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PowerPoint Slides
to accompany
Electric Machinery
Sixth Edition
A.E. Fitzgerald
Charles Kingsley, Jr.
Stephen D. Umans
Chapter 7
DC Machines
7-0
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7.1 INTRODUCTION
7-1
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7.1 INTRODUCTION
7-2
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7.1 INTRODUCTION
7-3
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7.1 INTRODUCTION
Tmech  Ka d ia
Constant determined by
the design of windings
7-4
Current in external
armature circuit
Direct axis air-gap flux per
pole
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7.1 INTRODUCTION
Rectified coil voltages and resultant voltage between brushes in a dc
machine.
ea  Ka  d m
Speed voltage
7-5
Angular speed
ea ia  Tmech m
Power
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7.1 INTRODUCTION
Typical form of magnetization curves of a dc machine.
ea
m
7-6
 Kad 
ea 0
m 0
 m 
ea 0
ea  
 m 0 
n
ea   ea 0
 n0 
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7.1 INTRODUCTION
Equivalent Circuit (Not in the textbook):
ia
Lf
if
Rf
Ra
La
 ea

For steady-state, current is dc,
therefore Lf and La can be
neglected.
7-7
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7.1 INTRODUCTION
Field-circuit connections of dc machines: (a) separate excitation, (b) series,
(c) shunt, (d) compound.
7-8
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7.1 INTRODUCTION
Volt-ampere characteristics of dc generators.
7-9
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7.1 INTRODUCTION
Speed-torque characteristics of dc motors.
7-10
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EXAMPLE (Final Exam 2006) :
Assume that a 240 V self-excited shunt
motor is supplied by a line current of 102.4
A when it is loaded with a full load at a
speed of 1000 rpm. The armature-circuit
resistance and the shunt-field circuit
resistance of the motor are 0.1 ohm and 100
ohm, respectively. Assume that a breaking
resistor of 1.05 ohm is used for dynamic
braking (breaking means that voltage
source is removed and immediately a
resistor is connected to the terminals of the
DC machine) and determine the following.
a)The value of counter emf Ea.
b)The full-load torque of the motor
c)The value of the armature winding current
at the time of initial breaking.
d)The value of initial dynamic breaking
(initial torque during breaking)
7-11
Ra
Rf
Ea
Vt
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The End of This Chapter
7-12
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Dc machine
armature winding
with commutator
and brushes.
(a), (b) Current
directions for two
positions of the
armature.
Figure 7.7
7-13
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Waveform of current in an armature coil
with linear commutation.
Figure 7.8
7-14
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Armature-mmf and flux-density distribution with brushes
on neutral and only the armature excited.
Figure 7.9
7-15
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Flux with only the armature excited and brushes
on neutral.
Figure 7.10
7-16
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Armature,
main-field,
and resultant
flux-density
distributions
with brushes
on neutral.
Figure 7.11
7-17
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Motor or generator connection diagram
with current directions.
Figure 7.12
7-18
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Short-shunt compound-generator connections.
Figure 7.13
7-19
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Magnetization
curves for a 250-V
1200-r/min dc
machine. Also
shown are fieldresistance lines
for the discussion
of self-excitation
in Section 7.6.1.
Figure 7.14
7-20
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Equivalent circuit for analysis of voltage buildup
in a self-excited dc generator.
Figure 7.15
7-21
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Cross section of a typical permanent-magnet motor.
Arrows indicate the direction of magnetization in the
permanent magnets.
Figure 7.17
7-22
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(a) Dimension definitions for the motor of Fig. 7.17.
(b) approximate magnetic equivalent circuit.
Figure 7.18
7-23
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Equivalent circuit of a permanent-magnet dc motor.
Figure 7.20
7-24
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Section of dc machine showing compensating winding.
Figure 7.22
7-25
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Schematic connection diagram of a dc machine.
Figure 7.24
7-26
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Series-connected universal machine.
Figure 7.25
7-27
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Typical torque-speed characteristics of a series
universal motor.
Figure 7.26
7-28
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1200 r/min
magnetization
curve for the
dc generator
of Problem 7.4.
Figure 7.27
7-29
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Series crane
motor (Problem
7.22): (a) hoisting
connection and
(b) lowering
connection.
Figure 7.28
7-30