15

ABSTRACTThis paper aims to determine the specifications and ratings of a DC Generator.INTRODUCTIONThe Direct Current GeneratorMost common electrical appliances (e.g., electric light-bulbs, and electric heating elements) work fine on AC electrical power. However, there are some situations in which DC power is preferable. For instance, small electric motors (e.g., those which power food mixers and vacuum cleaners) work very well on AC electricity, but very large electric motors (e.g., those which power subway trains) generally work much better on DC electricity. Let us investigate how DC electricity can be generated.

Figure 4.1:A split-ring commutator.

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. In this manner, the emfseen by the external circuit is always the same as the emfgenerated around the rotating coil. In a DC generator, the two ends of the coil are attached to different halves of a single split-ring which co-rotates with the coil. The split-ring is connected to the external circuit by means of metal brushes--see Fig.41. This combination of a rotating split-ring and stationary metal brushes is called acommutator. The purpose of the commutator is to ensure that the emfseen by the external circuit is equal to the emfgenerated around the rotating coil forhalfthe 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 emfgenerated around the coil goes through zero. In this special case, the emf seen in the external circuit is simply(218)

Figure4.2showsplotted 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. Thealternatorin 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.

Figure 4.2:Emf generated in a steadily rotating DC generator.

BODY PROCEDURE and DISCUSSION4.1 GeneralThe electrical machines in the FH2/3MkIV Electrical Machines Teaching System, as detailed in Section 3, include a wide range of different designs which enable a wide spectrum of investigations into many modes of operation. The individual machine characteristics make certain type of ideal for some applications but totally unsuitable for others. It is, therefore, necessary for engineers of all specializations and academic levels to appreciate the differences in characteristics so that they may, 1. Design or specify a piece of equipment and select the machine best suited for the applicationOr2. Maintain equipment and in particular identify whether the equipment is performing correctly or requires remedial action.The experiments detailed in this section are intended to allow investigation of commonly used industrial type electrical machines, motors, and generators, so that the required levels of understanding can be attained.The individual experiments include an experimental procedure and a suggested equipment list. These may both be modified to suit the particular requirements of the user and the equipment available.Also, the experiments have been designed with the minimum selection of instrument or control modules, see Section 2, to achieve the working test circuit. In the experiments most operating parameters are measured directly but others need to be calculated from measured data. Each experiment includes a suggested table of results.The scope of a particular experiment may be extended by using additional modules. For instance, instrumentation modules such as the Speed/Slip Indicator, Type S1, the Power Factor Meter, Type PF1, and Frequency Meter, Type F1, can be used in addition to the instruments recommended(the minimum required to achieve a viable test circuit) to reinforce other measurements or allow direct visual observation of the changing operating conditions and system performance.The traditional analogue or digital measuring instruments may be replaced by the Data Management System, Type DMS2, so that the flexibility and speed of a computer can enhance the experimental techniques. In particular, transient conditions may be investigated.Other important topics, such as transformer applications and power electronics drives, can be studied using additional, though fully compatible, modules. The relevant manuals supplied with these items need to be consulted to obtain the specialized information required for safe and meaningful operation.In each of the experimental procedures provided, the test machines are required to be run under load for a period of time. This is to ensure that the machines themselves achieve their normal operating temperatures before an experiment commences. In this way, the quality of the test results is not affected by such factors as changes in winding resistance and machine air gaps.Outline drawings, of the same type used in the experiments section, have been included in Appendix L at the rear of this manual. These may be used to create connection diagrams specific to local requirements.Whenever experiments are to be performed, it is recommended that the user becomes familiar with the equipment by reading Sections 2 and 3 of this manual and also any other manual pertaining to additional pieces of equipment being used.For a theoretical explanation of the individual machine operating principles and characteristics, it is recommended that the user refers to suitable text books or similar reference sources.NoteThe information given in the columns headed INITIAL SETTINGS is intended to give a starting point only, and it must not be assumed that the instrument ranges are necessarily the most appropriate to conduct the experiment.

4.2 D.C. MACHINE EXPERIMENTS: The basic D.C. machine consists of the following components,Figure 4.3 the Armature

1. THE ARMATURE. A complex arrangement of windings, connected in a series/parallel circuit, and mounted on a laminated core. The windings are terminated at an assembly called the COMMUTATOR. This whole assembly is supported on a shaft which is free to rotate within the frame of the machine. Electrical power is supplied to the armature via stationary brushes which are in sliding contact with the commutator.2. THE FIELD WINDINGS. These produce the magnetic flux necessary for the machine to operate as either a motor or as a generator. There are two types of field winding,a. SHUNT WINDING. A coil of many turns of thin copper wire. It has relatively high resistance and is connected across the armature or a separate supply. Current flowing through the Shunt Winding is generally considered to be constant and so the field strength may also be assumed constant for most applications.b. SERIES WINDING. A coil of few turns of thick copper wire which has a relatively low resistance. In most applications, this coil would be connected in series with the Armature circuit ad so the current, and the flux it produces, varies with changing operating Figure 4.4 The Shunt Generator conditions.It is possible to connect either the Shunt or the Series Field Windings to the Armature to obtain the required characteristic. Alternatively, both windings may be connected in a number of different combinations to produce a characteristic which combines the two effects. This is termed COMPOUNDING.

4.3 D.C. GENERATOR EXPERIMENTSThe basic construction of a D.C. Generator contains the components discussed in Section 4.2, and the operating characteristics will vary depending upon the contributions made by the armature and shunt and/or series field windings.The principal function of the experiments contained in this section is to investigate the changes in performance of a D.C. generator with different combinations of field windings. The experiments are:4.3.1. The Series Generator Load Characteristic4.3.2 The Shunt Generator Separately Excited field- Open circuit characteristic4.3.3 The Shunt Generator Load characteristics4.3.4 The Shunt Generator Self Excited Efficiency by direct loading4.3.5 The Compound Generator Separately-Excited shunt field Load characteristics in cumulative and differential connections4.3.6 The Compound Generator Load characteristics in cumulative connection4.3.7 Parallel Operation of GeneratorsNote: For clarity reasons, the voltmeter indicating the D.C. supply voltage is not known in many of the circuit and connection diagrams.

4..3.1.. THE D.C. SERIES GENERATORThe object of this experiment is to investigate the relationship between the output voltage of a D.C. series generator and its output current, when driven at constant speed.

EQUIPMENTINITIAL SETTINGSFH2 MkIV Test Bed Speed Range 1800rev/minD.C. Supply 110VField Rheostat to ZeroArmature Rheostat to infinity()START/STOP/RUN Switch t RUNFH50 D.C. Compound MachineTest Machine D.C. GeneratorFH50 D.C. Compound MachinePrime Mover- D.C. MotorFH3 MkIV Instrumentation FrameV2 D.C. Voltmeter15/75/150V Set to 15VA2 D.C. Ammeter1.5A RangeR1 Resistive Load50 Rheostat set to infinity()2000 Rheostat set to infinity()

ProcedurePosition the FH50 Mimic Diagram over the Machine Access Terminals of the FH2 MKIV Test BedMount the Test Generator FH50 into the Right-hand machine position, and the Prime Mover FH50 into the left-hand position. Locate the 16-Way plugs of the two machines into their respective sockets in the FH2 MKIV.Set up the equipment and connect as shown in the diagrams. Figure 4.3.1Note that the 2000 rheostat of R1 is connected in parallel with the 50 rheostat in order to act as a fine control.Switch on the FH2 MkIV at the Mains Switch and then press the Green ON Push button to engage the contactor.Start the prime mover by rotating the Armature Rheostat clockwise. Set the Armature Rheostat so that the machines rotate at 1500 rev/min. Adjust the 50 rheostat of R1 to give an output current of 500mA, and allow the machines to warm up for approximately 15 minutes.

NoteIf voltmeter and ammeter both show reverse indications proceed as follows: return controls to initial settings; apply 110V D.C. for a few seconds to the generator SHUNT field with the left-hand field connection positive(+); restart the prime mover and continue with the testTurn the R1 50 rheostat to zero and then to 50.R1 = 0, I = 1.125A R1 = 50, I = .00625ADecrease R1 to give a range of output current values, as indicated by the Results table, and record corresponding values of output current and output voltage. Use the 2000 rheostat of R1 as a fine control of load. Lower current readings may be obtained using the 250/500mA ranges of A2.It is recommended that output current settings are made by a series of small changes in R1 each accompanied by a correction of speed.Finally, set both R1 rheostats to infinity() and record the output voltage due to residual magnetism.3VNote. To obtain good results ensure that-1. The motor speed is maintained at 1500 rev/min precisely. This may require constant adjustment throughout the experiment to both the Armature and Field Rheostats on the FH2 MkIV.2. Throughout the experiment the R1 rheostat is turned in one direction only. If for any reason the rheostat is turned in the reverse direction the complete procedure should be repeated.3. Appropriate instrument ranges are used as the values change.

GraphPlot a graph of output voltage against load current

Figure 4.5 DC Series Generator Voltage in terms of current

RESULTS

Generator Speed = 1500rev/min(constant)Output Current(mA)Output Voltage (V)Output Current(mA)Output Voltage(V)

02.2550

502.5600

1003.40650

1504.0700

200750

25080012.20

30085012.80

35090012.80

400950

450100012.80

500

4.3.2.. D.C. SHUNT GENERATOR Open Circuit CharacteristicsThe object of this experiment is to investigate the relationship between the open-circuit voltage and field current for a DC shunt generator with the field separately excited and driven at constant speed.

EQUIPMENTINITIAL SETTINGSFH2 MkIV Test Bed Speed Range 1800rev/minD.C. Supply 110VField Rheostat to ZeroArmature Rheostat to infinity()START/STOP/RUN Switch t RUNFH50 D.C. Compound MachineTest Machine D.C. GeneratorFH50 D.C. Compound MachinePrime Mover- D.C. MotorFH3 MkIV Instrumentation FrameV2 D.C. Voltmeter15/75/150V Set to 15VA2 D.C. Ammeter250mA RangeR1 Resistive Load50 Rheostat set to infinity()2000 Rheostat set to infinity()R2 General Purpose RheostatFull in position, giving 110V on V

ProcedurePosition the FH50 Mimic Diagram over the Machine Access Sockets of the FH2 MkIV Test Bed.Mount the Test Generator into the right-hand machine position, and the Prime Mover FH50 into the left hand position. Locate the 16-way plugs of the two machines in their respective sockets in the FH2 MkIV Test Bed.Set up the equipment and connect as shown in the diagrams. Figure 4.3.2Start the prime mover by rotating the Armature Rheostat clockwise. Set the Armature Rheostat so that the machines rotate at 1500 rev/min. Allow the machines to warm up for approximately 15 minutes.Set both controls on R1 to maximum resistance() and record the output voltage. Next, set R2 to give a very low field supply voltage Vp and adjust R1 to produce a low setting of field current, check the speed, correct if necessary and record the output voltage.1. 30V 2. 2V

Take a set of readings of output voltage for increasing values of field current using both R1 and R2 for control of current.At higher current values it is recommended that R2 is placed at the Full in position(V, indicating 110V).Finally, take a set of readings of output voltage for increasing values of field current using both R1 and R2 for control of current.Note. To obtain good results ensure that 1)The Motor speed is maintained at 1500 rev/min precisely. This may require constant adjustment throughout the experiment to both the Armature and Field Rheostats of the FH2 MkIV.2)The R1 control is turned in one direction only. If for any reason the control is turned in the reverse direction the complete procedure must be repeated.3)Appropriate instrument ranges are used as the values change.

GraphsPlot graphs of output voltage against field current for both increasing and decreasing field current values.

Figure 4.6 DC Shunt Generator Voltage when increasing the field current

Below is the results on the experiment performed.

Figure 4.7 DC Shunt Generator Voltage when decreasing the field currentResultsGenerator Speed = 1500 rev/min (constant)

(a) Increasing field current(b) Decreasing field currentField Current(mA)Output Voltage (V)

02

108

2015

4032

6045

8060

10071

12082

14088

15094

180

200

220

240

260

Field Current(mA)Output Voltage (V)

260

240

220

200

180

15094

14091

12086

10077

8064

6050

4034

2018

1010

04

4.3.3 D.C SHUNT GENERATOR Self ExcitedThe object of this experiment is to investigate the relationship between the output voltage of a D.C. shunt generator and output current, when driven at constant speed.EQUIPMENTINITIAL SETTINGSFH2 MkIV Test Bed Speed Range 1800rev/minD.C. Supply 110VField Rheostat to ZeroArmature Rheostat to infinity()START/STOP/RUN Switch t RUNFH50 D.C. Compound MachineTest Machine D.C. GeneratorFH50 D.C. Compound MachinePrime Mover- D.C. MotorFH3 MkIV Instrumentation FrameV2 D.C. Voltmeter150V RangeA2 D.C. Ammeter250mA (AL) Range/1.5A Range(AF)R1 Resistive Load50 Rheostat set to infinity()2000 Rheostat set to infinity()ProcedurePosition the FH50 Mimic Diagram over the Machine Access Sockets of the FH2 MkIV.Mount the Test Generator in the right-hand machine position, and the Prime Mover in the left hand machine position. Locate the 16-way plugs of the two machines in their respective sockets in the FH2 MkIV.Set up the equipment and connect as shown in the diagrams of Figure 4.3.3.Switch on the FH2 MkIV at the Mains switch and then press the Green ON push-button to engage the contactor.Start the prime mover by rotating the Armature Rheostat clockwise. Set the Armature Rheostat so that the machines rotate at 1500 rev/min. Adjust the 2000 rheostat of R1 to give and output of 200mA and allow the machines to warm up for approximately 15 minutes.NoteIf the generator fails to excite, proceed as follows, return controls to initial settings; disconnect the SHUNT Field winding; apply 110V D.C. for a few seconds to the SHUNT field winding with the left-hand connection positive(+); reconnect shunt field; restart prime mover and continue to test.Set both of the R1 rheostats to minimum and then to maximum(). Increase the output current in steps, as indicated in the Results table, by rotating the R1 controls clockwise.Note that, at lower R1 resistance values, the load current will eventually start to decrease, and this will continue until it is possible to short-circuit the generator. It is difficult to take readings in this decreasing current range.Record corresponding values of field current, output voltage and output current.IT SHOULD BE NOTED THAT THE SELF-EXCITED SHUNT CHARACTERISTICS OF FH50 MACHINES VARY FROM UNIT TO UNIT AND, THEREFORE, THE CURRENT RANGE MAY NOT BE AS SUGGESTED IN THE TABLE BELOW.Note. To obtain good results ensure that-1. R1 rheostats are turned in ONE direction only. If the any reason the rheostat is turned in the reverse direction then the complete procedure must be repeated.2. The motor speed is maintained at 1500 rev/min precisely. This may require constant adjustment to both the Armature and Field Rheostats throughout the experiment.

GraphPlot a graph of output voltage against output current

Figure 4.8 DC Shunt Generator Self excited. Relationship between Voltage and Current.

ResultsGenerator speed = 1500 rev/min (constant)Output Current (mA)Output Voltage (V)Field Current (mA)

0790.1

40650.1

60640.1

80630.1

10061.50.1

120600.08

140590.075

16057.50.07

18050.50.06

200490.055

22042.50.05

240400.05

250380.05

300

320

340

360

V.ConclusionsAfter performing the experiments and gather the data we needed, we have concluded that for the series generator, as the output voltage is directly proportional to the output current. If we increase the voltage the output current also increases at a constant speed of 1500 rpm. For the open circuit characteristic of the DC Shunt generator, we have concluded that the relationship of the output voltage or the open circuit voltage is directly proportional to the field current. For the self-excited D.C. generator, as you increase the output current, it causes a decrease on the output voltage and field current.

VI. RecommendationsThe conducted experiment is highly recommended to be performed for students and hobbyists who seeks much knowledge of the operation of the D.C. generator, may it either be a series or a shunt D.C. Generator. We also recommend that the students and the hobbyists should be instructed well on how to operate the FH2 MkIV Test Bed to avoid mishandling the equipment since this equipment is worth a million to purchase.

REFERENCESWEBSITES:

[1]DC Machineshttp://farside.ph.utexas.edu/teaching/302l/lectures/node91.html [2]DC Generators and Its Typeshttp://www.cedengineering.com/upload/DC%20Generators%20and%20Motors.pdf

BOOKS:[1]Fitzgerald, Kingsley and Umans. Electric Machinery, 6th Edition. McGraw-Hill,Inc. 2003.[2]Chapman, Stephen J. Electric Machinery Fundamentals, 5th Edition. McGraw-Hill, Inc. 2012.