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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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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

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1. Press Start to energize mainrelay M.2. All M contacts close.

3. 1 AR, 2 AR, 3 AR Energized4. 1A energized

5. 2A energized6. 3A energized