Electrical Actuators - PowerPoint PPT Presentation

Title: Electrical Actuators


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Sensors and Actuators
Week 4 PE -4030 Electrical Actuators

• Prof. Charlton S. Inao
• Mechatronics
• Defence University
• College of Engineering
• Bishoftu, Ethiopia

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Topics
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Relay

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

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• 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.

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Relay
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Example omron Relay
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Simple Relay Circuit
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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)

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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.
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• Magnetic contactors are electromagnetically
  operated switches that provide a safe and
  convenient means for connecting and interrupting
  branch circuits.

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• 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.

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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.

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• 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.

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• 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.

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• 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
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Direct On Line(DOL)
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Direct On Line(DOL)
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Mini Circuit Breakers(MCB)
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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.
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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.
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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.
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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.
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Overload Relay(OLR)
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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

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• 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.

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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.

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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

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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

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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.

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• 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. 

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Commutator
Brush
Armature Winding
Armature
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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).

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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

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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.

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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

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Squirrel Cage Induction Motor
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• 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

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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

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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.

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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

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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.
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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

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• 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

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• 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
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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
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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
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MECHANICAL AIRFLOW CONTROL
AIRFLOW
Constant Speed
MECHANICAL SYSTEMS WORK BY RESTRICTING AIRFLOW !
Outlet Damper
AIRFLOW
Constant Speed
Inlet Guide Vane
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DRIVE AIRFLOW CONTROL
SYSTEM WORKS BY CONTROLLING SPEED, NOT
RESTRICTING AIRFLOW !
VARIABLE SPEED DRIVE
Transducer
Pressure or Volume Feedback
Speed Setpoint
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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.

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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.

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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.

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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

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Typical Stepping Motor Applications

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

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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

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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.
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• 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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