The Equivalent Circuit of a DC Generator
• Terminal relationship is given by Kirchhoff’s voltage law.
VT = EA - IARA
Chapter 8
DC Generators
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Figure 8-42
The equivalent circuit of a dc generator
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Figure 8-43
A simplified equivalent circuit eliminating the brush voltage drop and combining Radj with
the internal field resistance.
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Nonlinear Analysis of a Separately Excited DC Generator
Terminal Characteristic 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
Fnet = NF IF − FAR
IF* = IF −
FAR
NF
Figure 8-45
The 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
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2
Shunt DC Generator
Terminal Characteristic of a Shunt DC Generator
E A = Kφωm

τ ind = Kφ I A
V = E − R I
 T
A
A A
I A =IL + IF
IF =
VT
RF
↑,
↑,
↓,
↓,
∅ ↓, ↓,
↓
Thus the load voltage (Vt) drops off somewhat more
severely in a shunt connected DC generator as compared
to the separately excited generator.
Figure 8-52
The 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
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.
Figure 8-50
Voltage buildup on starting in a shunt dc generator.
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The Analysis of a Shunt DC Generator Including
Armature Reaction
The Analysis of a Shunt DC Generator
Figure 8-53
Graphical Analysis of a shunt dc generator with compensating windings.
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Figure 8-55
Graphical Analysis of a shunt dc generator with armature reaction.
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3
The Series DC Generator
Since
Terminal Characteristic of a Series DC Generator
= ∅ = = For no-load, = 0, = 0, = 0, ∅ = 0, = 0
With load
↑, ↑, ↑= − (" + "$ + " )
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Figure 8-57
Terminal Characteristic of a series dc generator
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Terminal Characteristic of a Series DC Generator
Including Armature Reaction
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The Cumulatively Compounded DC Generator
Figure 8-58
Terminal Characteristic of a series dc generator with large armature reaction effect,
suitable for electric Welders.
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The Analysis of a Cumulatively Compounded DC
Generator
Terminal Characteristic of a Cumulatively Compounded
DC Generator
Figure 8-63
Graphical Analysis of a cumulatively compounded dc generator.
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Terminal Characteristic of a Differentially Compounded
DC Generator
The Differentially Compounded DC Generator
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The Analysis of a Differentially Compounded DC Generator
Figure 8-67
Graphical Analysis of a differentially compounded dc generator.
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Hand operated starting resistors
DC Motor Starting Circuits
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Starting circuit components
1. Press Start to energize main
relay M.
2. All M contacts close.
3. 1TD Energized
4. 2TD energized
5. 3TD energized
a) Fuse b) push button c) relay d)TD relay e) Over load
1. Press Start to energize main
relay M.
2. All M contacts close.
3. 1 AR, 2 AR, 3 AR Energized
4. 1A energized
5. 2A energized
6. 3A energized
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