A machine with only amortisseur windings is called an induction
machine. Such machines are called induction machines because the rotor voltage
(which produces
the rotor current and the rotor magnetic field) is induced in
the rotor windings
rather than being physically connected by wires. The
distinguishing feature of an
induction motor is that no dc field current is required to run
the machine.
Although it is possible to use an induction machine as either a
motor or a
generator, it has many disadvantages as a generator and so is
rarely used in that
manner. For this reason, induction machines are usually referred
to as induction
motors.
INDUCTION MOTOR CONSTRUCTION
An induction motor has the same physical stator as a synchronous
machine, with
a different rotor construction. A typical two-pole stator is shown
in Figure 1. It
looks (and is) the same as a synchronous machine stator. There
are two different
types of induction motor rotors which can be placed inside the stator.
One is called
a cage rotor, while the other is called a wound rotor.
Figures 2 and 3 show cage induction motor rotors. A cage induction
motor rotor consists of a series of conducting bars laid into slots
carved in the face
of the rotor
and shorted at either end by large shorting rings. This design is referred
to as a cage rotor because the conductors, if examined by themselves,
would look like one of the exercise wheels that squirrels or hamsters
run on.
of three-phase windings that are mirror images of the windings
on the stator. The
three phases of the rotor windings are usually V-connected, and
the ends of the
three rotor wires are tied to slip rings on the rotor's shaft.
The rotor windings are
shorted through brushes riding on the slip rings. Wound-rotor
induction motors
therefore have their rotor currents accessible at the stator
brushes, where they can
be examined and where extra resistance can be inserted into the
rotor circuit. It is
possible to take advantage of this feature to modify the torque-
speed characteristic
of the motor. Two wound rotors are shown in Figure 4, and a
complete
wound-rotor induction motor is shown in Figure 5.
and they require much more maintenance
because of the wear associated
with their brushes and slip rings. As
a result, wound-rotor induction motors are
rarely used.
BASIC INDUCTION MOTOR CONCEPTS
Induction motor operation is basically
the same as that of amortisseur windings on
synchronous
motors. That basic operation will now be reviewed, and some importantinduction
motor terms will be defined.
TRENDS IN INDUCTION
MOTOR DESIGN
1880s by Nicola Tesla, who received a
patent on his ideas in 1888. At that time,
he presented a paper before the
American institute of Electrical Engineers [AIEE],
predecessor of today's Institute of
Electrical and Electronics Engineers (IEEE) in
which he described the basic
principles of the wound-rotor induction motor, along
with ideas for two other important ac
motors-the synchronous motor and the reluctance motor.
Although the basic idea of the
induction motor was described in 1888, the
motor itself did not spring forth in
full-fledged form. There was an initial period
of rapid development, followed by a
series of slow, evolutionary improvements
which have continued to this day.
The induction motor assumed recognizable
modem form between 1888 and
1895. During that period, two- and
three-phase power sources were developed to
produce the rotating magnetic fields
within the motor, distributed stator windings
were developed, and the cage rotor was
introduced. By 1896, fully functional and
recognizable three-phase induction
motors were commercially available.
Between then and the early 1970s,
there was continual improvement in the
quality of the steels, the casting
techniques, the insulation, and the construction
However, these improvements in induction
motor design did not necessarily
lead to improvements in motor operating
efficiency. The major design effort
was directed toward reducing the initial
materials cost of the machines, not toward
increasing their efficiency. The
design effort was oriented in that direction because
electricity was so inexpensive, making
the up-front cost of a motor the principal
criterion used by purchasers in its selection.
Since the price of oil began its spectacular
climb in 1973, the lifetime operating
cost of machines has become more and more
important, and the initial installation
cost has become relatively less important.
As a result of these trends,
new emphasis has been placed on motor efficiency
both by designers and by end
users of the machines.
New lines of high-efficiency induction
motors are now being produced by
all major manufacturers, and they are
fanning an ever-increasing share of the induction motor market. Several techniques
are used to improve the efficiency of
these motors compared to the traditional
standard-efficiency designs. Among
these techniques are:
1. More copper is used in the stator
windings to reduce copper losses.
2. The rotor and stator core length is
increased to reduce the magnetic flux density
in the air gap of the machine. This reduces
the magnetic saturation of the
machine, decreasing core losses.
3. More steel is used in the stator of
the machine, allowing a greater amount of
heat transfer out of the motor and
reducing its operating temperature. The rotor's
fan is then redesigned to reduce windage
losses.
4. The steel used in the stat or is a special
high-grade electrical steel with low
hysteresis losses.
5. The steel is made of an especially
thin gauge (i.e., the laminations are very
close together), and the steel has a
very high internal resistivity. Both effects
tend to reduce the eddy current losses
in the motor.
6. The rotor is carefully machined to produce
a uniform air gap, reducing the
stray load losses in the motor.
In addition to the general techniques described
above, each manufacturer
has his own unique approaches to improving
motor efficiency. A typical highefficiency induction motor is shown in Figure 32.
To aid in the comparison of motor
efficiencies, NEMA has adopted a standard
technique for measuring motor efficiency
based on Method B of the IEEE
Standard 112, Test Procedure for polyphase
Induction Motors and Generators.
NEMA has also introduced a rating called
NEMA nominal efficiency, which appears on the nameplates of design class A, B ,
and C motors.
Induction Generator Applications
Induction generators have been used since
early in the twentieth century, but by
the 1960s and 1970s they had largely
disappeared from use. However, the induction
generator has made a comeback since the
oil price shocks of 1973. With energy
costs so high, energy recovery became
an important part of the economics of
most industrial processes. The induction
generator is ideal for such applications
because it requires very little in the
way of control systems or maintenance.
Because of their simplicity and small size
per kilowatt of output power, induction
generators are also favored very strongly
for small windmills. Many commercial
windmills are designed to operate in
parallel with large power systems,
supplying a fraction of the customer's
total power needs. In such operation, the
power system can be relied on for
voltage and frequency control , and static capacitors
can be used for power-factor correction.
Induction Generator Applications
Induction generators have been used
since early in the twentieth century, but by
the 1960s and 1970s they had largely
disappeared from use. However, the induction
generator has made a comeback since
the oil price shocks of 1973. With energy
costs so high, energy recovery became
an important part of the economics of
most industrial processes. The
induction generator is ideal for such applications
because it requires very littIe in the
way of control systems or maintenance.
Because of their simplicity and small
size per kilowatt of output power, induction
generators are also favored very
strongly for small windmills. Many commercial
windmills are designed to operate in
parallel with large power systems,
supplying a fraction of the customer's
total power needs. In such operation, the
power system can be relied on for
voltage and frequency control , and static capacitors can be used for
power-factor correction.
A nameplate for a typical
high-efficiency integral-horsepower induction motor is
shown in Figure. The most important
ratings present on the nameplate are
1. Output power
2. Voltage
3.Current
4.Power
factor
5.Speed
6.Nominal efficiency
7.
NEMA design class
8. Starting code
except that it might not show a nominal efficiency.
The voltage limit on the motor is based on the maximum
acceptable magnetization
current flow, since the higher the voltage gets, the more
saturated the
motor's iron becomes and the higher its magnetization current
becomes. Just as in
the case of transformers and synchronous machines, a 60-Hz
induction motor may
be used on a 50-Hz power system, but only if the voltage rating
is decreased by an
amount proportional to the decrease in frequency. this derating
is necessary because
the flux in the core of the motor is. proportional to the
integral of the applied voltage. To keep the maximum flux in the core constant
while the period of integration is increasing, the average voltage level must
decrease.
The current limit on an induction motor is based on the maximum
acceptable
heating in the motor's windings, and the power limit is set by
the combination of
the voltage and current ratings with the machine's power factor
and efficiency.
NEMA design classes, starting code letters, and nominal
efficiencies were
discussed in previous sections of this subject.
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