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. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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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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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. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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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. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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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. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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Figure 8-55 Graphical Analysis of a shunt dc generator with armature reaction. 17

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The Series DC Generator

Since

Terminal Characteristic of a Series DC Generator

 = ∅ =    =   

For no-load,  = 0,  = 0,  = 0, ∅ = 0,  = 0 With load  ↑,  ↑,   ↑=  −  (" + "$ + " ) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Figure 8-57 Terminal Characteristic of a series dc generator 19

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. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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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. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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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. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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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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