15713 Instruments

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ELECTRICAL MEASURING

INSTRUMENTS

.



Analogue Instrument

An analogue instrument is one in which the

magnitude of the measured quantity is indicated by

means of a pointer. Instruments of this category

include moving coil instruments, oscilloscope etc.



Analogue Instrument



Digital Instruments

A digital instrument is one whose display is presented

in the form of a series of decimal values.



VOLTMETER

A voltmeter is used to measure the potential difference

between two points of a circuit. It is thus connected in parallel

with the circuit as shown. The voltmeter must have high

resistance so that the current flowing through the voltmeter and

the power absorbed by the circuit are minimum possible.



AMMETERS

An ammeter is used to measure the flow of current in a circuit.

It is thus connected in series with the circuit under test as shown,

So that current to be measured or a fraction of it passes

through the instrument itself. The ammeter must be capable of

carrying this current without injury to itself and without

abnormally increasing the resistance of the circuit into which is

inserted. For this reason, an ammeter is designed to have low

resistance.



Watt Meter

A wattmeter, as its name implies, measure electric power given

to or develop by an electronic apparatus or circuit.

In a d.c circuit, power (P = VI) can be easily determined from

voltmeter and ammeter readings.

But in an a.c circuit, power (P = VI Cos θ) depends not only on

voltage and current but also on the phase shift between them.



Energy Meters

WATT-HOUR METERS: The watt-hour meter is an instrument for

measuring energy. Since energy is the product of power and

time, the watt-hour meter must take into consideration both of

these factors.

In principle, the watt-hour meter is a small motor whose

instantaneous speed is proportional to the POWER passing

through it. The total revolutions in a given time are

proportional to the total ENERGY, or watt-hours, consumed

during that time.



Basic Principal

The basic principle of the ammeter and of the voltmeter is the

same, deflecting torque is produced when current flows through

their operating coils. In the ammeter, the deflecting torque is

produced by the current we wish to measure. In the voltmeter,

the deflecting torque is produced by a current which is

proportional to the potential difference to be measured.



Moving – Iron Type

Moving – Iron instruments depend for their action upon the

magnetic effect of current, and are widely used as indicating

instruments. In this type of instrument, the coil is stationary and

the deflection is caused by a soft-iron piece moving in the field

produced by the coil. This type of instrument is principally used

for the measurement of alternating currents and voltages.

There are two basic forms of moving – iron instruments.

Attraction type

Repulsion type



Attraction Type

Working Principle: When the current to be measured is passed

through the coil, a magnetic field is produced which attracts

the iron rod inwards, thereby deflecting the pointer which

moves over a calibrated scale.



Repulsion Type

Working Principle: When the current to be measured is passed

through the fixed coil, it set up its own magnetic field which

magnetizes the two rods with same polarity so that they repel

one another, with the result that the pointer is deflect and

causes the pointer to move from zero position.



Moving – Coil Instruments

It consists of

(a) A rectangular coil with many turns

(b) A powerful radial magnetic field between curved pole pieces Nand S and a soft iron cylinder

(c) Springs to control the angle of rotation of the coil(d) A uniform (linear) scale for measuring the current.

The moving coil instrument operates through the interaction of twomagnetic fields; the permanent magnet field and the field due to

the current flowing through a current carrying conductors The movingcoil instrument is commonly used in voltmeters, ammeters andohmmeters. It responds only to direct current. It is used in rectifier-type instruments to measure alternating current and voltage Thereare two types of moving coil instruments



Types of moving coil

PMMC(PERMANENT MAGNET MOVING COIL)

DYNAMOMETER TYPE



PMMC(PERMANENT MAGNET

MOVING COIL)

A permanent magnet moving coil instrument consists basically of a permanent

magnet to provide a magnetic field and a small lightweight coil is wound on a

rectangular soft iron core that is free to rotate around its vertical axis. When a

current is passed through the coil windings, a torque is developed on the coil by the

interaction of the magnetic field and the field set up by the current in the coil. The

aluminum pointer attached to rotating coil and the pointer moves around thecalibrated scale indicates the deflection of the coil



DYNAMOMETER TYPE MOVING COIL



DYNAMOMETER TYPE WATTMETER

The principle of operation of the electrodynamometer-type wattmeter is the same as that for dynamo-electric machines. The deflection torque is produced by the interaction of two magnetic fluxes. One of the

fluxes is produced by a fixed coil which carries a current proportional to the load current and therefore

called the current coil. The other flux is created by a movable coil which carries a current proportional to

the load voltage and thus called the voltage or potential coil. A high non-inductive resistance is connected

to the potential coil so that its current is almost in phase with the load voltage.

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