27-10-2012, 05:58 PM
PRINCIPLES OF OPERATION OF SYNCHRONOUS MACHINES
[attachment=37221]
ABSTRACT
The synchronous electrical generator (also called alternator) belongs to the
family of electric rotating machines. Other members of the family are the directcurrent
(dc) motor or generator, the induction motor or generator, and a number
of derivatives of all these three. What is common to all the members of this family
is that the basic physical process involved in their operation is the conversion
of electromagnetic energy to mechanical energy, and vice versa. Therefore, to
comprehend the physical principles governing the operation of electric rotating
machines, one has to understand some rudiments of electrical and mechanical
engineering.
Chapter 1 is written for those who are involved in operating, maintaining and
trouble-shooting electrical generators, and who want to acquire a better understanding
of the principles governing the machine’s design and operation, but
who do not have an electrical engineering background. The chapter starts by
introducing the rudiments of electricity and magnetism, quickly building up to
a description of the basic laws of physics governing the operation of the synchronous
electric machine, which is the type of machine all turbogenerators
belong to.
INTRODUCTION TO BASIC NOTIONS ON ELECTRIC POWER
Magnetism and Electromagnetism
Certain materials found in nature exhibit a tendency to attract or repeal each
other. These materials, called magnets, are also called ferromagnetic because
they include the element iron as one of their constituting elements.
Magnets always have two poles: one called north; the other called south. Two
north poles always repel each other, as do two south poles. However, north and
south poles always attract each other. A magnetic field is defined as a physical
field established between to poles. Its intensity and direction determine the forces
of attraction or repulsion existing between the two magnets.
Figures 1.1 and 1.2 are typical representations of two interacting magnetic
poles, and the magnetic field established between them.
Magnets are found in nature in all sorts of shapes and chemical constitution.
Magnets used in industry are artificially made. Magnets that sustain their magnetism
for long periods of time are denominated “permanent magnets.” These are
widely used in several types of electric rotating machines, including synchronous
machines. However, due to mechanical, as well as operational reasons, permanent
magnets in synchronous machines are restricted to those with ratings much
lower than large turbine-driven generators, which is the subject of this book.
Turbine-driven generators (for short: turbogenerators) take advantage of the fact
that magnetic fields can be created by the flow of electric currents in conductors.
See Figure 1.3.
THREE-PHASE CIRCUITS
The two-wire ac circuits shown above (called single-phase circuits or systems),
are commonly used in residential, commercial, and low voltage—low power
industrial applications. However, all electric power systems to which industrial
generators are connected are three-phase systems. Therefore any discussion in
this book about the “power system” will refer to a three-phase system. Moreover
in industrial applications the voltage supplies are, for all practical reasons,
balanced, meaning all three-phase voltages are equal in magnitude and apart by
120 electrical degrees. In those rare events where the voltages are unbalanced, its
implication into the operation of the generator will be discussed in other chapters
of this book.
Three-phase electric systems may have a fourth wire, called “neutral.” The
“neutral” wire of a three-phase system will conduct electricity if the source and/or
the load are unbalanced. In three-phase systems two sets of voltages and currents
can be identified. These are the phase and line voltages and currents.
Figure 1.11 shows the main elements of a three-phase circuit. Three-phase
circuits can have their sources and/or loads connected in wye (star) or in delta.
(See Fig. 1.12 for a wye-connected source feeding a delta-connected load.)
Almost without exception, turbine-driven generators have their windings connected
in wye (star). Therefore in this book the source (or generator) will be
shown wye-connected. There are a number of important reasons why turbogenerators
are star-connected.
THE SYNCHRONOUS MACHINE
At this point the rudiments of electromagnetism have been presented, together
with the four basic laws of physics describing the inherent physical processes
coexisting in any electrical machine. Therefore it is the right time to introduce
the basic configuration of the synchronous machine, which, as mentioned before,
is the type of electric machine that all large turbine-driven generators belong to.
Background
The commercial birth of the alternator (synchronous generator) can be dated back
to August 24, 1891. On that day, the first large-scale demonstration of transmission
of ac power was carried out. The transmission extended from Lauffen,
Germany, to Frankfurt, about 110 miles away. The demonstration was carried out
during an international electrical exhibition in Frankfurt. This demonstration was
so convincing about the feasibility of transmitting ac power over long distances,
that the city of Frankfurt adopted it for their first power plant, commissioned in
1894. This happened about one hundred and eight years before the writing of
this book (see Fig. 1.17).
The Lauffen-Frankfurt demonstration—and the consequent decision by the
city of Frankfurt to use alternating power delivery—were instrumental in the
adoption by New York’s Niagara Falls power plant of the same technology. The
Niagara Falls power plant became operational in 1895. For all practical purposes
the great dc versus ac duel was over. Southern California Edison’s history book
reports that its Mill Creek hydro plant is the oldest active polyphase (three-phase)
plant in the United States. Located in San Bernardino County, California, its first
units went into operation on September 7, 1893, placing it almost two years
ahead of the Niagara Falls project. One of those earlier units is still preserved
and displayed at the plant.
It is interesting to note that although tremendous development in machine ratings,
insulation components, and design procedures has occurred now for over
one hundred years, the basic constituents of the machine have remained practically
unchanged.
[attachment=37221]
ABSTRACT
The synchronous electrical generator (also called alternator) belongs to the
family of electric rotating machines. Other members of the family are the directcurrent
(dc) motor or generator, the induction motor or generator, and a number
of derivatives of all these three. What is common to all the members of this family
is that the basic physical process involved in their operation is the conversion
of electromagnetic energy to mechanical energy, and vice versa. Therefore, to
comprehend the physical principles governing the operation of electric rotating
machines, one has to understand some rudiments of electrical and mechanical
engineering.
Chapter 1 is written for those who are involved in operating, maintaining and
trouble-shooting electrical generators, and who want to acquire a better understanding
of the principles governing the machine’s design and operation, but
who do not have an electrical engineering background. The chapter starts by
introducing the rudiments of electricity and magnetism, quickly building up to
a description of the basic laws of physics governing the operation of the synchronous
electric machine, which is the type of machine all turbogenerators
belong to.
INTRODUCTION TO BASIC NOTIONS ON ELECTRIC POWER
Magnetism and Electromagnetism
Certain materials found in nature exhibit a tendency to attract or repeal each
other. These materials, called magnets, are also called ferromagnetic because
they include the element iron as one of their constituting elements.
Magnets always have two poles: one called north; the other called south. Two
north poles always repel each other, as do two south poles. However, north and
south poles always attract each other. A magnetic field is defined as a physical
field established between to poles. Its intensity and direction determine the forces
of attraction or repulsion existing between the two magnets.
Figures 1.1 and 1.2 are typical representations of two interacting magnetic
poles, and the magnetic field established between them.
Magnets are found in nature in all sorts of shapes and chemical constitution.
Magnets used in industry are artificially made. Magnets that sustain their magnetism
for long periods of time are denominated “permanent magnets.” These are
widely used in several types of electric rotating machines, including synchronous
machines. However, due to mechanical, as well as operational reasons, permanent
magnets in synchronous machines are restricted to those with ratings much
lower than large turbine-driven generators, which is the subject of this book.
Turbine-driven generators (for short: turbogenerators) take advantage of the fact
that magnetic fields can be created by the flow of electric currents in conductors.
See Figure 1.3.
THREE-PHASE CIRCUITS
The two-wire ac circuits shown above (called single-phase circuits or systems),
are commonly used in residential, commercial, and low voltage—low power
industrial applications. However, all electric power systems to which industrial
generators are connected are three-phase systems. Therefore any discussion in
this book about the “power system” will refer to a three-phase system. Moreover
in industrial applications the voltage supplies are, for all practical reasons,
balanced, meaning all three-phase voltages are equal in magnitude and apart by
120 electrical degrees. In those rare events where the voltages are unbalanced, its
implication into the operation of the generator will be discussed in other chapters
of this book.
Three-phase electric systems may have a fourth wire, called “neutral.” The
“neutral” wire of a three-phase system will conduct electricity if the source and/or
the load are unbalanced. In three-phase systems two sets of voltages and currents
can be identified. These are the phase and line voltages and currents.
Figure 1.11 shows the main elements of a three-phase circuit. Three-phase
circuits can have their sources and/or loads connected in wye (star) or in delta.
(See Fig. 1.12 for a wye-connected source feeding a delta-connected load.)
Almost without exception, turbine-driven generators have their windings connected
in wye (star). Therefore in this book the source (or generator) will be
shown wye-connected. There are a number of important reasons why turbogenerators
are star-connected.
THE SYNCHRONOUS MACHINE
At this point the rudiments of electromagnetism have been presented, together
with the four basic laws of physics describing the inherent physical processes
coexisting in any electrical machine. Therefore it is the right time to introduce
the basic configuration of the synchronous machine, which, as mentioned before,
is the type of electric machine that all large turbine-driven generators belong to.
Background
The commercial birth of the alternator (synchronous generator) can be dated back
to August 24, 1891. On that day, the first large-scale demonstration of transmission
of ac power was carried out. The transmission extended from Lauffen,
Germany, to Frankfurt, about 110 miles away. The demonstration was carried out
during an international electrical exhibition in Frankfurt. This demonstration was
so convincing about the feasibility of transmitting ac power over long distances,
that the city of Frankfurt adopted it for their first power plant, commissioned in
1894. This happened about one hundred and eight years before the writing of
this book (see Fig. 1.17).
The Lauffen-Frankfurt demonstration—and the consequent decision by the
city of Frankfurt to use alternating power delivery—were instrumental in the
adoption by New York’s Niagara Falls power plant of the same technology. The
Niagara Falls power plant became operational in 1895. For all practical purposes
the great dc versus ac duel was over. Southern California Edison’s history book
reports that its Mill Creek hydro plant is the oldest active polyphase (three-phase)
plant in the United States. Located in San Bernardino County, California, its first
units went into operation on September 7, 1893, placing it almost two years
ahead of the Niagara Falls project. One of those earlier units is still preserved
and displayed at the plant.
It is interesting to note that although tremendous development in machine ratings,
insulation components, and design procedures has occurred now for over
one hundred years, the basic constituents of the machine have remained practically
unchanged.