Download IM - Module 5

INDUCTION MACHINES
MODULE 5
Prepared by:
Aswathy Mohandas P
STEPPER MOTOR
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A stepper motor is an electromechanical device which
converts electrical pulses into discrete mechanical
movements. The shaft or spindle of a stepper motor rotates
in discrete step increments when electrical command
pulses are applied to it in the proper sequence.
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MAIN FEATURES
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The sequence of the applied pulses is directly
related to the direction of motor shafts rotation.
The speed of the motor shafts rotation is
directly related to the frequency of the input
pulses.
The length of rotation is directly related to the
number of input pulses applied.
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STEPPER MOTOR CHARACTERISTICS
Open loop
The motors response to digital input pulses provides open-loop
control, making the motor simpler and less costly to control.
Brushless
Very reliable since there are no contact brushes in the motor.
Therefore the life of the motor is simply dependant on the life of
the bearing.
Incremental steps/changes
The rotation angle of the motor is proportional to the input pulse.
Speed increases -> torque decreases
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DISADVANTAGES OF STEPPER MOTORS
There are two main disadvantages of stepper
motors:
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Resonance can occur if not properly controlled.
This can be seen as a sudden loss or drop in
torque at certain speeds which can result in missed
steps or loss of synchronism. It occurs when the
input step pulse rate coincides with the natural
oscillation frequency of the rotor. Resonance can
be minimised by using half stepping or
microstepping.
Not easy to operate at extremely high speeds.
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WORKING PRINCIPLE
Stepper motors consist of a permanent magnet
rotating shaft, called the rotor, and
electromagnets on the stationary portion that
surrounds the motor, called the stator.
When a phase winding of a stepper
motor is energized with current, a
magnetic flux is developed in the
stator. The direction of this flux is
determined by the “Right Hand
Rule”.
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UNDERSTANDING RESOLUTION
Resolution is the number of degrees rotated per
step.
Step angle = 360/(NPh * Ph) = 360/N
NPh = Number of equivalent poles per phase
= number of rotor poles.
Ph = Number of phases.
N = Total number of poles for all phases
together.
Example: for a three winding motor with a
rotor having 4 teeth, the resolution is 30
degrees.
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TYPES OF STEPPER MOTORS
There are three main types of stepper motors:
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Variable Reluctance stepper motor
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Permanent Magnet stepper motor
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Hybrid Synchronous stepper motor
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VARIABLE RELUCTANCE MOTOR
This type of motor consists of a soft iron multitoothed
rotor and a wound stator.
When the stator windings are energized
with DC Current, the poles become magnetized.
Rotation occurs when the rotor teeth
are attracted to the energized stator
poles.
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PERMANENT MAGNET MOTOR
The rotor no longer has teeth as with
the VR motor.
Instead the rotor is
magnetized with alternating north
and south poles situated in a straight
line parallel to the rotor shaft.
These magnetized rotor poles provide an increased
magnetic flux intensity and because of this
the PM motor exhibits improved torque characteristics
when compared with the VR type.
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HYBRID SYNCHRONOUS MOTOR
The rotor is multi-toothed like the VR motor and
contains an axially magnetized concentric
magnet around its shaft.
The teeth on the rotor provide an even
better path which helps guide the
magnetic flux to preferred locations in
the air gap.
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APPLICATIONS
Stepper motors can be a good choice whenever
controlled movement is required.
They can be used to advantage in applications
where you need to control rotation angle, speed,
position and synchronism.
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These include
printers
plotters
medical equipment
fax machines
automotive and scientific equipment etc.
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SWITCHED-RELUCTANCE MOTORS
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In principle, a switched-reluctance motor operates like a variablereluctance step motor discussed in the previous section.
However, the operation differs mainly in the complicated control
mechanism of the motor.
In order to develop torque in the motor, the rotor position should be
determined by sensors so that the excitation timing of the phase
windings is precise.
Although its construction is one of the simplest possible among
electric machines, because of the complexities involved in the
control and electric drive circuitry, switched-reluctance motors have
not been able to find widespread applications for a long time.
However, with the introduction of new power electronic and
microelectronic switching circuits, the control and drive circuitry of
a switched reluctance motor have become economically justifiable
for many applications where traditionally dc or induction motors
have been used.
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A switched-reluctance motor has a wound stator but has no
windings on its rotor, which is made of soft magnetic material
as shown in Figure.
The change in reluctance around the periphery of the stator
forces the rotor poles to align with those of the stator.
Consequently, torque develops in the motor and rotation takes
place.
The total flux linkages of phase-A in the following figure is la
= La(q) ia and of phase- B is lb = Lb(q) ib with the assumption
that the magnetic materials are infinitely permeable.
Since the magnetic axes of both windings
are orthogonal, no mutual flux linkages
are expected between them.
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BRUSHLESS DC MOTORS
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DC motors find considerable applications where
controlling a system is a primary objective.
However, electric arcs produced by the mechanical
commutator-brush arrangement are a major disadvantage
and limit the operating speed and voltage.
A motor that retains the characteristics of a dc motor but
eliminates the commutator and the brushes is called a
brushless dc motor.
A brushless dc motor consists of a multiphase winding
wound on a non-salient stator and a radially magnetized
PM rotor.
Figure shows a schematic diagram of a brushless dc motor.
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Voltage is applied to individual phase
windings through a sequential
switching operation to achieve the
necessary commutation to impart
rotation.
 The switching is done electronically
using power transistors or thyristors.
 For example, if winding 1 is
energized, the PM rotor aligns with
the magnetic field produced by
winding 1.
 When winding 1 is switched off
while winding 2 is turned on, the
rotor is made to rotate to line up with
the magnetic field of winding 2.
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As can be seen, the operation of a brushless dc
motor is very similar to that of a PM step motor.
The major difference is the timing of the
switching operation, which is determined by the
rotor position to provide the synchronism
between the magnetic field of the permanent
magnet and the magnetic field produced by the
phase windings.
The rotor position can be detected by using either
Hall-effect or photoelectric devices.
The signal generated by the rotor position sensor
is sent to a logic circuit to make the decision for
the switching, and then an appropriate signal
triggers the power circuit to excite the respective
phase winding.
The control of the magnitude and the rate of
switching of the phase currents essentially
determine the speed-torque characteristic of a
brushless dc motor, which is shown in Figure .
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PERMANENT-MAGNET MOTORS
The speed-torque characteristic of a PM motor can be
controlled by changing either the supply voltage or the
effective resistance of the armature circuit.
 The change in the supply voltage varies the no-load speed of
the motor without affecting the slope of the characteristic.
 Thus for different supply voltages, a set of parallel speedtorque characteristics can be obtained, as illustrated in Figure
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On the other hand, with the change in the effective resistance of
the armature circuit, the slope of the curve is controlled and the noload speed of the motor remains the same, as indicated in Figure
12.4.
Using magnets with different flux densities and the same crosssectional areas, or vice versa, there are almost infinite possibilities
for designing a PM motor for a given operating condition, as
shown in Figure 12.5.
From the same figure we can also conclude that an increase in
blocked-rotor torque can be achieved only at the expense of a
lower no-load speed.
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BASICS OF LINEAR MOTORS
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Analogous to Unrolled DC Motor
• Force (F) is generated
when the current (I)
(along vector L) and the
flux density (B) interact
• F = LI x B
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BENEFITS OF LINEAR MOTORS
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High Maximum Speed
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High Precision
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Response rate can be over 100 times that of a mechanical
transmission  faster accelerations, settling time (more
throughput)
Stiffness
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Accuracy, resolution, repeatability limited by feedback device,
budget
Zero backlash: No mechanical transmission components.
Fast Response
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Limited primarily by bus voltage, control electronics
No mechanical linkage, stiffness depends mostly on gain &
current
Durable
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Modern linear motors have few/no contacting parts  no wear
DOWNSIDES OF LINEAR MOTORS
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Cost
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Low production volume (relative to demand)
High price of magnets
Linear encoders (feedback) are much more expensive than rotary
encoders, cost increases with length
Higher Bandwidth Drives and Controls
 Lower force per package size
 Heating issues
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Forcer is usually attached to load  I2R losses are directly
coupled to load
No (minimal) Friction
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No automatic brake
COMPONENTS OF LINEAR MOTORS
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Forcer (Motor Coil)
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Windings (coils) provide current (I)
Windings are encapsulated within core
material
Mounting Plate on top
Usually contains sensors (hall effect
and thermal)
Magnet Rail
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Iron Plate / Base Plate
Rare Earth Magnets of alternating
polarity provide flux (B)
Single or double rail
F=
lI x B
MAGLEV SYSTEM- OPERATION
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The synchronous long stator linear motor of the
Transrapid maglev system is used both for propulsion
and braking. The motor generates an electro magnetic traveling field. The support magnets in the
vehicle function as the rotor (excitation portion) of the
electric motor . The primary propulsion component of
the Transrapid maglev system – the stator packs with
three-phase motor winding – are not installed in the
vehicle but in the guideway. By supplying alternating
current to the three-phase motor winding, an
electromagnetic traveling field is generated which
moves the vehicle, pulled along by its support
magnets which act as the excitation component. The
speed can be continuously regulated from standstill to
full operating speed by varying the frequency of the
alternating current.
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TYPES OF LINEAR MOTORS
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Iron Core
Coils wound around
teeth of laminations
on forcer
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Ironless Core
Dual back iron
separated by spacer
 Coils held together
with epoxy
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Slotless
Coil and back iron
held together with
epoxy
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