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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 Homeworkping.com Homework Help https://www.homeworkping.com/ Research Paper help https://www.homeworkping.com/ Online Tutoring https://www.homeworkping.com/ 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.
3. 3. 3 of 28 CHAPTER 2 CIRCUIT AND OPERATION
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.