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COURSE:
INTRODUCTION TO
ELECTRICAL MACHINES
Prof Elisete Ternes Pereira
SYNOPSIS

a)
b)
c)
d)
e)
f)
g)
Introducing the Basic types of Electric
Machines
A.C. Motors
Induction and Synchronous Motors
Ideal and Practical Transformers
D.C. Motors and Generators
Self and Separately Exited Motors
Stepper Motors and their characteristics
Assessment of Electric Motors
i.
ii.
iii.
iv.
v.
vi.
vii.
viii.
Efficiency
Energy losses
Motor load analysis
Energy efficiency opportunity analysis
Improve power quality
Rewinding
Power factor
Speed control
CONTENTS



1. INTRODUCTION:
Types of electric motors,
 Assessment of electric motors,
 Energy efficiency opportunities.
Classification of motors,
 AC
 DC
 Stepper motors
 Transformers
CONTENTS


2. AC MOTORS
AC motors
 Synchronous motors,
 Induction motors,
 Single and three phase motors.

AC motor components, its characteristics.

Torque-Current characteristics slip speed equation.

Assessments of electrical motors,
 Efficiency-load characteristics,
 Load equation
CONTENTS



3. DC MOTORS
DC motors
 Separately and self-excited series, shubt, hybrid motors
and their components.
Use of DC motors and its characteristics,




Speed control,
Service life,
Cost/benefit ration,
Torque equation,

Torque characteristics of series, shunt and hybrid – self excited
motors.
CONTENTS



4. GENERATOR AND TRANSFORMERS
Generators
 Types and characteristics
Transformers
 Types and characteristics,
CONTENTS


5. ENERGY ANALYSIS AND ASSESSMENT
Energy and power measurement,
 Energy efficiency,
 Proper utilization of electrical motors,

Improve power quality

Rewinding of motors.

Service and maintenance of electric motors.
INTRODUCTION TO
ELECTRICAL MACHINES
1
INTRODUCTION
TO
ELECTRICAL MACHINES

Essentially all electric energy is generated in a rotating machine: the
synchronous generator, and most of it is consumed by: electric
motors.
 In many ways, the world’s entire technology is based on these
devices.

The study of the behavior of electric machines is based on three
fundamental principles:
 Ampere’s law,
 Faraday’s law and
 Newton’s Law.
INTRODUCTION

TO
ELECTRICAL MACHINES
Various configurations result and are classified generally by the type
of electrical system to which the machine is connected:

direct current (dc) machines or

alternating current (ac) machines.
INTRODUCTION

TO
ELECTRICAL MACHINES
Machines with a dc supply are further divided into permanent
magnet and wound field types, as shown in Figure 4.1.
INTRODUCTION

ELECTRICAL MACHINES
The wound motors are further classified according to the
connections used:

The field and armature may have separate sources



(separately excited),
they may be connected in parallel

(shunt connected), or
they may be series


TO
(series connected).
See figure 4.2.
INTRODUCTION



TO
ELECTRICAL MACHINES
AC machines are usually
 single-phase or
 three-phase machines
and may be
 synchronous or
 asynchronous.
See figure next page.
INTRODUCTION

TO
ELECTRICAL MACHINES
Several variations are shown in Figure 4.2.
1.1 - BASIC ELECTROMAGNETIC LAWS:
AMPERE`S LAW AND FARADAY`S LAW

The two principles that describe the electromagnetic
behavior of electric machines are Ampere’s Law and
Faraday’s Law.


These are two of Maxwell’s equations.
Most electric machines operate by attraction or repulsion of
electromagnets and/or permanent magnets.
AMPERE`S LAW


Ampere’s law describes the magnetic field that can be produced
by currents or magnets.
In an electric machine, there will always be at least one set of
coils with currents.

A motor cannot be produced with permanent magnets alone.

 H  d  I enclosed
AMPERE`S LAW


Ampere’s law states that the line integral of the component of the
magnetic field along the path of integration is equal to the current
enclosed by the path.
This is exactly true for static fields and is a very good
approximation for the low-frequency fields dealt with in electric
machines:

 H  d  I enclosed

The right-hand side of the equation represents the current enclosed by
the integration path and is called the magnetomotive force (MMF).

AMPERE`S LAW  H  d  I enclosed




In electric machines, currents are
frequently placed in slots
surrounded by ferromagnetic
teeth.
The MMF corresponding to each
path is the total current enclosed
by the path.
If the slots contain currents that
are approximately sinusoidally
distributed , then the MMF will be
cosinusoidally distributed in space.
In this way, the magnetic field or
flux density in the air gaps of the
machine will often have a
sinusoidal or cosinusoidal
distribution.
An example illustrating the
determination of the MMF is shown
in Figure 4.3, where different
integration paths are shown by dotted
lines.
FARADAY`S LAW

&
EMF
Faraday’s law relates the induced voltage or electromotive force
(EMF) to the time rate of change of the magnetic flux linkage:
Vind

d

dt
Or:
 
d  
 E  d    dt  B  dS
where E is the electric field and B is the magnetic flux density.
FARADAY`S LAW
Vind



d

dt
 
d  
 E  d    dt  B  dS
This law states that the voltage induced in a loop is equal to the time
rate of change of the flux linking the loop.
The negative sign indicates that the voltage is induced such that the
current would oppose the change in flux linkage.
The change in flux linkage can be caused by a change in flux density
and/or a change in geometry.
MMF AND TRAVELING WAVES

Presented now is the important concept of rotating MMF
and fields in machines. Consider the following equation:
  F1 sin( t ) cos( P )

The equation (4.3) represents a standing wave.

The sine term describes the time variation, while the cosine
term describes the space variation.

P is the number of pole pairs around the machine.

At a fixed point on the wave  is constant, and the
amplitude is sinusoidally time varying.

The peak of the wave and the nodes or zero crossings,
however, are always located at the same position.
MMF AND TRAVELING WAVES

This equation
  F1 sin( t ) cos( P )
is contrasted with an expression of the form:
  F1 sin( t  P )



This equation represents a traveling wave.
If the behavior of a fixed point in space is considered, as
before, the amplitude varies sinusoidally in time.
However, it is no longer true that the peak or any point on
the wave remains in the same position.
MMF AND TRAVELING WAVES

To see this, consider the argument of the sine function, the
peak will be located at:
t  P 




2
To remain at the peak of the wave as time moves forward,
movement must be made in the positive  direction.
The expression therefore represents a wave traveling in the
positive  direction.
If the minus sign in the argument is replaced by a plus
sign, the wave travels backward or in the negative 
direction.
MMF AND TRAVELING WAVES

Function peaks at: t  P 

2
  F1 sin( t  P )




The speed of the traveling wave can be determined by
considering the argument of the sine function.
In one cycle, the t term increases by 2.
To remain at the same point on the wave, the space term
must also advance by 2. This means that  must increase
by (2/P).
Thus, the wave progresses one wavelength or two poles in
one cycle.
MMF AND TRAVELING WAVES

A standing wave can be decomposed into two traveling waves, one going forward and
one backward, each of half the amplitude of the standing wave.

This is seen by using a trigonometric identity:
1
sin t cos P  ((sin( t  P )  sin( t  P ))
2

The first term on the right-hand side of equation (4.5) represents a backward
traveling wave, and the second term represents a forward traveling wave.
MMF AND TRAVELING WAVES


Let us consider the case of a three-phase winding in which each
of the phases are displaced by 120 in space and the winding
currents are displaced by 120 in time.
For the first phase, phase a, the fundamental space component of
the MMF has the form:
K
a  K sin( t ) cos( P )  (sin( t  P )  sin( t  P ))
2

where K is a constant containing information on the winding
geometry.
MMF AND TRAVELING WAVES

Similarly, for phases b:
2
2
b  K sin( t  ) cos( P  )
3
3
K
4
b  (sin( t  P )  sin( t  P  ))
2
3
MMF AND TRAVELING WAVES

Similarly, for phases c:
4
4
c  K sin( t  ) cos( P  )
3
3
K
2
c  (sin( t  P )  sin( t  P  ))
2
3
MMF AND TRAVELING WAVES

Summarizing:
a 
K
(sin( t  P )  sin( t  P ))
2
b 
K
4
(sin( t  P )  sin( t  P  ))
2
3
c 
K
2
(sin( t  P )  sin( t  P  ))
2
3



The forward waves of all three
phases are the same, while the
backward waves are 120  out of
phase.

By adding the MMFs of
the three phases, a
resultant wave is obtained
in which the fundamental
space component travels
in the forward direction
and has a magnitude 3/2
times the peak MMF of
one phase alone.
The backward waves
cancel.
This is the most common
method of producing
traveling magnetic fields
in motors.
MMF AND TRAVELING WAVES

Figure 4.5 shows a simplified 18-slot machine with three-phase
sinusoidal currents.
MMF AND TRAVELING WAVES



Each phase is placed into three consecutive slots with the return nine
slots away.
The MMF at different instants of time is plotted, and the wave moves to
the right at a rate of one wavelength (two poles) per cycle.
The wave is distorted due to the space harmonics included. These have
little influence on the energy conversion process and are not dealt with
here.