electrical testing - part 1

8/10/2019 Electrical Testing - Part 1

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Course Outlines

1. Measurements and Testing

2. D.C Motors

3. Transformers

4. Transmission lines

5. Synchronous Alternators

6. Induction Motors


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Measurements

and Testing


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Why measuring and testing

Standards for testing

Types of testing

Testing procedures

Some measuring instruments


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The deterioration of electrical equipment

is normal and begins as soon as the

equipment is installed.

If deterioration is not checked, it can

cause electrical failures and malfunctions.

The purpose of an electrical testing and

measurements is to provide means for

such failures and malfunctions and

discovering hazards that can cause

failure of equipment or interruption of

electrical service.


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Testing of electrical equipment should

consist of conducting routine tests

inspections, repairs of electrical power

system apparatus such as generators,

transformers, circuit breakers, cables,

load motors, switchgear assemblies

along with associated equipment

comprised of control wiring, protective

devices and relays, supervisory

equipment and indicating and metering

instruments.


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Why measuring and testing?

Measuring and monitoring electrical

quantities enable the following :

Optimizing the fault prevention.

Scheduling the maintenance.

owing to early identification of

problems that results in greater

protection.

This is done not only of the plants but

also of the objects connected to them.


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An efficient system of measuring and

monitoring electrical quantities is important

for ensuring the success of all initiatives

that require :

Energy costs to be contained.

Quality energy supplies.

Continuity of service of the plants.

Achieving the above objectives requires the

activities to be implemented that are set out

in the following flow chart.


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Functions / Objectives of Electrical Measurements

Reducing

Energy Costs

Energy

Quality

Continuity of

Services

Functions :

Sub-metering

consumptions

and dividing

costs.

Monitoring load

patterns.

Managing

peaks.

Improving

power factor

corrections

Functions :

Analyzing

harmonics.Detecting over

voltages,

voltage

variations and

voltage gaps.Detecting

discharges from

steep transients.

Conformity of

supply.

Functions :

Control of plant

in real time.

Remote control.

Managing

alarms and

dividing costs. Preventive

maintenance and

maintenance in

the event of a

fault.


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Standards for testing :Legal standards are all those governing the

behavior of parties subject to the authority

of the state, including the directives that

are normally enacted in national legislation

through legislative decrees.

Technical standards are all the prescriptions

on the basis of which the machines, devices,materials and plants have to be designed,

built and tested to ensure their operating

efficiency and safety.


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The technical standards can be

subdivided into :

1.the International Electro Technical

Commission (IEC)

2.the American National Standards

Institute (ANSI)

3.the Institute of Electrical and

Electronics Engineers (IEEE)

4.Occupational Safety and Health

Administration (OSHA)


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IEC standards

Three committees are specifically

responsible for measuring instrumentation.

TC85 - Measuring equipment for

electrical and electromagnetic quantities.

TC66 - Safety of measuring, control and

laboratory instruments.

TC13 - Electrical energy measurement,

tariff and load control.


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Reference Standards for Measuring

Instruments

Electrical

energy

measurement

, tariff andload control.

Safety of

measuring,

control and

laboratoryinstruments.

Measuring

equipment

for electrical

and electro-magnetic

quantities.

TC 13 TC 66 TC 85


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Types of testing :

Types of testing methods include the four

categories of tests as well as the tests

themselves.

Categories of tests :

There are four categories of tests for

electrical equipment namely :

1.Factory tests.

2.Acceptance tests.

3.Routine maintenance tests.

4.Special maintenance tests.


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Factory tests : such tests are

performed at the factory to prove that the

equipment was manufactured properly

and meets specific design parameters.

The acceptance tests :such tests

are usually run at 80% of the factory test

voltage values to help indicating the

equipment deterioration without being

destructive.


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Commissioning tests are also proof tests

usually performed on new equipment and

systems after installation and beforeenergization, they are run to determine :

Whether the equipment is in compliance

with specifications to establish

benchmarks for future tests.

To determine the equipment was installed

correctly and without damage.

To verify whether the equipment meets

its intended design operation and limits.


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Routine and special maintenancetests : such tests are run after the

equipment has been in service for acertain amount of time to determine the

degree of deterioration of physical

parameters as operating time increases

and usually being run at 60% of the factory

tests.

Routine tests are performed on a periodic

basis and special tests are performed ondefective equipment to help determine the

cause of a failure and/or the extent of the

damage.


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Types of testing methods :

The testing of electrical power system

equipment involves :

Checking the insulation adequacy.

Electrical properties.

Protection and control.

Equipment operation.

Other items as they relate to the

overall system.


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Some of these checks are accomplished

using :

De-energized component tests.

Instrumentation and relay operation.

Calibration tests.

Energized functional testing of controlcircuits.

Megger testing of power circuits.

Phase out testing of power circuits.

Service testing.


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Testing procedures :

The testing procedure for electrical

equipment includes the following steps as

a flow chart :

1.

Identify the equipment or appliances

requiring testing and record them in

the Electrical Testing Register.

2.

Ensure that a qualified technician has

calibrated the test equipment within

the last 12 months.


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3. Undertake risk assessments to

identify the required frequency of

testing according to standards and

record in the Electrical Testing

Register.

4. Set the testing timeframes.

5. A competent personas a contractor or

trained staff member is engaged to

conduct testing of the equipment and

appliances.


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6. Perform the desired testing for the

identified equipment or appliances.

7. Record testing information and results

in the Electrical Testing Register.


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Safety

Some of the experiments involve voltages

that could conceivably lead to serious

injury or death.

Therefore strict adherence to the following

rules will greatly decrease the probability

that accidents will occur.

However, no set of rules can replace basiccommon sense, and all persons using the

laboratory are encouraged to constantly

THINK SAFETY !


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1.Always assume all circuits are

energized unless you know with

certainty that they are not.

2.Use one hand to make connections.

3.Never work on electrical circuits

with wet or moist hands.

4.Do not play with equipment notdirectly involved in your

experiment.


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5. When in the lab do not wear clothing

or jewelry which could constitute a

health hazard, Shoes are preferablyto be rubber soled ones and must be

worn in the lab, also, long hair

presents a hazard near moving parts

of machinery.

6. It is important for safety reasons for

anyone to easily trace out your test

circuit and, therefore do not work on

a cluttered bench.


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7. Never touch moving parts of

machinery.

8. Think out ahead of time the

consequences of closing or opening

a switch.

9. Never alter an energized circuit

unless you are certain of the

outcome.

10. If you know or suspect that an

accident is about to occur, take

immediate steps to prevent it.


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Inspections and component testing

of main power system

The following components are part of a

typical main power system. These should

be inspected and tested in accordance

with information specified below.

The inspection can be considered as apart of pre-checks and/or the functional

performance tests.


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Some measuring instruments

1.Voltmeter


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Voltmeter

to be used

on panels

for voltage

measure


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2.Ammeter


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Direct insertion for Ammeters & Voltmeters


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Indirect insertion for Ammeters & Voltmeters


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3.Watt meter


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4


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4.P.F meter


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Portable P.F meter

P F C ti U it


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P.F Correction Unit


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5 S h


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5.Synchroscope


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S h i i U it


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Synchronizing Unit


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7.

Electrical Torque Meter Set


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Load Resistor :

Load resistor contains three ganged

resistors with continuous spindleregulation.

The resistors are connected to terminals

for 3-ph, single phase or DC voltage.

The current in the resistor is limited by

tubular wire fuses in each phase.

The unit has handles and wheels for simple

and quick movement and is enclosed in a

perforated metal cabinet with cooling fan.


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Load Reactor

Enclosed in a strong

metal cabinet.

The front panel has

mimic diagram,terminals, fuses and

electrical data.

The unit can be used on

1- and 3-phase systems.

It has 12 step regulation.


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Load Capacitor

Housed in a metal

cabinet.

Electrical data and

symbols on the front

panel with terminalsand fuses.

This unit can be used

on 1- and 3-phasesystems.

It has 6 step regulation.

D C Motor


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D.C Motor

Slip Ring Induction Motor


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Slip Ring Induction Motor


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Squirrel Cage Induction Motor

Synchronous Machine


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Synchronous Machine

Three phase Transformer


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Three phase Transformer

Load Switches with Selectors


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Load Switches with Selectors


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Laboratory Flexes with Safety Plugs


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Lab System


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D.C.

Motors

Overview


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Overview

Introduction

D.C motor theory

D.C motor construction

D.C motor types and classification

D.C motor operation and control

D.C motor testing


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D C Motor Theory


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D.C Motor Theory

The operation of a dc motor is based on the

following principle :

A current carrying conductor placed in a

magnetic field, perpendicular to the lines

of flux, tends to move in a direction

perpendicular to the magnetic lines of flux.

There is a definite relationship between

the direction of the magnetic field, thedirection of current in the conductor, and

the direction in which the conductor tends

to move.

This relationship is best explained by using


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This relationship is best explained by using

the left hand rule for motors.

Consider a coil in a magnetic field of flux

densityB

, when the two ends of the coil

are connected across a D.C voltage

source, currentIflows through it.

A force is exerted on the coil as a result of

the interaction of magnetic field and

electric current, this force on the two

sides of the coil is makes the coil starts to

move in the direction of force.


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Fixed Magnets

Fleming's Left Hand (Motor) Rule


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Fleming s Left Hand (Motor) Rule

N

S

Direction of Rotation

Fixed Magnetic Field Direction

ConventionalCurrent Direction

SUse the Left Hand Rule to DetermineA


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NS

N

SUse the Left Hand Rule to Determine

the Rotation Direction of the

Armatures in A and B

Notice that when the

current through the

armature is reversed,

it moves (Rotates) in

the opposite direction

A

B

Hint: You will have to turn your left

hand upside down for example A


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a D C motor rotates as a result of two


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a D.C motor rotates as a result of two

magnetic fields interacting with each

other.

The armature of a D.C motor acts like an

electromagnet when current flows

through its coils.

Since the armature is located within the

magnetic field of the field poles, these

two magnetic fields interact.

The D C motor has field poles that are


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The D.C motor has field poles that are

stationary and an armature that turns on

bearings in the space between the field

poles.

The armature of a D.C motor has windings

on it connected to commutator segments.

The armature rotation can be illustrated as

follows :


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Counter E.M.F


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While a D.C motor is running, it acts

somewhat like a D.C generator wherethere is a magnetic field from the field

poles, and a loop of wire is turning and

cutting this magnetic field.

As the loop sides cut the magnetic field, a

voltage is induced in them, the same as it

was in the loop sides of the dc generator.

This induced voltage causes current to

flow in the loop and called back E.M.F (Eb).

This counter E.M.F cannot be equal to or


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q

greater than the applied battery voltage;

if it were, the motor would not run.

The counter E.M.F is always a little less,

however, it opposes the applied voltage

enough to keep the armature current

from the battery to a fairly low value.

If there were no such counter E.M.F,

much more current would flow throughthe armature and the motor would run

much faster, however, there is no way to

avoid that counter E.M.F.

The equations of the back e.m.f are :


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The equations of the back e.m.f are :

A

P.60

N.Z.=Eb

aabR.I-V=E

N.Eb


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Motor Loads

Motors are used to turn mechanical

devices, such as water pumps, grinding

wheels, fan blades, and circular saws.

For example, when a motor is turning a

water pump, the water pump is the load.

So, the definition of a motor load is themechanical device that the motor must

move.

The mechanical load connected to a D.C


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motor affects many electrical quantities

such as the power drawn from the line, theamount of current, the speed, the motor

efficiency, etc, which are all partially

controlled by the size of the load.

The physical and electrical characteristics

of the motor must be matched to the

requirements of the load if the work is to

be done without the possibility of damage

to either the load or the motor.

D.C Motor Construction


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Body / YokeBrushBearing


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Field Winding

Shaft

CommutatorArmature

Pulley

Brush

holder

Field Core

End Housing

1.Yoke which is a steel frame providing


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mechanical rigidity and also providing a path of

low magnetic reluctance between poles.

2.Pole Cores which are steel cores aroundwhich field coils are wound with adjacent poles

alternate in polarity like NSNS etc.

3.Pole Shoe which is a part of the pole steelstructure and conforms to the curvature of the

armature core in order to provide a uniform air

gap length.

4.Field Coils which are coils of insulatedcopper wire to provide the m.m.f for the

magnetic field.


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5.Armature core which is a stack of steel


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laminations mounted on the shaft of the

machine and representing a major part of

the magnetic circuit.

6.Armature coils which is a collection of

copper wires in which voltages are induced

and on which forces are produced by

current, such copper conductors are

placed in the slots.

7.Slots which are rectangular openings

around the periphery of the armature core

into which armature conductors are placed.

8.Commutator which is a ring of copper


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segments surrounding the shaft and

insulated from each other by strips of mica

where the ends of armature conductors are

connected to commutator segments.

9.Brushes which are stationary rectangularcarbon and graphite blocks that make

electrical contact with the rotating

commutator for the purpose of completing

the current path from the external

terminals through the armature conductors

and return.

10.Brush-rig which is a mechanical


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g

assembly which holds the brushes in

place and provides the adjusting tensionof the springs which push the brushes

against the commutator.

11.End bellswhich are steel structures on

both ends of the machine to provide

support for the bearings and brush

rigging.


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D.C Motor Types &


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Classification

DC Machines can be classified according

to the electrical connections of the

armature winding and the field windings.

The different ways in which these

windings are connected lead to machines

operating with different characteristics.

The field winding can be either self

excited or separately excited.

Further, in self excited motors, the field


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, ,

winding can be connected either in series

or in parallel with the armature winding.

These different types of connections give

rise to very different types of machines

as displayed in the following diagram.


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DC motors


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1. Permanent Magnet DC Motor

2. Separately Excited DC Motor3. Self Excited DC Motor

a) Shunt Wound DC Motor

b) Series Wound DC Motor

c) Compound Wound DC Motor

i. Cumulative compound DC motor

Long shunt cumulative DC motor

Short shunt cumulative DC motorii.Differential compound DC motor

Long shunt differential DC motor

Short shunt differential DC motor

Permanent Magnet DC Motor


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The permanent magnet D.C motor

consists of an armature winding as in

case of an usual motor, but does not

necessarily contain the field windings.

The construction of such a type of D.C

motors contains a radially magnetized

permanent magnets mounted on theinner periphery of the stator core to

produce the field flux.

The rotor on the other hand has a


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conventional D.C armature with

commutator segments and brushes.

The diagrammatic representation of a

permanent magnet D.C motor is :

Since the flux density are chosen at the


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time of construction and cant be

changed there after which means that :

= constant

Accordingly :

Eb = K . . N = K. N

i.e : V a. Ra = K. N

aa

K

RI-V=N

Moreover, the torque equation of dc


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motor suggests :

Ta = Ka. . a = KT. a

In this case the torque of D.C motor can

only be changed by controlling the

armature supply.

As a result, torque-speed characteristiccan be drawn as :

or N


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TTs1 Ts2 Ts3 Ts4

4

3

2

1

V4

V3

V2

V1

Separately excited machines


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In case of a separately excited DC motor

the supply is given separately to the fieldand armature windings.

The armature and field winding are

electrically separate from each other.

The field winding is excited by a separate

DC source.

So the torque and speed can be varied by

varying field flux , independent of the

armature electric current a


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Since the flux density can be varied by

i th fi ld t hi h


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varying the field current which means

that :

f Vf

Accordingly :

Eb = K . . N . N

i.e : V a. Ra . N

RI-VN

aa



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motor suggests :

Ta = Ka . . a . a

In this case the torque and speed of D.C

motor can be changed by controlling

either the field or the armature supply.

As a result, torque-speed characteristiccan be drawn as :

or N


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TTst

oVa

Self Excited DC Motor


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In case of self excited DC motor, the field

winding is connected either in series or

in parallel or partly in series, partly in

parallel to the armature winding and can

be classified as :

Shunt wound DC motor.

Series wound DC motor.

Compound wound DC motor.

Shunt wound DC motor


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In case of a shunt wound DC motor or

more specifically shunt wound selfexcited DC motor, the field windings are

exposed to the entire terminal voltage as

they are connected in parallel to the

armature winding.

To understand the characteristic of such

a type of DC motors, consider the basicvoltage equation given by :

V = Eb+ a. Ra


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where V , Eb , a , Ra are the supply

voltage back e m f armature current and


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voltage, back e.m.f, armature current and

armature resistance respectively.

Since back e.m.f increases with flux

and angular speed :

f

V

Accordingly :

Eb = K . . N . N

i.e : V

a. Ra . N

RI-VN

aa



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motor suggests :

Ta = Ka . . a . a


motor can be changed by controlling

either the field or the armature supply.

The DC shunt motor is a constant speedmotor, as the speed does not vary with

the variation of mechanical output load.

or N


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T

Slope = Ra/(Ka.)2

V/ Ka.

Shunt motor TorqueSpeed characteristics


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N

a

FULL LOAD

0

Amps)

Speed - armature current characteristics

T


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a

0

Amps)

Torque - armature current characteristics

Series wound DC motor


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In case of a series wound DC motor or

more specifically series wound selfexcited DC motor, the field windings are

connected in series to the armature

winding.

To understand the characteristic of such

a type of DC motors, consider the basic

voltage equation given by :

V = Eb+ a( Ra+ Rs)


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where V , Eb, a, Ra, Rsare the supply

voltage, back e.m.f, armature current,


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voltage, back e.m.f, armature current,

armature resistance and field resistance

respectively.

Since back e.m.f increases with flux and

angular speed :

f aAccordingly :

Eb = K . . N a. N

i.e : V

a(Ra+Rs) a. N

a

saa

I

)R+(RI-VN



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motor suggests :

Ta = Ka. . a (a)2


motor can be changed by controlling the

armature supply.

The DC series motor has a variable speedand a high starting torque, therefore it is

suitable in the field of electric traction.


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N

a

0

Amps)


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Compound Wound DC Motor


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The compound excitation characteristic

in a DC motor can be obtained by

combining the operational characteristic

of both the shunt and series excited DC

motors.

The compound DC motor essentially

contains the field winding connectedboth in series and in parallel to the

armature winding.


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The excitation of compound DC motor can

b f t t d di th t f


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be of two types depending on the nature of

compounding :

Cumulative Compound DC Motor

When the shunt field flux assists themain field flux, produced by the main

field connected in series to the

armature winding then its called

cumulative compound DC motor.

Differential Compound DC Motor


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In case of a differentially compounded

self excited DC motor, the arrangementof shunt and series winding is such that

the field flux produced by the shunt

field winding diminishes the effect offlux by the main series field winding.

The net flux produced in this case is

less than the original flux and hence

does not find much of a practical

application.


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E

R

a

I

L

V

I

a

I

SE

+

_

I

Sh

R

sh

SUPPLY

or N


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Compound motor TorqueSpeed characteristics

T

Differential

Compound

Cumulative

Compound

N


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0

I

a

Amps)

Differential

Compound

Shunt

Cumulative

Compound



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Both the cumulative compound and

differential compound DC motors can


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differential compound DC motors can

either be of short shunt or long shunt type

depending on the nature of arrangement.

Short Shunt DC Motor

If the shunt field winding is only parallel

to the armature winding and not the

series field winding then its known as

short shunt DC motor or more

specifically short shunt type compound

DC motor.

Long Shunt DC Motor


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If the shunt field winding is parallel to

both the armature winding and the

series field winding then itsknown as

long shunt type compounded DCmotor.


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electrical testing - part 1

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