Chapter 2-1 Dc Motors Compatibility Mode

8/11/2019 Chapter 2-1 Dc Motors Compatibility Mode

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ELECTRICAL MACHINES (DC Motors)

Contents

Introduction to Electrical & Direct Current Machines

Constructional Details of DC Machines Principle of Operation of DC Machines Types of DC Motor Power Flow Diagram Characteristics of DC Motors Armature Reaction Speed Control of DC Motors Applications

1

Chapter 2-1 (201405)


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Learning Outcomes

Upon completion of chapter-2 you should be able to:

State the principle by which machines convertmechanical energy to electrical energy and vice versa.

Discuss the operating differences between differenttypes of dc machines.

Explain the characteristics of dc machines.

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Introduction

Energy Conversion

Energy exists in many forms.

One form of energy can be obtained from the other

form with the help of converters.

Light bulbs and heaters require energy in electrical

form.

Electrical Machines:

Converters that translate an electrical input to a

mechanical output or vice versa are called the electric

machines.This process of translation is electromechanical energy

conversion.

The magnetic system acts as the link between the

electrical and mechanical systems


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4

Introduction . . .

Electromagnetic phenomena:Electrical machines use the following electromagnetic

phenomena for their electromechanical energy

conversion:

Whenever the field in the vicinity of a conductor changes

(or) flux linking a conductor changes, an emf is induced inthat conductor (Faradays Law).

Whenever a current-carrying conductor is placed in a

magnetic field, the conductor experiences a mechanical

force.


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Motor action


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Introduction . . . .

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Energy flow diagram

Gen Transformer

(step-up)Transmission

LineTransformer(step-down)

Distribution /Utilization: Loads could beMotors,Lighting,

Heaters, Coolers,

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Over view of DC Machines

Direct-current (DC) machines are divided into dc

generators and dc motors.DC Generator

A dc generator is a machine that converts mechanical

energy into electrical energy (dc voltage & current) by

using the principle of magnetic induction.

DC generators are not as common as they used to be,

because dc, when required, is mainly produced by

electronic rectifiers.

1010

A dc motor is a machine that converts electrical energy

into mechanical energy by supplying a dc power

(voltage and current).

DC motors are widely used in many applications.

DC motors are everywhere! (In house, office, )

DC Motor


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Over view of DC Machines . . .

1111


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CONSTRUCTIONAL DETAILS OF DC MACHINES

Stator: Field Poles (minimum 2) - magnetic flux Rotor : Armature - carries armature winding

Air gap (between poles and armature)

Commutator (ac to dc)

Carbon brushes (collects and carry current from

the commutator)

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Rotor of a DC machine

Tooth

Slot

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http://www.youtube.com/watch?v=Q4FlUP-kJe8


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Constructional details of dc machines . . .

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The ends of the windings are connected to the

commutator segments (built in copper and are very goodconductors).

Carbon brushes are placed over

commutator segments and serve as

leads for the electric connection.

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ARMATURE (Rotor)


The entire assembly of iron core, commutator, andwindings is called the armature.

The commutator is connected to the slotted iron core.

The windings of armatures are connected in differentways depending on the requirements of the machine.

More turns of conductor = higher rectified voltage 21


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There are two types of armature winding: Lap windingand Wave winding.

Lap Wound Armatures

are used in machines designed for low voltage and highcurrent

armatures are constructed with large wire because ofhigh current

Number of parallel paths = number of poles

ARMATURE . . .


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Wave Wound Armatures

are used in machines designed for high voltage and lowcurrent

are used in the small generator

No of parallel paths = 2

ARMATURE . . .


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Field winding


Most DC machines use electromagnets to provide themagnetic field.

Two types of field windings are used :

series field winding

shunt field winding

Series field windings

are so named because they are connected in series withthe armatureare made with relatively few turns of very large wire(sufficiently large to carry the current) and have a verylow resistance.

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Shunt field windingsHave relatively many turns of small wire, thus, it has amuch higher resistance than the series field.

is intended to be connected in parallel with, or shunt,the armature.

high resistance is used to limit current flow through thefield.

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When a DC machine uses both series and shunt fields,each pole piece will contain both windings.

The windings are wound on the pole pieces in such amanner that when current flows through the winding itwill produce alternate magnetic polarities.

Factors affecting the machine output

Speed

Field strength

No. of turns in the windings


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http://www.youtube.com/

watch?v=Ue6S8L4On-Y

http://www.youtube.com/wat

ch?v=0ajvcdfC65w

http://www.youtube.com/watch?v=M

FGqf6AfDB0


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DC motor principles . . .

The Advantages

The greatest advantage of DC motors may be speed

control.

Today, adjustable frequency drives can provideprecise speed control for AC motors, but they do so at

the expense of power quality, as the solid-state

switching devices in the drives produce a rich

harmonic spectrum. The DC motor has no adverse

effects on power quality.


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Power supply, initial cost, and maintenancerequirements are the drawbacks associated with DCmotors

Rectification must be provided for any DC motorssupplied from the grid. It can also cause power qualityproblems.

The construction of a DC motor is considerably morecomplicated and expensive than that of an AC motor,primarily due to the commutator, brushes, andarmature windings.

The drawbacks

DC motor principles . . .


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DC motor principle of operation . . .

All motors rely upon the force exerted by a magnetic fieldon a current-carrying conductor.

If a straight current carrying conductor is placed at rightangles to the uniform magnetic field existing between theNorth and south poles of a permanent magnet, the result

is shown in Fig.a. Two fields are present: the uniform field due to the

magnet with lines of force that are straight and parallel,and the circular field around the current-carryingconductor, shown dotted.

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As the lines of force above the conductor in Fig.a point in

the same direction, they add together, and as the lines offorce below the conductor oppose each other, theysubtract. The resultant magnetic field is shown in fig. b.

Because the field is strong above the conductor andweak below the conductor, the distorted lines of forcetend to straighten like stretched elastic bands.

A force is thus exerted on the conductor, tending tomove it down, as indicated by the arrow.

If it were free to move, the conductor would leave themagnetic field.

If the current is reversed through the conductor, thecircular field around the conductor will also reverse.Hence, the conductor will tend to move in the oppositedirection, i.e., upwards.

Similarly, if the polarity of the main magnetic field is

reversed, the direction of conductor motion will change. 34


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N

S

Into the plane is denoted by a cross (X)and out of the plane is denoted by a dot (.)


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Torque

It is the turning or twisting force about an axis.

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sin

Force

ont

h

e

conductor

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Counter emf(Back emf) in dc motors

0 60E Zn P A

When a dc supply is connected to the dc motor, a

large current will flow through the armature conductors

because its resistance is very low.

Each current carrying conductor experiences a force

(because they are immersed in the magnetic field).

These forces add up to produce a powerful torque,causing the armature to rotate.

As soon as the armature begins to turn, a 2nd

phenomenon takes place: the generator effect. With

the armature rotating in the magnetic field, the

armature conductors generate an emf. This generated (induced) emf is proportional to the

speed of rotation of the motor and the flux per pole,

and is as follows:


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0SI E E R

Counter emf(Back emf) in dc motors . . .

Where, Z = total number of armature conductors

= effective flux per pole (Wb)n = speed of rotation (rpm)

P = no. of poles

A = no. of parallel paths

The generated voltage opposes the supply voltage, thuslimiting the armature current.

In case of a motor, the induced voltage, E0 is called

counter emf because it opposes the source voltage.

The armature current is given by

where ES = line supply voltage & E0 = counter

(generated) emf.


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Counter emf(Back emf) in dc motors . . .

When the motor is at rest, the counter emf (cemf) is zero

and so the starting current is given by: I = (ES-0) / R.

As the speed increases, the cemf increases, with the

result that the value of armature current diminishes.

When a motor runs at no-load, the counter-emf must be

slightly less than ES, so as to enable a small current to

flow, sufficient to produce the required torque.


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Mechanical power and torque

The electrical power supplied to the armature, which is

converted to mechanical power (mechanical powerdeveloped), is

where, P =mechanical power developed by the motor (W)

E0= induced voltage in the armature (cemf) (V)

I = total current supplied to the armature (A)

The mechanical power P is also given by the expression,

where n is the speed of rotation.

Combining the above two equations for P,

0P E I

9.55P T nT

09.55 6.28PZ InT E I T A


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where, T = torque developed (N-m)

Z = total number of armature conductors

F = effective flux per pole (Wb)

I = armature current (A)

6.28 = constant, to take care of units (=2)

Mechanical power and torque . . .

09.55 6.28P

Z InT E I T A

Speed of rotation

0 60E Zn P A

0, 60 ( )Speed n E A Z P

Chapter 2-1 Dc Motors Compatibility Mode

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