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# 1 - principles of electrical machines and power electronics p_c_sen

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```or vice versa are called electric
machines. The process of translation is known as electromechanical energy
conversion. An electric machine is therefore a link between an electrical
system and a mechanical system, as shown in Fig. 4.1. In these machines r
the conversion is reversible. If the conversion is from mechanical to electri- i
cal, the machine is said to act as a generator. If the conversion is from 1 .
electrical to mechanical, the machine is said to act as a motor. Hence, the I
same electric machine can be made to operate as a generator as well as a
motor. Machines are called ac machines (generators or motors) if the electri-
cal system is ac and de machines (generators or motors) if the electrical I
system is de. I
Note that the two systems in Fig. 4.1, electrical and mechanical, are differ-
ent in nature. In the electrical system the primary quantities involved are
voltage and current, while the analogous quantities in the mechanical system
are torque and speed. The coupling medium between these different systems
is the field, as illustrated in Fig. 4.2.
4.1 ELECTROMAGNETIC CONVERSION
Three electrical machines (de, induction, and synchronous) are used exten-
sively for electromechanical energy conversion. In these machines, conver-
sion of energy from electrical to mechanical form or vice versa results from
the following two electromagnetic phenomena:
1. When a conductor moves in a magnetic field, voltage is induced in
the conductor.
2. When a current-carrying conductor is placed in a magnetic field, the
conductor experiences a mechanical force.
These two effects occur simultaneously whenever energy conversion takes
place from electrical to mechanical or vice versa. In motoring action, the
electrical system makes current flow through conductors that are placed in
the magnetic field. A force is produced on each conductor. If the conductors
121
122 chapter 4 DC Machines
Electrical
system
e. i ·
Generator
Energy flow
Mechanical
system
T.n
FIGURE 4.1 Electromechanical en-
ergy conversion.
are placed on a structure free to rotate, an electromagnetic torque will be
produced, tending to make the rotating structure rotate at some speed. If
the conductors rotate in a magnetic field, a voltage will also be induced in
each conductor. In generating action, the process is reversed. In this case,
the rotating structure, the rotor, is driven by a prime mover (such as a steam
turbine or a diesel engine). A voltage will be induced in the conductors that
are rotating with the rotor. If an electrical load is connected to the winding
formed by these conductors, a current i will flow, delivering electrical power
to the load. Moreover, the current flowing through the conductor will inter-
act with the magnetic field to produce a reaction torque, which will tend to
oppose the torque applied by the prime mover. Note that in both motoring
and generating actions, the coupling magnetic field is involved in producing
a torque and an induced voltage.
The basic electric machines (de, induction, and synchronous), which de-
pend on electromagnetic energy conversion, are extensively used in various
power ratings. The operation of these machines is discussed in detail in this
and other chapters.
Motional Voltage, e
An expression can be derived for the voltage induced in a conductor moving
in a magnetic field. As shown in Fig. 4.3a, if a conductor of length l moves
at a linear speed v in a magnetic field B, the induced voltage in the conductor
is
e =Blv (4.1)
where B, l, and v are mutually perpendicular. The polarity of the induced
voltage can be determined from the so-called right-hand screw rule.
The three quantities v, B, and e are shown in Fig. 4.3b as three mutually
perpendicular vectors. Turn the vector v toward the vector B. If a right-hand
screw is turned in the same way the motion of the screw will indicate. the
direction of positive polarity of the induced voltage.
FIGURE 4.2 Coupling field be-
tween electrical and mechanical
systems.
p
I
J...
T Electromagnetic Conversion
l + + X X e X X e fiB X X X X / / l v /
X X
1
X X / /
X X X X /
B -v
X indicates B into the paper
(a) (b)
FIGURE 4.3 Motional voltage. (a) Conductor moving in
the magnetic field. (b) Right-hand screw rule.
Electromagnetic Force, f
123
For the current-carrying conductor shown in Fig. 4.4a, the force (known as
Lorentz force) produced on the conductor is
{=Eli (4.2)
where B, l, and i are mutually perpendicular. The direction of the force can
be determined by using the right-hand screw rule, illustrated in Fig. 4.4b.
Turn the current vector i toward the flux vector B. If a screw is turned in
the same way, the direction in which the screw will move represents the
direction of the force f.
Note that in both cases (i.e., determining the polarity of the induced
voltage and determining the direction of the force) the moving quantities
(v and i) are turned toward B to obtain the screw movement.
Equations 4.1 and 4.2 can be used to determine the induced voltage and
the electromagnetic force or torque in an electric machine. There are, of
course, other methods by which these quantities (e and f) can be determined.
Basic Structure of Electric Machines
The structure of an electric machine has two major components, stator and
rotor, separated by the air gap, as shown in Fig. 4.5.
X X l X X X X X X /B f l /
X X
1
X X / /
X X X X /
f
(a) (b)
FIGURE 4.4 Electromagnetic force. (a) Current-carrying conductor
moving in a magnetic field. (b) Force direction.
r
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124 chapter 4 DC Machines
(a) (b)
FIGURE 4.5 Structure of electric machines. (a) Cy-
lindrical machine (uniform air gap). (b) Salient pole
machine (nonuniform air gap).
Stator: This part of the machine does not move and normally is the
outer frame of the machine.
Rotor: This part of the machine is free to move and normally is the
inner part of the machine.
Both stator and rotor are made of ferromagnetic materials. In most ma-
chines slots are cut on the inner periphery of the stator and outer periphery
of the rotor structure, as shown in Fig. 4.5a. Conductors are placed in these
slots. The iron core is used to maximize the coupling between the coils
(formed by conductors) placed on the stator and rotor, to increase the flux
(a) (b)
FIGURE 4.6 Laminations. (Courtesy of Westinghouse Canada Inc.) (a) Stator.
(b) Rotor.
p
Electromagnetic Conversion 12 5
density in the machine and to decrease the size of the machine. If the stator
or rotor (or both) is subjected to a time-varying magnetic flux, the iron core
is laminated to reduce eddy current losses. The thin laminations of the iron
core with provisions for slots are shown in Fig. 4.6.
The conductors placed in the slots of the stator or rotor are interconnected
to form windings. The winding in which voltage is induced is called the
armature winding. The winding through which a current is passed to produce
the primary source of flux in the machine is called the field winding. Perma-
nent magnets are used in some machines to provide the major source of
flux in the machine.
Rotating electrical machines take many forms and are known by many
names. The three basic and common ones are de machines, induction ma-
chines, and synchronous machines. There are other machines, such as per-
manent magnet machines, hysteresis machines, and stepper machines.
DC Machine
In the de machine, the field winding is placed on the stator and the armature
winding on the rotor. These windings are shown in Fig. 4.7. A de current
is passed through the field winding to produce flux in the machine. Voltage
(a) (b)
(c)
FIGURE 4.7 DC machine. (a) Stator. (b) Rotor. (Courtesy of General Electric
Canada Inc.) (c) Schematic cross-sectional view for a 2-pole machine.
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126 chapter 4 DC Machines
induced in the armature winding is alternating. A mechanical commutator
and a brush```
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