Operating Characteristics of DC-AC Three-Phase Generators and Motors - Assignment Example

1- Executive Summary: - DC and AC machines both Motors and Generators have been used for the purpose of converting Electric power to mechanical and mechanical power to electric power respectively. There are two types of these machines AC and DC. Similarly they can be single phase or three phased. A DC or AC machine can be used as a motor or a generator. If the electric power is fed to the armature of an electric machine it will produce flux. The flux produced by that reacts with the field flux. When these fluxes react lines of forces are cut and an induced torque is produced and the rotor of the machine starts rotating. In such condition the machine is called a Motor. Similarly if the field flux is present and the rotor conductors are moved inside that field an induced voltage is produced in them which produce terminal voltage i.e. electric power is generated. In such situation machine is called a Generator. In this assignment first DC series generator is examined. Then Series motor characteristics are studied. Then two types of induction motors i.e. capacitor start and cage rotor motors are studied and we find that the difference in the two motors is that of the capacitor which helps to rotate the motor shaft in the beginning when there is not enough torque to start the Motor. In all these experiments relating to motors we have considered the effects of changing load torque on speed, output power, power factor and efficiency. 2- Aims and Objectives: - Objective of this assignment is to enable students to understand the basic concepts underlying the effect of increasing load on electric machines. Series Generators and different types of Motors are studied with the effects of changing loads on their characteristics. All these machines show change in their behavior when load is increased. Studying that behavior is the main Aim of this assignment. 3- Experiments: - a- Experiment # 1 Title: - The object of this experiment is to investigate the relationship between the output voltage of a DC series generator and its output current, when driven at constant speed. Equipment Required: - Equipment Initial settings Specifications Test Bed Speed range 1800 rev/min, D.C. supply 110 V, Field rheostat to 0, Armature Rheostat to ∞, START/STOP/RUN switch to RUN. The Test Bed (FH2) holds and gives power for most of the machines, except the Stepper Motor (FH150). This machine needs the Stepper Motor System (SMS2) to drive it. FH50 compound machine Test Machine DC generator Works as a shunt, series and compound motor and also works as a generator. Approximate ratings are: As a motor: 100 W (shunt), 110 V, 1500 rev.min–1 As a generator: 40 W (shunt), 110 V, 1500 rev.min–1 FH50 compound machine Prime mover DC motor Works as a shunt, series and compound motor and also works as a generator. Approximate ratings are: As a motor: 100 W (shunt), 110 V, 1500 rev.min–1 As a generator: 40 W (shunt), 110 V, 1500 rev.min–1 Instrumentation frame V2 DC voltmeter 15/75/150V set to 15 A2 DFC ammeter 1.5A range R1 resistive load 50Ώ rheostat set to ∞, 2000Ώrheostat set to ∞. Circuit diagram: - Procedure: - After wiring the circuit as shown in the circuit schematic, I set up DC series generator. I measured the output current using Ammeter which was connected at the output in series and voltage was measured by connecting voltmeter in parallel with the output load (R1). I varied the load R1 in small steps to obtain the given values of current which I recorded on the Ammeter. The output voltage values were recorded using voltmeter. These are given in the table below. Results: - Output current (mA) Output voltage (V) Output current (mA) Output voltage (V) 0 1.5 600 8.7 50 1.6 650 9.5 100 1.8 700 10.5 150 2.2 750 11.4 200 2.8 800 12.2 250 3.4 850 13 300 3.9 900 13.2 350 4.7 950 13.2 400 5.4 1000 12.8 450 6.2 1100 12 500 7 1200 11.2 550 7.9 Graph of output voltage and output current: - Following graph shows the relationship between V and I at output terminals, Explanation of results: - As the field windings are in series with armature windings an increase in armature current (as load increases armature current increases) will also increase the field current which in turn will produce more induced voltage and output voltage will increase as a result. Conclusions: - Voltage characteristics of series generators are steep as shown in the graph, so they are used only where these steep voltage characteristics of the motor can be exploited. An example of such applications is arc wielding. b- Experiment # 8 Title: - The object of this experiment is to investigate the relationships between the speed, power output and efficiency of a DC series motor and the torque produced by the motor. Equipment Required: - Equipment Initial settings Specifications Test Bed Speed range 1800 rev/min, D.C. supply 110 V, Field rheostat to 0, Armature Rheostat to ∞, START/STOP/RUN switch to RUN. The Test Bed (FH2) holds and gives power for most of the machines, except the Stepper Motor (FH150). This machine needs the Stepper Motor System (SMS2) to drive it. FH50 compound machine Prime mover DC motor Works as a shunt, series and compound motor and also works as a generator. Approximate ratings are: As a motor: 100 W (shunt), 110 V, 1500 rev.min–1 As a generator: 40 W (shunt), 110 V, 1500 rev.min–1 Instrumentation frame V2 DC voltmeter 15/75/150V set to 15 A2 DFC ammeter 1.5A range Circuit Diagram: - Procedure: - After wiring the circuit as shown in the circuit schematic diagram I set up the DC series Motor. Following steps were carried out: - I placed the FGH50 mimic diagram over the machine access terminals of the Test bed. Then I mounted the Test Generator into the right hand machine position and the prime mover FH 50 into the left hand position. Then I connected the circuit as shown in the circuit schematic diagram. Then I started the motor by reducing the armature rheostat to zero. Motor supply was adjusted to 110V. Voltmeter and Ammeter were used for the purpose of measuring currents and voltages. Following equations were used to make Calculations. Results: - Torque Nm Speed (rev/min) Voltage (v) Current (A) Input power (W) Output power (W) Efficiency (p.u) 0.05 2550 116 1.1 127.6 13.345 0.104585 0.1 2300 110 1.25 137.5 24.073333 0.175079 0.15 2150 110 1.35 148.5 33.755 0.227306 0.2 2000 110 1.45 159.5 41.866667 0.262487 0.25 1850 110 1.5 165 48.408333 0.293384 0.3 1700 110 1.7 187 53.38 0.285455 0.35 1500 110 1.85 203.5 54.95 0.270025 0.4 1400 110 1.95 214.5 58.613333 0.273256 0.45 1300 110 2.01 221.1 61.23 0.276934 0.5 1200 110 2.12 233.2 62.8 0.269297 0.55 1100 110 2.3 253 63.323333 0.25029 0.6 950 110 2.4 264 59.66 0.225985 0.65 850 110 2.5 275 57.828333 0.210285 0.7 750 110 2.6 286 54.95 0.192133 0.75 650 110 2.7 297 51.025 0.171801 Graph of torque against speed, output power and efficiency, Explanation of Results: - Induced torque in series motor is = K×φ×IA as load torque increases motor demands more current to produce its induced torque. As input power has not changed so current in armature remains the same and induced torque reduces and the motor slows down as shown in graph. Also efficiency is the ratio of output power to input power and as output power is directly related to speed so efficiency decreases after a certain speed limit. Conclusions: - From above experiment the disadvantage of series motors is seen that when torque on this motor goes to zero its speed goes to infinity. In practice this does not happen but the motor can turn very fast in such situations and can damage itself. So the series motor should never be operated at no-load. c- Experiment # 16 Title: - The object of this experiment is to investigate the relationships between the speeds, current, power factor and efficiency of a capacitor start induction motor and the torque produced by the motor. Equipment Required: - Equipment Initial settings Specifications Test Bed Speed range 1800 rev/min, D.C. supply 110 V, Field rheostat to 0, Armature Rheostat to ∞, START/STOP/RUN switch to RUN. The Test Bed (FH2) holds and gives power for most of the machines, except the Stepper Motor (FH150). This machine needs the Stepper Motor System (SMS2) to drive it. FH80 capacitor motor Test machine Single-phase capacitor start/run induction motor, Four-pole, 180 W, 230 V, 50/60 Hz Nominal speed: 1490/1780 rev.min–1 Instrumentation frame V3 AC voltmeter 250V range A3 AC ammeter 3A range W1 AC Wattmeter 500 Watts C1Capacitive load Set to 15 mF Circuit Diagram: - Procedure: - After wiring the circuit as shown in the circuit schematic diagram I set up the capacitor start induction motor. Following steps were carried out: - I placed the FH80 mimic diagram over the machine access terminals of the Test bed. Then I mounted the FH80 Test Generator into the right hand machine position. Then I connected the circuit as shown in the circuit schematic diagram. Then I started the motor by reducing the armature rheostat to zero. Motor supply was adjusted to 110V. Voltmeter and Ammeter were used for the purpose of measuring currents and voltages. START/STOP/RUN button was set to RN before the Test and was set to STOP after the Test. Following equations were used to make Calculations. Results: - Graph of torque against speed, current, power factor and efficiency of a capacitor start induction motor. Following graphs show the these relationships, Explanation of Results: - Capacitor start motors are induction motors with a capacitor which helps to run the load in starting after that the motor behaves as any other induction motor so the graphs for this experiment are same as for experiment no 21. The same reasons are applied to these results as explained in the next experiment. Conclusions: - For some cases the starting torque supplied by motor is insufficient to start the load on motor. In those cases capacitor start motors are used. A capacitor is placed in series with the auxiliary windings of the motor. The m.m.f of starting current in those windings is adjusted by suitably selecting the capacitor value. The capacitor only helps in starting and after the motor continues to run it behaves as any other induction motor. d- Experiment # 21 Title: - The object of this experiment is to investigate the relationships between the speed, current, output power, power factor and efficiency of a cage rotor induction motor and the torque developed by the motor. Equipment Required: - Equipment Initial settings Specifications Test Bed Speed range 1800 rev/min, D.C. supply 110 V, Field rheostat to 0, Armature Rheostat to ∞, START/STOP/RUN switch to RUN. The Test Bed (FH2) holds and gives power for most of the machines, except the Stepper Motor (FH150). This machine needs the Stepper Motor System (SMS2) to drive it. FH90 capacitor motor Cage rotor induction motor Test Machine Three-phase, four-pole induction motor 120 W, 230 V (Delta), 50/60 Hz Nominal speed: 1473/1770 rev.min–1 Star or delta operation Instrumentation frame V3 AC voltmeter 250V range A3 AC ammeter 2A range W1 AC Wattmeter 500 Watts Circuit Diagram: - Procedure: - After wiring the circuit as shown in the circuit schematic diagram I set up the cage rotor induction motor. Following steps were carried out. I placed the FH90 mimic diagram over the machine access terminals of the Test bed. Then I mounted the FH90 Test Generator into the right hand machine position. Then I connected the circuit as shown in the circuit schematic diagram. Then I pressed the green ON pushbutton to activate the contactor. Motor supply was adjusted to 110V. Voltmeter and Ammeter were used for the purpose of measuring currents and voltages. START/STOP/RUN button was set to RN before the Test and was set to STOP after the Test. Following equations were used to make the calculations Results: - Torque (Nm) Speed (rev/min) output power (w) wattmeter Wa(W) wattmeter Wb (w) Input Power (w) Line current (A) Line Voltage Volt amperes (VA) Power factor Efficiency (p.u) 0.1 1500 15.71 5 76 81 0.4 245 169.7 0.4773 0.19395 0.2 1500 31.41 8 82 90 0.4 245 169.7 0.5303 0.349 0.3 1480 46.49 18 90 108 0.4 245 169.7 0.6364 0.43046 0.4 1460 61.15 27 100 127 0.45 245 190 0.6684 0.4815 0.5 1450 75.9 38 110 148 0.45 245 190 0.7789 0.51284 0.6 1445 90.8 47 120 167 0.5 245 212.2 0.7871 0.54371 0.7 1420 104.1 58 130 188 0.55 245 233.4 0.8055 0.55372 0.8 1400 117.28 70 140 210 0.6 245 254.6 0.8248 0.55848 0.9 1380 130.06 80 150 230 0.65 245 275.8 0.8339 0.56548 1 1350 141.37 92 170 262 0.7 245 297 0.882 0.53958 1.1 1320 152.05 110 185 295 0.76 245 322.5 0.9147 0.51542 1.2 1280 160.85 120 210 330 0.85 245 360.7 0.9149 0.48742 1.3 1200 163.36 140 240 380 1 245 424.4 0.8955 0.42989 Graphs of torque against speed, output power, current, power factor and efficiency Following graphs show these relationships, Explanation of Results: - For induction motor induced torque and speed of rotation are inversely related. The induced torque of motor is zero at synchronous speed so an induction motor cannot reach synchronous speed it is nearly synchronous. The graph tells us that from no load to full load speed increases but than decreases again because induced torque is zero and motor slows down. Conclusions: - An induction motor does not have a separate field circuit. It depends on transformer action to induce voltages and currents in its field circuit. So it can be called a rotating transformer. This motor normally operates at a speed nearly equal to synchronous speed but it can never operate at synchronous speed as found in the experiment and explained in the results. e- Experiment # 25 Title: - The object of this experiment is to investigate the relationships between speed, input power, output power, current, power factor and efficiency of a synchronous motor and the torque produced by the motor. Equipment Required: - Equipment Initial settings Specifications Test Bed Speed range 1800 rev/min, D.C. supply 110 V, Field rheostat to 0, Armature Rheostat to ∞, START/STOP/RUN switch to RUN. The Test Bed (FH2) holds and gives power for most of the machines, except the Stepper Motor (FH150). This machine needs the Stepper Motor System (SMS2) to drive it. FH100 slip ring machine Test Machine 120 W, 230 V (Delta), 50/60 Hz, three-phase Synchronous speed: 1500/1800 rev.min–1 • Synchronous generator: 80 W, 230 V (Delta), 50/60 Hz, three-phase Synchronous speed: 1500/1800 rev.min–1 FH50 DC compound Machine Starting Motor Works as a shunt, series and compound motor and also works as a generator. Approximate ratings are: As a motor: 100 W (shunt), 110 V, 1500 rev.min–1 As a generator: 40 W (shunt), 110 V, 1500 rev.min–1 Instrumentation frame V2 DC voltmeter 150 V range V3 AC voltmeter 250V range A3 AC ammeter 2A range W1 AC Wattmeter 500 Watts R1 resistive load 50Ώ set to ∞ and 2000Ώ set to 400 Ώ. Circuit Diagram: - Procedure: - After wiring the circuit as shown in the circuit schematic diagram I set up the synchronous motor. Following steps were carried out: - I Placed the FH100 mimic diagram over the machine access terminals of the Test bed. Then I mount the slip ring machine into the right hand machine position and the prime mover FH50 into the left hand position. Then I connected the circuit as shown in the circuit schematic diagram. I used Voltmeter and Ammeter for the purpose of measuring currents and voltages while Watt meters measuring powers. Following equations were used to make the Calculations, Results: - Torque (Nm) Speed (rev/min) output power (w) Wattmeter Wa(w) wattmeter Wb (w) Input Power (w) Line current (A) Line Voltage Volt amperes (VA) Power factor Efficiency (p.u) 0.05 1500 7.85 30 28 58 0.1 245 42.39 1.3684 0.13534 0.1 1500 15.7 36 30 66 0.15 245 63.58 1.0381 0.23788 0.2 1500 31.4 45 40 85 0.2 245 84.77 1.0027 0.36941 0.3 1500 47.1 52 50 102 0.24 245 101.7 1.0027 0.46176 0.4 1500 62.8 60 60 120 0.26 245 110.2 1.0889 0.52333 0.5 1500 78.5 70 70 140 0.32 245 135.6 1.0322 0.56071 0.6 1500 94.2 80 85 165 0.38 245 161.1 1.0244 0.57091 0.7 1500 109.9 85 102 187 0.44 245 186.5 1.0027 0.5877 0.8 1500 125.6 90 128 218 0.5 245 211.9 1.0287 0.57615 Relationship of torque with speed, input power, output power, current, power factor and efficiency Following graphs show these relations, Explanation of Results: - For a synchronous motor, input power is the applied electric power (P = I × V). As the torque is directly proportional to current so the graph is a straight line. Similarly consider the following equation for synchronous motor, I = Pin / ((3)1/2 × V× cos (ωt)) From this equation as ‘I’ is inversely related to Power factor so the Pf decreases in the beginning with torque (torque is directly related to ‘I’). Conclusions: - Frequency, terminal voltage and speed of synchronous motors are fixed so these can be used in power systems where generators are much larger than motors and constant speed is required. Speed of these motors remains constant from no load to full load. f- 3.6- Experiment # 26 Title: - The object of this experiment is to investigate the relationships between the current driving a synchronous motor and the excitation voltage applied to the rotor, for a range of torque values. Equipment Required: - Equipment Initial settings Specifications Test Bed Speed range 1800 rev/min, D.C. supply 110 V, Field rheostat to 0, Armature Rheostat to ∞, START/STOP/RUN switch to RUN. The Test Bed (FH2) holds and gives power for most of the machines, except the Stepper Motor (FH150). This machine needs the Stepper Motor System (SMS2) to drive it. FH100 slip ring machine Test Machine 120 W, 230 V (Delta), 50/60 Hz, three-phase Synchronous speed: 1500/1800 rev.min–1 • Synchronous generator: 80 W, 230 V (Delta), 50/60 Hz, three-phase Synchronous speed: 1500/1800 rev.min–1 FH50 DC compound Machine Starting Motor Works as a shunt, series and compound motor and also works as a generator. Approximate ratings are: As a motor: 100 W (shunt), 110 V, 1500 rev.min–1 As a generator: 40 W (shunt), 110 V, 1500 rev.min–1 Instrumentation frame V2 DC voltmeter 150 V range V3 AC voltmeter 250V range A3 AC ammeter 2A range R1 resistive load 50Ώ set to ∞ and 2000Ώ set to 400 Ώ. Circuit Diagram: - Procedure: - After wiring the circuit as shown in the circuit schematic diagram I set up the synchronous motor. Following steps were carried out: - I Placed the FH100 mimic diagram over the machine access terminals of the Test bed. Then I mounted the slip ring machine into the right hand machine position and the prime mover FH50 into the left hand position. Then I connected the circuit as shown in the circuit schematic diagram. After setting the torque to 0.1Nm I used Voltmeter and Ammeter for the purpose of measuring currents and voltages while Watt meters for measuring powers. I repeated the above steps for other values of torque i.e. 0.15, 0.20, 0.25, 0.30, 0.35 and 0.40 Results: - Excitation voltage(V) Line Curent Torque 0.1 Nm Torque=0.15 Nm Torque=0.2 Nm Torque=0.25 Nm Torque=0.3 Nm Torque=0.35Nm Torque=0.4Nm 120 0.23 0.24 0.25 0.26 0.28 0.29 0.32 110 0.2 0.21 0.23 0.24 0.26 0.28 100 0.18 0.2 0.21 0.23 0.24 0.27 90 0.16 0.18 0.19 0.22 0.24 80 0.15 0.16 0.18 0.21 70 0.13 0.15 0.18 60 0.13 0.15 50 0.13 40 30 Relationship of torque with current driving a synchronous motor and the excitation voltage applied to the rotor: - 1- Following graph shows the relation between current and excitation voltage for torque 0.1 Nm. 2- Relation between current and excitation voltage for torque 0.15 Nm 3-Relation between current and excitation voltage for torque 0.3 Nm Explanation of Results: - Excitation Voltage and current relation means that the motor draws more current when load torque is increased. The reason is that as load torque is increased motor induces its own torque to maintain the constant speed and that induced torque increases with the increase in armature current which is shown in the graph. Conclusions: - In a synchronous motor as load or torque changes, motor will develop its induced torque so that load and rotor rotate at same speed. Speed is constrained to be constant by the input power supply. As the power supply has not been changed so the motor will continue to rotate at synchronous speed butt armature current is increased. g- Discussions and conclusions: - An electric machine is a device that can convert either electric energy into mechanical energy or mechanical energy into electrical energy. When it is used to convert electric energy into mechanical energy it is called a Motor and when it converts mechanical energy into electrical energy it is called Generator. DC machines are those where electric power fed to them or generated by them is on DC. And AC machines are those where electric power fed to them or drawn from them is AC. AC machines used are generally three phase. From the above experiments we conclude that all except synchronous motors have their speed decreased when the load is increased. Capacitor start motor was used when starting torque was required to run the motor in the beginning. Synchronous motor showed very stable and smooth power factor characteristics. While other motors have shown an improving power factor with increase in load torque. Efficiency of these machines was found to be less than 100 because no practical machine is perfect and there are some losses always present. Only series Generator type is studied for generators and we found that it’s V-I characteristics are steeper and can be applied in arc wielding. 4- References [1]- Electric Machinery fundamentals by Chapman [2]- Power Systems - Design and Analysis, 4th Edn., J. DUNCAN GLOVER, MULUKUTLA S. SARMA, THOMAS J. VERBYE. [3]- http://www.tecquipment.com/Datasheets/FH%20Machines_0808.pdf [4]- Electric Power Systems, 4th Edn., B.M WEEDY AND B.J. CORY, 2001, JOHN WILEY & SONS LTD 5- Appendix M-Files for the graphs drawn above are as: - Experiment 8: - Relation between torque and efficiency. t=[0.05,0.1,0.15,0.2,0.25,0.3,0.35,0.4,0.45,0.5,0.55,0.6,0.65,0.7,0.75] %specify the values obtained for torque e=[0.104584639,0.175078788,0.227306397,0.262486938,0.293383838,0.285454545,0.27002457,0.273255633,0.276933514,0.269296741,0.250289855,0.225984848,0.210284848,0.192132867,0.171801347] %specify the values of efficiency calculated figure(1) %draw figure plot(t,e,r,linewidth,5) %plots the efficiency against torque with line width of 5 set(gca,fontsize,14) %font size of labeling is set to 15 on current axis xlabel(Torque (Nm)) %specify independent varialble ylabel(Efficiency (p.u)) %specify dependent variable title(Relation between Torque and Efficiency) %give title axis([-0.5,1,0.1,0.3]) %apply grid to graph M-File for experiment NO 26 t=[120,110,100,90,80,70,60,50] %specify the values obtained for torque e=[0.23,0.2,0.18,0.16,0.15,0.13,0.13,0.13] %specify the values of efficiency calculated %apply grid to graph r=[120,110,100,90,80,70,60] %specify the values obtained for torque h=[0.24,0.21,0.2,0.18,0.16,0.15,0.15] %specify the values of efficiency calculated %draw figure a=[120,110,100,90,80,70] b=[0.24,0.21,0.2,0.18,0.16,0.15] c=[120,110,100,90,] d=[0.24,0.21,0.2,0.18,] plot(t,e,r,h,a,b,c,d,linewidth,5) legend(0.1Nm,0.15Nm,0.2Nm,0.3Nm) %plots the efficiency against torque with line width of 5 set(gca,fontsize,14) %font size of labeling is set to 15 on current axis xlabel(Current (A)) %specify independent varialble ylabel(Excitation Voltage) %specify dependent variable title(Relation between Current and Excitation Voltage) %give title axis([50,150,0.1,0.3]) %apply grid to graph Read More