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Unit 3- DC Machines.pdf
1. EVEN SEMESTER
2017-2018
P. MARIA SHEEBA
ASSISTANT PROFESSOR /ECE
MOUNT ZION COLLEGE OF ENGINEERING AND TECHNOLOGY
PUDUKKOTTAI
1
BE8254- BASIC ELECTRICAL AND INSTRUMENTATION
ENGINEERING
2. m
d d dx
Blx Bl
dt dt dt
dx
Blv Bl
dt
E
m
Therefore,
d
dt
E
CONCLUSION: to produce emf one should make ANY
change in a magnetic flux with time!
Faraday’s Law
2
3. Changing magnetic flux produces an emf (or changing B-
Field produces E-Field)
The rate of change of magnetic flux is required
Faraday’s Law
3
4. The direction of the emf
induced by changing flux
will produce a current
that generates a
magnetic field opposing
the flux change that
produced it.
Lenz’s Law
4
5. Lenz’s Law: emf appears and current flows that creates a magnetic field that
opposes the change – in this case an decrease – hence the negative sign in
Faraday’s Law.
B, H
N S
V+, V-
Lenz’s Law
5
6. Lenz’s Law: emf appears and current flows that creates a
magnetic field that opposes the change – in this case an
increase – hence the negative sign in Faraday’s Law.
B, H
N S
V-, V+
Lenz’s Law
6
9. Claim: Direction of induced current must be so as to
oppose the change; otherwise conservation of
energy would be violated.
• Why???
– If current reinforced the change, then the
change would get bigger and that would in
turn induce a larger current which would
increase the change, etc..
– No perpetual motion machine!
Conclusion: Lenz’s law results from energy
conservation principle.
Lenz’s Law
9
10. Induced Current – quantitative
Suppose we pull with velocity
v a coil of resistance R through
a region of constant magnetic
field
v
w
x
x x x x x x
x x x x x x
x x x x x x
x x x x x x
We must supply energy to produce the current
and to move the loop (until it is completely out of
the B-field region). The work we do is exactly
equal to the energy dissipated in the resistor, i.e.
W=I2Rt 10
17. • Balanced phase voltages are equal in magnitude and are out of phase
with one another by 120 degrees.
• Phase voltages sum up to zero.
• Two possible combinations:
abc or (+) sequence acb or () sequence
Three phase circuits
17
18. Y-connected Load D-connected Load
A balanced load is one in which the phase impedances are equal
in magnitude and in phase.
Three phase circuits
18
19. Construction of DC machines
An electrical machine deals with the energy transfer either
from mechanical to electrical or electrical to mechanical
form. This process is called electromechanically energy
conversion. The device which is used for electromechanical
energy conversion is known as machine.
Machine
Generator - mechanical to
electrical
Motor- electrical to
mechanical
19
20. Construction of DC machines
• The machine is DC generator or motor the
constructions are remain same.
• The main parts of de machine are:-
Yoke
Pole
Field winding
Armature
Commutator
Brushes
20
23. Yoke
• Functions :-
It serves the purpose of outermost cover
of D.C. machine.
It provide mechanical support to the pole.
It provide low reluctance path.
23
24. Yoke
Choice of material:-
To provide low reluctance path, it must be
made up of some magnetic material.
It is prepared by cost iron because it is
cheapest.
For large machine rolled steel, cast steel,
silicon steel is used.
24
26. Functions :-
Poles
Choices of material:-
Cost iron.
Pole core basically carries a field
winding.
It directs the flux.
Pole shoe enlarge the area of armature
core to come across the flux.
26
27. • The field winding is wound on the pole core
with a definite direction.
Functions:-
• To carry the current due to pole core, on
which the field winding is placed.
Choice of material:-
• It has carry the current hence its made up of
copper.
Field Winding
27
28. It is further divided in to two parts namely,
1. Armature core
2. Armature winding.
Armature core :
Armature core is cylindrical in shape.
It consists of slots on its periphery.
It has air ducts to permit the air flow through
armature for cooling purpose.
Armature
28
29. Functions:
• Armature core provides house for armature
winding.
Choice of material:
• Cost iron or cost steel.
Armature core
29
30. Functions:
Generation of EMF take place in the armature
winding in case of generator.
To carry the current supply in case of d.c.
motors.
Choice of material:
Copper.
Armature winding
30
32. Brushes
Bearings are stationary and resting on the surface of
the commutator.
Functions:
To collect the current from commutator.
Choice of material:
Carbon.
32
33. Bearing
• Bearing :-
For heavy duty machines roller bearings are
preferred.
• Types of armature winding:-
Lap winding Wave winding
33
34. Mechanical energy is converted to electrical
energy
Three requirements are essential
1. Conductors
2. Magnetic field
3. Mechanical energy
DC Generator
34
35. All generators work on a principle of dynamically induced
EMF, this is also known as Faraday’s law of
electromagnetic induction.
• Whenever the number of flux linked with the coil
changes , an electromotive force is set up in that coil.
Generator action requires following basic components.
The conductor
The flux
The relative motion between conductor and flux.
Theory operation of DC generators
35
36. • Fleming’s right hand rule is also known as
generator rule.
Fleming’s right hand rule
36
42. • In Separately Exited Generator, a separate d.c
supply is used to provide exciting current
through the field winding.
• The d.c generator produces d.c voltage. If this
generated voltage itself is used to excite the
field winding of same d.c generator, it is called
Self Excited Generator.
42
45. Self exited generator
Based on how field winding is connected to the
armature to derive its excitation, this is further
divided in to following three types:
1. Shunt generator.
2. Series generator.
3. Compound generator.
45
46. Shunt Generator
When the field winding is
connected in parallel with
the armature and the
combination across the
load then the generator is
called shunt generator.
46
48. Series generator
• When the field winding
is connected in series
with the armature and
the combination across
the load then the
generator is called
series generator.
48
50. A 250 V, 10 kW, separately exited generator has
an induced e.m.f. of 250 V at full load. If the
brush drop is 2 V per brush, calculate the
armature resistance of the generator.
Example 2
Solution
50
51. Note that 250 V, 10kW generator means the full
load capacity of generator is to supply 10 kW
load at a terminal voltage =250 V.
51
53. • A short shunt compound d.c. generator supplies a current of 75 A at a
voltage of 225 Calculate the generated voltage if the resistance of
armature, shunt field and series field windings are 0.04 ohms , 0.90 ohms
and 0.02 ohms respectively.
Example 3
Solution
Consider a short shunt generator
53
60. Now there are two fluxes present,
1. The flux produced by the permanent magnet
called main flux.
2. The flux produced by the current current
carrying conductor.
60
63. • In the practical d.c. motor
the permanent magnet is
replaced by a field
winding which produces
the required flux called
main flux and all the
armature conductors,
mounted on the
periphery of the armature
drum, get subjected to
the mechanical force.
• Due to this , overall
armature experiences a
twisting, force called
torque and armature of
the motor starts rotating.
63
64. Direction of rotation of motor
• The magnitude of the force experienced by the
conductor in a motor is given by,
B= flux density.
L= length of conductor.
I= Magnitude of current.
64
65. • In the figure it is shown
that, a portion of a
conductor of length L
placed vertically in a
uniform horizontal
magnetic field strength
H, produced by two
magnetic poles N and S.
If i is the current flowing
through this conductor,
Fleming Left Hand Rule
65
68. There are 2 types of winding
- Lap and Wave winding
Lap winding
A = P
The armature windings are divided into
no. of sections equal to the no of poles
Wave winding
A = 2
It is used in low current output and high
voltage.
2 brushes
Armature windings DC motors
68
69. • It is seen in the generating action, that when a conductor cuts
the lines of flux, e.m.f. gets induced in the conductor. After a
motering action, armature starts rotating and armature
conductors cut the main flux. So there is a generation action
existing In a motor, the e.m.f induced due to generating action
is known as back e.m.f
Back EMF
69
72. • A 220 V, d.c. motor has an armature resistance of 0.75 ohms.
It is drawing an armature current of 30 A, driving a certain
load. Calculate the induced e.m.f. in the motor under this
condition.
Example 4
Solution
72
73. Power equation of DC motor
• The voltage equation is;
• Multiplying by Ia
• The above equation is called power equation of DC motor
73
76. • N= speed, then angular
speed is
• Work done in one
revolution is
• W= F * distance traveled in
one revelation
• W= F * 2 𝜋R
Torque Equation of DC Motor
76
79. • DC shunt motor
• DC series motor
• DC compound motor
Types of DC Motor
79
80. Shunt Motor
• The parallel
combination of two
windings is connected
across a common dc
power supply.
• The resistance of shunt
field winding is always
higher than the
armature winding.
80
82. Series Motor
• The field winding is
connected in series with
the armature.
• The current passing
through the series
winding is same as the
armature current.
82
85. Long Shunt compound motor
• In this the series
winding in series with
the armature winding
and the shunt winding
is connected in parallel
with the armature
connection.
85
86. Short Shunt compound motor
• In this the series
winding is series with
the parallel
combination of
armature winding and
the shunt winding.
• It has good starting
torque and constant
speed characteristic.
86
87. Cumulative compound DC motors
• If the two field windings i.e. series and shunt
are wounded in such a way that the fluxes
produced by them add each other.
Differential compound DC motors
• If the two field windings i.e. series and shunt
are wounded in such a way that the fluxes
produced by them airways try to cancel each
other.
87
95. • The speed regulation for a d.c. motor is
defined as the ratio of change in speed
corresponding to no load and full load
condition to speed corresponding to full load.
Speed Regulation
95
97. DC Shunt Motor Characteristics
Torque (vs) armature current characteristics
97
98. • From the speed equation we get,
• When the load increases the
armature current increases and
hence drop IR also increases.
DC Shunt Motor Characteristics
Speed – Armature current characteristics
98
99. • This curve shows that
the speed remains
constant when the
torque from no load to
full load.
DC Shunt Motor Characteristics
Speed – torque characteristics
99
102. DC Shunt Motor Characteristics
Speed – torque characteristics
102
103. DC Shunt Motor Characteristics
Speed – torque characteristics
103
104. • Flux Control Method
• Armature Control Method
• Voltage Control Method
Speed DC Motor
104
105. • It is seen that speed of the
motor is inversely
proportional to flux. Thus by
decreasing flux speed can
be increased and vice versa.
• To control the flux, a
rheostat is added in series
with the field winding, as
shown in the circuit
diagram. Adding more
resistance in series with
field winding will increase
the speed, as it will
decrease the flux.
Flux Control Method
105
106. • Field current is relatively
small and hence I2R loss is
small, hence this method is
quiet efficient.
• Though speed can be
increased by reducing flux
with this method, it puts a
limit to maximum speed as
weakening of flux beyond
the limit will adversely
affect the commutation.
Flux Control Method
106
107. • Speed of the motor is
directly proportional to the
back emf Eb and Eb = V-
IaRa.
• That is when supply voltage
V and armature resistance
Ra are kept constant, speed
is directly proportional to
armature current Ia.
• Thus if we add resistance in
series with armature,
Ia decreases and hence
speed decreases.
Armature Control Method
107
108. • A) Multiple voltage control:
• B) Ward-Leonard System
Multiple voltage control:
• In this method the, shunt filed is connected to a fixed exciting voltage, and
armature is supplied with different voltages.
• Voltage across armature is changed with the help of a suitable switchgear.
The speed is approximately proportional to the voltage across the
armature.
Voltage Control Method
108
109. This system is used where very sensitive speed control of motor is required
(e.g electric excavators, elevators etc.) The arrangement of this system is as
required in the figure beside.
Ward-Leonard System
M2 is the motor whose speed control is required.
M1 may be any AC motor or DC motor with constant speed.
G is the generator directly coupled to M1.
109
110. .
Ward-Leonard System
In this method the output from the generator G is fed to
the armature of the motor M2 whose speed is to be
controlled.
The output voltage of the generator G can be varied from
zero to its maximum value, and hence the armature voltage
of the motor M2 is varied very smoothly. Hence very
smooth speed control of motor can be obtained by this
method
110
111. Type of Motor Applications
shunt 1. Blowers
2. Fans
3. Lath machines
4. Drilling machines
series 1. Cranes
2. Trolleys
3. Conveyers
Cumulative compound 1. Rolling mills
2. elevators
Differential compound Not suitable for any
practical applications
Applications of DC Motor
111
112. Type of Generator Applications
Separately exited
Generator
1. Electro plating
2. Electro refining
Shunt Generator 1. Battery charging
2. Ordinary lighting purpose
Series Generator 1. Welding
2. Arc lamp
Cumulative
compound Generator
1. Domestic lamp
Differential
compound Generator
1. Electric arc welding
Applications of Generator
112
