Chapter 4
DC Machines
Presentation Outline
 Construction
 Principles of operation
 Equivalent circuit and e.m.f equation
 Characteristics of D.C Generator
 Torque, power and voltage equations
 Speed equation of D.C motor
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DC Machines
• The direct current (dc) machine can be used as a motor
or as a generator
• The major advantages of dc machines are
Provides a wide range of speed control
Has high starting torque
Quick starting and stopping and acceleration
Application areas are :mills, mines and trains
 Even today the starter of a car is a series dc motor
 DC generators are not widely used due to solid state
rectifier
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Construction
DC machine contains two basic parts
1. Stationary part:
 Stator is the outer stationary part of the machine and
contains field winding, stator poles and iron core
 It is designed mainly for producing magnetic flux
2. Rotating part:
 It is called aramture.
 Rotating part contains ball bearing, rotor coil (armature), iron
core, commutation coil and brushes.
 It is a place where electrical energy is converted to mechanical
energy(electric motor) or mechanical energy is converted to
electrical energy(generator)
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Construction ...
General arrangement of a dc machine
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Working principle
Generator Principle:
 whenever a conductor cuts magnetic flux, dynamically
induced e.m.f. is produced in it according to Faraday’s
Laws of Electromagnetic Induction.
 This e.m.f. causes a current to flow if the conductor circuit
is closed.
Basic essential parts of an electrical generator are
(i) a magnetic field and
(ii) a conductor or conductors which can move so as to
cut the flux.
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Simple Loop Generator
Working principle
 Consider a single turn loop ABCD rotating clockwise in a
uniform magnetic field with a constant speed as shown in
fig. above
 As the loop rotates, the flux linking the coil sides AB and CD
changes continuously .
 Hence alternating e.m.f. induced in these coil
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• Working Principle…
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Working principle…
In the next quarter revolution i.e. from 90º to 180º,
the rate of change of flux linkages decreases.
Hence, the induced e.m.f. decreases gradually till in
position 5 of the coil, it is reduced to zero value.
In the next half revolution i.e. from 180º to 360º, the
variations in the magnitude of e.m.f. are similar to
those in the first half revolution.
Its value is maximum when coil is in position 7 and
minimum when in position 1.
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Fleming’s Right hand rule
Used to determine the direction of the induced current
 For the first half revolution of the coil ,the direction
of current flow is ABMLCD and the current flows
through the load resistance R in the direction M to L.
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Working principle…
For the second half revolution of the coil ,the direction of
current flow is DCLMBA , which is just the reverse of the
previous direction of flow.
 the current which obtained from such a simple generator
reverses its direction after every half revolution.
 Such a current undergoing periodic reversals is known as
alternating current(AC).
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Working principle…
• For making the flow of current unidirectional in the external
circuit, the slip-rings are replaced by split-rings
 In the first half revolution current flows along (ABMLCD) 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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Working principle…
In the next half revolution, 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 the segment which is positive i.e. segment ‘b’ in this
case
Fig. waveform of the current through the external circuit
 In both cases current in the load resistance flows from M to L
 The current is unidirectional but not continuous like pure direct
current.
 The current is due to the rectifying action of the split-rings (also
called commutator) that it becomes unidirectional in the external
circuit.
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Construction of DC Machines
DC generators and DC motors have the same general
construction
All DC machines have five basic principal components
i. Field system
ii. Armature core
iii. Armature winding
iv. Commutator
v. Brushes
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Construction…
i.Field system
Used to produce uniform magnetic field
Consists of Yoke(frame),field poles and field coils
Field coils are mounted on the poles & carry d.c exciting
current
The field coils are connected in such a way that adjacent
poles have opposite polarity
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Construction…
ii. Armature core
 The purpose of armature core is to hold the armature
winding and provide low reluctance path for the flux
through the armature from N pole to S pole.
 It is keyed to the machine shaft and rotates between field
poles
 Consists of slotted soft-iron laminations stacked to form
cylindrical core
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Construction…
iii. Armature winding
 These are insulated conductors placed in the slots of
armature core
 The armature windings are connected in a symmetrical
manner forming a closed loop or series of closed loops
 The armature of a DC machine have lap or wave winding
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Construction…
Lap Wound Armatures
 The windings of a lap wound armature are connected in
parallel.
 This permits the current capacity of each winding to be added
and provides a higher operating current
 Number of current path, A=P ; p=no of poles
 are used in machines designed for low voltage and high
current.
 armatures are constructed with large wire because of high
current
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Construction…
Wave Wound Armatures
 The windings are connected in series for this kind of armature
winding.
 Coils are laid out in a wave pattern and cross all the poles.
 When the windings are connected in series, the voltage of each
winding adds, but the current capacity remains the same
 Number of current path, A=2 (always)
 are used in machines designed for high voltage and low current
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Construction…
iv. Commutator
Convert the alternating current induced in the
armature conductors into unidirectional current in
the external load circuit(mechanical rectifier)
It is of cylindrical structure made from copper
segments insulated from each other mica sheets
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Construction…
v. Brushes
Ensure electrical connection between the rotating
commutator and stationary external load circuit
 Function to collect current from commutator
Made of carbon or graphite
Brushes
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Generated E.M.F Equation of a DC generator
Let, Ø= flux per pole in weber
Z = Total number of conductor
= number of slots* number of conductors/slot
P = Number of poles
A = Number of parallel paths
N =armature speed in rpm
Eg = generated e.m.f per parallel path in armature
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Armature resistance, Ra
The resistance offered by the armature circuit is known
as armature resistance (Ra) and includes:
(i) resistance of armature winding
(ii) resistance of brushes
The armature resistance depends upon the
construction of machine.
 Except for small machines, its value is generally less
than 1Ω
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Types of DC Generators
The magnetic field in a DC generator is normally produced by
electromagnets rather than permanent magnets.
Based on the method of field excitation, DC generators are
generally classified in to two classes:
(i) Separately excited d.c. generators
(ii) Self-excited d.c. generators
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Separately Excited D.C. Generators
 It is a generator whose field magnet winding is supplied from an
independent external d.c. source (e.g., a battery etc.)
 The voltage output depends upon the speed of rotation of armature
and the field current(Eg= ФPZN/60A)
Fig. Separately Excited D.C. Generators
 Separately excited d.c. generators are rarely used in practice.
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Open Circuit Characteristic…
Fig Open circuit characteristic
Notes:
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Armature reaction
The armature current produces a flux that counteracts the main flux
produced by the field poles.
This flux has the following effects collectively called armature reaction.
I.
Flux weakening
II.
Neutral plane shift
● Flux weakening: The direction of the flux produced by the armature
current is opposite to the main flux. Hence the magnitude of the main flux
will reduce as a result of this flux.
Compensating windings connected in series with the rotor windings are used
to improve flux weakening. These windings produce a flux equal but opposite
to the reaction flux to cancel it.
When the load changes in the rotor, the current in the compensating winding
changes too.
● Neutral plane shift: The neutral plane is the plane where the magnetic
flux density of the main flux is zero. It lies in the region where there is no
field poles.
Due to the reaction flux, the flux density at the ideal neutral plane (the plane
perpendicular to the main field plane) is not zero. The actual neutral plane
will shift.
Interpole windings are connected in series with the rotor windings to avoid
neutral plane shift caused by the reaction flux.
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Self-Excited D.C. Generators
 It is a generator whose field magnet winding is supplied by current
from the output of the generator itself
 depending upon the manner in which the field winding is connected to
the armature , Self-Excited D.C. Generators are classified as
(i) Series generator;
(ii) Shunt generator;
(iii) Compound generator
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Fig Series dc generator
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Characteristics of series dc generator
 The curve AB in above figure identical to open circuit characteristic
(O.C.C.) curve.
 here load current is similar to field current (i.e. IL=If)
 In a DC series generator, terminal voltage increases with the load
current. This is because, as the load current increases, field current
also increases.
 However, beyond a certain limit, terminal voltage starts decreasing
with increase in load. This is due to excessive demagnetizing effects
of the armature reaction.
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(ii) Shunt generator
 In a shunt generator, the field winding is connected in parallel with the armature
winding so that terminal voltage of the generator is applied across it.
 part of armature current flows through shunt field winding and the rest flows
through the load
Fig. Shunt generator
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Characteristics of shunt dc generator
 To determine the internal and external load characteristics of a DC
shunt generator the machine is allowed to build up its voltage before
applying any external load.
 During a normal running condition, when load resistance is decreased,
the load current increases, the terminal voltage also falls
 Beyond certain value further decrease in load resistance results
drastic drop down of terminal voltage due to excessive armature
reaction at very high armature current
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(iii) Compound generator
 In a compound-wound generator, there are two sets of field windings on each pole—one is in
series and the other in parallel with the armature.
 A compound wound generator may be:
(a) Short Shunt in which only shunt field winding is in parallel
with the armature winding
(b) Long Shunt in which shunt field winding is in parallel with both series
field and armature winding
Fig. Short Shunt generator
Fig. long Shunt generator
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Voltage Regulation
The change in terminal voltage of a generator between
full and no load (at constant speed) is called the voltage
regulation, usually expressed as a percentage of the
voltage at full-load.
where
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DC motor operation
 In a dc motor, the stator poles are supplied by dc excitation
current, which produces a dc magnetic field.
 The rotor is supplied by dc current through the brushes,
commutator and coils.
 The interaction of the magnetic field and rotor current
generates a Lorntez force (F= IlBsinϴ) that drives the motor.
 This force is perpendicular to both the magnetic field and
conductor
Animation
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DC motor...
 The generated force turns the rotor until the coil reaches the neutral
point between the poles.
 At this point, the magnetic field becomes practically zero together
with the force.
 However, inertia drives the motor beyond the neutral zone where the
direction of the magnetic field reverses.
 To avoid the reversal of the force direction, the commutator changes
the rotor current direction when the coil passes the neutral zone.
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Fig. Basic operation of DC motor
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Fig. shunt wound motor
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Starting methods of a DC motor(Reading Assignment)
 During starting DC motors develop large starting current.
 To avoid damage of the armature circuit, due to this
excessive starting current, dc motor starters are used
Types: i. 3 Point Starter
ii. 4 Point Starter
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Voltage Equation of D.C. Motor
Let in a DC motor
The armature current Ia is given by;
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Power Equation
If we multiply the above voltage eqn. by Ia throughout, we get
Condition for Maximum Power
Hence mechanical power developed by the motor is maximum when back e.m.f.
is equal to half the applied voltage.
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Armature Torque of D.C. Motor
Torque is the turning moment of a force about an axis
𝑻=𝒓×𝑭
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Armature Torque…
Torque eqn. of DC motor
Since Z, P and A are fixed for a given machine,
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Armature Torque…
we get the expression of Ta as:
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Shaft Torque
 The torque which is available at the motor shaft for doing useful work is known as
shaft torque. It is represented by Tsh.
 part of the armature torque is lost in overcoming the iron and frictional losses in the
motor
= Lost torque
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Types of D.C. Motors
There are three types of d.c. motors characterized by the connections of field winding in
relation to the armature
i. Series-wound motor:
 Here the field winding is connected in series with the armature
ii.
Shunt-wound motor:
 Here the field winding is connected in parallel with the armature
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Types of D.C. Motors…
iii. Compound-wound motor:
 It has two field windings; one connected in parallel with the armature
and the other in series with it
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Power stages in DC Motor
Fig. power flow in DC motor
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Speed of a D.C. Motor
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Speed Relations
Speed Regulation
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