element: a small piece of soft iron in the form of a

vane or rod

Coil: to produce the magnetic field due to current flowing through it and also to magnetize the iron pieces.

In repulsion type, a fixed vane or rod is also used and magnetized with the same polarity.

Control torque is provided by spring or weight (gravity)

Damping torque is normally pneumatic, the damping device consisting of an air chamber and a moving vane attached to the instrument spindle.

Deflecting torque produces a movement on an aluminum pointer over a graduated scale.


Repulsion Type Attraction Type

Electrodynamic type instruments are similar to the PMMC-

type elements except that the magnet is replaced by two

serially connected fixed coils that produce the magnetic field

when energized.

The movable coil carries a pointer, which is balanced by

counter weights and controlled by springs.

The motor torque is proportional to the product of the

currents in the moving and fixed coils

ELECTRO-DYNAMIC MEASURING

INSTRUMENTS

ELECTRO-DYNAMIC MEASURING

INSTRUMENTS

This type of instruments can be used to measure AC or DC

current or voltage.

However, the major application of this instrument is as the

wattmeter for power measurement.

SINGLE PHASE INDUCTION TYPE

ENERGY METER/WATT HOUR-METER

An energy meter or watt-hour meter is an instrument that is

used to measure either quantity of electricity or energy over

a period of time.

The unit of energy is joule or watt second. The commercial

unit of electrical energy is kilowatt hour (KWh).

For measurement of energy in AC circuit, the meter used is

based on “electro-magnetic induction” principle. They are

known as induction type of instruments.



Induction type energy meter essentially consists of following

components

(a) Driving system (b) Moving system (c) Braking system and

(d) Registering system.



The Driving systemis consists of two electromagnets, called

“shunt” magnet and “series” magnet, of laminated

construction

The moving system essentially consists of a light rotating

aluminium disk mounted on a vertical spindle or shaft.

In th Braking system the damping of the disk is provided by a

small permanent magnet, located diametrically opposite to

the AC magnets.

The registering or counting system essentially consists of

gear train, driven either by pinion gear on the disc shaft,

which turns pointers that indicate on dials the number of

times the disc has turned

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Measuring Instruments