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Chapter Outline
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Introduction
Induced voltage
Electromagnetic force, f
Simple loop generator
The voltage induced in a rotating loop
The induced torque in the rotating loop
Practical generator
Armature winding
Problems with commutation in real machines
Solutions to the problems with commutation
The internal generated voltage and induced torque equations of real dc machines
Power flow and losses in dc machines
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Introduction
 Electric machines convert electrical energy(power) to
mechanical energy(power) or vice versa.
 This process of conversion is known as
electromechanical energy conversion.
 An electric machine is therefore a link between an
electrical system and a mechanical system.
 In these machines the conversion is reversible.
 If the conversion is from mechanical to electrical
energy, the machine is said to act as a generator.
 If the conversion is from electrical to mechanical
energy, the machine is said to act as a motor.
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Introduction cont…
Conversion of energy from electrical to
mechanical form or vice versa results from the
following
 When a conductor moves in a magnetic field, voltage is induced in the
conductor. (Generator action)
 When a current carrying conductor is placed in a magnetic field, the conductor
experiences a mechanical force. (Motor action)
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Introduction cont…
 These two effects occur simultaneously whenever energy
conversion takes place from electrical to mechanical or
vice versa.
 In motoring action
 the electrical system makes current flow through conductors
that are placed in the magnetic field.
A force is produced on each conductor.
If the conductors are placed on a structure free to
rotate, an electromagnetic torque will be produced,
tending to make the rotating structure rotate at some
speed.
If the conductors rotate in a magnetic field, a voltage
will also be induced in each conductor.
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Introduction cont…
 In generating action,
 the process is reversed.
 The rotating structure, the rotor, is driven by a prime mover
(such as a steam turbine or a diesel engine).
 A voltage will be induced in the conductors that are rotating with
the rotor.
 If an electrical load is connected to the winding formed by these
conductors, a current i will flow, delivering electrical power to
the load.
 Moreover, the current flowing through the conductor will
interact with the magnetic field to produce a reaction torque,
which will tend to oppose the torque applied by the prime mover.
 Note: In both motor and generator actions, the coupling
magnetic field is involved in producing a torque and an
induced voltage.
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Introduction cont…
DC machines are generators that convert
mechanical energy to DC electric energy and
motors that convert DC electric energy to
mechanical energy.
Most DC machines are like AC machines in that
they have AC voltages and currents within them
DC machines have a DC output only because a
mechanism exists that converts the internal ac
voltages to dc voltages at their terminals. Since
this mechanism is called a commutator, dc
machinery is also known as commutating
machinery.
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Induced voltage
Voltage induced in a conductor moving at speed v
in a magnetic field ,B is given as.
eind = ( v x B) • I
Where
v is speed
B is magnetic field
I is length
If all the three are
perpendicular to each other
as in this figure,
eind = B lv
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Electromagnetic force, f
 For the current-carrying conductor shown in Fig.3.3(a),
the force (known as Lorentz force) produced on the
conductor can be determined from the following
equation:
F = i(lxB)
Where
i is current
B is magnetic field
I is length
If B is perpendicular
to l as in this figure,
f = Bli
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Simple loop generator
Fig 1.1
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Simple loop generator
Fig 1.2
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Simple loop generator
In the figure (fig 1.1 and 1.2)is shown a single
turn rectangular copper coil ABCD moving about
its own axis, a magnetic field provided by either
permanent magnets or electromagnets.
The two ends of the coil are joined to two sliprings or discs a and b which are insulated from
each other and from the central shaft.
Two collecting brushes (of carbon or copper) (1
and 2)press against the slip rings.
Their function is to collect the current induced in
the coil and to convey it to the external load
resistance R.
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Working Theory
Imagine the coil to be rotating in clockwise
direction
As the coil assumes successive positions in the
field, the flux linked with it changes.
Hence, an EMF is induced in it which is
proportional to the rate of change of flux linkages.
When the coil plane is perpendicular to the lines
of the flux(position 1),flux linked with the coil is
maximum but the rate of change of flux linkage is
minimum.
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Working Theory cont..
Fig 1.2
At this position 1,
the coil sides AB and CD run parallel to the line of
the flux(slide along).
The induces emf=NdΦ/dt=0
Take this as start position(angle of rotation θ=00)
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Working Theory cont..
Until position 3,dΦ/dt increases.
At position 3:
Θ=900
The coil is horizontal(parallel to flux lines)
AB and CD run perpendicular to the line of the
flux
The flux linked with the coil is minimum
But rate of change of flux linkage is maximum
So,maximum emf is induced
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Working Theory cont..
From 900 to 1800,the flux linked with the loop
gradually increases but dΦ/dt decreases
So the emf decreases gradually and is zero at
position 5(1800).
During this half cycle,
The emf is from A to B and C to D.
Direction of current ABMLCD
Current through R is M to L
Half revolution of the coil completes
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Working Theory cont..
In the next half revolution(i,e 1800 to 3600)
The variations in emf are similar to those in first
half
Maximum in position 7
Minimum in position 1
But the direction of induced current is reversed (D
to C and B to A)
Path of current DCLMBA (reversed)
The current through R is L to M(reversed)
i.e The current through R is AC
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Commutation
Fig 1.3
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Commutation
Fig 1.4
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Commutation
 For making the flow of current unidirectional in the
external circuit, the slip rings are replaced by split rings
which shown in Fig 1.3 and 1.4.
 The split rings are made out of a conducting cylinder
which is cut into two halves or segments insulated from
each other by a thin sheet of mica or some other
insulating material.
 As before, the coil ends are joined to these segments on
which rest the carbon or copper brushes.
 In the practical generator which has more than two
poles and more than one coil the split rings has not just
two halves but has many parts as shown in Fig1.4 (b).
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Commutation
It is seen (Fig13.(a) to Fig1.3 (b)) that in the
first half revolution current flows along
ABXYCD i.e. the brush No. 1 in contact with
segment a acts as the positive end of the
supply and b as the negative end.
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Commutation
In the next half revolution (fig 1.5), the
direction of the induced current in the coil has
reversed.
But at the same time, the positions of segments
a and b have also reversed with the result that
brush No. 1 comes in touch with that segment
which is positive i.e segment b in this case.
Hence, the current in the load resistance again
flows from X to Y.
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Commutation
The waveform of the current through the external
circuit is as shown in Fig1.6.
This current is unidirectional but not continuous
like pure direct current.
It should be noted that the position of brushes is
so arranged that the changeover of segments a and
b from one brush to the other takes place when
the plane of the rotating coil is at right angles to
the plane of the flux lines.
It is so because in that position, the induced EMF
in the coil is zero.
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Commutation
Fig 1.5
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Voltage o/p of simple loop gen
Fig 1.6
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The voltage induced in a rotating loop
Fig 1.7
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The voltage induced in a rotating loop
If the rotor of this machine is rotated, a voltage
will be induced in the wire loop(fig 1.7).
The voltage on each segment isgiven by
Equation
eind = ( v x B) • I
Segment BA: eind = vBl under the pole
eind = 0 beyond the pole adges
Segment BC and DA : eind = 0 under the pole
eind = 0 beyond the pole adges
Segment CD: eind = vBl under the pole
eind = 0 beyond the pole adges
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The voltage induced cont…
 The total induced voltage on the loop eind is given by
 When the loop rotates through 180°, segment ab is
under the north pole face instead of the south pole face.
 At that time(180°), the direction of the voltage on the
segment reverses, but its magnitude remains constant.
 The resulting voltage etot is shown as a function of time
in Figure 8- 3.
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The voltage induced cont…
Fig 1.8
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The voltage induced cont…
Notice that the tangential velocity v of the
edges of the loop can be expressed as v = rω
where r is the radius from axis of rotation out to
the edge of the loop and w is the angular velocity
of the loop.
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The voltage induced cont…
Notice also from Figure 8-4 that the rotor
surface is a cylinder, so the area of the rotor
surface A is just equal to 2πrl.
Since there are two poles, the area of the rotor
under each pole (ignoring the small gaps
between poles) is Ap = πrl.
Therefore,
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The voltage induced cont…
Since the flux density B is constant
everywhere in the air gap under the pole faces,
the total flux under each pole is just the area of
the pole times its flux density:
Therefore, the final form of the voltage
equation is
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The voltage induced cont…
Thus, the voltage generated in the machine is
equal to the product of the flux inside the
machine and the speed of rotation of the
machine, multiplied by a constant representing
the mechanical construction of the machine.
In general, the voltage in any real machine will
depend on the same three factors:
1. The flux in the machine
2. The speed of rotation
3. A constant representing the construction of the
machine
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DC voltage of single loop…
The induced voltage is converted in to DC in
the external circuit by the commutator (split
ring) and brushes
Fig 1.9
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DC voltage of single loop…
Fig 1.10
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The Induced Torque in the Rotating
Loop
Fig 1.11
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The Induced Torque in the Rotating
Loop
Suppose a battery is now connected to the
machine
The approach to take in determining the torque on
the loop is to look at one segment of the loop at a
time and then sum the effects of all the individual
segments.
The force on a segment of the loop is given by
Equation
The torque on the segment is given by
where θ is the angle between r and F.
The torque is essentially zero whenever the loop is
beyond the pole edges.
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The Induced Torque cont…
While the loop is under the pole faces, the
torque is
Segment BA:
Segment BC and DA
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The Induced Torque cont…
Segment AC and DA
The resulting total induced torque on the loop
is given by
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The Induced Torque cont…
 By using the facts that Ap=πrl and ϕ = ApB, the torque expression
can
be reduced to
 Thus, the torque produced in the machine is the product of the flux
in the
 machine and the current in the machine, times some quantity
representing the mechanical construction of the machine (the
percentage of the rotor covered by pole faces).
 In general, the torque in any real machine will depend on the same
three factors:
1.
2.
3.
The flux in the machine
The current in the machine
A constant representing the construction of the machine
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The Induced Torque cont…
 Example 1-1. (next figure)
The next figure shows a simple rotating loop between
curved pole faces connected to a battery and a resistor
through a switch. The resistor shown models the total
resistance of the battery and the wire in the machine.
The physical dimensions and characteristics of this
machine are:
r=0.5m,R=0.3Ω,l=1.0m,VB=120v(battery
voltage),B=0.25T
a) What happens when the switch is closed?
b) What is the machine's maximum starting current?
What is its steady-state angular
velocity at no load?
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The Induced Torque cont…
c)
Suppose a load is attached to the loop, and the resulting load
torque is 10 N· m.
What would the new steady-state speed be? How much power is
supplied to the
shaft of the machine? How much power is being supplied by the
battery? Is this
machine a motor or a generator?
d) Suppose the machine is again unloaded, and a torque of 7.5 N • m
is applied to
the shaft in the direction of rotation. What is the new steady-state
speed? Is this
machine now a motor or a generator?
e) Suppose the machine is running unloaded. What would the final
steady-state
speed of the rotor be if the flux density were reduced to 0.20 T?
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The Induced Torque cont…
Fig 1.12
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Practical generator
 The simple loop generator has been considered in detail merely to
bring out the basic principle underlying the construction and
working of as actual generator illustrated in Fig.3.13 which consists
of the following essential parts:
i.
Magnetic Frame or Yoke
ii. Pole-cores and Pole-shoes
iii. Pole Coils or Field Coils
iv. Armature Core
v. Armature Windings or
vi. Commutator
vii. Brushes and Bearings
Of these, the yoke, the pole cores, the armature core and air gaps
between the poles and the armature core form the magnetic circuit
whereas the rest form the electrical circuit.
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Practical generator cont…
Fig 1.13
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Practical generator cont…
i. Yoke
The outer frame or yoke serves
 Provides mechanical support for the poles and acts as
a protecting cover for the whole machine
 Caries the magnetic flux produced by poles
ii. Pole cores and pole shoes
 The field magnets consist of pole cores and pole
shoes.
 The pole shoes serve two purposes
1.
2.
Spread out the flux in the air gap and being of larger
crossection ,reduce the reluctance of the magnetic path
Support field coils
 They are made of lamminated steel
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Practical generator cont…
iii. Pole coils or field coils
 Consist of copper wires are former wound for
correct dimension
 The former is removed and the wound coil is put
in to place over the core
 When current passes through these coils,
electromagnetic poles formed to produce flux to
be cut by the armature conductors
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Practical generator cont…
iv. Armature core
 Houses armature conductors(coils) and cause
them to rotate and hence cut the magnetic flux
produced by the poles.
 Slots to place armature coils
 Lamminated steel discs(to reduce eddy current
loss) and perforated (for cooling)
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Practical generator cont…
v. Armature windings
 Usually former wound
 Then put in the armature slots lined with tough
insulating material
 The conductors of the coil insulated from each
other
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Practical generator cont…
vi. Commutator
 The function of the commutator is to facilitate
collection of current from the armature
conductors i.e. converts the alternating current
induced in the armature conductors into
unidirectional current in the external load circuit.
 It is of cylindrical structure and is built up of
wedge-shaped segments of high conductivity
hard-drawn or drop-forged
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Practical generator cont…
vii. Brushes and bearings
 Brush:






Function to collect current from commutator
Made of carbon or graphite
In the shape of rectangular box
Adjustable spring tension to press it against the
commutator
Flexible copper at the top to carry current from brushes to
holder
Number of brushes per spindle is according to amount of
current to be collected
 Bearing

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To couple the moving part(armature) and stationary
part(stator)
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Armature winding
 Two types of windings mostly employed for the
armatures of DC machines are known as Lap
Winding and Wave Winding.
 The difference between the two is merely due to
the different arrangement of the end connections
at the front or commutator end of armature.
 The coils of rotor part or armature are arranged
in many options which influence the performance
of DC machines.
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Armature Windings cont…
 But before doing this, we have to explain many
terms as shown in the following items
 Pole-pitch
 The periphery of the armature divided by the
number of poles of the generate.
 i.e. the distance between two adjacent poles.
 It is equal to the number of armature conductors
(or armature slots) per pole.
 Example:If there are 400 conductors and 4 poles,
then pole pitch is 400/4= 100 conductor
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Armature Windings cont…
 Conductors:








See fig 1.13
The conductors AB and CD along with their ends
constitute one coil of the winding
The coil maybe single turn(fig 1.13 a) or multi turn(fig
1.13b)
Single turn coil has two conductors per coil
Multi turn coil has many conductors per coil side
In multi turn conductors are wrapped with a tape as a
single unit(coil side) and placed in armature slot
Number of commutator bars = nuber of coils for both lap
and wave windings
Coil side of a coil(1-turn or multi) is called winding
element
Number coilside=twice number of coils
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Armature Windings cont…
 fig 1.14
A
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b
c
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Armature Windings cont…
 Coil span or Coil pitch(Ys)








It is the distance, measured in terms of armature slots (or
armature conductors), between two sides of a coil.
It is, in fact, the periphery of the armature spanned by the two
sides of the coil as shown in Fig.3.15.
If coil span = pole pitch then full pitch winding
Full pitch means coil span is 1800 electrical degrees
In full pitch, coil sides lie under opposite poles and their
induced emf is scalar sum
So maximum emf is induced in full pitche winding
If coil span is less than pole pitch, fractional pitched winding
In fractional pitched winding, induced voltage around the coil
is vector sum of the coilside voltage so less than full pitched
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Armature Windings cont…
Coil-pitch/coil span
Ys =integer(no of slots/no of poles)
If no of slots/no of poles is integer, full pitch
Example:
If 36 slot and 4 poles,Ys=36/4=9
full pitch
If 35 slot and 4 poles,Ys=int(35/4)
=int(8+3/4)=8
fractional pitch
Fig 1.15
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Armature Windings cont…
 Back Pitch
 The distance, measured in terms of the armature
conductors(slots) which a coil advances on the
back of the armature is called back pitch and is
denoted by Yb.
 In other words, it is the number difference of the
conductors connected together at the back end of
the armature.
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Armature Windings cont…
 Front Pitch
 The number of armature conductors or elements
spanned by a coil on the front (or commutator end of
an armature) is called the front pitch and is
designated by Yf.
 Alternatively, the front pitch may be defined as the
distance (in terms of armature conductors) between
the second conductor of one coil and the first
conductor of the next coil which are connected
together to commutator end of the armature.
 Both front and back pitches for lap and wavewinding are shown in Fig. 3.16 and Fig.3.17
respectively
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Lap winding
Fig 1.16
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Wave winding
Fig 1.17
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Armature Windings cont…
 Resultant Pitch
 It is the distance between the beginning of one
coil and the beginning of the next coil to which it
is connected as shown in Fig.3.16 and Fig.3.17.
 As a matter of precaution, it should be kept in
mind that all these pitches, though normally
stated in terms of armature conductors(coil
sides), are also sometimes given in terms of
armature slots or commutator bars.
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Armature Windings cont…
 Commutator Pitch (Yc)
 It is the distance (measured in commutator bars
or segments) between the segments to which the
two ends of a coil are connected as shown in
Fig.3.18.
 Equal to number of bars b/n coil leads
 For lap winding, Yc = Yb - Yf and equal to its
‘plex’,
 For wave winding: Yc = Yb + Yf
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Armature Windings cont…
Fig 1.18
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b)
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Armature Windings cont…
 Single-layer Winding


It is that winding in which one conductor or one coil side is
placed in each armature slot as shown in Fig 1.19.
Such a winding is not much used.
 Two-Layer Winding




In this type of winding,' there are two conductors or coil sidesper slot arranged in two layers.
Usually, one side of every coil lies in the upper half of one
slot and other side lies in the lower half of some other slot at a
distance of approximately one pitch away (Fig 1.19).
The transfer of , the coil from one slot to another is usually
made in a radial plane by means of a peculiar bend or twist at
the back
Coil sides(slots) at upper side are numbered odd, but those in
the lower are numbered even
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Armature Windings cont…
Fig 1.19
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Armature Windings cont…
 Multiplex Winding
 In such winding, several sets of complete closed and
independent windings
 Increases number of parallel paths in a winding
 Increases current rating
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Armature Windings cont…
 Lap and wave windings





The winding types of the armature of the DC machines
can be divided to two main types, Lap and Wave
windings.
The main difference between them is the method of
connecting the terminals of coils to the commutator
segments.
In case of lap wound there are two kinds, the simplex lap
winding and multiple lap winding.
For simple lap winding the two terminals of one coil is
connected to adjusent two commutator segments as
shown in Fig 1.18.
Each of them can be arranged progressively or
retrogressively
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Armature Windings cont…
 The following rules apply to both
 Yf and Yb are approximately equal to pole
pitch(to get full pitch)
 Yf and Yb should be odd
 Number of commutator segment=no of slot=no
of coils=1/2 no of coil sides
 Winding must close up on itself
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Simplex lap winding
1. Yb and Yf are odd and opposite sign, can’t be
equal, differ by two or multiples of two
2. Yb and Yf nearly equal to pole pitch
3. Average pitch YA=(YB+YF)/2=pole pitch=Z/p
4. YC  1 (in general YC  m) m is multiplici ty
5. YR=YB-YF is even ,i.e YR=2m
6. The number of slots for 2-layer =no of coils(half
of no of coil sides)=no of commutator segments
7. Number of parallel paths=mp.
m=multiplicity
p=no of poles
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Simplex lap winding
For a simplex lapwinding
Z
P
Z

P
Z

P
Z

P
YF 
YB
YF
YB

 1


 1



 1


 1


for progressiv e winding
for retrogress iv e winding
Z
must be ev en to mak ethe winding possible
P
So the winding table for simplex lap winding is
Back connection
Front connection
1to (1  YB )  C 2
C 2 to (C 2  YF )  C3
Where
C3 to (C3  YB )  C 4
C 4 to (C 4  YF )  C5
.
.
.
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Simplex lap winding
 Example
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Position and number of brushes
 It is very important that brushes are in correct position
relative to the field poles.
 By right-hand rule is used to identify the direction of
e.m.f. in each conductor
 A positive brush will be placed on that commutator
segment where the currents in the coils are meeting to
flow out of the segment.
 A negative brush will be placed on that commutator
segment where the currents in the coils are meeting to
flow in.
 in a simplex lap winding, the number of brushes is
equal to the number of poles.
 Brushes of the same polarity are connected together,
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Position and number of brushes
 Therefore, in a simplex lap winding, the
number of parallel paths is equal to the
number of poles.
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Equalizing Connections






Because of wear in the bearings, and for other reasons, the air gaps
in a generator become unequal and, therefore, the flux in some
poles becomes greater than in others.
This causes the voltages of the different parallel paths to be
unequal and unequal additional circulating currents in the brush.
To relieve the brushes of these circulating currents, points on the
armature that are at the same potential are connected together by
means of copper bars called equalizer rings.
The equalizers provide a low resistance path for the circulating
current. As a result, the circulating current due to the slight
differences in the voltages of the various parallel paths passes
through the equalizer rings instead of passing through the brushes.
Equalizer rings should be used only on windings in which the
number of coils is a multiple of the number of poles.
Satisfactory results are obtained by connecting about every third
coil to an equalizer ring.
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Simple lap winding
 Example:
Draw a developed diagram of a simple 2-layer lap winding
for a 4-pole generator with 12 coils.
Solution:
The number of commutator segments  12
Number of coil sides Z  12 * 2  24
POle pitch  24 / 4  6
Note that YB and YF shoul be odd and differ by 2 or multiples of it
For progressiv e winding
Z
YF   1  5
P
Z
YB   1  7
P
The winding table is
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Example
Back Connection
Front Connection
1 to (1+7) = 8
To
8 to (8-5) = 3
3 to (3+7) = 10
To
10 to (10-5) = 5
5 to (5+7) = 12
To
12 to (12-5) = 7
7 to (7+7) = 14
To
14 to (14-5) = 9
9 to (9+7) = 16
To
16 to (160-5) = 11
11 to (11+7) = 18
To
18 to (18-5) = 13
13 to (13+7) = 20
To
20 to (20-5) = 15
15 to (15+7) = 22
To
22 to (22-5) = 17
17 to (17+7) = 24
To
24 to (24-5) = 19
19 to (19+7) = 26 = 26-24 = 2
To
2 to (26-5) = 21
21 to (21+7) = 28 = 28-24 = 4
To
4 to (28-5) = 23
23 to (23+7) = 30 = 30-24 = 6
To
6 to (30-5) = 25-24 = 1
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Example
Equalizer Bars
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Example
Ia/4
Ia/2
Ia/4
Ia
Ia/2
Ia/4
Ia/4
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Simplex Wave Winding


Assuming a two layer winding and supposing that conductor AB
lies in the upper half of the slot ,then going once around the
armature, the winding ends at A’B’ which must be at the upper
half of the slot at the left (retrogressive) or right (prograssive).
Counting in terms of conductors, it means that AB and A’B’ differ
by two conductors(conductor sides) although they differ by one
slot.
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Simplex Wave cont…
If P  no of poles , then
YB  back pitch 
 nearly equal to the pole pitch
YF  front pitch
Then average pitch YA 
YB  YF
2
If Z is no of coil sides
Z 2
P
Since P is always even, Z  PYA  2 must also be even
Then
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Design of Simplex Wave Winding
i.
Both pitches YB and YF are odd and are of the same
sign.
ii. Average pitch,YA, should be an integer,
iii. Both YB and YF are nearly equal to pole pitch and
may be equal or differ by 2. If they differ by 2, they
are one more and one less than YA.
iv. Commutator pitch is given by;
+sign for progressive winding
- negative for retrogressive winding.
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Design of Simplex Wave Winding
v. Average pitch is given by
Since YA must be a whole number, there is a
restriction on the value of Z.
vi.
vii.Number of parallel paths = 2m (m is
multiplicity)
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Example of wave winding
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Dummy windings
 In a simplex wave winding, the average pitch YA (or
commutator pitch YC) should be a whole number.
 Sometimes the standard armature punchings available
in the market have slots that do not satisfy the wavw
winding requirement so that more coils (usually only
one more) are provided than can be utilized.
 These extra coils are called dummy or dead coils.
 The dummy coil is inserted into the slots in the same
way as the others to make the armature dynamically
balanced but it is not a part of the armature winding.
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Applications of Lap and Wave Windings
 A wave winding has a higher terminal voltage than a
lap winding because it has more conductors in series.
 A lap winding carries more current than a wave
winding because it has more parallel paths.
 In small machines, the current-carrying capacity of
the armature conductors is not critical and in order to
achieve suitable voltages, wave windings are used.
 In large machines suitable voltages are easily obtained
because of the availability of large number of
armature conductors and the current carrying capacity
is more critical. lap windings are used
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COMMUTATION PROBLEMS IN
REAL MACHINES
 The commutation process is not as simple in
practice as it seems in theory, because two
major effects occur in the real world to dist
urb it:
1. Armature reaction
2. Ldi/dt voltages
 This section explores the nature of these
problems and the solutions employed to
mitigate their effects.
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Armature Reaction
 If the terminals of a DC generator is
connected to load armature current flows in
the windings
 This current flow will produce a magnetic
field of its own, which will distort the
original magnetic field from the machine 's
poles.
 This distortion of the flux in a machine as the
load is increased is called armature reaction.
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Armature Reaction cont…
 Two serious problems of Armature Reaction
 The first problem caused by armature reaction is
neutral-plane shift.



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The magnetic neutral plane is defined as the plane
within the machine where the velocity of the rotor
wires is exactly parallel to the magnetic flux lines, so
that eind in the conductors in the plane is exactly zero.
The brushes are placed at neutral point(zero emf).
When neutral point shifts, the brush point is at non
zero emf so arcing and sparking at the brushes occur.
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Fig 1.20
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Armature Reaction cont…
 The second major problem caused by armature
reaction is called flux weakening.






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At locations on the pole surfaces where the rotor
magnetomotive force adds to the pole magnetomotive
force, only a small increase in flux occurs.
But at locations on the pole surfaces where the rotor
magnetomotive force subtracts from the pole
magnetomotive force, there is a larger decrease in flux.
1he net result is that the total average flux under the entire
pole face is decreased
Flux weakening causes problems in both generators and
motors.
In generators, it simply to reduce the voltage produced
In motors, it increases speed and could cause a runaway
condition
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Fig 1.21
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Fig 1.22
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L di/dt Voltages
 The second major problem is the Ldi/dt voltage that
occurs in commutator segments being shorted out by
the brushes, sometimes called inductive kick.
 Notice that when a commutator segment is shorted
out, the current flow through that commutator
segment must reverse.
 With even a tiny inductance in the loop, a very
significant inductive voltage kick v = Ldi/dt will be
induced in the shorted commutator segment.
 This high voltage naturally causes sparking at the
brushes of the machine, resulting in the same arcing
problems that the neutral-plane shift causes.
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L di/dt Voltages cont…
 Assuming that the current in the brush is 400 A,
the current in each path is 200 A.
 Assuming that the machine is turning at 800
r/min and that there are 50 commutator
segments (a reasonable number for a typical
motor), each commutator segment moves under
a brush and clears it again in t = 0.00 15 s.
 Therefore, the rate of change in current with
respect to time in the shorted loop must average
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Fig 1.23
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Solutions to the Problems with
Commutation
 Three approaches have been developed to
partially or completely correct the problems
of armature reaction and L di/dt voltages:
1. Brush shifting
2. Commutating poles or interpoles
3. Compensating windings
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0
BRUSH SHIFTING.



Someone had to adjust the brushes to the new neutral point every time
the load on the machine changed.
shifting the brushes may have stopped the brush sparking, but it actually
aggravated the flux-weakening effect of the armature reaction in the
machine.
This is true because of two effects:
1.
2.



The rotor magnetomotive force now has a vector component that opposes the
magnetomotive force from the poles (see Figure 8- 27).
The change in armature current distribution causes the flux to bunch up even
more at the saturated parts of the pole faces.
Another slightly different approach sometimes taken was to fix the
brushes in a compromise position (say, one that caused no sparking at
two-thirds of full load).
In this case, the motor sparked at no load and somewhat at full load
Today, brush shifting is only used in very small machines that always run
as motors.
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1
Fig 1.24
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COMMUTATING POLES OR
INTERPOLES
 Because of the requirement that a person must adjust the
brush positions of machines as their loads change, another
solution to the problem of brush sparking was developed.
 The basic idea behind this new approach is that if the
voltage in the wires undergoing commutation can be made
zero, then there will be no sparking at the brushes.
 To accomplish this, small poles, called commutating poles
or interpoles, are placed midway between the main poles.
 These commutating poles are located directly over the
conductors being commutated.
 By providing a flux from the commutating poles, the
voltage in the coils undergoing commutation can be
exactly canceled.
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INTERPOLES cont…
 If the cancellation is exact, then there will be no
sparking at the brushes.
 The commutating poles do not otherwise change the
operation of the machine, because they are so small
that they affect only the few conductors about to
undergo commutation.
 Notice that the armature reaction under the main pole
faces is unaffected, since the effects of the
commutating poles do not extend that far.
 This means that the flux weakening in the machine is
unaffected by commutating poles.
 They are connected in series with the windings on the
rotor.
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INTERPOLES cont…







As the load increases and the rotor current increases, the magnitude
of the neutral-plane shift and the size of the Ldi/dt effects increase
too.
Both these effects increase the voltage in the conductors undergoing
commutation.
However, the interpole flux increases too, producing a larger voltage
in the conductors that opposes the voltage due to the neutral-plane
shift.
The net result is that their effects cancel over a broad range of loads.
The interpoles must induce a voltage in the conductors undergoing
commutation that is opposite to the voltage caused by neutral-plane
shift and L di/dt effects.
Note that interpoles work for both motor and generator operation
The current both in its rotor and in its interpoles reverses direction.
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INTERPOLES cont…
 The interpoles must be of the same polarity as the next upcoming main
pole in a generator.
 The interpoles must be of the same polarity as the previous main pole
in a motor.
Fig 1.25
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Fig 1.26
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COMPENSATING WINDINGS
 For very heavy, severe duty cycle motors, the flux-weakening problem can
be very serious.
 To completely cancel armature reaction and thus eliminate both neutralplane shift and flux weakening, a different technique was developed.
 This third technique involves placing compensating windings in slots
carved in the faces of the poles parallel to the rotor conductors, to cancel
the distorting effect of armature reaction.
 These windings are connected in series with the rotor windings, so that
whenever the load changes in the rotor, the current in the compensating
windings changes, too.
 Notice that the magnetomotive force due to the compensating windings is
equal and opposite to the magnetomotive force due to the rotor at every
point under the pole faces.
 The resulting net magnetomotive force is just the magnetomotive force due
to the poles, so the flux in the machine is unchanged regardless of the load
on the machine.
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COMPENSATING WINDINGS cont
 The major disadvantage of compensating windings is that
they are expensive, since they must be machined into the
faces of the poles.
 Any motor that uses them must also have interpoles, since
compensating windings do not cancel Ldi/dt effects.
 The interpoles do not have to be as strong, though, since
they are canceling only L dildt voltages in the windings, and
not the voltages due to ne utral-plane shifting.
 Because of the expense of having both compensating
windings and interpoles on such a machine, these windings
are used only where the extremely severe nature of a motor
's duty demands them.
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9
Simplex lap winding
Fig 1.27
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0
Simplex lap winding
Fig 1.28
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1
End
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2