This document describes an experiment conducted on a self-excited DC shunt generator. The objectives were to investigate the relationship between output voltage and current of the generator when driven at a constant speed. The experiment involved connecting a DC generator and motor, and recording voltage and current values across various loads. The results showed a nearly linear relationship between output voltage and current, with voltage decreasing as current increased. Some inaccuracies may have been due to old equipment. In conclusion, the experiment demonstrated the relationship between output voltage and current of a self-excited DC shunt generator driven at a constant speed.

1. UNIVERSITY OF BOTSWANA
FACULTY OF ENGINEERING AND TECHNOLOGY
DEPARTMENT OF ELECTRICAL
ELECTRICAL MACHINES (EEB 326)
LAB 2: SELF EXCITED D.C SHUNT GENERATOR
AUTHOR: BOSA THEOPHILUS NTSHOLE
STUDENT ID: 201301848

2. Contents
OBJECTIVES........................................................................................................................................3
INTRODUCTION..................................................................................................................................3
DC MACHINES.................................................................................................................................3
THEORY..............................................................................................................................................4
DC SHUNT GENERATOR ON NO LOAD...............................................................................................4
DC SHUNT GENERATOR ON LOAD....................................................................................................4
Equipment used .................................................................................................................................6
PROCEDURE.......................................................................................................................................6
RESULTS.............................................................................................................................................7
DISCUSSION .......................................................................................................................................8
CONCLUSIONS....................................................................................................................................8
REFFERENCES.....................................................................................................................................8
GROUP MEMBERS ..............................................................................................................................9

3. OBJECTIVES
The main objective of this experiment is to investigate the relationship between the output
voltage of a direct current shunt generator and output current when driven at a constant speed.
INTRODUCTION
DC MACHINES
DC machines are usually built with the field magnetic circuit in the stator, through the yoke, pole
and body and pole shoe. Field windings are round the pole body. Depicted below is the figure of
a typical DC machine;
Figure 1: DC MACHINE
Rotor is made up of stampings that are the laminations of low hysteresis steel. Dynamo sheet
steel is often used. The armature windings are on the rotor and are put in slots and laminated core
and held in place by the strips of insulating material. Each slot contains two layers, half a coil in
each layer. The coil may be anything from single conductor to a multitude of turns. The whole
assembly is then impregnated with vanish to remove air and prevent movement. The rotor
windings are connected to the commutator. The commutator is made up of copper strips that
have a wedge shaped cross section. These are stacked together with insulation between the strips.
The assembly is clamped together onto the shaft between two discs each having a tapered flange.
Each segment has a strip of copper or steel soldered in a slot in the segment to connect the
segment to the windings, an illustration is demonstrated on figure 2 that follows on the next
page;

4. Figure 2: ILLUSTRATIONS IN WINDING INSULATION OF A MACHINE
THEORY
DC SHUNT GENERATOR ON NO LOAD
The output voltage of a DC shunt generator depends on the field strength. This depends on the
output voltage and field resistance. If the field is supplied from another DC source, voltage will
follow the saturation curve for a magnetic circuit since v is proportional to the flux. Illustration is
depicted below;
Figure 3: CIRCUIT DIAGRAM FOR DC SHUNT GENERATOR AND CORRESPONDING GRAPH OF VOLTAGE VS CURRENT
Thus if the field resistance R1 in the diagram, then the voltage will be only due to the residual
flux that is nearly zero. If the resistance is reduced to R2, the voltage will rise suddenly to V2.
Further the reduction in the field resistance to R3 will cause the voltage to rise to V3. Thus R2 is
the critical resistance, any resistance above this and the output voltage will be zero.
DC SHUNT GENERATOR ON LOAD
Neglectingarmature reactiontobrushdrop
Thus Ia, the relationship between E and If is linear, it is depicted on the next figure;

5. Figure 4: THE RELATIONSHIP OF If AND E AT SPEED N
A shunt generator will fail to excite if
- The field resistance is above critical value
- The speed is too low
- There is no residual magnetism
- The field connections or rotation are reversed
- If the machine is run up with load resistance too low

6. Equipment used
EQUIPMENT INITIAL SETTINGS
Test Bed Speed range 1800 rev/min
DC supply to 110 V
Field rheostat to zero
Armature rheostat to infinity
START/STOP/RUN switch to RUN
FH50 DC Compound Machine Test machine- DC Generator
FH50 DC Compound Machine Prime mover- DC motor
Instrument Frame
V2 DC Voltmeter 150 V range
A2 DC Ammeter 250 mA (AL) range/1.5 A range (AF)
R1 Resistive load 50Ω rheostat set to zero
2000 Ω rheostat set to infinity
PROCEDURE
The FH50 mimic diagram was positioned over the machine access sockets of the test bed. The
test generator was mounted in the right hand position of the machine, and the prime mover was
mounted in the left hand side of the machine. The 16-way plugs of the two machines were then
located in their respective sockets in the test bed. The equipment was set up as in the following
figure;
Figure 5: self excited d.c shunt generator connection circuit

7. Before the test was run, it was made sure that the machines were cool. After acquiring positive
assurance the test bed was switch on at the main switch and then the green ON was pressed to
engage the contactor. The prime mover was then started by rotating the armature rheostat
clockwise. The armature rheostat was then set so that the machines rotate at 1500 rev/min. the
2000 Ω rheostat of R1 was adjusted to give an output current of 200mA and both machines were
allowed to warm up for 5 minutes. The corresponding values of field current were then recorded
together with output voltage values. Results were then tabulated below and a graph was plotted
(graph of output voltage against output current).
RESULTS
OUTPUT CURRENT(mA) OUTPUT VOLTAGE(V) FIELD CURRENT (mA)
0 84 0.100
40 81 0.100
60 80 0.100
80 78 0.100
100 75 0.100
120 72 0.100
140 70 0.075
160 67 0.070
180 64 0.065
200 62 0.060
220 58 0.055
240 53 0.053
260 45 0.051
280 42 0.050
0
10
20
30
40
50
60
70
80
90
100
0 50 100 150 200 250 300
OUTPUTVOLTAGE(V)
OUTPUT CURRENT(mA)
GRAPH OF OUTPUT VOLTAGE AGAINST OUTPUT
CURRENT

8. DISCUSSION
This experiment involved a lot of electrical equipment which might have as well contributed to
inaccurate results. The equipment might have achieved this because of overheating and being so
old. The coils of the machines must have lost their strength to carry all the flux that was
generated. Also the apparatus were set up in very confined space which had led to delaying set
up to start up the experiment.
CONCLUSIONS
The relationship between the output voltage and output current was successfully investigated.
The output voltage and output current was found o be having a nearly linear relationship with a
negative gradient when the self excited DC generator is driven at constant speed.
REFFERENCES
1. “Electromagnetic and Electromechanical Machines”, Matsch, Leander W, Intext
Educational Publishers, 1972.
2. “Electromechanical Devices for Energy Conversion and Control Systems”, DelToro,
Vincent, Prentice-Hall, Inc., 1968.
3. Electrical-power.pdf

9. GROUP MEMBERS
Refilwe Moseki
Bosa Ntshole
TapologoRamasesane
GilbertGababonwe
JosephChaenda
OrapelengOnkabetse
One ColleenMontsho
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