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146686534 ee-410-lab1-fall10-1305411901


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146686534 ee-410-lab1-fall10-1305411901

  1. 1. 1 of 28 Get Homework Done Homework Help Research Paper help Online Tutoring click here for freelancing tutoring sites CHAPTER 1 INTRODUCTION 1.1 OVERVIEW
  2. 2. 2 of 28 Single-phase rectifiers are commonly used for power supplies for domestic equipment. However, for most industrial and high-power applications, three-phase rectifier circuits are the norm. As with single-phase rectifiers, three-phase rectifiers can take the form of a half-wave circuit, a full- wave circuit using a center-tapped transformer, or a full-wave bridge circuit. Thyristors are commonly used in place of diodes in order to allow the output voltage to be regulated. Many devices that generate alternating current (some such devices are called alternators) generate three-phase AC. For example, an automobile alternator has six diodes inside it to function as a full-wave rectifier for battery charging applications. Three phase rectifier have main advantages which include cost, simplicity and the ease of adding multiple outputs. Before starting the transformer design it is important to define the power supply parameters such as input voltage, power output, minimum operating frequency, and maximum duty cycle. From there we can calculate the transformer parameters, and select an appropriate core. This report is intended to introduce information about designing a basic Three phase rectifier converter. This report presents the design for a simple Three phase rectifier converter, which is analyzed via simulations using OrCad PSpice software. Descriptions of the basic circuit operation and simulation findings are followed by discussion in this report. This report includes several major components, including mathematical equation, waveforms simulated and datasheets for components that have been chosen for the design. 1.2 OBJECTIVES i. Study and analyze the performance and characteristics of Three phase rectifier . ii. Explain the principle of operation of Three phase rectifier circuit. iii. Design and simulate a simple Three phase rectifier circuit by using OrCad PSpice software. iv. Determine the design criteria for basic Three phase rectifier. v. Develop calculations for determining relative circuit parameters for the design of Three phase rectifier.
  4. 4. 4 of 28 2.1 BASIC TOPOLOGY Parallel connection via interphase Transformers permits the Implementation award of rectifiers for high current applications. Series connection for high voltage is also Possible, as shown in the figure of 12.12 fullwave rectifiers. With this arrangement, it cans be seen that the three common- cathode valves RESPECT generate a positive voltage to the neutral position, and the three common-anode valves Produce a negative voltage. The result is a dc voltage twice the value of the half wave rectifier. Each half of the bridge is a three-pulse converter group. This bridge connection is a two-way connection, and alternating Currents flow in the valve-side transformer windings Sulawesi Lingo half periods, avoiding dc components into the windings, and saturation in the transformer magnetic core. Characteristics These also made the Graetz Bridge Called The Most Used widely line commutated thyristor rectifiers. The configuration does not need any special transformers, and works as a six-pulse rectifiers. The characteristic of this series rectifier produces a dc voltage twice the value of the half-wave rectifiers 2.2 PRINCIPLE OF OPERATION A good rule-of-thumb for determining the connections on diode rectifiers is that the ac input voltage will be connected to the bridge where the anode & cathode of any two diodes are joined. Since this occurs at two points in the bridge, in a four diode bridge the two ac lines will be connected there without respect to polarity since the incoming ac voltage does not have a specific polarity. The positive terminal for the power supply will be connected to the bridge where the two cathodes of the diodes are joined, & the negative terminal will be connected to the bridge where the two anodes of the diodes are joined 2.3 THEORETICAL WAVEFORMS Figure 1 shows the electrical diagram for a three-phase bridge rectifier. From this diagram, notice that the secondary winding of a three-phase transformer is shown connected to the diode rectifier. Phase A of the three-phase voltage from the transformer is connected to the point where the cathode of diode 1D is connected to the anode of diode 2D. Phase B is connected to the point where the cathode of diode 3D is connected to the anode of diode 4D, & phase C is connected to the point where the cathode of diode 5D is connected to the anode of diode 6D. The anodes of diodes 1D, 3D, & 5D are connected together to provide a common point for the dc negative
  5. 5. 5 of 28 terminal of the output power. The cathodes of diodes 2D, 4D, & 6D are connected to provide a common point for the dc positive terminal of the output power. Figure 1 :a) Three-Phase Full-wave Rectifier with Resistive Load Figure3 :b) Source and output voltage . c)current for resistive load
  6. 6. 6 of 28 CHAPTER 3 DESIGN PROCEDURES 3.1 GENERAL DESIGN PROCEDURES 3.1.1 Defining three phase rectifier Converter The three-phase full-wave bridge rectifier is used where the required amount of dc power is high & the transformer efficiency must be high. Since the output waveforms of the half-waves overlap, they provide a low ripple percentage. In this circuit, the output ripple is six times the input frequency. Since the ripple percentage is low, the output dc voltage is usable without much filtering. This type of rectifier is compatible with transformers that are wye or delta connected THREE-PHASE VOLTAGES figure 3: Electrical displacement & generation of a three-phase voltage
  7. 7. 7 of 28 figure 4: Three-phase internal generator connections & a stationary armature with a rotating dc field. Three phase is the most common polyphase electrical system. Poly means more than one. It is, in this instance, a system having three distinct voltages that are out of step with one another. There are 120 degrees between each voltage. figure 3 shows sine waves taken on an electrical oscillo- graph instrument trace. This display shows the voltage relationships of the windings. This can be taken at any point in a three-phase system. The three phases are generated by placing each phase coil in the alternator 120 degrees apart, mechanically. A rotating dc magnetic field will then cut each phase coil in succession, inducing a voltage in each armature coil, out of step with each other. Armatures are the electrical components of the ac generator that have voltage induced in them. Armatures may be either the rotating piece of the alternator or the stationary component of the alternator. These armature coils may be connected internally or externally in a delta or a wye (star) connection. Rotating fields are more commonly used than stationary fields because generating large amounts of current would require larger sizes of conductors & iron to rotate. Therefore, it's more practical to make the armature stationary. Wye (star) & delta connections are shown in figure 4. These connections are shown in more detail under the heading of transformers. ALTERNATOR TYPES
  8. 8. 8 of 28 Two principal types of synchronous alternators are: (1) the revolving-armature alternator & (2) the revolving-field alternator. figure 5: Parts of an alternator of the revolving-armature type figure 5 illustrates an alternator with a stationary field, a revolving armature, & the elementary wiring symbol for a three-phase alternator. The armature consists of the windings into which current is induced. The magnetic field for this type of alternator is established by a set of stationary field poles mounted on the periphery of the alternator frame. The field flux created by these poles is cut by conductors inserted in slots on the surface of the rotating armature. The armature conductors are arranged in a circuit which terminates in slip rings. Alternating current induced in the armature circuit's fed to the load circuit by brushes which make contact with the slip rings. The revolving-armature alternator generally is used for low-power installations. The fact that the load current must be conducted from the machine through a sliding contact at the slip rings poses many design problems at higher values of load current & voltage. One alternator design has semiconductor rectifier diodes installed on the exciter field, thus eliminating the brushes & sliprings for the revolving field alternator (see Brushless Generators). FIELD EXCITATION Direct current (dc) must be used in the electromagnetic field circuit of an alternator. As a result, all types of alternators must be supplied with field current from a dc source, except for small permanent magnet fields. The dc source may be a dc generator operated on the same shaft as the alternator. In this case, the dc generator is called an exciter, shown on the self-excited
  9. 9. 9 of 28 synchronous alternator in figure 6A. The circuit diagram for this alternator is shown in figure 6B. In installations where a number of alternators require excitation power, this power is supplied by a dc generator driven by a separate prime mover. The output terminals of this generator connect to a dc exciter bus from which other alternators receive their excitation power by means of brushes & slip rings for the revolving field alternator. figure :6 (A) Self-excited synchronous alternator (Image General Electric Company) (B) Circuit Diagram FIELD DISCHARGE CIRCUIT A field discharge switch is used in the excitation circuit of an alternator. This switch eliminates the potential danger to personnel & equipment resulting from the high inductive voltage created when the field circuit's opened. Figure 7 illustrates the connections for the field circuit of a separately excited alternator. With the discharge switch closed, the field circuit's energized & the field discharge switch functions as a normal double-pole, single-throw switch. The discharge switch shown in figure 8 has an auxiliary switch blade at A in addition to the normal blades at C & D (figure 7).
  10. 10. 10 of 28 figure 7: Field discharge circuit figure 8: Field discharge switch When it's desired to open the field circuit, the following actions must take place. • Before the main switch contacts open, switch blade A meets contact B & thus pro vides a second path for the current through the field discharge resistor. • When the main switch contacts C-D open (figure 7) high inductive voltage is created in the field coils by the collapsing magnetic field. • This high voltage is dissipated by sending a current through the field discharge resistor. • This procedure eliminates the possibility of damage to the insulation of the field windings as well as danger to anyone opening the circuit using a standard double pole switch. A field circuit's used with all types of alternators.
  11. 11. 11 of 28 figure9: Field discharge circuit FREQUENCY The frequency of an alternator is a direct function of (a) the speed of rotation of the armature or the field & (b) the number of poles in the field circuit. The frequency commonly used in the United States is sixty cycles per second or hertz (Hz). Power companies are particularly concerned with maintaining a constant frequency for their energy output since many devices depend on a constant value of frequency. This constant value is achieved by sensitive control of the prime mover speed, driving the alternator. If the number of field poles in a given alternator is known, then it's possible to deter mine the speed required to produce a desired frequency. One cycle of voltage is generated each time an armature conductor passes across two field poles of opposite magnetic polarity. The frequency in cycles per second or hertz is the number of pairs of poles passed by the conductor in a second. Since the speed of rotating machinery is given in revolutions per minute (r/min), the speed in revolutions per second is obtained by dividing the speed (r/min) by 60. In a two-pole alternator the frequency is: f = (pairs of poles / 2) ((rev/min)/60) or: f = poles x RPM / 120 Where f = frequency in hertz (formerly cycles per second) p = number of poles
  12. 12. 12 of 28 RPM = speed in revolutions per minute 120 = conversion factor The formula for frequency can be rearranged so that the speed required to give a desired frequency can be obtained. RPM = 120 x f / P If a two-pole alternator is to be operated at a frequency of 60 Hz, the correct speed is obtained from the formula RPM = (120 x f)/p RPM = 120 x 60 / 2 = 3,600 RPM For a four-pole alternator operated at a frequency of 60 Hz, the required speed is: f = 120x60/4 = 1800RPM The two examples given illustrate the previous statement that the frequency of an alternator is a direct function of the speed of rotation & the number of poles in the alternator field circuit. VOLTAGE CONTROL The voltage output of an alternator increases as the speed of rotation accelerates, thus increasing the lines of force cut per second. As the field excitation increases, this increases the magnetic fields to the point of magnetic saturation of the field poles. For practical purposes, an alternator must be operated at a constant speed to maintain a fixed frequency. Thus, the only feasible method of controlling the voltage output is to vary the field excitation. Field rheostats are used to vary the resistance of the total field circuit. This variation of resistance, in turn, changes the value of field current (figure 6B). • A low value of field current results in less flux & less induced voltage at a given speed.
  13. 13. 13 of 28 • A high field current results in greater field flux & a higher induced voltage at a given speed. • The value of flux at which the field poles saturate determines the maximum voltage obtainable at a fixed speed & frequency. ROTATING-FIELD ALTERNATORS Rotating-field alternators are used extensively because of the ease with which a high-load current can be taken from the machine. The load isn't connected through the use of slip rings or sliding contacts. Thus, the use of rotating-field alternators results in a savings in initial cost & fewer maintenance requirements. Stator Winding figure 8 illustrates the stator (stationary or nonmoving) windings of a rotating field, three-phase alternator. The three-phase armature windings are embedded 120 degrees from one another in the slots of a laminated steel core which is clamped securely to the alternator frame. Output leads from the stator emerge from the bottom of the stator & connect directly to the load circuit. It can be seen that slip rings & brushes are not required in a stationary winding of this type. As a result, higher values of output voltage & current are possible. Standard values of voltage output for a rotating-field & alternator are as high as 11,000 to 13,800 volts. figure 10: Stator winding of an alternator (Photo General Electric Company) Rotating Field
  14. 14. 14 of 28 The rotating portion of a rotating-field alternator consists of field poles mounted on a shaft which is driven by the prime mover. The magnetic flux established by the rotating field poles cuts across the conductors of the stator winding to produce the induced output voltage of the stator. The following comparison can be made between the rotating-armature alternator & the rotating- field alternator. In the rotating-armature alternator, the armature conductors cut the flux established by stationary field poles. For the rotating-field alternator, the motionless conductors of the stator winding are cut by the flux established by rotating field poles. In each case an induced voltage is generated. figure 11 shows a salient field rotor for low-speed, three-phase alternators. For this type of rotor, the field poles protrude from the rotor support structure which is of steel construction & commonly consists of a hub, spokes, & rim. This support structure is called a spider. Each of the field poles is bolted to the spider. The field poles may be dove- tailed to the spider in some alternators to provide a better support for the poles against the effects of centrifugal force. figure 11: Alternator rotor, salient field type (Photo General Electric Company) figure 12 shows a non-salient rotor. This type of rotor has a smooth cylindrical surface. The field poles (usually two or four) don't protrude above this smooth surface. Non-salient rotors are used to decrease windage losses on high-speed alternators, & to improve balance & reduce noise.
  15. 15. 15 of 28 figure 12: Alternator rotor, non-salient field type Power Supply for Rotor The field windings of both salient & non-salient rotors require dc power. Slip rings & brushes are used to feed the current to the windings at a potential of 100 to 250 volts dc. The brushes & rings are easily maintained because of the low values of field current encountered. TERMINAL MARKINGS A standard system of marking leads for field circuits has been established by the ANSI (American National Standards Institute). The field leads for both alternators & generators are indicated by the markings F1 & F2. In addition, the F1 lead always connects to positive bus of the dc source. (See I figure 7 & 9.) ALTERNATOR REGULATION Regardless of the type of generator or alternator used in a system, the terminal output voltage of the machine varies with any change in the load current. The impedance of the windings & the power factor of the load Circuit both influence the regulation of an alternator. An increase in load current in a pure resistive load circuit causes a decrease in output voltage. A voltage drop of approximately 10 percent is common when going from a condition of no-load to full-load in a typical alternator. For an inductive load, an increase in load current will cause a greater voltage drop than is obtained with a pure resistive load. A load with a low value of lagging power factor produces a large drop in output voltage. A capacitive load circuit produces the opposite effect. In other words, the output voltage rises above the no-load value with an increase in load current & is high at a low value of leading power factor.
  16. 16. 16 of 28 AUTOMATIC VOLTAGE CONTROL Unlike dc generators, alternators cannot be compounded to alter the voltage-load characteristic. Moreover, output voltage variations are more likely to be severe because of changes in the load power factor. As a result, automatic voltage regulators generally are used with alternators. Automatic voltage regulators change the alternator field current to compensate for any increase or decrease in the load voltage. A relay is used to increase or decrease the field resistance through contactors bridged across a field circuit resistor. As the ac line voltage falls, the relay bypasses sections of the field resistor to cause an increase in the flux & thus increase the induced voltage. An increase in the ac line voltage causes the relay to open contactors across the field resistor to decrease the field current, flux, & induced voltage. Power companies stabilize voltage by using a type of varying ratio transformer as a voltage regulator. BRUSHLESS EXCITERS WITH SOLID-STATE VOLTAGE CONTROL figure 13 Diagram of an exciter with permanent magnet generator (Electric Machinery, Turbodyne Division, Dresser Industries, Inc.) The permanent magnet generator (figure 13) supplies high-frequency ac power input to the voltage regulator. Voltage & reactive current feedback information is provided to the regulator from potential & current transformers. Using these feedback signals & a reference point established by setting the voltage adjusting rheostat, the voltage regulator (which has a transfer switch allowing the operator to select automatic regulator control or manual control) provides a
  17. 17. 17 of 28 controlled dc output. The dc is fed to the field of the rotating exciter; the three-phase, high- frequency ac output is then rectified by a full-wave bridge. This rectified signal is applied to the main generator field. Fully rated, parallel, solid-state diodes with indicating fuses are provided to permit full load generation with a diode (rectifier) out of service. The use of a stroboscope light permits the indicating fuses to be viewed during operation to determine if a diode has failed. figure 14 shows a cutaway view of a brushless exciter. figure 15 shows the rotating components of a brushless excitation system. figure 14 :Cutaway view of a brushless exciter showing the components (Electric Machinery, Turbodyne Division, Dresser Industries, Inc.) figure :15 Rotating components of the brushless excitation system: EXCITER FIELD; PERMANENT MAGNET GENERATOR; DIODE WHEEL (Electric Machinery, Turbodyne Division, Dresser Industries, Inc.)
  18. 18. 18 of 28 Diode Rectifier Circuits Laboratory – A Power Electronics Laboratory Experiment Purpose: This lab seeks to illustrate the fundamental operation of uncontrolled rectifier circuits. The waveforms associated with these circuits are visualized using simulation and experimentation. The circuit operating values obtained using theoretical equations are challenged through simulation and experimentation. 1. LIST OF EQUIPMENT  PC Computer with ORCAD PSPICE Schematic Capture and Simulation 4. 3.2 DESIGN CALCULATION For this report, we are designing a basic Three phase rectifier converter based on an pre- assumed problem. The calculations and simulations for the design are all done based on the problem, because the thorough analysis on the design of Three phase rectifier converter is too complex. 4.1 Prelab Calculations 1.2 For the three-phase full-wave rectifier, calculate: a. The average and total rms values of the output voltage when the phase voltage (line to neutral) AC input is 100 Vac-rms.  VR  3VmLL   3( 2 3100V)   234 V  ˜VR  0.956 VmLL  0.956  2 3 100V  234 V b. The ratio of (average/total rms).
  19. 19. 19 of 28  Average TotalRMS  234 V 234 V 100% c. The input power factor assuming ideal diodes, and Rload = 150 .  pfin  Pinout Sin  ˜Vout ˜Iout 3 ˜VLN ˜I  0.956 2 3V 3 2 3  V  0.956 1.3 Which rectifier circuit do you suggest has better dc output performance? Better AC input performance? The 3- rectifier circuit has better DC output performance due to the increased ripple frequency. By increasing the ripple frequency, filtering becomes easier as well as the average DC value increases. The 1- rectifier circuit has better AC input performance due to its processing of a single component of input current. Since the source is delivering power on both the positive and negative half cycles, the source has a zero distortion power factor. This is seen in the power factor calculation above in question 1.1.
  20. 20. 20 of 28 RESULTS AND DISCUSSION 4.1 SIMULATION RESULTS The simulation is done using OrCad PSpice Software ver. 9.1. The simulation of the design is based on the pre-assumed problem as stated in the previous section. The resultant waveforms and data are obtained and compared with the theoretical knowledge of Three phase rectifier converter. DATA AND OBSERVATIONS Resistive Load With Capacitor Theoretical Simulated Simulated Three Phase Vrms (V) 234.0 234.4 211.65 Vdc-rms (V) 234.0 232.2 210.27 pf 0.956 0.93 0.298 Table 1: Output Voltage Summary for Theoretical Calculations, Simulated, and Experimental Results Resistive Load With Capacitor Theoretical Simulated Simulated Three Phase Vdc-rms/Vrms 1.000 0.990 1.000 Table 2: Calculated Efficiencies for Output Voltages of Table 1 Three-Phase Full-Wave Rectifier # of pulses 6
  21. 21. 21 of 28 Peak-to-peak ripple 38 Table 3: Experimental Results for Output Voltage Pulses Per Single Input Voltage Period The efficiency of the single-phase rectifier as asked in procedure step f) is the following:  %Efficiency Pout Pin  60.14W 63.40W  94.9% DNR = Did Not Record Quantitatively NA = Non-Applicable or Not Procedurally Called for Measurement
  22. 22. 22 of 28 6. GRAPHS Graph 3: Simulated Voltage Waveforms for 3-Full-wave rectifier with resistive load no cap. Top (Vout) Bottom (Vin) Time 150.0ms 160.0ms 170.0ms 180.0ms 190.0ms141.2ms 198.7ms V(D8:2) V(D9:2) V(D7:1) -100V 0V 100V -150V V(D6:2,R2:1) 200V 225V 250V SEL>>
  23. 23. 23 of 28 Graph 4: Simulated Current Waveforms for 3-Full-wave rectifier with resistive load no cap. Top Graph (Iout) Bottom (Iin) Time 200ms 210ms 220ms 230ms 240ms 250ms 260ms 270ms 280ms 290ms 300ms I(V2) -2.0A 0A 2.0A SEL>> -I(R2) 1.4A 1.5A 1.6A 1.7A
  24. 24. 24 of 28 Graph 5: Simulated Voltage Waveforms for 3-Full-wave rectifier with cap in parallel with resistor. Top (Green=Output Voltage Red=Icap) Bottom (Vin) Time 19.90s 19.91s 19.92s 19.93s 19.94s 19.95s 19.96s 19.97s 19.98s 19.99s 20.00s V(V1:+,0) -200V 0V 200V V(D1:2,R1:1) - I(C2)+139 140 150 160 135 165 SEL>> Time 19.90s 19.91s 19.92s 19.93s 19.94s 19.95s 19.96s 19.97s 19.98s 19.99s 20.00s -I(R1) 920mA 924mA 928mA 932mA I(V1) -40A 0A 40A SEL>>
  25. 25. 25 of 28 Graph 6: Simulated Current Waveforms for 3-Full-wave rectifier with capacitor in parallel with resistor. Bottom (Iout) Top (Iin) Graph 7: Simulated Voltage Waveforms for 3-Full-wave rectifier with cap in parallel with resistor. Top (Green=Output Voltage Red=Icap) Bottom (Vin) Time 19.90s 19.91s 19.92s 19.93s 19.94s 19.95s 19.96s 19.97s 19.98s 19.99s 20.00s V(D8:2) V(D9:2) V(D7:1) -200V 0V 200V V(D7:2,D8:1) - I(C1)+234 230 240 250 260 SEL>>
  26. 26. 26 of 28 Graph 8: Simulated Current Waveforms for 3-Full-wave rectifier with capacitor in parallel with resistor. Bottom (Iout) Top (Iin) CHAPTER 5 DISCUSSION ANDCONCLUSION Time 19.90s 19.91s 19.92s 19.93s 19.94s 19.95s 19.96s 19.97s 19.98s 19.99s 20.00s I(R2) -1.622A -1.620A -1.618A -1.616A SEL>> - I(V2) -40A 0A 40A
  27. 27. 27 of 28 5.1. DISCUSSION Three-Phase Full-Wave Rectifier For the three-phase full-wave rectifier circuit it can be seen that the input current waveform in Graph 11 looks very similar to the simulated result in Graph 4. The non-sinusoidal nature of this graph, as indicated by a small dip in the middle of the conducting period, shows that this circuit has contributed a harmonic to the input current signal. The creation of this harmonic is due to the switching of the input current from a conducting to an off state almost instantaneously. From the output voltage waveform shown in Graph 14, it is seen that the output voltage has 6 pulses per one period of input voltage. It is also shown that the output voltage following the three-phase line-to-line voltage of the transposition of the three voltage phasors. 5.2 CONCLUSION Altogether, this lab illustrated the operation and behavior of uncontrolled diode rectifier circuits namely, the single-phase full wave rectifier and the 6-pulse three-phase full wave rectifier circuit. Through theoretical calculations the team was able to predict the circuit operating values and roughly sketch the types of expected waveforms. While this analytical method proved to be useful in establishing the mathematical principles, it was the simulation that truly gave the group the visual grasp of the circuit’s operational behavior. Finally, through experimentation the circuit was realized and implemented using real electronic circuit components. By implementing these circuits in laboratory, we were able to confirm the design and operation of the circuits from a high level understanding to a device performance level as seen in the non-ideal characteristics of the diodes (dead time and forward voltage drop). This lab also let us accomplish prerequisite learning objectives such as reviewing our understanding of three-phase power concepts and the use of current and voltage probes for oscilloscope analysis. Another important understanding we gained was that of electrical safety. By working with capacitors and high AC voltage, we learned the proper attitudes and respect to have when dealing with >120V AC.
  28. 28. 28 of 28