Electrical Machine DC By Fekade Walle

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CHAPTER FOUR DC MACHINCES INTRODUCTION

• By Fekade Walle 2016

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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.

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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.

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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).

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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

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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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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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CONTD
Figure 4.2 Flux paths in a 6-pole dc machines
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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.
•  

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Sectional view of a DC machine

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. 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

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Rotor and rotor winding

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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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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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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.
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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.

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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

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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
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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

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Separately-Excited and Self-Excited DC Generators
If

Separately-Excited
Self-Excited
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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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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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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
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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
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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

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Power Division in DC Machines
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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.
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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.

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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

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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.

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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

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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),

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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.

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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

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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

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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.

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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.

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Circuit diagram and magnetization
characteristics of shunt DC generator
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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.

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• 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.

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DC Generator Characteristics
The terminal voltage of a dc generator is given
by
Open-circuit and load characteristics
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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.

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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
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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
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Speed Control in DC Motors
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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.
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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.
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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.
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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
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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.
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Speed Control in Series DC Motors
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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.