Experiment to obtain Generator External Characteristics

Design Experiment to obtain Generator External Characteristics
Wong Yew Chun, Tong Janice, Thomas Tan Wan Kiat, Gun Wei Hwa and Thiruselvan a/l Muniandy
UniMAP, School of Mechatronics Engineering, Penang , Malaysia. Email:[e-mail]
UniMAP, School of Mechatronics Engineering, Johor, Malaysia. Email:[e-mail]
UniMAP, School of Mechatronics Engineering, Sarawak,Malaysia. Email: [e-mail]
UniMAP, School of Mechatronics Engineering, Kedah, Malaysia. Email: [e-mail]
UniMAP, School of Mechatronics Enginerring,Penang, Malaysia.Email:[e-mail]

ABSTRACT This paper is a design of experiment to obtain saturation curve of separately-excited DC generator and generator external characteristic. Four types of generators were used during the experiment, which are self-excited generator, series wound generator, shunt wound generator and compound generator. Our major result is to obtain a graph of output voltage-load current characteristic at 700 rpm which is 50% rated speed for all the generators. From the results, we analyze the characteristics of each type of generator. The separately excited, is driven at constant speed and has a fixed field current, the output voltage will decrease with increased load current. Moreover for series wound generator, the external characteristic curve shows, the voltage output starts at current equal to zero and increase until a peak point. In a shunt excited generator, it can be seen that the output voltage decreases faster than with separate excitation. As the output voltage decreasing exciting current falls which further weakens the field. Therefore, the output voltage in a dc shunt-wound generator varies inversely as load current varies. Lastly a compound generator is nearly ideal any increase in the excitation current, speed of rotation and also increase in load will not make any changes to the output or just slightly increase or decrease the output value only.
1.0 INTRODUCTION
DC Generator A generator is a machine that converts mechanical energy into electrical energy by using the principle of magnetic induction. This principle is explained as follows: Whenever a conductor is moved within a magnetic field in such a way that the conductor cuts across magnetic lines of flux, voltage is generated in the conductor. A DC machine can run either as a motor or as a generator. A motor converts electrical power into mechanical power while a generator converts mechanical power into electrical power. A generator must, therefore, be mechanically driven in order that it may produce electricity. A simple DC generator consists of the same basic elements as a simple AC generator: i.e., a multi-turn coil rotating uniformly in a magnetic field. The main difference between a DC generator and an AC generator lies in the manner in which the rotating coil is connected to the external circuit containing the load. In an AC generator, both ends of the coil are connected to separate slip-rings which co-rotate with the coil, and are connected to the external circuit via wire brushes. This combination of a rotating split-ring and stationary metal brushes is called a commutator. The purpose of the commutator is to ensure that the emf seen by the external circuit is equal to the emf generated around the rotating coil for half the rotation period, but is equal to minus this emf for the other half (since the connection between the external circuit and the rotating coil is reversed by the commutator every half-period of rotation). The positions of the metal brushes can be adjusted such that the connection between the rotating coil and the external circuit reverses whenever the emf generated around the coil goes through zero. In this special case, the emf seen in the external circuit is simply

Figure below shows plotted as a function of time, according to the above formula. The variation of the emf with time is very similar to that of an AC generator, except that whenever the AC generator would produce a negative emf the commutator in the DC generator reverses the polarity of the coil with respect to the external circuit, so that the negative half of the AC signal is reversed and made positive. The result is a bumpy direct emf which rises and falls but never changes direction. This type of pulsating emf can be smoothed out by using more than one coil rotating about the same axis, or by other electrical techniques, to give a good imitation of the direct current delivered by a battery. The alternator in a car (i.e., the DC generator which recharges the battery) is a common example of a DC generator of the type discussed above. Of course, in an alternator, the external torque needed to rotate the coil is provided by the engine of the car.

Application
The application of a DC generator is a dynamo. The dynamo was the first electrical generator capable of delivering power for industry. The dynamo uses electromagnetic induction to convert mechanical rotation into direct current through the use of a commutator. The dynamo is fundamentally based on Faraday 's law of induction, which states, "The induced electromagnetic force or EMF in any closed circuit is equal to the time rate of change of the magnetic flux linking the circuit." Basically, this means a current in a closed circuit can be induced when mechanical force is applied against the magnetic field linking the circuit, as in a generator, or vice versa, as in an engine. This generator led the first steps into the use of electricity in industry. Larger and larger ones were built, linked together in a series. The dynamo was not only the first commercially useful electrical generator, but also one of the first motors, which was discovered by accident. Today, it is mainly remembered as a simple device on which more complex, later electrical devices were based, such as the electric motor, the alternating-current alternator, and the rotary converter. Dynamo still used in some low power applications, particularly where low voltage DC is required, since an alternator with a semiconductor rectifier can be inefficient in these applications. Hand cranked dynamos are used in clockwork radios, hand powered flashlights, mobile phone rechargers and other human powered equipment to recharge batteries.

2.0 THEORETICAL CONFIGURATION
2.1 Separately Excitation generator
Since the field winding is an electromagnet, current flow through it to produce a magnetic field. This current is called the excitation current, and can be supplied to the field winding in one of two ways: it can come from a separate, external dc source, in which case the generator is called a separately-excited generator. In other words, when the DC field current in such a generator is supplied by an independent source, the generator is also said to be separately excitation. [1]
Figure 1: Connection diagram of a separately excited generator

2.2 Self Excitation
2.2.1 Series Wound Generator
In the series-wound generator, shown in figure, the field windings are connected in series with the armature. Current that flows in the armature flows through the external circuit and through the field windings. The external circuit connected to the generator is called the load circuit. A series-wound generator uses very low resistance field coils, which consist of a few turns of large diameter wire. The voltage output increases as the load circuit starts drawing more current. Under low-load current conditions, the current that flows in the load and through the generator is small. Since small current means that a small magnetic field is set up by the field poles, only a small voltage is induced in the armature. If the resistance of the load decreases, the load current increases. Under this condition, more current flows through the field. This increases the magnetic field and increases the output voltage. A series-wound dc generator has the characteristic that the output voltage varies with load current. This is undesirable in most applications. For this reason, this type of generator is rarely used in everyday practice.

Figure 2: Connection diagram of a series wound generator

2.2.2 Shunt Wound Generator
In this field winding is connected in parallel with the armature conductors and have the full voltage of the generator applied across them.The field coils consist of many turns of small wire. They are connected in parallel with the load. In other words, they are connected across the output voltage of the armature. Current in the field windings of a shunt-wound generator is independent of the load current (currents in parallel branches are independent of each other). Since field current, and therefore field strength, is not affected by load current, the output voltage remains more nearly constant than does the output voltage of the series-wound generator. In actual use, the output voltage in a dc shunt-wound generator varies inversely as load current varies. The output voltage decreases as load current increases because the voltage drop across the armature resistance increases (E = IR). In a series-wound generator, output voltage varies directly with load current. In the shunt-wound generator, output voltage varies inversely with load current. A combination of the two types can overcome the disadvantages of both. This combination of windings is called the compound-wound dc generator.

Figure 3: Connection diagram of a shunt wound generator

2.2.3 Compound Generator
Compound-wound generators have a series-field winding in addition to a shunt-field winding, as shown in figure. The shunt and series windings are wound on the same pole pieces. They can be either short-shunt or long-shunt as shown in figures. In a compound generator, the shunt field is stronger than the series field. When series field aids the shunt field, generator is said to be commutatively-compounded. On the other hand if series field opposes the shunt field, the generator is said to be differentially compounded. In the compound-wound generator when load current increases, the armature voltage decreases just as in the shunt-wound generator. This causes the voltage applied to the shunt-field winding to decrease, which results in a decrease in the magnetic field. This same increase in load current, since it flows through the series winding, causes an increase in the magnetic field produced by that winding. By proportioning the two fields so that the decrease in the shunt field is just compensated by the increase in the series field, the output voltage remains constant. This is shown in figure, which shows the voltage characteristics of the series-, shunt-, and compound-wound generators. As you can see, by proportioning the effects of the two fields (series and shunt), a compound-wound generator provides a constant output voltage under varying load conditions. Actual curves are seldom, if ever, as perfect as shown.

Figure 4: Connection diagram of a compound generator

2.3 Ideal Output The curve in figure 5 shows the relationship between output voltage and output current which is known as the external characteristic. If a separately excited generator is driven at constant speed and has a fixed field current, the output voltage will decrease with increased load current. This decrease is due to the armature resistance and armature reaction effects. If the field flux remained constant, the generated voltage would tend to remain constant and the output voltage would be equal to the generated voltage minus the IR drop of the armature circuit. However, the demagnetizing component of armature reactions tends to decrease the flux, thus adding an additional factor, which decreases the output voltage. In a shunt excited generator, it can be seen that output voltage decreases faster than with separate excitation. This is due to the fact that since the output voltage is reduced because of the armature reaction effect and armature IR drop, the field voltage is also reduced which further reduces the flux. It can also be seen that beyond a certain critical value, the shunt generator shows a reversal in trend of current values with decreasing voltages. This point of maximum current output is known as the breakdown point. For the series wound generator, the armature is in series with the field, load current must be flowing to obtain flux in the field. As the voltage and current rise the load resistance may be increased to its normal value. The compound generator gives the best external characteristic.

Figure 5: Ideal Generator Curve Graph

3.0 EXPRIMENTAL SETUP Procedures
1. All switches were ensured to off and control knob fully turned counter -clockwise. The circuit was connected as shown in figures below and all the ground were connected.

2. The circuit in figure above shows unloaded connection. The schematic diagram below was referred to know how to connect voltage and current for measurement in loaded connection.
3. Diagram also shows that external excitation voltage were connected to field winding E1 - E2 each for generator and driving component. The field winding resistance was measured for both.
4. Multimeter was connected and set for current measurement [use scale Ampere (A)] at point between excitation voltage source E1. DC Meter and DC Motor Control Unit were switched ON.
5. The DC voltage was adjusted from Variable DC Voltage Board until the Rotate Speed Meter showed 300rpm.
6. The values of DC Current Meter and DC Voltage Meter shown in DC Motor Control Unit were recorded for input value and the value of output (current and voltage) were recorded using DC meter.
7. Step 4 and 5 were repeated for 500 and 700rpm. Note that since fixed value of excitation was used, field parameter needed to be recorded only once and need not be repeated. The multimeter was disconnected from field measurement before the experiment was ran for different speed.
8. The bulb was engaged as load. The circuit in diagram was referred for connection. The experiment was started with no bulb and as additional bulbs were used as increasing load, the bulbs were connected in parallel.
9. The results are recorded in the table respectively. Table 1 is for separated-excited generator, table 2 is for series wound generator, table 3 is for shunt wound generator and table 4 is for compound generator.
10. The graph of generated voltage (V) versus load (A) for the generator speed at 300rpm was plotted using MATLAB software. The same parameter for generator running at speed of 500 and 700 rpm were also plotted on the same graph.
11. The observations were recorded based from the experimental data. The differences between the experimental data with the theoretical data that being thought in theory classes had been compared and identified.

4.0 RESULTS AND DISCUSSION
4.1Results:
4.1.1 Tables
A. Separately-excited generator:
Table 1(a): No load
Input Value
Theoretical
Rpm
Experimental
rpm
Output Value
Voltage (V)
Current (A)

Voltage (V)
Current (A)
48
0.167
300
316
51
0
75
0.206
500
504
82
0
105
0.238
700
708
115
0

Table 1(b): 1 load
Input Value
Theoretical
Rpm
Experimental
rpm
Output Value
Voltage(V)
Current(A)

Voltage (V)
Current (A)
48
0.274
300
301
44
0.104
78
0.340
500
500
75
0.137
101
0.410
700
700
105
0.164

Table 1(c): 2 loads
Input Value
Theoretical
Rpm
Experimental
rpm
Output Value
Voltage(V)
Current(A)

Voltage (V)
Current (A)
48
0.270
300
301
42
0.144
77
0.317
500
501
72
0.182
106
0.360
700
701
102
0.216

Table 1(d): 3 loads
Input Value
Theoretical
Rpm
Experimental
rpm
Output Value
Voltage(V)
Current(A)

Voltage (V)
Current (A)
50
0.395
300
301
39
0.212
80
0.413
500
502
70
0.270
108
0.445
700
700
99
0.320

Table 1(e): 4 loads
Input Value
Theoretical
Rpm
Experimental
rpm
Output Value
Voltage(V)
Current(A)

Voltage (V)
Current (A)
52
0.423
300
303
37
0.279
81
0.509
500
503
66
0.352
112
0.550
700
708
96
0.420

B. Series wound generator:
Table 2(a): No load
Input Value
Theoretical
Rpm
Experimental
rpm
Output Value
Voltage(V)
Current(A)

Voltage (V)
Current (A)
45
0.105
300
302
4
0
74
0.108
500
501
6
0
102
0.112
700
701
10
0

Table 2(b): 1 load
Input Value
Theoretical
Rpm
Experimental
rpm
Output Value
Voltage(V)
Current(A)

Voltage (V)
Current (A)
45
0.110
300
300
5
0.014
73
0.113
500
502
7
0.023
101
0.121
700
705
12
0.059

Table 2(c):2 loads
Input Value
Theoretical
Rpm
Experimental
rpm
Output Value
Voltage(V)
Current(A)

Voltage (V)
Current (A)
44
0.108
300
300
7
0.054
73
0.125
500
504
9
0.72
101
0.136
700
700
14
0.116

Table 2(d): 3 loads
Input Value
Theoretical
Rpm
Experimental
rpm
Output Value
Voltage(V)
Current(A)

Voltage (V)
Current (A)
44
0.113
300
300
8
0.086
73
0.135
500
500
10
0.102
101
0.161
700
701
15
0.172

Table 2(e)-4 loads
Input Value
Theoretical
Rpm
Experimental
rpm
Output Value
Voltage(V)
Current(A)

Voltage (V)
Current (A)
44
0.115
300
300
9
0.100
73
0.151
500
501
11
0.124
101
0.188
700
702
16
0.206

C. Shunt wound generator:
Table 3(a): No load
Input Value
Theoretical
Rpm
Experimental
rpm
Output Value
Voltage(V)
Current(A)

Voltage (V)
Current (A)
45
0.088
300
302
3
0
73
0.094
500
501
5
0
87
0.095
700
603
7
0

Table 3(b): 1 load
Input Value
Theoretical
Rpm
Experimental
rpm
Output Value
Voltage(V)
Current(A)

Voltage (V)
Current (A)
45
0.093
300
301
2
0.021
73
0.095
500
502
4
0.030
102
0.100
700
701
6
0.035

Table 3(c): 2 loads
Input Value
Theoretical
Rpm
Experimental
rpm
Output Value
Voltage(V)
Current(A)

Voltage (V)
Current (A)
45
0.095
300
302
2
0.036
73
0.097
500
503
3
0.053
101
0.103
700
701
5
0.066

Table 3(d): 3 loads
Input Value
Theoretical
Rpm
Experimental
rpm
Output Value
Voltage(V)
Current(A)

Voltage (V)
Current (A)
45
0.095
300
300
2
0.047
73
0.102
500
503
3
0.071
101
0.105
700
702
4
0.090

Table 3(e): 4 loads
Input Value
Theoretical
Rpm
Experimental
rpm
Output Value
Voltage(V)
Current(A)

Voltage (V)
Current (A)
44
0.088
300
301
1
0.055
73
0.098
500
500
2
0.086
101
0.101
700
700
3
0.099

D. Compound generator:
Table 4(a); No load
Input Value
Theoretical
Rpm
Experimental
rpm
Output Value
Voltage(V)
Current(A)

Voltage (V)
Current (A)
44
0.094
300
302
2
0
72
0.097
500
501
4
0
100
0.101
700
701
6
0

Table 4(b): 1 load
Input Value
Theoretical
Rpm
Experimental
rpm
Output Value
Voltage(V)
Current(A)

Voltage (V)
Current (A)
44
0.096
300
300
2
0.029
72
0.100
500
500
4
0.042
100
0.109
700
700
6
0.050

Table 4(c): 2 loads
Input Value
Theoretical
Rpm
Experimental
Rpm
Output Value
Voltage(V)
Current(A)

Voltage (V)
Current (A)
44
0.102
300
300
2
0.048
72
0.107
500
500
4
0.078
100
0.117
700
700
6
0.098

Table 4(d): 3 loads
Input Value
Theoretical
Rpm
Experimental
rpm
Output Value
Voltage(V)
Current(A)

Voltage (V)
Current (A)
44
0.103
300
300
1
0.064
72
0.112
500
500
3
0.109
100
0.130
700
702
6
0.145

Table 4(e): 4 loads
Input Value
Theoretical
Rpm
Experimental
rpm
Output Value
Voltage(V)
Current(A)

Voltage (V)
Current (A)
44
0.105
300
300
1
0.073
72
0.120
500
500
3
0.137
101
0.146
700
700
6
0.193

4.1.2 Graph:

Figure 6: Graph of Separately-excited generator

Figure 7: Graph of Series Wound Generator

Figure 8: Graph of Shunt Wound Generator

Figure 9: Graph of Compound Generator

Figure 10: Graph of Output Voltage – Load Current Characteristic at 700 rpm ( 50% Rated Speed)

4.2 Discussion:
As shown in figure 10, in a shunt excited generator, it can be seen that the output voltage decreases faster than with separate excitation. This is due to the theoretical that, the output voltage decreases as load current increases because the voltage drop across the armature resistance increases (E=IR). As the output voltage decreasing exciting current falls which further weakens the field. Therefore, the output voltage in a dc shunt-wound generator varies inversely as load current varies. For separated-excited generator, the output voltage will decrease with increased load current as shown. This decrease is due to the armature resistance and armature reaction effects. If the field flux remained constant, the generated voltage would tend to remain constant and the output voltage would be equal to the generated voltage minus the IR drop of the armature circuit. . As the voltage and current rise the load resistance may be increased to its normal value. Beside that for series wound generator, the external characteristic curve shows, the voltage output starts at current equal to zero and increase until a peak point. Finally, compound generator shows constant output voltage when we increase the load current. This is because the voltage drop, which occurs in the shunt machine, is compensated for by the voltage rise, which occurs in the series machine. The addition of a sufficient number of series turns offsets the armature IR drop and armature reaction effect, resulting in a flat-compound generator, which has a nearly constant voltage. If more series turns are added, the voltage may rise with load and the machine.
The rated voltage of the generator is 240V while the rated current is 1.25A.
For separately-excited dc generator:
i. A generator, which is separately excited, is driven at constant speed and has a fixed field current, the output voltage will decrease with increased load current as shown. This decrease is due to the armature resistance and armature reaction effects. If the field flux remained constant, the generated voltage would tend to remain constant and the output voltage would be equal to the generated voltage minus the IR drop of the armature circuit. ii. By comparing the theoretically graph and experiment graph, there are slightly different, the graph for theoretically is curving down but the graph we obtained by experiment is slightly straight down. This may be due to some restriction in the generator. iii. The output voltage is directly proportional to the speed of the rotation and the strength of the magnetic field. As the speed of the armature is increased, the output voltage will also increase. There is, however, a limit to the safe operating speed of the rotor before physical damage occurs. Likewise, the output voltage can be controlled up to a point by adjusting the field current. An increase in field current increases the output voltage only if the field poles are not saturated. Field control of the output voltage is accomplished by varying the total resistance of the field circuit. Reversing the polarity of the exciting current will reverse the flux and thus the polarity of induced voltage. iv. From the experiment we just able to obtain the result from no-load to full-load using 50% rated speed of the generator that is 700rpm. Therefore the output regulation of the generator is:

Regulation = x 100% = x 100% = 19.79%

v. The obvious disadvantage of a separately excited DC generator is that we require an external DC source for excitation. But since the output voltage may be controlled more easily and over a wide range (from zero to a maximum), this type of excitation finds many applications. Series-wound generator: i. To build up the voltage on a series generator, the external circuit must be connected and its resistance reduced to a comparatively low value. Since the armature is in series with the field, load current must be flowing to obtain flux in the field. As the voltage and current rise the load resistance may be increased to its normal value. As the external characteristic curve shows, the voltage output starts at current equal to zero and increase until a peak point. Theoretically, the voltage output starts at zero, reaches a peak, and then falls back to zero. ii. The exciting current through the field winding of a series generator is the same current the generator delivers to the load. If the load has high resistance, only a minimum output voltage can be generated because of minimum output voltage due to its residual magnetism. If the load draws current increases, the excitation current increases, the magnetic field becomes stronger and the generated voltage is high. iii. From the experiment we just able to obtain the result from no-load to full-load using 50% rated speed of the generator that is 700rpm. Therefore the output regulation of the generator is:

Regulation = x 100% = x 100% = -37.5%

Iv. Therefore, in a series wound generator, changes in load current greatly affect the generator output voltage. A series generator has very poor voltage regulation and is not recommended for use as a power source.
v. A generator 's voltage output must be constant under various load conditions
Shunt generator:
i. In a shunt excited generator, it can be seen that the output voltage decreases faster than with separate excitation. This is due to the fact that, the output voltage decreases as load current increases because the voltage drop across the armature resistance increases (E=IR). As the output voltage decreasing exciting current falls which further weakens the field. Therefore, the output voltage in a dc shunt-wound generator varies inversely as load current varies. ii. From the experiment we just able to obtain the result from no-load to full-load using 50% rated speed of the generator that is 700rpm. Therefore the output regulation of the generator is:

Regulation = x 100% = x 100% = 133.33%

iii. Self-excited shunt generators have the disadvantage in the changes in their load current from no-load to full-load cause their output voltage to change also. Their poor voltage regulation is due to three factors: a) The magnetic field strength drops as the armature voltage drops, which further reduce the magnetic field strength, which in turn reduces the armature voltage etc.
b) The armature voltage drop (I2R losses) from no-load to full-load. c) The running speed of the driving motor may change with load. (This is particularly true of internal combustion engines and induction motors.)
Compound generator:
i. Combination of a shunt field and a series field gives the best external characteristic as illustrated in Figure. The voltage drop, which occurs in the shunt machine, is compensated for by the voltage rise, which occurs in the series machine. The addition of a sufficient number of series turns offsets the armature IR drop and armature reaction effect, resulting in a flat-compound generator, which has a nearly constant voltage. If more series turns are added, the voltage may rise with load and the machine. ii. From the experiment we just able to obtain the result from no-load to full-load using 50% rated speed of the generator that is 700rpm. Therefore the output regulation of the generator is:

Regulation = x 100% = x 100% = 0%

iii. A compound generator is nearly ideal any increase in the excitation current, speed of rotation and also increase in load will not make any changes to the output or just slightly increase or decrease the output value only.

5.0 CONCLUSION
According to the result we obtained, the graph is slightly different with the ideal graph. From the result, we can compute that, compound generator is the best generator because the percentage regulation is 0%, follow by separately-excited generator which is 19.79%. Series-wound generator and shunt-wound generator has very poor voltage regulation, changes in load current greatly affect the generator output voltage.
6.0 RECOMMENDATION
Ways to improve the output regulation of the separately-excited generator is to increase the speed of the rotating armature and strength of the magnetic field. To compensate for various voltages, a dummy load, called a "ballast" load, is used to keep output voltage constant. Having a ballast load is an absolute requirement of a series-wound generator. For shunt wound generator, to regulate the output, again a ballast load is used to compensate for varying load conditions. Because the voltage of this kind of generator also is also difficult to control, a shunt-wound generator is seldom used.

7.0 ACKNOWLEDGMENT First of all, the special thank goes to our helpful lecturer, Mr. Ahmad Firdaus Bin Ahmad Zaidi and our Vocational Training Officer (PLV), Mr. Mohd Azri Bin Abd Aziz. They gave us this opportunity to design this experiment. They gave us fully support by helping us in the progression and smoothness of the experiment. The co-operation are much indeed appreciated.
Next, a hundreds grateful thanks to our group members , Wong Yew Chun, Tong Janice, Thomas Tan Wan Kiat , Gun Wei Hwa and Thiruselvan a/l Muniandy. Each of them contributed their hard time and energy during this few weeks when completing this experiment. All experiments would be nothing without the enthusiasm, imagination and co-operation from all of them. Last but not least, we would like to thank all my course-mates and friends for kindly helping us when we seek for help or facing problem when design the experiment.

8.0 REFERENCES

[1] Theodore Wildi, Electrical Machines, Drives and Power Systems, sixth edition, Pearson Prentice Hall: Part 2. Electrical Machines and Transformers, pp.71-117
[2] M. Young, The Technical Writer’s Handbook. Mill Valley, CA: University Science, 1989.
[3] Admin, “Generator Characteristic”, [online database], 28 March 2007http://powerelectrical.blogspot.com/2007/03/generator-chracteristics.html [retrieved 6 November 2011]
[4] Ahmad Firdaus Bin Ahmad Zaidi, Lecture Notes DC generators and DC motor, 2011, UniMAP.
[5] Kothari and Nagrath, Electric Machines, 2010, fourth edition, McGraw Hill
[6] ‘Electrical Machines- Generators (Description and Applications) http://www.mpoweruk.com/generators.htm
[7] ‘Dynamo’, http://en.wikipedia.org/wiki/Dynamo [retrieved 14 November 2011]
[8] ‘DC Generator construction’ http://www.tpub.com/content/doe/h1011v2/css/h1011v2_93.htm

References: [1] Theodore Wildi, Electrical Machines, Drives and Power Systems, sixth edition, Pearson Prentice Hall: Part 2. Electrical Machines and Transformers, pp.71-117 [2] M. Young, The Technical Writer’s Handbook. Mill Valley, CA: University Science, 1989. [3] Admin, “Generator Characteristic”, [online database], 28 March 2007http://powerelectrical.blogspot.com/2007/03/generator-chracteristics.html [retrieved 6 November 2011] [4] Ahmad Firdaus Bin Ahmad Zaidi, Lecture Notes DC generators and DC motor, 2011, UniMAP. [5] Kothari and Nagrath, Electric Machines, 2010, fourth edition, McGraw Hill [6] ‘Electrical Machines- Generators (Description and Applications) http://www.mpoweruk.com/generators.htm [7] ‘Dynamo’, http://en.wikipedia.org/wiki/Dynamo [retrieved 14 November 2011] [8] ‘DC Generator construction’ http://www.tpub.com/content/doe/h1011v2/css/h1011v2_93.htm