03dee1012 Topic 2_part 1

TOPIC 2:

DC AND AC METERS

PART 1

DEE 1012

1

2.1 KNOW THE BASIC PRINCIPLE OF DC

AND AC METER

2

Outcomes

Draw the construction of Permanent Magnet Moving Coil

(PMMC).

Describe the basic construction and operating principle of

PMMC.

Draw basic configuration of PMMC AC meter

moving Coil

(Electrodynamometer) moving Iron

Describe basic principle of PMMC for AC meter

3

INTRODUCTION

In electrical analog instruments, uses different types of

principle such as

PMMC, Moving Iron, Electrodynamometer, Hot Wire,

Thermocouple, Induction Type and Electrostatic.

PMMC type can be used for dc measurement only

Induction type for ac measurement only

The others types can be used for both

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Indicating instrument

Electro-

static

Induction Moving coil

Permanent Magnet Dynamo-

meter

( Fix coil)

Moving Iron

Repulsion type Attraction

type

Hotwire Thermo

couple

5

Indicating instrument

Moving coil

Permanent Magnet

Can work in DC only

Dynamo-meter

( Fix coil)

Can work in DC and AC

Moving Iron

Repulsion type


Attraction type


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Introduction

There are two types of moving coil instruments:

1. Permanent magnet type (PMMC)

2. Dynamometer type

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INTRODUCTION

A permanent magnet moving coil (PMMC) is one such

instrument which is popularly used and has various

applications

Moving Coil Instruments are used for measuring DC

quantities but can be used on AC systems when fed

through bridge rectifiers. For example in construction of

analog multimeter.

PMMC Darsonval meter movement

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The construction of Permanent Magnet

Moving Coil (PMMC)

9

Cross section view Top view

The Permanent-Magnetic Moving-Coil Movement

Used In A Meter

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Principle operation of PMMC

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Principle operation of PMMC

1. When a current flow in the coil, it generates a magnetic field which is proportional to the current.

2. The electromagnetic action of the current in the coil and the magnetic field will produced the deflecting torque (causes the coil rotate)

3. The pointer deflections are directly proportional to the current passing through the coil.

4. The controlling torque is provided by spiral springs. These springs serve as a flexible connection to the coil conductors.

5. Damping torque is caused by the eddy current set up in the aluminum coil which prevents the oscillation of the coil.

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The PMMC has a variety of uses such as ammeter,

voltmeter, ohmmeter and galvanometer

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PMMC Instrument

Deflecting System Controlling System Damping System

Permanent

MagnetMoving Coil Iron Core

Control

Springs

Eddy

Currents

Developed

in the

Aluminium

former

Of enameled

copper wire of thin

cross section and

about 50-100 turns

Wound on a

rectangular aluminium

former which is pivoted

on the spindle

Free to rotate in

the gap of core

and permanent

magnet

has

Consists of Consists of Consists of

is are

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Deflecting System This system provides the deflecting

torque proportional to the quantity to be measured and moves

the pointer from its zero position when a current flows.

Controlling System The controlling force is equal and

opposite to the deflecting torque in order to make the deflection

of the pointer proportional to the magnitude of the quantity to

be measured. The force also brings the pointer back to zero

position when the force is removed.

Damping System provides the damping torque so that the

pointer quickly comes to the final steady state position without

any swing or oscillations.

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Deflecting & Controlling torque

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Damping torque

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Damping Curve 20

Under damp

-The pointer will oscillate in long time

before it stop. User difficult to read the

measured value accurately.

Over damp

-The pointer will move slowly to final

value. The measured value will less than

expected value and not accurate.

Critical damp

-The pointer will stop at absolute value

in short time without resonant. The

measured value accurate and fast.

TORQUE EQUATION

The Deflecting or Operating Torque (Td) is produced by

effect such as Magnetic.

The controlling or restoring Torque (Tc) oppose the Td and

increase with the deflection of the moving system.

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At rest Td = Tc

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deflectionAngular =

constant spring =K

KTc

BANITd

AcoilmovingtheinCurrentI

mwxlareacoileffectiveA

coiltheofturnsofnumberN

Tm

WbgapairindensityfluxB

NmTorquedeflectingTdWhere

2)(

/2

1m = 100cm

10mm = 1cm

1m=1000mm

Example 1

A PMMC instrument with a 100 turns coil has a magnetic

flux density in its air gaps of B=0.2 T. The coil dimension

are w=1 cm and l=1.5 cm. Calculate the torque on the

coil for a current of 1 mA.

= =

0.2 1 1

100 2.5

1

100 100 1103

23

NmNm

TBANId

T

3/6103

310110021012105.12.0

Example 2

A PMMC meter with a 200 turns coil having flux density

force as much as 0.5 T. Coil diameter is 1mm and length

is 2.5cm. Calculate Deflecting torque when current

through coil is 2mA

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B = 0.5 T

A = (1 X 10-3m) X(2.5 X 10-2 m) = 2.5 X 10-5 m2

N = 200

I = 2 X 10-3 A

Td = BANI

= 0.5 (2.5 X 10-5)(200)(2 X 10-3 )

= 5 X 10-6 Nm

= 5Nm

Electrodynamometer

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PMMC AC meter

moving coil((Electrodynamometer))

The electrodynamometer movement has the same

basic operating principle as PMMC meter movement,

except that the permanent magnet is replaced by fixed

coils.- (air core electromagnet)

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Basic configuration of PMMC AC meter


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Basic configuration of PMMC AC meter


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Hair spring

Fix Coil

Produce the necessary field that required by the pointer to

deflect from the initial place

Moving Coil

Same as the fixed coil unless the facts that is will rotating

during the flowing of the current that flow inside it

Pointer

The moving coil is mounted on an aluminum spindle .

It consist of pointer and counter weight

The suspension used in cased of high accuracy desired

Controlling torque is provided by the hairspring

The hairspring is the element that will push the pointer to

its origin

Hair spring

Introduction

Electrodynamometer meter movements use stationary

coil (fix coil) and moving coils to develop interacting

magnetic fields (that is the electrodynamometer uses two

electromagnetic fields in its operation)

One field is created by the current flowing through a pair

of series-connected stationary coils.

The other field is caused by current flowing through a

movable coil that is attached to the pivot shaft.

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If the current in the coils are in the correct directions, the

pointer rotates clockwise.

The rotational torque on the movable coil is caused by

the opposing magnetic forces of the three coils.

They respond to alternating current because the a.c.

reverses direction simultaneously in all three coil and

also can operates on direct current.

These principle are used in wattmeter.

Electrodynamometer meters have low sensitivity and high

accuracy

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Operating principle

1. The operating principle is the interaction between the

currents in the moving coil, mounted on a shaft, and the

fixed coils.

2. That is, the deflecting torque is produced by the reaction between the magnetic field set up by the current in the moving coils and the magnetic field set up by current in the fixed coil.

3. When the two coils are energized, their magnetic fields will interact as a result of mechanical force exists between the coils and the resulting torque will tend to rotate the moving coil and cause the pointer attached to it to move over the scale.

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4) Since there is no iron, the field strength is proportional

to the current in the fixed coil and therefore, the

deflecting torque is proportional to the product of the

currents in the fixed coils and the moving coil.

The force of attraction or repulsion between the fixed and

moving coils is directly proportional to the product of

ampere turns of fixed coils and the moving coils

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Deflecting Torque

The controlling torque is produced by two control springs,

which also act as leads to the moving coil

This system provides for air damping

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Control System

Damping System

Application of Electrodynamometer

instrument

Electrodynamometer of wattmeter

Measurement of Power

Electrodynamometer of Ammeter

Measurement of Current

Electrodynamometer of Voltmeter

Measurement of voltmeter

Moving iron instrument

MOVING IRON INSTRUMENTS

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,

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This type of instrument is principally used for the

measurement of alternating currents and voltages, though

it can also be used for D.C measurements but is then

liable to small errors due to remanent magnetism in the

iron

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moving iron instruments

Attraction type

Repulsion type

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Attraction Type- singleIron

The basic working principle of attraction type moving

iron instruments is illustrated in fig below

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In this system, when current flows through the coil, a

magnetic field is produced at its centre.

A soft iron rod fixed to the spindle becomes magnetized

and is pulled inside the coil, the force of attraction being

proportional to the strength of the field inside the coil,

which again is proportional to the strength of the current.

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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.

Working Principle

The deflecting torque is due to the force of attraction

between the field of the coil and the iron disc.

The magnetization of the iron disc is proportional to the

field strength H.

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Deflecting Torque

The force F pulling the disc inwards is proportional to the

magnetization M of disc and field strength H.

Deflecting torque (Td) MH

But M H

H I

: Td I2

Thus, the deflecting torque is proportional to the square of

the current passing through the coil.

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The controlling torque is achieved by gravity control, but

now spring control is used almost universally.

In these instrument, the damping of the moving system is

obtained by air damping, in which a light aluminum

piston moves freely inside the curved cylinder closed at

one end.

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Controlling Torque

Damping Torque

Repulsion Type - DoubleIron

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(Rod A)

(Rod B)

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(Rod A)

(Rod B)

It consists of a fixed coil inside which two soft iron and

are arranged parallel to one another and along the axis

of the coil

Repulsion Type

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(Rod A)

(Rod B)

One of these rods A, is fixed to the coil frame, while the

other rod B is moving and is mounted on the spindle.

Repulsion Type

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(Rod A)

(Rod B)

The moving rod carries a pointer which moves over a

calibrated scale.

Repulsion Type

In this type of movement, the coil which receives the

current to be measured is stationary.

The field set up by the coil magnetizes two iron vanes,

which then becomes temporary magnets.

Since the same field magnetizes both vanes, both vanes

have the same magnetizes polarity.

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Consequently, there is a force of repulsion between the

two vanes. One of the vanes (stationary vane) is attached

to the coil form.

The other vane (the moving vane) is mounted on the pivot

shaft to which the meter pointer is attached.

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Thus, the magnetic force of repulsion forces the moving vane away from the stationary vane.

Of course, this force is offset by the counter torque of the spiral springs attached to the pivot shaft.

The greater the current through the coil in, the strength the magnetic repelling force; thus, the farther the moving vane rotates and the more current the pointer indicates.

The iron vane meter movement can operate on either a.c or d.c

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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.

The force of repulsion is approximately proportional to

the square of the current passing through the coil.

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The deflecting torque results due to the repulsion between

the two similarly magnetized (charged) soft iron rods.

Instantaneous torque repulsive force and repulsive

force to the product of pole strengths M1 and M2 of two

vanes.

Pole strengths are magnetizing force H of the coil and

H current passing through the coil

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Deflecting Torque

Therefore, the instantaneous torque, which is the

deflecting torque, is given as

Instantaneous torque I2

i.e. Td I2

Hence, deflecting torque is proportional to the square of

the current when used in an A.C circuit; the instrument

reads the r.m.s value of the electrical quantity

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In this type of instrument, controlling torque is obtained

either with a spring or by gravity.

In these case , spring has been used for the controlling

torque.

In this type of instrument, pneumatic type damping is

used.

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Controlling Torque

Damping Torque

Application of moving iron meter

Moving iron instruments are used as Voltmeter and

Ammeter only.

Both can work on AC as well as on DC.

03dee1012 Topic 2_part 1

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