Expt 2 DC Compound Generator Using Simulink [PDF]

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Department of Electrical Engineering



Laboratory Manual for Electrical Machines 1



Experiment No. 2 DC COMPOUND GENERATOR – SELF EXCITED



OBJECTIVE To investigate the relationship between the output voltage and the output current of a DC generator with a shunt connected field and a cumulatively connected series field, and driven at constant speed BACKGROUND INFORMATION The field windings of a self-excited generator may be supplied in three different ways. The field windings may be connected in shunt with the armature resulting in a shunt generator. If the field windings are in series with the armature, the result is a series generator. Finally, the field windings may consist of two sections, one of which is connected in series and the other in shunt or parallel with the armature, resulting in a compound generator. As in the case of a self-excited shunt generator, the residual flux must be present at the poles of the machine to get the self-excitation process started. Compound generators are normally connected as cumulatively compound so that magneto motive force (mmf) of the series field windings aids that of the shunt field windings. The advantage is that through the action of the series field windings, the flux per pole can increase with load, resulting in a voltage output, which is nearly constant, or which even rises somewhat as load increases. The shunt field windings usually contain many turns of relatively small wire while the series field windings, wound on the outside, consist of a few turns of a comparatively heavy conductor because it must carry the full load armature current of the machine. The voltage of a cumulative compound generator may also be controlled over reasonable limits by means of rheostat in the shunt field circuit. The external characteristic of a cumulatively compound generator depends upon the relative strengths of the shunt and series field windings. As the load current increases, the series field magneto motive force increases and tends to increase the flux, and therefore, the generated voltage. In some generators, the increase in generated voltage is greater than the IaRa drop so that instead of decreasing, the terminal voltage increases as shown in Figure 3.1. Generators having this characteristic are called over-compounded generators. The eventual drop of the characteristic curve after full-load condition occurs because of saturation of the magnetic circuit. A flat compounded generator is one having a full load terminal voltage equal to the no-load terminal voltage as indicated in Fig. 3.1. The series windings of this generator are weaker than the one in the over-compounded machine and therefore, do not increase the flux as much for a given armature current. Still weaker series windings give the machine the characteristic of an under-compounded generator where the full load terminal voltage is less than the no-load voltage. When the series field windings are reversed from that of the cumulative compound generator, the mmf of the series field windings weakens that of the shunt winding thereby reducing the flux per pole. The result is a differential compound generator whose characteristic is similar to that of the self-excited shunt generator but more drooping as shown in Fig. 3.1.



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Department of Electrical Engineering



Laboratory Manual for Electrical Machines 1



EQUIPMENT



INITIAL SETTINGS (double check)



FH2 MkIV Test Bend



Speed Range: 1800 rev/min DC Supply: 110 V Field Rheostat: zero Armature Rheostat: Infinite START/STOP/RUN: Switch to Run



FH50 DC Compound Machine FH50 DC Compound Machine FH3 MkIV Instrumentation Frame V2 DC Voltmeter A2 DC Ammeter R1 Resistive Load (as diverter) Resistor (Decade Resistor or Additional R1 Resistive load) FH210 Automatic Constant Speed Unit



Test Machine – DC Generator Prime Mover – DC Motor 75 V Range 1.5 A Range 50 ohms Rheostat set to infinity 2000 ohms Rheostat set to infinity 1000 ohms, 5 W (see later note)



1500 rev/min (FH210 is optional)



PROCEDURES 1. Open the Simulink as circuit simulator that can be found on the upper tab of the Matlab, click the blank model in order to design the required circuit. 2. Look for the library browser that can be found on upper left tab of the Simulink. Look for the following equipment and add the block to the model: DC machine, DC voltage source, series RLC branch (double click to choose the branch type to R or resistor), voltage, measurement, current measurement, bus selector, scope, constant and powergui. 3. Set up the equipment and connect the wiring diagram as shown in Fig. 3.2(a) or Fig. 3.2(b). Make sure that the equipment is properly connected. Notes: a. The 100-ohm resistor is included in order to reduce the effect of the shunt field and thus emphasize the effect of compounding. b. The 50-ohm rheostat of R1 is used as a series field diverter, and the 2000 ohm rheostat as the generator load. 4. For the DC machine, double click it to change the properties, change the mechanical input from Torque TL to Speed W click apply and ok. Then connect the constant equipment to the DC machine having a constant value of 25 rps. 5. For the powergui, double click it to change it properties to Continuous as simulation type. 6. For the bus selector, connect the DC machine to the bus selector. Double click the bus selector to remove the “???signal1 and ???signal2” and select the “Speed, armature current, field current and electrical torque” that can be found on the left tab of the block parameters: Bus selector. 7. Once the circuit is properly connected, try to run the circuit. If there is a misconnection the Simulink will notify the user. Otherwise if none the user will be able to obtain the results. 8. Set the 50 ohm rheostat to 10 ohms. Turn the 2000-ohm rheostat of R1 to zero and then to maximum resistance (∞). 9. In steps, gradually decrease the 2000-ohm rheostat to produce the current values shown in the Results table and record corresponding values of output voltage and output current. 10. Repeat the whole procedure with the R1 50-ohm rheostat set to 40 ohms and then zero.



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Department of Electrical Engineering



Laboratory Manual for Electrical Machines 1



11. It is recommended that output current settings are made by a series of small changes in R1, each accompanied by a correction of speed. 12. Record the results in Table 3.1. 13. Plot graphs of output voltage against output or load current for the four different conditions. In order to allow a direct comparison, it is recommended that all graphs be plotted on a common set of axes.



WIRING DIAGRAM



Figure 3.2(a) SCHEMATIC DIAGRAM Series Field



IL AL



If



IA



Shunt Field Ia



Armature



Diverter R1 (50 ohms)



VL



R1 as load 2000 ohms



R1 or decade resistor (Set to 100 ohms)



Figure 3.2(b)



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Department of Electrical Engineering



Laboratory Manual for Electrical Machines 1



Applicable Equations: VL = Eg – IARA – ISRS



IA = If + IL



IS = IL when there is no diverter resistor



CURVES Over-compounded VL



Flat-compounded Under-compounded Differential Compounded



Separately-excited shunt



Self-excited shunt



IL Figure 3.1 – Typical Characteristic Curves of Different DC Generators



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Department of Electrical Engineering



Laboratory Manual for Electrical Machines 1



PRELIMINARY DATA SHEET Experiment 2 - DC COMPOUND GENERATOR – SELF EXCITED



NAME: ______________________________ COURSE/SECTION: __________________



DATE: ________________ GROUP NO.: __________



Table 3.1 Compound Generator with Compounding Output Current IL (mA) 0 50 100 150 200 250 300 350 400 450 500 550



Output Voltage VL (V) Diverter Diverter Diverter at ∞ at 40 Ω at 10 Ω



Compound Generator without compounding Output Voltage Output VL (V) Current Diverter IL (mA) at zero Ω 0 30 50 70 100 110 120 130 120 110 80 60



_________________________ Instructor’s Signature



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