Characteristics of Three Phase Synchronous Generators - Lab Report Example

Characteristics of Three Phase Synchronous Generators Lab Report Term Project Author Name Name of Affiliation Abstract A three Phase Synchronous Generator is loaded to its full load so as to know the efficiency that is output power/ input power and voltage regulation. As is obvious, the power wasted in this test is considerable, but the results are obtained under the actual operating conditions, and hence are actual results. This makes study of their characteristics of Three Phase Synchronous Generators successful. Table of Contents Abstract 2 Introduction 4 Theoretical background on synchronous machines 4 Construction 5 Synchronous motor 6 Starting of synchronous motor 7 Experiment methodology 8 Results/ summaries of results 10 Open and Short Circuit Characteristics 10 Voltage Regulation of Synchronous Generators 13 Synchronization of Synchronous Generators 15 Discussion/ experimental results comparison with the theory 19 Conclusion 21 References 22 Introduction When loading a three Phase Synchronous Generator, its terminal voltage changes and there are load-dependent losses in its windings. The change in terminal voltage depends on the magnitude and the power factor of the current, in other words, it depends on the load impedance. When for example a load current of 5 amp at a given power frequency will change the three Phase Synchronous Generator terminal voltage that is from its no load or open circuit value more than a current of 3 amp at the same power frequency, will be less with a unity power frequency current, and the drop may be even negative with a leading power factor. Theoretical background on synchronous machines Synchronous machines are available in three forms as motor as alternator and as synchronous convertors. The armature winding of synchronous machines, which is generally on stator, is similar to induction motor armature winding. When used as motor, three-phase supply is given to armature, which produces rotating magnetic field as in the induction motor. The speed of this rotating magnetic field is synchronous speed, Ns=120f/P where P is the number of poles on the machine and f=Frequency(Godsell, 2014a). These machines are constructed in two general forms salient pole and cylindrical rotor the field winding is generally put on rotor. Two slip rings are provided to carry D.C. field current. Same machine can be used as an alternator or a motor. When working as alternator, the machine is coupled to prime mover and is run at synchronous speed. When field winding is excited with D.C. current, three phase voltage is generated and is available at armature terminals. The frequency of generated voltage is f=Ns. P/120Hz. When used as motor, the machine armature is connected to a three-phase balanced supply of rated voltage. If the frequency of supply Hz, the motor runs at the constant speed Ns, which is the synchronous speed, for the same frequency, a machine with higher number of poles will have a lower speed(Keler, 2015). Construction Salient pole synchronous machines are constructed in all sizes for small and medium speeds. An elementary 4 pole, 3 phase machine. Stator has two coils in each phase which are connected in series. The three phases may be connected in star or delta. The pole faces are so shaped that the radial air gap length increases from the pole centre to pole tips. This creates the sinusoidal flux density in the air gap. Damper winding is provided in the pole faces. This consists of copper bars short-circuited at both ends, placed in the specially provided holes. Field windings are connected to two slip-rings mounted on rotor shaft (Godsell, 2014b). Cylindrical rotor machines are used for high speeds. In the se machines, the diameter of the rotor is kept smaller. This reduces the peripheral sped. The fields winding is placed in slots provided on rotor. A three phase balanced supply when connected to the stator of this machine produces a two-pie pole fields which rotates at synchronous speed. If the field winding is excited with a suitable direct current, thus creating two pole fields, and the rotor is run at a synchronous speed, a three phase voltage will be generated in armature. The frequency of this voltage will be f=Ns.P/120. The voltage generated in each phase will be sinusoidal as flux density wave-form is sinusoidal and in related by the expression E=4.44 , where E=r.m.s value of generated per phase (volts), , F is frequency (Hz) and T is number of effective turns per phase(Godsell, 2014b). Further the voltages in three phases will differ in time phase. If the rotor is rotated in anticlockwise direction. A. the voltage in phase b will lag behind voltage in phase a by 120 degrees (.2and voltage in phase C will be behind the voltage in phase b by 120 degrees(Keler, 2015). The windings of alternator are not concentrated but are distributed. To take into account the effect this, the voltage expression is multiplied by a factor known as distribution factor (Kd). further, if the pitch of the coil is not kept full, but is its fraction the output voltage reduces and a factor is introduced in the voltage equation to account for this reduction. This in known as pitch factor (Kp). Modified expression for voltage can be given as a follows E=4.44, where Synchronous motor Construction of synchronous motor is similar to that of alternator. If the armature of the synchronous machine is connected to a balanced three phase supply, a rotating magnetic field is produced. The stator produces a two pole field, which is rotating in clockwise direction. If the rotor is excited at this instant such that the poles are created on rotor with the polarity, the rotor will experience a torque in anticlockwise direction due to the interaction between the two fields. If the rotor is stationary, this torque will be there for half the revolution of a stator field for stator field rotation from point to point in the next half revolution, the rotor will experience a torque in opposite direction. This will repeat every half a revolution i.e., the torque on rotor will be pulsating in nature and will change the direction every half revolution. Due to the inertia, the rotor does not move in any direction. Hence, the synchronous motor is not self-starting(Godsell, 2014a).. Now consider that the rotor is rotated in the direction of the rotating magnetic field at the same speed with the help of some prime mover, and then excited such that polarities created on rotor. The rotor will now experience a torque in the direction of rotation of rotation and as the field and rotor both are moving in the same direction with the same speed, this torque will be unidirectional. If now the prime mover of rotor is cut off, the rotor will continue to rotate in the same direction with the same speed under the influence of the above said torque. The rotor will lag behind the stator field by some angle, . This angle is known as torque angle (Irwin and Nelms, R. M. 2008. Starting of synchronous motor Synchronous motors can be started in by means of an auxiliary motor or with the help of damper winding. In the first method, an auxiliary motor is coupled to synchronous motor and the set is run up to synchronous speed with the help of this motor. At this speed, supply is switched on to synchronous motor and its field is excited. The rotor is pulled into synchronism. After this, the supply to auxiliary motor is switched off. The motor having damper windings on rotor poles can be started like induction motor. No auxiliary motor is required and so the method is also known as self-starting of synchronous motor (Keler, 2015). A motor having damper windings on rotor pole faces, when connected to supply, with its field winding open, runs as induction motor. When the speed is near synchronous speed, the field winding on the poles is excited and the motor is pulled into synchronism i.e. the stator field and rotor field are locked. Now that the rotor runs at synchronous speed, there is no current in damper winding and motor action is there (Alexander and Sadiku, 2009). In this method, with the motor started as induction motor using the role of a damper winding starting current will be highs, if full voltage is impressed on stator at the time of the starting. Also, high voltage will be induced in the field-winding which may cause damage to the insulation of field winding. To protect the motor against these, the motor is started with reduced voltage supply like induction motor and during starting field winding is disconnected from the D.C. supply and is short circuited, through the field discharge resistor. This protects field winding against high induced voltage (Nilsson and Riedel 2008). Experiment methodology Different laboratory experimental designs were carried out analyzed. The aim was to determine characteristics of Three Phase Synchronous Generators. The first experiment was about Open and short circuit characteristics where a generator was set to rotate at 3000 rev/min and results was recorded. The second recording under Open and short circuit characteristics was effect of speed variation on output voltage and frequency. The results were recorded. Under this the synchronous impedance was calculated using Then various graphs were plot to those effects The second part experiments were to determine voltage regulation of synchronous generators. Resistive load, inductive loads and capacitive loads were connected to the generator and generator was set to rotate at 1500 rev/min. The same was repeated for generator rotating at 1500 rev/min. Different recording was done for different loading and graphs were plotted. The third part of experiments was synchronization of synchronous generators. The first experiment was done for synchronization of synchronous generators by Setting the speed of the generator at 3000 rev/min and DC field voltage. The results was measured and recorded. The values of power of output power and efficiency were calculated using; Where N is the speed in rev/min and T is the torque in Nm. Efficiency = This was recorded and graphs to the results were plotted. The last experiment under synchronization of synchronous generators was done for power factor control a three phase synchronous motor. The same procedure was followed but speed/torque variable control is set fully counter-clockwise to power interlock position. The results was recorded and plot graphs of field current (rotor) against the (stator) current were made. Results/ summaries of results Open and Short Circuit Characteristics The field voltage in steps of 10 V up to 50V and the corresponding values of open circuit line voltage was measured and recorded for a generator to rotating at 3000 rev/min. Figure 1 shows the results of the experiment. The observations for line output voltage as well as field voltage against field current largely suggest closely similar relationship for the view factor although some deviations are encountered. The relationship between voltage and current is plotted in the Graph. The resulting plot assumes a linear approach as the magnitude of voltage begins to rise; this is an indication of the success of the experiment. Figure 1: voltage Vs field current The short circuit operation results are plotted in figure 2 shown below Figure 2: current output vs field current As the field current increases the current output increases. There are differences in the magnitude of the results produced. But the changes are in the same direction. Figure 3: Line output voltage and frequency Vs Torque As the rotating speed reduces from 3000rev/min to 1000rev/min, it can be observed that line output voltage reduces as well as frequency. This means that the rotating speed is critical in determining line output voltage and frequency. Thus an increase in the speed will lead to increase line output voltage and frequency of a Three Phase Synchronous Generators From the results above the synchronous impedance = = 118.91 From the graph it can be noted that find the field current when the open circuit voltage is the normal operating level of 220V is 220V = 470.2 field current + 16.51. This gives 0.4323A the short circuit line current at this value of the field current will be calculated using The equation is y=2.327x+0.008 where y is short circuit line current and x field current which is 0.43. y=2.327(0.43)+0.008 = 1.0086A From the equation for synchronous impedance at the normal operating voltage (220V) of the generator = = 511.63 Voltage Regulation of Synchronous Generators The capacitor, resistor and inductor were used to determine their impact on voltage. Figure 4: Output Line Voltage (Volts) vs Load Resistance (ohms In all the experiments the generator was set to rotate at 1500 rev/min as well as at 1000 rev/min. the results were taken and the graphs below shows the relationship between the variables and voltage. The graph below shows that the speed of rotation does not affect voltage when using resistors. This is because the graphs have the same shape and the graph has the same trend at almost at equal points in every measurement. Looking at voltage regulation with inductive load one will note in figure 5 that the voltage vs load inductance has the same trend. But there is a peak when relationship changes. It can be noted that when the inductance decreases voltage takes the same trend. Figure 5: Output Line Voltage (Volts) vs Load inductance The resistance of the inductances determines the shape of this graph. At low voltage, the resistance of the inductances much less. At extreme voltage, either high or low, the impedance is larger and the amplitude of the current is therefore small. In the case of Capacitor resistance a graph has been made as shown in figure 6. From the graph it can be noted that at both torque have the same trend. However a higher torque process slightly higher Output Line Voltage. Figure 6: Output Line Voltage(Volts) vs Load Capacitance Synchronization of Synchronous Generators The next was synchronization of synchronous generators where two experiments were carried out. The Load Resistance Torque and Input Power were recorded, then output power and efficiency were calculated as where N is the speed in rev/min and T is the torque in Nm. Efficiency = This values have been used to plot the graphs below. Figure 7: output power & efficiency against torque The graph above show how that output power & efficiency against torque have the same trend . The figure above shows that when there is a load increase, the voltage at end of feeder will drop. This means that voltage is regulated when load on feeder varies, this ensured by load tap-changing transformers which can be inform of feeder or fixed or switched shunt capacitors, bus voltage regulators and line voltage regulators. When there is power is leaving the sgenerator there is a meter which measures amount of power that leaves to ensure the maximum is not exceeded or the minimum is not reached. Second graph Figure 8: output power & efficiency against torque(b) Both graphs indicate that output power & efficiency against torque have the same trend. This means that both output power & efficiency are affected by torque. The figures below shows plot graphs of field current (rotor) against the (stator) current. Figure 9: field current (rotor) against the (stator) current Figure 10: field current (rotor) against the (stator) current Figure 11: field current (rotor) against the (stator) current Discussion/ experimental results comparison with the theory Theoretically, A three phase synchronous generators receives three phase currents for its armature and a direct content for its field winding. The electrical power relieved from the source of excitation can be utilized only for production of the field flux by the electro-magnets, and the power is wasted as i2R loss in the field. Thus, it cannot contribute to the shaft-power that is the mechanical output of the motor. The A.C. armature currents, on the other hand, are responsible for producing the mechanical power, thus, the three-phase A.C. side must feed an active component of current to the motor. If the conditions are such that the A.C. side feeds only the active component, the power factor is unity. There is a particular field current which corresponds to such a condition. For the mechanical output kept constant, the active component requirement is fixed. In such a situation, if the field current is increased, let us see its effect. On the A.C. side, since the supply voltage is constant, the air gap flux must remain substantially constant. The most notable and observable difference is that the figures gave a less difference in these variations. This showed an improved efficiency in not only timing but also giving the output of required variable. Similar results to this setting were obtained in the sixth experiment within the Output Line Voltage vs Load Resistance; although the highest variable was achieved at same as the other, another difference is the presence of a wider gap between the functions that represents the utility of the applied variables under control and comparison. In the above expression, the leakage impedance drop in the armature winding is ignored only for simplicity. If the air-gap flux must remain substantially constant, and if the increased field current tries to increase it, then the only way out is that the surplus field ampere-turns are nullified. It is possible if the power factor on the A.C. side becomes leading. For the decreased field current and a corresponding lagging armature current, as an effect of variation of the field current, the power factor and the A.C. current both vary. The active current remaining constant for a given mechanical output. The power factor has a maximum value of unity and hence, the current then is minimum and equals I. The plot of the power factor-Vs-field current has a shape V-curve and from that shape, it is known as the ‘inverted V-curve.’ which is not in our case. The plot of armature current-Vs-field current has a shape and from that shape; it is known as the “V” valve. This is different from the case done here where it is straight line. The curves for different values of the mechanical output will be different but will be the family of “V” curves. For different output, the active currents are different, as it is obvious. The field currents for unity power frequency. are different in from what we got in our experiment due to the impedance drop. Considering the above variation, an important feature of the synchronous generators is pointed out, by variation of the field current, the power factor can be controlled and the A.C. line current also changes, accordingly. Thus, constant speed operation at the synchronous speed and control of power factor are the distinct advantages of the synchronous motor over an induction motor. The presence of positive sequence currents in the armature of a synchronous machine produce armature m.m.f. which rotates at synchronous speed in the same direction as that of the field. As such the armature m.m.f. and field m.m.f. are stationary with respects to each other and have no induction effect under steady state (Zuo, Fang and Muhammad 2003). The presence of negative sequence currents in the armature produce armature m.m.f rotating in a direction opposite to that field. With respect to field, it rotates at double the syn. Speed in the direction of the opposite to normal direction of rotation. It will induce a voltage in damper bars as well as in field at double the line frequency. Conclusion The results above indicates that three phase synchronous generators when is loaded to the efficiency that is output power/ input power and voltage regulation are known. It can be seen that the theoretical values and the experimentally determined values are not similar. This does not authenticate the derived results. Graph depicts this variation between the theory and the experimentally determined values. The obtained linearity again suggested the successful determination of the calculated values of plot. The plot is largely linear although at larger distances, assumes an exponential form References Alexander, C. & Sadiku, M. 2009. Fundamentals of Electric Circuits. New York: McGraw-Hill. Allen, S., 2014. Electric Circuits. New York: mazon Digital Services, Inc. Bayliss, C. & Hardy, B., 2012. Transmission and Distribution Electrical Engineering. New York: Newnes Blume, S. 2010. Electric Power System Basics for the Nonelectrical Professional. New York: Wiley-IEEE Press; Butler, A., 2014.Basic Electrical Formulae. New York : Amazon Digital Services, Inc Godsell, G, 2014a. Electrical Machine Principles: A Must Have Guide for Students and Professionals. London: RG Kindle Publishing. Godsell, G, 2014b: Electricity Power Systems: A Comprehensive Guide for Students and Professionals (Electrical Engineering Book 3) Godsell, G, 2014c. Electrical Transformers: A Top Graded Study for Students and Professionals. London: RG Kindle Publishing Grigsby, L. 2007. Electric Power Generation, Transmission, and Distribution. New York. CRC Press. Hambley, A.,2012. Electrical Engineering: Principles & Applications. Ne York: Prentice Hall Irwin, J. D. & Nelms, R. M. 2008. Basic Engineering Circuit Analysis. New York: John-Wiley. Keler, V., 2015. Everything Electrical: How To Test Circuits Like A Pro: Part 1. Amazon Digital Services, Inc Maxfied, C., Bird, J., Williams, T., Kester, W. & Bensky D., 2012.. Electrical Engineering: Know It All. New York: Newnes McDonald, J. 2007. Electric Power Substations Engineering. New York: CRC Press Nilsson, J. & Riedel, S. 2008. Electric Circuits. New York: Prentice Hall, 2008. Scherz. P. & MONK, S., 2011. Practical Electronics for Inventors, Third Edition. New York: Newnes Singh, S, Basic Electrical Engineering. New Delhi: PHI Usna, 2010. AC synchronous generators Read More

Construction Salient pole synchronous machines are constructed in all sizes for small and medium speeds. An elementary 4 pole, 3 phase machine. Stator has two coils in each phase which are connected in series. The three phases may be connected in star or delta. The pole faces are so shaped that the radial air gap length increases from the pole centre to pole tips. This creates the sinusoidal flux density in the air gap. Damper winding is provided in the pole faces. This consists of copper bars short-circuited at both ends, placed in the specially provided holes.

Field windings are connected to two slip-rings mounted on rotor shaft (Godsell, 2014b). Cylindrical rotor machines are used for high speeds. In the se machines, the diameter of the rotor is kept smaller. This reduces the peripheral sped. The fields winding is placed in slots provided on rotor. A three phase balanced supply when connected to the stator of this machine produces a two-pie pole fields which rotates at synchronous speed. If the field winding is excited with a suitable direct current, thus creating two pole fields, and the rotor is run at a synchronous speed, a three phase voltage will be generated in armature.

The frequency of this voltage will be f=Ns.P/120. The voltage generated in each phase will be sinusoidal as flux density wave-form is sinusoidal and in related by the expression E=4.44 , where E=r.m.s value of generated per phase (volts), , F is frequency (Hz) and T is number of effective turns per phase(Godsell, 2014b). Further the voltages in three phases will differ in time phase. If the rotor is rotated in anticlockwise direction. A. the voltage in phase b will lag behind voltage in phase a by 120 degrees (.

2and voltage in phase C will be behind the voltage in phase b by 120 degrees(Keler, 2015). The windings of alternator are not concentrated but are distributed. To take into account the effect this, the voltage expression is multiplied by a factor known as distribution factor (Kd). further, if the pitch of the coil is not kept full, but is its fraction the output voltage reduces and a factor is introduced in the voltage equation to account for this reduction. This in known as pitch factor (Kp).

Modified expression for voltage can be given as a follows E=4.44, where Synchronous motor Construction of synchronous motor is similar to that of alternator. If the armature of the synchronous machine is connected to a balanced three phase supply, a rotating magnetic field is produced. The stator produces a two pole field, which is rotating in clockwise direction. If the rotor is excited at this instant such that the poles are created on rotor with the polarity, the rotor will experience a torque in anticlockwise direction due to the interaction between the two fields.

If the rotor is stationary, this torque will be there for half the revolution of a stator field for stator field rotation from point to point in the next half revolution, the rotor will experience a torque in opposite direction. This will repeat every half a revolution i.e., the torque on rotor will be pulsating in nature and will change the direction every half revolution. Due to the inertia, the rotor does not move in any direction. Hence, the synchronous motor is not self-starting(Godsell, 2014a).. Now consider that the rotor is rotated in the direction of the rotating magnetic field at the same speed with the help of some prime mover, and then excited such that polarities created on rotor.

The rotor will now experience a torque in the direction of rotation of rotation and as the field and rotor both are moving in the same direction with the same speed, this torque will be unidirectional. If now the prime mover of rotor is cut off, the rotor will continue to rotate in the same direction with the same speed under the influence of the above said torque. The rotor will lag behind the stator field by some angle, . This angle is known as torque angle (Irwin and Nelms, R. M. 2008. Starting of synchronous motor Synchronous motors can be started in by means of an auxiliary motor or with the help of damper winding.

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