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Electrical Actuators

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Title: Electrical Actuators


1
Sensors and Actuators
Week 4 PE -4030 Electrical Actuators
  • Prof. Charlton S. Inao
  • Mechatronics
  • Defence University
  • College of Engineering
  • Bishoftu, Ethiopia

2
Topics
3
Relay
  • A relay is an electrical switch that opens and
    closes under the control of another electrical
    circuit.

4
  • RELAYS
  • Relays are electrically operated switches in
    which changing a current in one electrical
    circuit switches a current on or off in another
    circuit..
  • When there is a current through the solenoid of
    the relay, a magnetic field is produced which
    attracts the iron armature, moves the push rod,
    and so close the normally open(NO) switch
    contacts and opens the normally closed(NC) switch
    contacts.

5
Relay
6
Example omron Relay
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Simple Relay Circuit
9
Motor Controller
  • A motor controller is a device or group of
    devices that serves to govern in some
    predetermined manner the performance of an
    electric motor
  • include a manual or automatic means for starting
    and stopping the motor, selecting forward or
    reverse rotation, selecting and regulating the
    speed, regulating or limiting the torque, and
    protecting against overloads and faults

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Contactor
  • Contactors are used by electrical equipment that
    is frequently turned off and on (opening and
    closing the circuit), such as lights, heaters,
    and motors
  • to make and break all power supply lines running
    to a load
  • to repeatedly establish and interrupt an
    electrical power circuit (NEMA)

12
CONTACTORS
  • contactor is an electrically controlled switch
    used for switching an electrical power circuit,
    similar to a relay except with higher current
    ratings.

A magnetic contactor is operated
electromechanically without manual
intervention Can be operated remotely, without
the need for putting a person in a potentially
dangerous location Magnetic contactors use a
small control current to open and close the
circuit.
13
  • Magnetic contactors are electromagnetically
    operated switches that provide a safe and
    convenient means for connecting and interrupting
    branch circuits.

14
  • Contactors come in many forms with varying
    capacities and features.
  • Unlike a circuit breaker, a contactor is not
    intended to interrupt a short circuit current..
  • Contactors are used to control electric motors, 
  • lighting, 
  • heating, capacitor banks,
  • thermal evaporators,
  • and other electrical loads.

15
Magnetic Contactor Operation
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DOL
  • The simplest form of motor starter for the
    induction motor is the Direct On Line starter.
    The Direct On Line Motor Starter (DOL) consist a
    MCCB or Circuit Breaker, Contactor and an
    overload relay for protection. Electromagnetic
    contactor which can be opened by the thermal
    overload relay under fault conditions.

21
  • Principle of Direct On Line Starter (DOL)
  • To start, the contactor is closed, applying full
    line voltage to the motor windings. The motor
    will draw a very high inrush current for a very
    short time, the magnetic field in the iron, and
    then the current will be limited to the Locked
    Rotor Current of the motor. The motor will
    develop Locked Rotor Torque and begin to
    accelerate towards full speed.

22
  • As the motor accelerates, the current will begin
    to drop, but will not drop significantly until
    the motor is at a high speed, typically about 85
    of synchronous speed. The actual starting current
    curve is a function of the motor design, and the
    terminal voltage, and is totally independent of
    the motor load.
  • The motor load will affect the time taken for the
    motor to accelerate to full speed and therefore
    the duration of the high starting current, but
    not the magnitude of the starting current.

23
  • Provided the torque developed by the motor
    exceeds the load torque at all speeds during the
    start cycle, the motor will reach full speed. If
    the torque delivered by the motor is less than
    the torque of the load at any speed during the
    start cycle, the motor will stops accelerating.
    If the starting torque with a DOL starter is
    insufficient for the load, the motor must be
    replaced with a motor which can develop a higher
    starting torque.

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Components of DOL Starter
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Working Principle
30
Direct On Line(DOL)
31
Direct On Line(DOL)
32
Mini Circuit Breakers(MCB)
33
There are two arrangement of operation of
miniature circuit breaker. One due to thermal
effect of over current and other due to
electromagnetic effect of over current. The
thermal operation of miniature circuit breaker is
achieved with a bimetallic strip whenever
continuous over current flows through MCB, the
bimetallic strip is heated and deflects by
bending. This deflection of bimetallic strip
releases mechanical latch.
34
As this mechanical latch is attached with
operating mechanism, it causes to open the
miniature circuit breaker contacts . But during
short circuit condition, sudden rising
of electric current, causes electromechanical
displacement of plunger associated with tripping
coil or solenoid of MCB. The plunger strikes
the trip lever causing immediate release of latch
mechanism consequently open the circuit breaker
contacts.
35
Operating Mechanism of Miniature Circuit
Breaker The Operating Mechanism of Miniature
Circuit Breaker provides the means of manual
opening and closing operation of miniature
circuit breaker. It has three-positions ON,
OFF, and TRIPPED. The external switching
latch can be in the TRIPPED position, if the
MCB is tripped due to over-current. When
manually switch off the MCB, the switching latch
will be in OFF position. In close condition of
MCB, the switch is positioned at ON. By
observing the positions of the switching latch
one can determine the condition of MCB whether it
is closed, tripped or manually switched off.
36
The MCB has some advantages compared to 
fuse.1. It automatically switches off the
electrical circuit during abnormal condition of
the network means in over load condition as well
as faulty condition. The fuse does not sense
but Miniature Circuit Breaker does it in more
reliable way. MCB is much more sensitive to over
current than fuse.2. Another advantage is, as
the switch operating knob comes at its off
position during tripping, the faulty zone of the
electrical circuit can easily be identified. But
in case of fuse, fuse wire should be checked by
opening fuse grip or cutout from fuse base, for
confirming the blow of fuse wire.3. Quick
restoration of supply can not be possible in case
of fuse as because fuses have to be rewirable or
replaced for restoring the supply. But in the
case of MCB, quick restoration is possible by
just switching on operation.4. Handling MCB is
more electrically safe than fuse.Because of to
many advantages of MCB over fuse units, in modern
low voltage electrical network, Miniature Circuit
Breaker is mostly used instead of backdated fuse
unit.
37
Overload Relay(OLR)
38
Overload (OL) Protection/Overload Relays(OLR)
  • Overload protection prevents an electric motor
    from drawing too much current, overheating, and
    literally burning out
  • Most commonly used OL is the overload relay

39
  • an overload protection device is required that
    does not open the circuit while the motor is
    starting, but opens the circuit if the motor gets
    overloaded and the fuses do not blow
  • An overload relay consists of
  • A current sensing unit (connected in the line to
    the motor).
  • A mechanism to break the circuit, either directly
    or indirectly.

40
Mechanism Device
  • Eutectic (melting alloy)
  • Bimetallic
  • Solid State

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Characteristics
  • Many overload protection devices have a trip
    indicator built into the unit
  • Overload relays can have either a manual or an
    automatic reset
  • Overload relays also have an assigned trip class.
    The trip class is the maximum time in seconds at
    which the overload relay will trip when the
    carrying current is at 600 of its current rating.

47
Electrical Actuators
  • AC Motors
  • DC Motors
  • Stepper Motors
  • Servo motors
  • Linear motors

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Motors
  • DC Motors
  • 1. Series Motor
  • 2. Shunt Motor
  • 3. Compound
  • AC Motors
  • 1. Induction Motors
  • 2. Synchronous Motors
  • Servo Motors
  • Stepper Motors

50
DC Motor
  • When current flows in a conductor it produces a
    magnetic field about it.
  • when the current-carrying conductor is within an
    externally generated magnetic field, the fields
    interact and a force is exerted on the conductor.
  • Therefore if a conductor lies within a magnetic
    field
  • motion of the conductor produces an electric
    current
  • an electric current in the conductor will
    generate motion
  • The reciprocal nature of this relationship means
    that, for example, the DC generator above will
    function as a DC motor

51
Construction
  • 1) Stator The static part that houses the field
    windings and receives the supply and2) Rotor
    The rotating part that brings about the
    mechanical rotations.

52
  • 3) Yoke of dc motor.(cast iron or steel, provides
    protective cover)
  • 4) Poles of dc motor.(cast iron, slotted)
  • 5) Field winding of dc motor.(copper wire)
  • 6) Armature winding of dc motor.( Copper wire)
  • 7) Commutator of dc motor. (copper segments
    stacked together, it commutates or relay the
    supply current from the mains to the armature
    windings housed over a rotating structure through
    the brushes of dc motor.)
  • 8) Brushes of dc motor. The brushes of dc
    motor are made with carbon or graphite
    structures, making sliding contact over the
    rotating commutator. The brushes are used to
    relay the electric current from external circuit
    to the rotating commutator form where it flows
    into the armature windings. 

53
Commutator
Brush
Armature Winding
Armature
54
Applications of series, shunt and compound
  • Series Motor Armature and field connected in a
    series circuit.
  • Apply for high torque loads that do not require
    precise speed regulation. Useful for high
    breakaway torque loads.
  • locomotives, hoists, cranes, automobile starters
  • Starting torque
  • 300 to as high as 800 of full load torque.
  • Shunt Motor Field coil in parallel (shunt) with
    the armature.
  • Current through field coil is independant of the
    armature.
  • Result excellent speed control.
  • Apply where starting loads are low
  • fans, blowers, centrifugal pumps, machine tools
  • Starting torque
  • 125 to 200 full load torque (300 for short
    periods).

55
Compound Wound Motor
  • Performance is roughly between series-wound and
    shunt-wound
  • Moderately high starting torque
  • Moderate speed control
  • Inherently controlled no-load speed
  • safer than a series motor where load may be
    disconnected
  • e.g. cranes

56
AC Motors
  • AC motors can be divided into two main forms
  • synchronous motors
  • induction motors
  • High-power versions of either type invariably
    operate from a three-phase supply, but
    single-phase versions of each are also widely
    used particularly in a domestic setting.

57
3 phase Induction Motor
  • Stator
  • As its name indicate stator is a stationary part
    of induction motor. A three phase supply is given
    to the stator of induction motor.
  • Rotor
  • The rotor is a rotating part of induction motor.
    The rotor is connected to the mechanical load
    through the shaft. The rotor of the three phase
    induction motor are further classified as
  • Squirrel cage rotor Slip ring rotor or wound
    rotor or phase wound rotor

58
Squirrel Cage Induction Motor
59
  • Advantages of squirrel cage induction rotor
  • 1. Its construction is very simple and rugged2.
    as there are no brushes and slip ring, these
    motors requires less maintenance.
  • ApplicationsSquirrel cage induction motor is
    used in lathes, drilling machine, fan, blower
    printing machines etc

60
Synchronous Motor
  • When a 3 phase electric conductors are placed in
    a certain geometrical positions (In certain angle
    from one another) there is an electrical field
    generate.
  • The rotating magnetic field rotates at a certain
    speed, that speed is called synchronous speed.
  • Now if an electromagnet is present in this
    rotating magnetic field, the electromagnet is
    magnetically locked with this rotating magnetic
    field and rotates with same speed of rotating
    field.
  • Synchronous motors is called so
  • because the speed of the rotor of
  • this motor is same as the rotating
  • magnetic field. It is basically a fixed
  • speed motor because it has only
  • one speed, which is synchronous
  • speed and therefore no intermediate
  • speed is there or in other words
  • its in synchronism with the supply
  • frequency. Synchronous speed is given by
  • Ns120f/P

61
Principle of Operation Synchronous Motor
  • Synchronous motor is a doubly excited machine i.e
    two electrical inputs are provided to it. Its
    stator winding which consists of a 3 phase
    winding is provided with 3 phase supply and rotor
    is provided with DC supply.
  • The 3 phase stator winding carrying 3 phase
    currents produces 3 phase rotating magnetic flux.
    The rotor carrying DC supply also produces a
    constant flux.
  • At a particular instant rotor and stator poles
    might be of same polarity (N-N or S-S) causing
    repulsive force on rotor and the very next second
    it will be N-S causing attractive force. But due
    to inertia of the rotor, it is unable to rotate
    in any direction due to attractive or repulsive
    force and remain in standstill condition. Hence
    it is not self starting.

62
Application of Synchronous Motor
  • Synchronous motor having no load connected to
    its shaft is used for power factor improvement.
    Owing to its characteristics to behave at any
    power factor, it is used in power system in
    situations where static capacitors are
    expensive. Synchronous motor finds application
    where operating speed is less (around 500 rpm)
    and high power is required. For power requirement
    from 35 kW to 2500KW, the size, weight and cost
    of the corresponding induction motor is very
    high. Hence these motors are preferably used. Ex-
    Reciprocating pump, compressor, rolling mills etc

63
Flemings Right Hand Rule
Fleming's right hand rule (for generators) shows
the direction of induced current flow when a
conductor moves in a magnetic field.
The right hand is held with the thumb, first
finger and second finger mutually at right
angles, as shown in the diagram .
  1. The Thumb represents the direction of Motion of
    the conductor.
  2. The First finger represents the direction of the
    Field.
  3. The Second finger represents the direction of the
    induced or generated Current (in the classical
    direction, from positive to negative).

These mnemonics are named after British engineer
John Ambrose Fleming, who invented them.
64
A. Nameplate Information
Motor Data Characteristics
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  • Universal Electric Motor indicates that this is
    a standard replacement motor
  • Thermally Protected motor is equipped with
    devices designed to disconnect the current flow
    if insulating materials become too hot
  • SER 12P 14666J manufacturers serial number
  • MOD HE3E207N manufacturers model number
  • STK. NO. 619 manufacturers stock number
  • VOLTS 208-230 motor can operate on either 208
    or 230 volts
  • HZ 60 frequency for which the motor is designed
    to operate

67
  • AMPS 1.2 motor draws 1.2 amperes when operating
    at full load capacity
  • RPM-1025 indicates motor turns 1025 revolutions
    per minute when pulling its rated load
  • PH1 motor runs on single-phase power
  • CAP5MFD370VAC motor is equipped with a
    continuous-operation run capacitor, rated at 5
    microfarads and 370 volts AC
  • INS CL B motor has class B insulation,
    providing protection up to 130oC (266oF)
  • AMB 60oC motor is rated to work at an ambient
    of 60oC (140oF)

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  • HP-1/5 motor is designed to pull a 1/5 Hp load
    when operated at the rated voltage and cycle
  • CONT continuous duty
  • Motor will pull rated load under rated conditions
    continuously and not overheat
  • May have INT intermittent duty rated for 5,
    15, 30, or 60 minute operating times
  • AO air-over ventilation is used to cool this
    motor
  • ROT REV indicates direction of rotation of the
    shaft
  • BRG SLV motor has sleeve bearings

69
  • FRAME A48 designation that gives motor
    dimensions based on NEMA standards
  • Two-digit frame numbers 16 distance in inches
    from centerline of shaft to foot of base
  • TYPE FH indicates motor is a fractional-horsepow
    er motor
  • SF 1.35 indicates motor will tolerate a 35
    overload for extended periods
  • HSG OPEN indicates type of motor enclosure
  • CONNECTIONS wiring diagrams for installation or
    changing direction of rotation

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How does an electric motor work?
stator
rotor
stator
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Elements of an Induction Motor The Rotor
No direct electrical connections are made to the
rotor. All forces are magnetically induced by
the stator, via the air gap.
Rotor Bar Current
Cast aluminum rotor bars
Carry induced current (skewed bars shown)
Cast aluminum end rings
Laminations of high-silicon content steel
Electrically joins rotor bars at both motor ends
Low-eddy current loss magnetic medium
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AC MOTOR FORMULAS
SYNCHRONOUS SPEED
VOLTS / HERTZ
Motor Line Volts Motor Frequency
V/Hz
Example 460 V, 60 Hz motor V/Hz 460/60
7.66 V/Hz
MOTOR SLIP
VOLTS FREQUENCY V/Hz 460 60
7.66 345 45
7.66 230 30
7.66 115 15 7.66
7.66 1 7.66
78
AC MOTOR SIZE
Frame size is directly related to base RPM, for
a given Horsepower
Example 15 HP motors of different base speeds
15 HP
15 HP
15 HP
1200 (6-pole) 284 67.5 lb-ft 19.3
3600 (2-pole) 215 22.5 lb-ft 18.5
1800 (4-pole) 254 45 lb-ft 18.7
Base RPM Frame Size Torque Amps
79
DC motor
How is the direction of the current switched??
Commutator
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Variable Torque applications for AC Drives
Pump Control
Fan Control
81
MECHANICAL AIRFLOW CONTROL
AIRFLOW
Constant Speed
MECHANICAL SYSTEMS WORK BY RESTRICTING AIRFLOW !
Outlet Damper
AIRFLOW
Constant Speed
Inlet Guide Vane
82
DRIVE AIRFLOW CONTROL
SYSTEM WORKS BY CONTROLLING SPEED, NOT
RESTRICTING AIRFLOW !
VARIABLE SPEED DRIVE
Transducer
Pressure or Volume Feedback
Speed Setpoint
83
DC motors
  • A DC motor is a mechanically commutated electric
    motor powered from direct current (DC). The
    stator is stationary in space by definition and
    therefore the current in the rotor is switched by
    the commutator to also be stationary in space.
    This is how the relative angle between the stator
    and rotor magnetic flux is maintained near 90
    degrees, which generates the maximum torque.

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  • DC motors have a rotating armature winding
    (winding in which a voltage is induced) but
    non-rotating armature magnetic field and a static
    field winding (winding that produce the main
    magnetic flux) or permanent magnet. Different
    connections of the field and armature winding
    provide different inherent speed/torque
    regulation characteristics.

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  • The speed of a DC motor can be controlled by
    changing the voltage applied to the armature or
    by changing the field current.
  • The introduction of variable resistance in the
    armature circuit or field circuit allowed speed
    control.
  • Modern DC motors are often controlled by power
    electronics systems called DC drives.

86
Application
  • The introduction of DC motors to run machinery
    eliminated the need for local steam or internal
    combustion engines, and line shaft drive systems.
  • DC motors can operate directly from rechargeable
    batteries, providing the motive power for the
    first electric vehicles.
  • Today DC motors are still found in applications
    as small as toys and disk drives, or in large
    sizes to operate steel rolling mills and paper
    machines.

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Brushless DC Motor
  • The brushed DC electric motor generates torque
    directly from DC power supplied to the motor by
    using internal commutation, stationary magnets
    (permanent or electromagnets), and rotating
    electrical magnets.
  • Like all electric motors or generators, torque is
    produced by the principle of Lorentz force, which
    states that any current-carrying conductor placed
    within an external magnetic field experiences a
    torque or force known as Lorentz force.
    Advantages of a brushed DC motor include low
    initial cost, high reliability, and simple
    control of motor speed.
  • Disadvantages are high maintenance and low
    life-span for high intensity uses. Maintenance
    involves regularly replacing the brushes and
    springs which carry the electric current, as well
    as cleaning or replacing the commutator. These
    components are necessary for transferring
    electrical power from outside the motor to the
    spinning wire windings of the rotor inside the
    motor. Brushes are made of conductors.

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Brushless DC motors
  • Typical brushless DC motors use a rotating
    permanent magnet in the rotor, and stationary
    electrical current/coil magnets on the motor
    housing for the rotor, but the symmetrical
    opposite is also possible. A motor controller
    converts DC to AC.
  • This design is simpler than that of brushed
    motors because it eliminates the complication of
    transferring power from outside the motor to the
    spinning rotor.
  • Advantages of brushless motors include long life
    span, little or no maintenance, and high
    efficiency. Disadvantages include high initial
    cost, and more complicated motor speed
    controllers. Some such brushless motors are
    sometimes referred to as "synchronous motors"
    although they have no external power supply to be
    synchronized with, as would be the case with
    normal AC synchronous motors.

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Uncommutated
  • Other types of DC motors require no commutation.
  • Homopolar motor A homopolar motor has a
    magnetic field along the axis of rotation and an
    electric current that at some point is not
    parallel to the magnetic field. The name
    homopolar refers to the absence of polarity
    change.
  • Homopolar motors necessarily have a single-turn
    coil, which limits them to very low voltages.
    This has restricted the practical application of
    this type of motor.
  • Ball bearing motor A ball bearing motor is an
    unusual electric motor that consists of two ball
    bearing-type bearings, with the inner races
    mounted on a common conductive shaft, and the
    outer races connected to a high current, low
    voltage power supply. An alternative construction
    fits the outer races inside a metal tube, while
    the inner races are mounted on a shaft with a
    non-conductive section (e.g. two sleeves on an
    insulating rod). This method has the advantage
    that the tube will act as a flywheel. The
    direction of rotation is determined by the
    initial spin which is usually required to get it
    going.

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Connection Types
  • A series DC motor connects the armature and field
    windings in series with a common D.C. power
    source. The motor speed varies as a non-linear
    function of load torque and armature current
    current is common to both the stator and rotor
    yielding I2 (current) squared behavior A series
    motor has very high starting torque and is
    commonly used for starting high inertia loads,
    such as trains, elevators or hoists.This
    speed/torque characteristic is useful in
    applications such as dragline excavators, where
    the digging tool moves rapidly when unloaded but
    slowly when carrying a heavy load.
  • With no mechanical load on the series motor, the
    current is low, the counter-EMF produced by the
    field winding is weak, and so the armature must
    turn faster to produce sufficient counter-EMF to
    balance the supply voltage. The motor can be
    damaged by over speed. This is called a runaway
    condition.
  • Series motors called "universal motors" can be
    used on alternating current. Since the armature
    voltage and the field direction reverse at
    (substantially) the same time, torque continues
    to be produced in the same direction. Since the
    speed is not related to the line frequency,
    universal motors can develop higher-than-synchrono
    us speeds, making them lighter than induction
    motors of the same rated mechanical output. This
    is a valuable characteristic for hand-held power
    tools. Universal motors for commercial power
    frequency are usually small, not more than about
    1 kW output. However, much larger universal
    motors were used for electric locomotives, fed by
    special low-frequency traction power networks to
    avoid problems with commutation under heavy and
    varying loads.

91
Shunt connection
  • A shunt DC motor connects the armature and field
    windings in parallel or shunt with a common D.C.
    power source. This type of motor has good speed
    regulation even as the load varies, but does not
    have the starting torque of a series DC motor. It
    is typically used for industrial, adjustable
    speed applications, such as machine tools,
    winding/unwinding machines and tensioners.

92
Compound connection
  • A compound DC motor connects the armature and
    fields windings in a shunt and a series
    combination to give it characteristics of both a
    shunt and a series DC motor.
  • This motor is used when both a high starting
    torque and good speed regulation is needed. The
    motor can be connected in two arrangements
    cumulatively or differentially.
  • Cumulative compound motors connect the series
    field to aid the shunt field, which provides
    higher starting torque but less speed regulation.
    Differential compound DC motors have good speed
    regulation and are typically operated at constant
    speed.

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Servomotors
  •  Servomotor is normally a simple DC motor which
    is controlled for specific angular rotation with
    help of additional servomechanism (a typical
    closed loop feedback control system).
  • A Servo is a small device that incorporates a
    three wire DC motor, a gear train, a
    potentiometer,an integrated circuit, and an
    output shaft bearing (Shown in Figure). Of the
    three wires that stick out from the motor casing,
    one is for power, one is for ground, and one is a
    control input line. The shaft of the servo can be
    positioned to specific angular positions by
    sending a coded signal. As long as the coded
    signal exists on the input line, the servo will
    maintain the angular position of the shaft. If
    the coded signal changes, then the angular
    position of the shaft changes.

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Stepper motors
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Stepper motors
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  • Features of Stepping Motors
  • Digital control of speed and position.
  • Open loop system with no position feedback
    required.
  • Excellent response to acceleration, deceleration
    and step commands.
  • Noncumulative positioning error ( 5 of step
    angle).
  • Excellent low speed/high torque characteristics
    without gear reduction.
  • Inherent detent torque.
  • Holding torque when energized.
  • Bidirectional operation.
  • Can be stalled without motor damage.
  • No brushes for longer trouble free life.
  • Precision ball bearing

99
Typical Stepping Motor Applications
  • For accurate positioning of X-Y tables, plotters,
    printers, facsimile machines, medical
    applications, robotics, barcode scanners, image
    scanners, copiers, etc.

100
Stepper motor
  • motor-bipolar stepperSTH-55D226-03 Shinano
    Kenshi Stepping motor with metal gearThis is a
    1.8 degree stepper motor with a metal gear. It
    was pulled from a printer assembly. It is perfect
    for CNC, automation robotics or remote control
    applications. Specifications
  • Nominal Voltage5
  • Current0.5
  • Wires4
  • Condition pulled
  • Steps / Revolution200
  • Step Size (degrees)1.8
  • NEMA frame size 23
  • QTY available1

101
Construction
  • There are three basic types of step motors
    variable reluctance (VR), permanent magnet (PM)
    and hybrid.
  • The hybrid type step motor design it has some of
    the desirable features of both the VR and PM. It
    has high resolution, excellent holding and
    dynamic torque and can operate at high stepping
    rate.
  • In Fig. 5-1 construction of SKC stepping motor is
    shown.
  • In Fig. 5-2 the detail of rotor construction is
    shown.

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  • The hybrid rotor has 2 sets (stacks) of
    laminations separated by a permanent magnet. Each
    set of lams has 50 teeth and are offset from each
    other by 1/2 tooth pitch. This gives the rotor 50
    N and 50 S poles at the rotor O.D.
  • Fig. 6-3 illustrates the movement of the rotor
    when the phase sequence is energized.

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  • In step 1, phase A is excited so that the S pole
    of the rotor is attracted to pole 1,5 of the
    stator which is now a N pole, and the N pole of
    the rotor is attracted to pole 3,7 of the stator
    which is a S pole now. At this point
  • there is an angle difference between the rotor
    and stator teeth of ¼ pitch (1.8 degrees). For
    instance, the stator teeth of poles 2,6 and 4,8
    are offset 1.8 degrees from the rotor teeth.
  • In step 2, there is a stable position when a S
    pole of the rotor is lined up with pole 2,6 of
    the stator and a N pole of the rotor lines up
    with pole 4,8 of stator. The rotor has moved 1.8
    degrees of rotation from step 1. The switching of
    phases per steps 3, 4 etc. produces 1.8 degrees
    of
  • rotation per step.

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Step Motor Load Calculations and Selection
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Sample Problem
  • 1) How many steps are required to achieve
  • a) one complete revolution(360 )
  • b) half revolution and(180 )
  • c) quarter of a revolution (90 )
  • If the a certain stepper motor has a a step size
    of 7.5 degree/step.

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Sample Problem
  • 2) How many steps are required to achieve
  • a) one complete revolution(360 )
  • b) half revolution and(180 )
  • c) quarter of a revolution (90 )
  • If the a certain stepper motor has a a step size
    of 1.8 degree/step.
  • SOLUTION

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SERVOMOTOR
  • A servomotor is a rotary actuator that allows for
    precise control of angular position, velocity and
    acceleration. It consists of a suitable motor
    coupled to a sensor for position feedback. It
    also requires a relatively sophisticated
    controller, often a dedicated module designed
    specifically for use with servomotors.
  • Servomotors are not a different class of motor,
    on the basis of fundamental operating principle,
    but uses servomechanism to achieve closed loop
    control with a generic open loop motor.
  • Servomotors are used in applications such
    as robotics, CNC machinery or automated
    manufacturing.

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  • As the name suggests, a servomotor is
    a servomechanism. More specifically, it is
    a closed-loop servomechanism that uses position
    feedback to control its motion and final
    position. The input to its control is some
    signal, either analogue or digital, representing
    the position commanded for the output shaft.
  • motor stops.

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The motor is paired with some type of encoder to
provide position and speed feedback. In the
simplest case, only the position is measured.
The measured position of the output is compared
to the command position, the external input to
the controller. If the output position differs
from that required, an error signal is generated
which then causes the motor to rotate in either
direction, as needed to bring the output shaft to
the appropriate position. As the positions
approach, the error signal reduces to zero and
the
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  • A servomotor is a specific type of motor and
    rotary encoder combination that forms a
    servomechanism. This assembly may in turn form
    part of another servomechanism. The encoder
    provides position and usually speed feedback,
    which by the use of a PID controller allow more
    precise control of position and thus faster
    achievement of a stable position (for a given
    motor power).

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  • Servomotors are used for both high-end and
    low-end applications, although the mid-range is
    generally handled by stepper motors. Most
    servomotors, are precision industrial components.

A servomechanism, sometimes shortened to servo,
is an automatic device that uses error-sensing
negative feedback to correct the performance of a
mechanism and is defined by its function. It
usually includes an in-built encoder.
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  • The term correctly applies only to systems where
    the feedback or error-correction signals help
    control mechanical position, speed or other
    parameters.
  • For example, an automotive power window control
    is not a servomechanism, as there is no automatic
    feedback that controls positionthe operator does
    this by observation.
  • By contrast a car's cruise control uses closed
    loop feedback, which classifies it as a
    servomechanism.

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SERVO MOTORS
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  • How is the servo controlled?
  • Servos are controlled by sending an electrical
    pulse of variable width, or pulse width
    modulation (PWM), through the control wire. There
    is a minimum pulse, a maximum pulse, and a
    repetition rate.
  • A servo motor can usually only turn 90 degrees
    in either direction for a total of 180 degree
    movement. The motor's neutral position is defined
    as the position where the servo has the same
    amount of potential rotation in the both the
    clockwise or counter-clockwise direction.
  • The PWM sent to the motor determines position of
    the shaft, and based on the duration of the pulse
    sent via the control wire the rotor will turn to
    the desired position. The servo motor expects to
    see a pulse every 20 milliseconds (ms) and the
    length of the pulse will determine how far the
    motor turns.
  • For example, a 1.5ms pulse will make the motor
    turn to the 90-degree position. Shorter than
    1.5ms moves it to 0 degrees, and any longer than
    1.5ms will turn the servo to 180 degrees, as
    diagramed below

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  • When these servos are commanded to move, they
    will move to the position and hold that position.
  • If an external force pushes against the servo
    while the servo is holding a position, the servo
    will resist from moving out of that position.
  • The maximum amount of force the servo can exert
    is called the torque rating of the servo.
  • Servos will not hold their position forever
    though the position pulse must be repeated to
    instruct the servo to stay in position.

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Types of Servo Motors
There are two types of servo motors - AC and
DC. AC servo can handle higher current surges and
tend to be used in industrial machinery. DC
servos are not designed for high current surges
and are usually better suited for smaller
applications. Generally speaking, DC motors are
less expensive than their AC counterparts. These
are also servo motors that have been built
specifically for continuous rotation, making it
an easy way to get your robot moving. They
feature two ball bearings on the output shaft for
reduced friction and easy access to the
rest-point adjustment potentiometer. 
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Servo Motor Applications Servos are used in CNC
machine tool, radio-controlled airplanes to
position control surfaces like elevators, rudders,
walking a robot, or operating grippers. Servo
motors are small, have built-in control circuitry
and have good power for their size. They can
also be used to operate remote-controlled or
radio-controlled toy cars, robots and airplanes. S
ervo motors are also used in industrial
applications, robotics, in-line manufacturing,
pharmaceutics and food services  In food
services and pharmaceuticals, the tools are
designed to be used in harsher environments,
where the potential for corrosion is high due to
being washed at high pressures and temperatures
repeatedly to maintain strict hygiene standards.
Servos are also used in in-line manufacturing,
where high repetition yet precise work is
necessary. 
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Applications
Servo motors (or servos) are self-contained
electric devices that rotate or push parts of a
machine with great precision. Servos are found
in many places from toys to home electronics to
cars and airplanes. If you have a
radio-controlled model car, airplane, or
helicopter, you are using at least a few
servos. In a model car or aircraft, servos move
levers back and forth to control steering or
adjust wing surfaces. By rotating a shaft
connected to the engine throttle, a servo
regulates the speed of a fuel-powered car or
aircraft. Servos also appear behind the scenes
in devices we use every day.
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Electronic devices such as DVD and Blu-ray
DiscTM players use servos to extend or retract
the disc trays. In 21st-century automobiles,
servos manage the car's speed The gas pedal,
similar to the volume control on a radio, sends
an electrical signal that tells the car's
computer how far down it is pressed. The car's
computer calculates that information and other
data from other sensors and sends a signal to the
servo attached to the throttle to adjust the
engine speed. Commercial aircraft use servos
and a related hydraulic technology to push and
pull just about everything in the plane.
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How does a servo motor work?
The simplicity of a servo is among the features
that make them so reliable. The heart of a servo
is a small direct current (DC) motor, similar to
what you might find in an inexpensive toy. These
motors run on electricity from a battery and spin
at high RPM (rotations per minute) but put out
very low torque (a twisting force used to do
work you apply torque when you open a jar). An
arrangement of gears takes the high speed of the
motor and slows it down while at the same time
increasing the torque. (Basic law of physics
work force x distance.) A tiny electric motor
does not have much torque, but it can spin really
fast (small force, big distance). The gear design
inside the servo case converts the output to a
much slower rotation speed but with more torque
(big force, little distance). The amount of
actual work is the same, just more useful. Gears
in an inexpensive servo motor are generally made
of plastic to keep it lighter and less costly On
a servo designed to provide more torque for
heavier work, the gears are made of metal and
are harder to damage.
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Types of servo motors
  • Servos come in many sizes and in three basic
    types positional rotation, continuous rotation,
    and linear.
  • Positional rotation servo This is the most
    common type of servo motor. The output shaft
    rotates in about half of a circle, or 180
    degrees. It has physical stops placed in the gear
    mechanism to prevent turning beyond these limits
    to protect the rotational sensor. These common
    servos are found in radio-controlled cars and
    water- and aircraft, toys, robots, and many other
    applications.

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  • Continuous rotation servo This is quite similar
    to the common positional rotation servo motor,
    except it can turn in either direction
    indefinitely. The control signal, rather than
    setting the static position of the servo, is
    interpreted as the direction and speed of
    rotation. The range of possible commands causes
    the servo to rotate clockwise or counterclockwise
    as desired, at varying speed, depending on the
    command signal. You might use a servo of this
    type on a radar dish if you mounted one on a
    robot. Or you could use one as a drive motor on a
    mobile robot.
  • Linear servo This is also like the positional
    rotation servo motor described above, but with
    additional gears (usually a rack and
    pinion mechanism) to change the output from
    circular to back-and-forth. These servos are not
    easy to find, but you can sometimes find them at
    hobby stores where they are used as actuators in
    larger model airplanes.

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Linear Motors
  • Linear motors are electric induction motors that
    produce motion in a straight line rather than
    rotational motion. In a traditional electric
    motor, the rotor (rotating part) spins inside
    the stator (static part)
  • in a linear motor, the stator is unwrapped and
    laid out flat and the "rotor" moves past it in a
    straight line. Linear motors often
    use superconducting magnets, which are cooled to
    low temperatures to reduce power consumption.
  •  

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Artwork Top Normal motor The rotor spins
inside the stator and the whole motor is fixed in
place. Bottom A linear motor is like a normal
electric motor that has been unwrapped and laid
in a straight line. Now the rotor moves past the
stator as it turns.
142
  • A linear motor is effectively an AC induction
    motor that has been cut open and unwrapped. The
    "stator" is laid out in the form of a track of
    flat coils made from aluminum or copper and is
    known as the "primary" of a linear motor. The
    "rotor" takes the form of a moving platform known
    as the "secondary."
  • When the current is switched on, the secondary
    glides past the primary supported and propelled
    by a magnetic field.

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  • Traditional linear motors were basically a
    permanent-magnet rotary motor rolled out and laid
    flat. Imagine the stator and rotor being cut
    along a radial plane and then unrolled so that
    they could provide linear thrust. Energizing the
    stationary part of the motor causes motion in the
    moving part, which typically contains some type
    of conductive material.

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Advantages
  • Linear motors have a number of advantages over
    ordinary motors.
  • Most obviously, there are no moving parts to go
    wrong. As the platform rides above the track on a
    cushion of air, there is no loss of energy to
    friction or vibration (but because the air-gap is
    greater in a linear motor, more power is required
    and the efficiency is lower).

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Uses
  • Their practical uses include magnetic levitation,
  • linear propulsion,
  • and linear actuators.
  • They have also been used for pumping liquid
    metals.

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Application of Linear Motors
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Principles of Magnetic Levitation
  • Maglev (derived from magnetic levitation) is a
    method of propulsion that uses magnetic
    levitation to propel vehicles with magnets rather
    than with wheels, axles and bearings. With
    maglev, a vehicle is levitated a short distance
    away from a guide way using magnets to create
    both lift and thrust.

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R-Maglev EDS suspension is due to the magnetic
fields induced either side of the vehicle by the
passage of the vehicle's superconducting magnets.
EDS Maglev propulsion via propulsion coils
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How it works
  • A MagLev is constantly kept afloat by
    electromagnets on the track (also called a
    guideway) and on the train's underside.
  • The opposing polarities of magnets are attracted
    to each other and the same polarities oppose each
    other.

So a MagLev would be levitated with the track's
and the train's magnets facing each other on the
opposing sides.
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1) The Levitation System
  • Support electromagnets built into the
    undercarriage and along the entire length of the
    train pull it up to the guideway electromagnets,
    which are called ferromagnetic reaction
    rails(NeFeBo).
  • The guidance magnets placed on each side of the
    train keep it centered along the track and guide
    the train along.
  • All the electromagnets are controlled
    electronically in a precise manner. It ensures
    the train is always levitated at a distance of 8
    to 10 mm from the guideway even when it isn't
    moving.
  • This levitation system is powered by onboard
    batteries, which are charged up by the linear
    generator when the train travels.
  • The generator consists of additional cable
    windings integrated in the levitation
    electromagnets. The induced current of the
    generator during driving uses the propulsion
    magnetic field's harmonic waves, which are due to
    the side effects of the grooves of the long
    stator so the charging up process does not
    consume the useful propulsion magnetic field.
  • The train can rely on this battery power for up
    to one hour without an external power source. The
    levitation system is independent from the
    propulsion system.

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Opposite poles on magnets keep train above
track Train is propelled by electro-magnetic
system in the sides of the "guideway" instead of
onboard engine Top speed (with passengers) -
450km/h (280mph) Developed by Transrapid Int in
Germany Operating commercially in Shanghai Test
facility in Emsland, northern Germany, is longest
of its kind at 31.5km (19.5 miles)
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(EMS) "Transrapid International"
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The Maglev Track
  • The magnetized coil running along the track,
    called a guideway, repels the large magnets on
    the train's undercarriage, allowing the train to
    levitate between 0.39 and 3.93 inches (1 to 10
    cm) above the guideway. Once the train is
    levitated, power is supplied to the coils within
    the guideway walls to create a unique system of
    magnetic fields that pull and push the train
    along the guideway.

The electric current supplied to the coils in the
guideway walls is constantly alternating to
change the polarity of the magnetized coils. This
change in polarity causes the magnetic field in
front of the train to pull the vehicle forward,
while the magnetic field behind the train adds
more forward thrust.
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2) The Propulsion System
  • For propulsion and braking of a MagLev, a long
    electromagnetic stator is installed underneath
    both sides of the guideway facing the train's
    support electromagnets, which resemble a motor's
    rotor. The construction of this system looks like
    the stator of a rotating motor was cut open and
    stretched along the guideway undersides and the
    rotor part is built into the undercarriage of a
    train.

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