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Electrical Machine DC By Fekade Walle

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The DC machines are versatile (adaptable) and flexible machine. It is extensively used in industry and found in a wide variety of volt-ampere, torque-speed characteristics and various connections of the field winding. DC machines can work as generators, motors & brakes. In the generator mode the machine is driven by a prime mover (such as a steam turbine or a diesel engine) with the mechanical power converted into electrical power. – PowerPoint PPT presentation

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Title: Electrical Machine DC By Fekade Walle


1
CHAPTER FOUR DC MACHINCES INTRODUCTION
  • By Fekade Walle 2016

2
CHAPTER FOUR DC MACHINCES INTRODUCTION
  • The DC machines are versatile (adaptable) and
    flexible machine. It is extensively used in
    industry and found in a wide variety of
    volt-ampere, torque-speed characteristics and
    various connections of the field winding. DC
    machines can work as generators, motors brakes.
    In the generator mode the machine is driven by a
    prime mover (such as a steam turbine or a diesel
    engine) with the mechanical power converted into
    electrical power. In the motor mode, the machine
    drives a mechanical load with the electrical
    power supplied converted into mechanical power.
    In the brake mode, the machine decelerates on
    account of the power supplied or dissipated by it
    and, therefore, produces a mechanical braking
    action.

3
DC MACHINCES APPLICATIONS
  • No doubt, application like Aircrafts, ships and
    road mounted vehicles which are isolated from
    land based A.C networks employ DC sources
    including DC generators and secondary batteries
    for power supply but the modern trend is to use
    AC generators with the DC supply being obtained
    by rectification with the help of static power
    rectifiers. D.C. generators are still being used
    to produce power in small back-up and stand-by
    generating plants driven by windmill and mountain
    streams (mini-hydro-electric plants) to provide
    uninterrupted power supply.

4
DC MACHINCES APPLICATIONS
  • Apart from DC generators, the DC motors are
    finding increasing applications, especially where
    large magnitude and precisely controlled torque
    is required. Such motors are used in rolling
    mills, in overhead cranes and for traction
    purpose like in forklift trucks, electric
    vehicles, and electric trains. They are also used
    in portable machine tools supplied from
    batteries, in automotive vehicles as starter
    motors, blower motors and in many control
    applications as actuators and as speed and
    position sensing device (tacho-generators for
    speed sensing and servomotors for positioning and
    tracing).

5
CONSTRUCTION OF DC MACHINES
  • The DC machines used for industrial applications
    have essentially three major parts
  • Field system (stator) b)
  • Armature (Rotor) and
  • c) Commutator

6
DC Machine Construction
  • The stator of the dc machine has poles, which are
    excited by either dc current or permanent magnets
    to produce magnetic fields.
  • In the neutral zone, in the middle between the
    poles, commutating poles are placed to reduce
    sparking of the commutator.
  • Compensating windings are mounted on the main
    poles. These reduces flux weakening commutation
    problems.

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9
Field System
  •  The field system is located on the stationary
    part of the machine called stator. The field
    system is designated for producing magnetic flux
    and, therefore, provides the necessary excitation
    for operation of machine. Figure 4.2 shows that
    the main flux f paths which starts from a North
    pole, crosses the air gap and then travels down
    to the armature core. There, it divides into two
    equal (f/2) halves, each half enter the nearby
    South Pole so as to complete the flux. Each flux
    line crosses the air-gap twice. Some flux lines
    may not enter the armature this flux, called the
    leakage flux, is not shown in Figure 4.2.

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12
CONTD
Figure 4.2 Flux paths in a 6-pole dc machines
13
Contd
  • The stator of dc machines comprises of 
  • 1. Main poles These poles are
    designed to produce the main magnetic flux
  • 2. Frame These provide support for the
    machine. In many machines the frame is also a
    part of the magnetic circuit. 
  • 3. Interpoles These poles are designed to
    improve commutation conditions to ensure sparkles
    operation of machine.
  •  

14
Sectional view of a DC machine

15
. Armature
  • The armature is the rotating part (rotor) of the
    dc machine where the process of electromechanical
    energy conversion takes pace. The armature is a
    cylindrical body, which rotates between the
    magnetic poles. The armature and the field system
    are separated from each other by an air gap. The
    armature consists of
  • Armature core with slots and
  • Armature winding accommodated in slots

16
Rotor and rotor winding

17
DC Machine Construction
  • The rotor coils are connected in series through
    the commutator segments.
  • The ends of each coil are connected to a
    commutator segment.
  • The commutator consists of insulated copper
    segments mounted on an insulated tube.
  • Two brushes are pressed to the commutator to
    permit current flow and they are placed in
    neutral zone.

18
DC Machine Construction
  • The rotor coils are connected in series through
    the commutator segments.
  • The ends of each coil are connected to a
    commutator segment.
  • The commutator consists of insulated copper
    segments mounted on an insulated tube.
  • Two brushes are pressed to the commutator to
    permit current flow and they are placed in
    neutral zone.

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20
.
Commutator   It is mounted on the rotor of a dc
machine and it performs with help of brushes a
mechanical rectification of power from ac to dc
in case of generators and dc to ac in case of
motors. The ends of armature coils are connected
to the commutator, which together with the
brushes rectifies the alternating e.m.f induced
in the armature coils and helps in the collection
of current. It is cylindrically shaped and is
placed at one end of the armature. The
construction of the commutator is quite
complicated because it involves the combination
of copper, iron and insulating materials. The
connection of armature conductors to the
commutator is made with the help of risers.
21
Brushes and Brush Holder
  • Brushes are needed to collect the current from
    the rotating commutator or to lead the current to
    it. Normally brushes are made up of carbon and
    graphite, so that while in contact with the
    commutator, the commutator surface is not
    spoiled. The brush is accommodated in the brush
    holder where a spring presses it against the
    commutator with pressure of 1.5 to 2.0 Ncm2 (see
    Figure 4.6). A twisted flexible copper conductor
    called pigtail securely fixed in to the brush is
    used to make the connection between the brush and
    its brush holder. Normally brush holders used in
    dc machines are of box type. The numbers of brush
    holders usually equal to the number of main poles
    in dc machines.

22
EMF equation
  • Let,
  • Ø flux per pole in weber
  • Z Total number of conductor
  • P Number of poles
  • A Number of parallel paths
  • N armature speed in rpm
  • Eg emf generated in any on of the parallel path

23
Flux cut by 1 conductor in 1 revolution
P f Flux cut by 1 conductor in 60
sec P f N /60 Avg emf generated in
1 conductor PfN/60 Number of conductors
in each parallel path Z /A Eg
PfNZ/60A
24
TYPES OF DC GENERATORS
  • The field winding and the armature winding can be
    interconnected in various ways to provide a wide
    variety of performance characteristics. This can
    be taken as outstanding advantages of a dc
    machines. A dc machine can work as an
    electromechanical energy converter only when its
    field winding is excited with direct current,
    except for small dc machines employing permanent
    magnets. According to the method of their field
    excitation dc generators are classified into the
    following group
  •  a) Separately excited and
  • b) Self excited

25
Separately-Excited and Self-Excited DC Generators
If

Separately-Excited
Self-Excited
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27
Self Excitation
  • When the field winding is excited by its own
    armature, the machines is said to be a self
    excited dc machine. In these machines, the field
    poles must have a residual magnetism, so that
    when the armature rotates, a residual voltage
    appears across the brushes. This residual voltage
    should establish a current in the field winding
    so as to reinforce the residual flux. According
    the connection of the field winding with the
    armature winding, a self-excited dc machine can
    be sub-divided as follows

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30
Compound Excitation
  • A compound excitation involves both series-exited
    winding and the shunt-excited winding. From the
    view point of connections, a dc compound machine
    may have short- shunt connection or a long shunt
    connection. In short shunt connection of Figure
    4.15 (a) the shunt field or voltage excited
    winding is connected across the armature
    terminals. In long-shunt connection, the shunt
    field is connected across
  • the series connection of the armature and series
    winding or the machine or line terminals as shown
    in Figure 4.15 (b).
  • However there is appreciable difference in the
    operating characteristics of short-shunt and long
    shunt. The choice between the two types depends
    on mechanical considerations of connections or
    reversing switches.

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32
The Induce Torque Equations Of Real Machines
  • The torque in any dc machine depends on three
    factors
  • The flux F in the machine
  • The armature (or rotor) current IA in the machine
  • A constant depending on the construction of the
    machine

The torque on the armature of a real machine the
number of conductors Z x the torque on each
conductor
33
Power Flow and Losses in DC Machines
  • Electrical or copper losses (I2 R losses)
  • Brush losses
  • Core losses
  • Mechanical losses
  • Stray load losses

Brush losses
Core losses
Copper losses
the hysteresis losses and eddy current losses
occurring in the metal of the motor. These losses
vary as B2 and, for the rotor, as the (n1.5)
Armature loss Field loss
34
Power Flow and Losses in DC Machines
  • Mechanical losses
  • Friction losses are losses caused by the friction
    of the bearings in the machine
  • Windage losses are caused by the friction between
    the moving parts of the machine and the air
    inside the motor's casing
  • Stray losses
  • Unknown losses
  • By convention to be 1 percent of full load

35
Power Division in DC Machines
36
Efficiency
The losses are made up of rotational losses
(3-15), armature circuit copper losses (3-6),
and shunt field copper loss (1-5). The voltage
drop between the brush and commutator is 2V and
the brush contact loss is therefore calculated as
2Ia.
37
ARMATURE REACTION
  • If a load is connected to the terminals of the dc
    machine, a current will flow in its armature
    windings. This current flow will produce a
    magnetic field of its own, which will distort the
    original magnetic field from the machines field
    poles. This distortion of the magnetic flux in a
    machine as the load is increased is called the
    armature reaction.

38
ARMATURE REACTION
  • By armature reaction is meant the effect of
    magnetic field set up by armature current on the
    distribution of flux under main poles. In other
    words armature reaction is meant the effect of
    armature ampere-turns upon the value and the
    distribution of the magnetic flux entering and
    leaving the armature core.
  • It demagnetizes or weakens the main flux
  • It cross magnetizes or distorts it

39
COMMUTATION
  • Commutation is a process of converting AC
    armature voltage into DC or vice-versa with the
    aid of mechanical switching device called
    commutator. Armature conductors carry current in
    one direction when they are under N-pole and in
    opposite direction when they are under the
    influence of S-pole. So when the conductors come
    under the influence of the S-pole from the
    influence of N-pole, the direction of flow of
    current in them is reversed. The process of
    reversal of current in a coil is termed as
    commutation.

40
COMMUTATION contd
  • The period during which the coil remains
    short-circuited is called commutation period, Tc.
    This commutation period is very small and is in
    the order of 0.001 to 0.003s. Good commutation
    means no sparking at the brushes. A machine is
    said to have poor commutation if there is
    sparking clearly seen at the brushes and the
    commutator surface. This leads to damage the
    machine during operation. Poor commutation may be
    caused by electrical or mechanical reasons.
  • The mechanical reasons may be due to non-
    uniform brush pressure, uneven commutator
    surface, uneven air gap due to damage of ball
    bearings, etc.
  • The electrical reasons are due to Armature
    reaction effect and Self induced emf in the
    armature windings

41
Methods of improving commutation
  • There have been adapted two practical ways of
    improving commutation i.e. of making current
    reversals in the short-circuited coil as sparkles
    as possible. The two methods are
  • By using inter poles,
  • Compensating windings and
  • Brush shifting (for small machines),

42
Commutating poles or interpoles
  • These are small poles fixed to the yoke and
    spaced in between the main poles. They are wound
    with comparatively few heavy gauge copper wire
    turns and are connected in series with the
    armature so that they carry full armature
    current. Their polarity, in the case of a
    generator, is the same as that of the main pole
    and For a motor, the polarity of the interpole
    must be the same as that of the main pole.

43
Solutions to Problems with Commutation in Real
Machines
  • Commutating poles or interpoles
  • It cancels the voltage in the coils undergoing
    commutation
  • interpole windings are in series with the rotor
    windings
  • as the rotor current incleases flux produced by
    interpole also inceases
  • producing an oppssing effect to that of neutral
    plan shift

44
Compensating winding
  • The effect of cross-magnetization can be
    neutralized using compensating winding. These are
    conductors embedded in pole faces, connected in
    series with the armature windings and carrying
    current in an opposite direction to that flowing
    in the armature conductors under the pole face.
  • Compensating winding
  • Solves the problem of flux weakening and neutral
    plane shift
  • Compensating windings are in series with the
    rotor windings placing in slots carved in the
    faces of the poles parallel to the rotor
    conductors

45
DC Generator Characteristics
  • In general, three characteristics specify the
    steady-state performance of a DC generators
  • Open-circuit characteristics generated voltage
    versus field current at constant speed.
  • External characteristic terminal voltage versus
    load current at constant speed.
  • Load characteristic terminal voltage versus
    field current at constant armature current and
    speed.

46
Open-circuit characteristics generated
voltage versus field current at constant speed.
  • Consider, the emf generated in the armature
    winding of a DC machine under no load condition.
    It is given by
  • Since, P, Z and a are constants for a particular
    generator, hence at constant given speed.
  • The generated emf is directly proportional to the
    flux per pole (speed being constant), which in
    turns depends upon the field current If.
  • The characteristic curve plotted between
    generated emf Eg and the field current If at
    constant speed of rotation is called the
    magnetization curve or O.C.C. of the DC
    generator.
  • The magnetization characteristics of a separately
    excited generator or shunt generator can be
    obtained as explained below.

47
Circuit diagram and magnetization
characteristics of shunt DC generator
48
Internal characteristics
  • Internal characteristics gives the relation
    between the induced armature e.m.f, E and the
    armature current Ia.
  • Let us consider a separately-excited generator
    giving its rated no-load voltage of E0 for a
    certain constant field current. If there were no
    armature reaction and armature voltage drop, then
    this voltage would have remained constant as
    shown in figure by the horizontal line 1. But
    when the generator is loaded, the voltage falls
    due to these two causes, thereby giving slightly
    dropping characteristics.

49
  • If we subtract from E0 the values of voltage
    drops due to armature reaction for different
    loads, then we get the value of E-the e.m.f
    actually induced in the armature under load
    conditions. Curve 2 is plotted in this way and is
    known as the internal characteristic.

50
DC Generator Characteristics
The terminal voltage of a dc generator is given
by
Open-circuit and load characteristics
51
External characteristics
  • External characteristic terminal voltage versus
    load current at constant speed.
  • The armature reaction
  • voltage drop in the armature winding, series ,
    inter pole and compensating windings
  • voltage drop at the brush contact( 0.8- 1,0-V per
    brush ) and
  • The drop in terminal voltage due to (i) and (ii)
    results in a decreased field current which
    further reduces the induced emf.

52
DC Generator Characteristics
It can be seen from the external characteristics
that the terminal voltage falls slightly as the
load current increases. Voltage regulation is
defined as the percentage change in terminal
voltage when full load is removed, so that from
the external characteristics,
External characteristics
53
Self-Excited DC Shunt Generator
Maximum permissible value of the field resistance
if the terminal voltage has to build up.
Schematic diagram of connection
Open-circuit characteristic
54
Speed Control in DC Motors
55
Speed Control in Shunt DC Motors
Armature Voltage Control Ra and If are kept
constant and the armature terminal voltage is
varied to change the motor speed. For
constant load torque, such as applied by an
elevator or hoist crane load, the speed will
change linearly with Vt. In an actual
application, when the speed is changed by varying
the terminal voltage, the armature current is
kept constant. This method can also be applied to
series motor.
56
Speed Control in Shunt DC Motors
Field Control Ra and Vt are kept constant,
field rheostat is varied to change the field
current. For no-load condition, Te0. So,
no-load speed varies inversely with the field
current. Speed control from zero to base speed
is usually obtained by armature voltage control.
Speed control beyond the base speed is obtained
by decreasing the field current. If armature
current is not to exceed its rated value (heating
limit), speed control beyond the base speed is
restricted to constant power, known as constant
power application.
57
Speed Control in Shunt DC Motors
Armature Resistance Control Vt and If are kept
constant at their rated value, armature
resistance is varied. The value of Radj can
be adjusted to obtain various speed such that the
armature current Ia (hence torque, TeKafdIa)
remains constant. Armature resistance control is
simple to implement. However, this method is less
efficient because of loss in Radj. This
resistance should also been designed to carry
armature current. It is therefore more expensive
than the rheostat used in the field control
method.
58
Speed Control in Series DC Motors
Armature Voltage Control A variable dc voltage
can be applied to a series motor to control its
speed. A variable dc voltage can be obtained from
a power electronic converter. Torque in a
series motor can be expressed as
59
Speed Control in Series DC Motors
Field Control Control of field flux in a sries
motor is achieved by using a diverter
resistance. The developed torque can be expressed
as.
60
Speed Control in Series DC Motors
61
Speed Control in Series DC Motors
Armature Resistance Control Torque in this case
can be expressed as Rae is an external
resistance connected in series with the
armature. For a given supply voltage and a
constant developed torque, the term
(RaRaeRsKwm) should remain constant.
Therefore, an increase in Rae must be accompanied
by a corresponding decrease in wm.
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