Electrical Machines IWeek 1: Overview, Construction and EMF equation

Course Contents

Definition of the magnetic terms, magnetic materials and the

B-H curve.

Magnetic circuits principles.

Electromechanical Energy Conversion Principles.

Force and torque equations in magnetic circuits.

Construction of a DC machine.

EMF and torque equations in dc machines.

Armature windings and commutator design.

Armature reaction and compensation techniques.

Self excitation of dc generators.

External characteristics of dc generators.

Kinds of losses and efficiency of dc machines.

Torque and speed characteristics of dc motors.

Speed control of dc motors.

Starting of dc motors.

DC Motor electrical braking techniques.

Electrical Machines

I

Study

Understand

Lab

work

ReportsExamples

Ask

Read

Course Work:

� Course work:

1- Every week assignment (solve questions related to the lecture): to behanded in every week

for points

2- points are transformed to marks if you are consistent in delivering your reports

3- NO late submission are allowed

� Lab reports:

1- Contribute to almost 10 marks – related to your physical presence in lab

� Quizes:

1- 7th, 12th, ….etc.

� Final

Its not about marks

in tests.. Its about

continuously working

hard all semester!

Introduction

Machines are called

� AC machines (generators or motors) if the electrical system is AC.

� DC machines (generators or motors) if the electrical system is DC.

Ele

ctr

ical M

achin

es

DC machines

Motor

Generator

AC MachinesTransformers

Induction motor

Synchronous generator

Special Machines

Faraday's Law

Direct Current (DC) Machines Fundamentals

� Generator action: An emf (voltage) is induced in a conductor if it

moves through a magnetic field.

� Motor action: A force is induced in a conductor that has a current

going through it and placed in a magnetic field.

Any DC machine can act either as a generator or

as a motor. Not all machines have this feature

except for the DC machine

Lets formulate this in a more “scientific way”

Applications of DC Motors:

1. D.C Shunt Motors: It is a constant speed motor. Where the speed is required to remain almost constant

from no-load to full load. Where the load has to be driven at a number of speeds and any

one of which is nearly constant.

• Lathes

• Drills

• Boring mills

• Shapers

• Spinning and Weaving machines.

2. D.C Series motor:It is a variable speed motor. The speed is low at high torque. At light or no load ,the

motor speed attains dangerously high speed.

• Electric traction

• Cranes

• Elevators

• Air compressor

• Vacuum cleaner

• Hair drier

• Sewing machine

LETS BRAIN STORM!!!!

WHAT DO YOU THINK IS

INSIDE THE MACHINE????

Construction of DC machine

Rotor: rotating part of the

machine

Stator: Stationary part of the machine

Two electrical circuits present in

the dc machine:

1- Field circuit

2- Armature circuit

Stator

1- Stator:

Frame: provides physical support

Poles: projects

inwards and

provides a

path for the

magnetic flux

Poles: the end of the poles

that are close to rotor

“spread out” over the rotor

surface to distribute flux

evenly over the rotor

surface. We call the end as

“pole shoe”. Due to their

spread out they are often

called Salient Poles.

Field windings: windings responsible for

magnetic flux production

Air gap

Air gapAir gap

Inter Poles: located

between poles and

used to overcome

armature reaction

THE STATOR COULD BE

LAMINATED OR MADE OF

SINGLE CAST PIECE OF

METAL

Armature

2- Rotor: Rotating part of machineRotor of dc machine is often called “armature” as it holds the armature windings

THE ROTOR IS COMPOSED OF MANY LAMINATIONS

STAMPED FROM A STEEL PLATE.

Commutator: built on the shaft of the rotor at one end of

the core. Made of copper bars insulated by mica (ورنیش ). Mica is very hard and is harder than the commutator

material itself for good sticking. Serves as a “mechanical

rectifier”.

Brushes: made of carbon, graphite or a mixture

of both. They have high CONDUCTIVITY and low

friction coefficient to reduce the wear but they

are softer than commutator to avoid

commutator wear. It is very much affected by

the current flowing in them and how they are

adjusted.

Armature winding: carries current crossing the

field, thus creating shaft torque in a rotating

machine or force in a linear machine as well as

generate an electromotive force (EMF). Some

call it “The power-producing component” of an

alternator, generator, dynamo or motor.

Faraday Laws

1- If a flux passes through a turn of coil of a wire, a voltage will be

induced in the turn of wire that is directly proportional to the rate of

change in flux with respect to time.

tN

∆

∆Φ−=e

e= average emf (V)

N= number of turns

ф = flux passing through the turn

t= time

-ve sign is an expression of Lenz’s law: The direction of

the voltage buildup in a coil is such that if the coil end

were short cct, it would produce current that would

cause a flux opposing the original flux changeф�

Opposing flux

I

e

+

-

If a flux is increasing in strength, then the voltage

built up in the coil will tend to establish a flux that

will oppose the increase

HOW CAN MAGNETIC FIELD AFFECT THE

SURROUNDING

في حاله وجود ملف في مجال مغناطیسي،سیالحظ وجود فرق جھد حثي علي اطراف الملف و ھذا الجھد سیؤدي

لتولید مجال اخر عكس اتجاه المجال االساسي

FLUX ALREADY

PRESENT

FLUX CREATED BY

EMF

Faraday Laws HOW CAN MAGNETIC FIELD AFFECT THE SURROUNDING

2- Magnetic field induces a force on a current carrying wire within the

field.

�

iL

XX

XX

XX

XX

XX

XX

XX

XX

�B= magnetic flux density

(wb/m2)

i= current (A)

F= force induced (N)

L= length of conductor (m)

Force direction is

given by the left-hand rule

� � ��B sin Θ

Field into

the page

� � �� X B)

Θ = angle between the

wire and the flux density

vector

The induction of a force in a wire by a current in the presence of a magnetic field is the basis of the

motor action.

+ تیار : بالعربي كدهقوة لتحريك = مجال

الملف

MOTOR

ACTION

Faraday Laws HOW CAN MAGNETIC FIELD AFFECT THE SURROUNDING

3- If a wire moves through magnetic field, a voltage is induced in it

= velocity of wire

B= magnetic flux density

(wb/m2)

L= length of conductor (m)

e= voltage induced

Force direction is given by the right-hand rule

� � X B) . L

L

XX

XX

XX

XX

XX

XX

XX

XX

�+

-

e

+ ++

- --

• A potential difference is maintained across the conductor as long as there is motion through

the field

• If motion is reversed, polarity of potential difference is also reversed

The induction of voltages in a wire moving in a magnetic field is the fundamental aspect of operation

of all types of generators. That’s why it is called generator action

: بالعربي كده= مجال + حركة

EMF

Force direction is

given by the right-hand rule

GENERATOR

ACTION

The EMF equation :

Let,

ф= flux per pole in weber

Z = Total number of conductor

P = Number of poles

a = Number of parallel paths: This describes the way the machine's

armature conductors are connected relative to each other and to the

number of poles. The two basic ways of connecting these conductors are

called 'lap' and 'wave', but it gets more complicated.

n =armature speed in rpm

e = emf generated in any on of the parallel path

a=P

(lap)

a=2

(wave)

Assume one

coil only now

One coil = 2 conductor

EMF is induced in the conductor according to Faraday's law.

The average value of e.m.f. induced in each armature conductor is,

� � −�ф

��

Consider one revolution of conductor. In one revolution, conductor will cut total

flux produced by all the poles i.e. ф * P. ( األقطاب كل من طالع اللي المجال كل )

• The time required to complete one revolution is 60/n seconds as speed is n

r.p.m. Hint: rpm (revolutions per minute)

The EMF equation :

n (rev) 1 min * 60 (sec)

1 rev ????? (sec)

����� ��

�

= ф P �

��

Now the conductors in one parallel path are always in

series. There are total Z conductor with a parallel paths,

hence Z/a number of conductors are always in series and

e.m.f. remains same across all the parallel paths.

EMF produced by

one conductor

������ �ф P �

��x �

�

Total EMF produced

by armature

conductors

P, Z, a: design parameters

N, ф: control parameters

������ ∝k ф nEMF is

proportional to the

field and speed of

rotation

MOST IMPORTANT

EQUATION IN DC

GENERATORS

� � − !ф

!"= 1 * change of flux / time

Numerator= !#

denomenator=!"

Types of dc motor and generator:

• Separately excited dc

motor

• Shunt dc motor

• Permanent magnet dc

motor

• Series dc motor

• Compound dc motor

• Separately excited dc

generator

• Shunt dc generator

• Series dc generator

• Compound dc generator

MOTOR GENERATOR

1. Separately Excited: Field and armature windings are either connected separate.

2. Shunt: Field and armature windings are either connected in parallel.

3. Series: Field and armature windings are connected in series.

4. Compound: Has both shunt and series field so it combines features of series and shunt motors.

Questions

• Explain and describe using drawings the construction of dc machine

• What is the function of the following in dc machines:

a- armature winding

b- field winding

• Explain how dc machines can work as generator and motor

• State some applications and types of connections of dc machines

(generator and motor)

• Derive the EMF equation for dc machines