chapter 8 dc generators - department of electrical and ...ece.citadel.edu/barsanti/elec316/l7_dc...
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Chapter 8
DC Generators
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The Equivalent Circuit of a DC Generator
• Terminal relationship is given by Kirchhoff’s voltage law.
VT = EA - IARA
Figure 8-42
The equivalent circuit of a dc generator
Figure 8-43
A simplified equivalent circuit eliminating the brush voltage drop and combining Radj with
the internal field resistance.2Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Terminal Characteristic of a Separately Excited DC Generator
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Figure 8-45The terminal characteristic of a separately dc generator (a) with and (b) without compensating windings.
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Nonlinear Analysis of a Separately Excited DC Generator
• The net mmf and the equivalent field current of the generator in the presence of the armature reaction are
given by
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net F F ARF N I F= −
* AR
F F
F
FI I
N= −
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Nonlinear Analysis of a Separately Excited DC Generator
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7Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 8Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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9Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 10Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Shunt DC Generator
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A m
ind A
T A A A
A L F
T
F
F
E K
K I
V E R I
I =I I
VI
R
φω
τ φ
=
= = −
+
=
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���� ↑, �� ↑,↓, � ��� ↓, ∅ ↓,��↓,↓
Thus the load voltage (Vt) drops off somewhat more severely in a shunt connected DC generator as compared
to the separately excited generator.
Terminal Characteristic of a Shunt DC Generator
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Figure 8-52The terminal Characteristic of a Shunt DC Generator.
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Voltage Buildup in a Shunt Generator
• Requires residual flux in the poles of the generator
• The field resistance should be less than Rcritical
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Figure 8-50Voltage buildup on starting in a shunt dc generator.
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Figure 8-51
The effect of the shunt field resistance on no-load terminal voltage in a dc
generator. If RF > R2 = Rcritical, then generator’s voltage will never build up.16Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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The Analysis of a Shunt DC Generator
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Figure 8-53Graphical Analysis of a shunt dc generator with compensating windings.
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The Analysis of a Shunt DC Generator Including Armature Reaction
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Figure 8-55Graphical Analysis of a shunt dc generator with armature reaction.
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The Series DC Generator
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Since �� = �∅� = ������ = ������
For no-load, �� = 0, �� = 0, �� = 0, ∅ = 0, �� = 0
With load �� ↑, �� ↑, ����� ↑= �� − ��("� + "$ + "�)
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Terminal Characteristic of a Series DC Generator
Figure 8-57Terminal Characteristic of a series dc generator
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Terminal Characteristic of a Series DC Generator Including Armature Reaction
Figure 8-58Terminal Characteristic of a series dc generator with large armature reaction effect, suitable for electric Welders.
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The Cumulatively Compounded DC Generator
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Terminal Characteristic of a Cumulatively Compounded DC Generator
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The Analysis of a Cumulatively Compounded DC Generator
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Figure 8-63Graphical Analysis of a cumulatively compounded dc generator.
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The Differentially Compounded DC Generator
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Terminal Characteristic of a Differentially Compounded DC Generator
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The Analysis of a Differentially Compounded DC Generator
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Figure 8-67Graphical Analysis of a differentially compounded dc generator.
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DC Motor Starting Circuits
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Hand operated starting resistors
Starting circuit components
a) Fuse b) push button c) relay d)TD relay e) Over load
1. Press Start to energize mainrelay M.2. All M contacts close.3. 1TD Energized4. 2TD energized5. 3TD energized