POWER FACTOR CORRECTION OF A 3-PHASE 4- SWITCH INVERTER FED BLDC MOTOR

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POWER FACTOR CORRECTION OF A 3-PHASE 4- SWITCH INVERTER FED BLDC MOTOR

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POWER FACTOR CORRECTION OF A 3-PHASE 4- SWITCH INVERTER FED BLDC MOTOR

  1. 1. 1 CHAPTER 1 INTRODUCTION 1.1GENERAL 1.1.1 Brushless DC motors Brushless DC motors also known as electronically commutated motors are electric motors powered by direct-current (DC) electricity and having electronic commutation systems, rather than mechanical commutator and brushes. The current-to-torque and frequency-to-speed relationships of BLDC motors are linear. BLDC motors may be described as stepper motors, with fixed permanent magnets and possibly more poles on the rotor than the stator, or reluctance motors. The latter may be without permanent magnets, just poles that are induced on the rotor then pulled into alignment by timed stator windings. However, the term stepper motor tends to be used for motors that are designed specifically to be operated in a mode where they are frequently stopped with the rotor in a defined angular position; this page describes more general BLDC motor principles, though there is overlap. The maximum power that can be applied to a BLDC motor is exceptionally high, limited almost exclusively by heat, which can weaken the magnets. Magnets demagnetize at high temperatures, the Curie point, and for neodymium-iron-boron magnets this temperature is lower than for other types. A BLDC motor's main disadvantage is of higher cost, which arises from two issues. First, BLDC motors require complex electronic speed controllers to run. Brushed DC motors can be regulated by a comparatively simple controller, such as a variable resistor.
  2. 2. 2 1.2 LITERATURE SURVEY The selection of this project and its completion needs so many Surveys from various books, technical papers and journals. The main idea using more number of switches complexities of control, so that the 3-Phase 4-Switch technology has been used, which reduces number of switches, and thereby the switching losses can be minimized. Using PWM Boost Converter the line harmonics is reduced and the line current becomes sinusoidal and will be in phase with the applied voltage. The methodology of speed and power factor control is modified to bring better output. Power Factor Correction using Boost Converter reduces the line harmonics and hence, the line current becomes sinusoidal in shape and will be in same phase with the voltage. The efficiency of the utility system will be high so that the total working of the system will get better and hence, the motor can have better control. The performance of the drive system was evaluated through digital simulation by Mat lab/simulink, the electromagnetic torque characteristics, rotor speed, PWM pulse but brushless D.C. motor is nonlinear function of the phase current and rotor position. Obtaining the mathematical model is not an easy task, because the magnetic circuit operates at varying level of saturation under operating conditions. To reduce the complexity, the linearised models have been used for designing for controller and performance analysis. A brushless DC motor is simulated for the analysis. All simulations are completely documented by their block diagram, corresponding special mat lab functions & parameters.
  3. 3. 3 CHAPTER 2 BRUSHLESS DC MOTOR 2.1 INTRODUCTION TO BLDC MOTOR The need for reliable electrical contact no brush sparking, no electrical interference and a long working life spawned the aerospace industry to develop the first brushless D.C. motors in the 1960s. While they have not totally replaced the brushed type, the advantages of brushless D.C. motors are apparent in terms of speed, response, reliability and power density. The primary advantages of brushless D.C. motors are:  Low Maintenance  No Brush Sparking  High Operating Speeds  High Efficiency  Compact Size  Fast Response But brushless D.C. motors are not without their drawbacks. They are more expensive due to their complex control system, the use of rare earth magnets and the need for rotor position sensors. Higher costs are overcome by a longer life and lower maintenance costs. They are primarily used in fractional horsepower applications; however, they have been used as drives in electric vehicles up to 100KW. Brushless D.C. motors are widely used in the fractional horsepower range for disk or tape drives for computer peripherals as well as in CD/DVD players, cooling fans, laser printers and photocopiers. In addition, they are
  4. 4. 4 often used as spindle drives in machine tools because they can be driven at very high speeds with fast acceleration, deceleration and reversing responses. The larger horsepower sizes have found application in electric vehicles. Since they do not have brushes, hence, are not susceptible to arcing, brushless D.C motors are safe for environments with flammable gases, such as in the petrochemical industries. 2.2 CONSTRUCTION AND OPERATION OF BLDC MOTOR 2.2.1 Construction of BLDC MOTOR Brushless D.C. motors are most commonly constructed with a radically magnetized permanent magnet rotor, mounted on a steel cylinder, and phase windings wound on a slotted, laminated, non-salient stator. There are usually multiple phase windings; however, a single winding can be wound so that it’s distributed over the stator core. This is the reverse of brushed D.C. motors allows brushless D.C. motors to have less internal resistance and much better heat dissipation in the stator coils resulting in higher operating efficiencies since heat can more efficiently dissipate via the stationary motor housing. Brushless D.C. motors do not use carbon brushes or a mechanical commutator; rather, commutation is done via a complex electronic controller used in conjunction with a rotor position sensor. This is the primary reason why brushless D.C. motors are low maintenance and non-sparking. In addition, without brushes or a mechanical commutator, they have less shaft friction or inertia, less audible noise and much better torque to weight ratios hence; they are much smaller in size than a comparable brushed D.C. motor.
  5. 5. 5 Figure 2.1 The four poles on the stator of a two-phase BLDC motor. This is part of a computer cooling fan; the rotor has been removed. BLDC motors can be constructed in several different physical configurations: In the 'conventional' configuration, the permanent magnets are part of the rotor. Three stator windings surround the rotor. In the 'out runner' configuration, the radial-relationship between the coils and magnets is reversed; the stator coils form the center of the motor, while the permanent magnets spin within an overhanging rotor which surrounds the core. The flat type, used where there are space or shape limitations, uses stator and rotor plates, mounted face to face. Out runners typically have more poles, set up in triplets to maintain the three groups of windings, and have a higher torque at low RPMs. In all BLDC motors, the coils are stationary. There are also two electrical configurations having to do with how the wires from the windings are connected to each other. The delta configuration connects the three windings to each other in a triangle-like circuit, and power is applied at each of the connections. The wye ("Y"-shaped) configuration, sometimes called a star winding, connects all the windings to a central point and power is applied to the remaining end of each winding.
  6. 6. 6 A motor with windings in delta configuration gives low torque at low rpm, but can give higher top rpm. Wye configuration gives high torque at low rpm, but not as high top rpm. Although efficiency is greatly affected by the motor's construction, the wye winding is normally more efficient. In delta-connected windings, half voltage is applied across the windings adjacent to the undriven lead, increasing resistive losses. In addition, windings can allow high- frequency parasitic electrical currents to circulate entirely within the motor. A wye-connected winding does not contain a closed loop in which parasitic currents can flow, preventing such losses. From a controller standpoint, the two styles of windings are treated exactly the same, although some less expensive controllers are designed to read voltage from the common center of the wye winding. 2.2.2 OPERATIONS OF BLDC MOTOR Brushless D.C. motors operate in conjunction with an electronic controller and a rotor position feedback sensor. Based upon the actual rotor position, the controller sequentially energizes or switches “on” the stator’s phase windings so that torque is continuously generated as the permanent magnet (PM) rotor rotates. This switching action is called electronic commutation. To sense the rotor’s angular position, position sensors are used. When the PM rotor passes one set of phase windings, a signal from the position sensor is sent to the controller which then sends a signal to switch “on” the next set of phase windings so that the magnetic fields of the rotor and the stator’s phase windings remain synchronized. The torque/speed characteristic of the motor is determined by the magnitude of the signal and the switching rate of the controller. For example, in a 2-phase motor, when the phase 1 winding is energized, the PM rotor will rotate to align itself with magnetic field produced by the phase 1 winding. When the phase 1 winding is turned “off”, the phase
  7. 7. 7 2 winding is turned “on” and the rotor will continue to rotate to align itself with the magnetic field of the phase 2 winding. This “on” and “off” switching of the phase windings will maintain torque of the PM rotor. Figure 2.2 BLDCs use electronic commutation to control the power distribution to the motor, using Hall-effect sensors to measure the motor’s position. The number of windings and the type of switching sequence determines the phase winding utilization and the torque response of the motor. Fewer windings and fewer switching pulses give the motor less winding utilization, more inertia to overcome and a poorer torque response. The most common configuration of a brushless D.C. motor is “three phase six pulse.” In this configuration, the stator has three phase windings connected in a delta or star configuration with no neutral. The windings are excited by 6 pulses sequentially with each winding being switched by pulses of opposite polarity. This delivers a linear torque speed characteristic, similar to a brushed D.C. motor. Figure 2.3 Schematic for delta and wye winding styles
  8. 8. 8 The BLDC motor is an AC synchronous motor with permanent magnets on the rotor and windings on the stator. Permanent magnets create the rotor flux and the energized stator windings create electromagnet poles. The rotor is attracted by the energized stator phase. By using the appropriate sequence to supply the stator phases, a rotating field on the stator is created and maintained. This action of the rotor - chasing after the electromagnet poles on the stator is the fundamental action used in synchronous permanent magnet motors. The lead between the rotor and the rotating field must be controlled to produce torque And this synchronization implies knowledge of the rotor position. On the stator side, three Phase motors are the most common. Figure 2.4 A three-phase synchronous motor with a one permanent magnet pair pole rotor Permanent magnet synchronous motors can be classified in many ways, one of these is that it is of Particular interest tous is that depending on back emf profiles. BLDC and PMSM. This terminology defines the shape of the back-emf of the synchronous motor. Both BLDC and PMSM motors have permanent magnets on the rotor but differ in the flux distributions and back-emf profiles. To get the best performance out of the synchronous motor.
  9. 9. 9 2.2.3 BLDC Motor Control The BLDC motor is characterized by a two phase ON operation to control the inverter. In this control scheme, torque production follows the principle that current should flow in only two of the three phases at a time and that there should be no torque production in the region of back EMF zero crossings. The following figure describes the electrical wave forms in the BLDC motor in the two phases ON operation. This control structure has several advantages  Only one current at a time needs to be controlled.  Only one current sensor is necessary or none for speed loop only, as detailed in the next sections.  The positioning of the current sensor allows the use of low cost sensors. We have seen that the principle of the BLDC motor is, at all times, to energize the phase Pair which can produce the highest torque. To optimize this effect the Back EMF shape is trapezoidal. The combination of a DC current with a trapezoidal Back EMF makes it theoretically possible to produce a constant torque. In practice, the current cannot be established instantaneously in a motor phase; as a consequence the torque ripple is present at each 60 degree phase commutation. 2.2.4 Two Phase ON Operation If the motor used has a sinusoidal Back EMF shape, this control can be applied but the produced torque is: Firstly, not constant can be made up from portions of a sine wave.
  10. 10. 10 This is due to its being the combination of a trapezoidal current control strategy and of a sinusoidal Back EMF. Bear in mind that a sinusoidal Back EMF shape motor controlled with a sine wave strategy produces a constant torque. Secondly, the torque value produced is weaker. Figure 2.5 Electrical Waveforms in the Two Phase ON Operation and Torque Ripple 2.2.5 Torque Ripple in a Sinusoidal Motor Controlled as a BLDC Figure 2.6 Torque Ripple in a Sinusoidal Motor Controlled as a BLDC
  11. 11. 11 2.2.6 Transport High power BLDC motors are found in electric vehicles and hybrid vehicles. These motors are essentially AC synchronous motors with permanent magnet rotors. A number of electric bicycles use BLDC motors that are sometimes built into the wheel hub itself, with the stator fixed solidly to the axle and the magnets attached to and rotating with the wheel. The bicycle wheel hub is the motor. This type of electric bicycle also has a standard bicycle transmission with pedals, sprockets, and chain that can be pedaled along with, or without, the use of the motor as need arises. 2.2.7 Heating and ventilation There is a trend in the HVAC and refrigeration industries to use BLDC motors instead of various types of AC motors. The most significant reason to switch to a BLDC motor is the dramatic reduction in power required to operate them versus a typical AC motor. While shaded-pole and permanent split capacitor motors once dominated as the fan motor of choice, many fans are now run using a BLDC motor. Some fans use BLDC motors also in order to increase overall system efficiency. In addition to the BLDC motor's higher efficiency, certain HVAC systems use BLDC motors because the built-in microprocessor allows for programmability, better control over airflow, and serial communication.
  12. 12. 12 2.2.8 Stepper motor The stepper motor is used in microprocessor and microcontroller-based and robotic equipment, as it consumes less power and provides accurate movement of robotic arms. Semiconductor producers include Infineon Technologies, Texas Instruments and Microchip. Infineon offers so-called LIN stepper motors used in applications such as instrumentation and gauges, CNC machining, multi-axis positioning, printers and surveillance equipment. 2.2.9 Applications BLDC motors fulfill many functions originally performed by brushed DC motors, but cost and control complexity prevents BLDC motors from replacing brushed motors completely in the lowest-cost areas. Nevertheless, DC motors have come to dominate many applications, particularly devices such as computer hard drives and CD/DVD players. Small cooling fans in electronic equipment are powered exclusively by BLDC motors. They can be found in cordless power tools where the increased efficiency of the motor leads to longer periods of use before the battery needs to be charged. Low speed, low power BLDC motors are used in direct-drive turntables for "analog" audio discs.
  13. 13. 13 CHAPTER 3 PROJECT DESCRIPTION 3.1 BLOCK DIAGRAM The power factor correction circuit has been designed for a nominal mains voltage of 230 Vrms, 50 - 60 Hz. Basically, the circuit consists of several sections they are given bellow. Figure 1 shows the block diagram of the circuit. Figure 3.1 Block diagram of the proposed project
  14. 14. 14 3.1.1 POWER SUPPLY It has unit which has to drive a circuit. It has basic unit for all electrical circuits. The follings are required for construction of a power circuit. Figure 3.2 power supply regulator The linear supply The component blocks of a linear supply are common to all variants, and can be described as follows: Input circuit Conditions the input power and protects the unit, typically voltage selector, fuse, on-off switching, filter and transient suppressor. Transformer Isolates the output circuitry from the ac input, and steps down or up the voltage to the required operating level. Rectifier and reservoir Converts the ac transformer voltage to dc, reduces the ac ripple component of the dc and determines the output hold-up time when the input is interrupted. Regulation Stabilizes the output voltage against input and load fluctuations.
  15. 15. 15 3.2 CURRENT TRANSFORMER In electrical engineering, a current transformer is used for measurement of electric currents. Current transformers, together with voltage transformers, are known as instrument transformers. When current in a circuit is too high to directly apply to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. A current transformer also isolates the measuring instruments from what may be very high voltage in the monitored circuit. Current transformers are commonly used in metering and protective relays in the electrical power industry. Figure 3.3 current transformer coil Current transformers used in metering equipment for three-phase 400 ampere electricity supply. Like any other transformer, a current transformer has a primary winding, a magnetic core, and a secondary winding. The alternating current flowing in the primary produces a magnetic field in the core, which then induces a current in the secondary winding circuit. Primary objective of current transformer design is to ensure that the primary and secondary circuits are efficiently coupled, so that the secondary current bears an accurate relationship to the primary current. The most common design of CT consists of a length of wire wrapped many times around a silicon steel ring passed over the circuit being measured. The CT's primary circuit therefore consists of a single 'turn' of conductor, with a secondary of many tens or hundreds of turns. The primary winding
  16. 16. 16 may be a permanent part of the current transformer, with a heavy copper bar to carry current through the magnetic core. Window-type current transformers are also common, which can have circuit cables run through the middle of an opening in the core to provide a single-turn primary winding. When conductors passing through a CT are not centered in the circular opening, slight inaccuracies may occur. Shapes and sizes can vary depending on the end user or switchgear manufacturer. Typical examples of low voltage single ratio metering current transformers are either ring type or plastic molded case. High-voltage current transformers are mounted on porcelain bushings to insulate them from ground. Some CT configurations slip around the bushing of a high-voltage transformer or circuit breaker, which automatically centers the conductor inside the CT window. The primary circuit is largely unaffected by the insertion of the CT. The rated secondary current is commonly standardized at 1 or 5 amperes. For example, a 4000:5 CT would provide an output current of 5 amperes when the primary is passing 4000 amperes. The secondary winding can be single ratio or multi ratio, with five taps being common for multi ratio CTs. The load, or burden, of the CT should be of low resistance. If the voltage time integral area is higher than the core's design rating, the core goes into saturation towards the end of each cycle, distorting the waveform and affecting accuracy. Figure 3.4 current transformer coil connection
  17. 17. 17 3.2.1 Usage Current transformers are used extensively for measuring current and monitoring the operation of the power grid. Along with voltage leads, revenue-grade CTs drive the electrical utility's watt-hour meter on virtually every building with three-phase service and single-phase services greater than 200 amps. The CT is typically described by its current ratio from primary to secondary. Often, multiple CTs are installed as a "stack" for various uses. For example, protection devices and revenue metering may use separate CTs to provide isolation between metering and protection circuits, and allows current transformers with different characteristics to be used for the devices. 3.2.2 Safety precautions Care must be taken that the secondary of a current transformer is not disconnected from its load while current is flowing in the primary, as the transformer secondary will attempt to continue driving current across the effectively infinite impedance. This will produce a high voltage across the open secondary, which may cause arcing. 3.2.3 Accuracy Accuracy classes for various types of measurement are set out in IEC 60044-1, Classes 0.1, 0.2s, 0.2, 0.5, 0.5s, 1, and 3. The class designation is an approximate measure of the CT's accuracy. The ratio error of a Class 1 CT is 1% at rated current; the ratio error of a Class 0.5 CT is 0.5% or less. Errors in phase are also important especially in power measuring circuits and each class has an allowable maximum phase error for specified load impedance. Current
  18. 18. 18 transformers used for protective relaying also have accuracy requirements at overload currents in excess of the normal rating to ensure accurate performance of relays during system faults 3.2.4 Knee-point voltage The knee-point voltage of a current transformer is the magnitude of the secondary voltage after which the output current ceases to follow linearly the input current. This means that the one-to-one or proportional relationship between the input and output is no longer within declared accuracy. In testing, if a voltage is applied across the secondary terminals the magnetizing current will increase in proportion to the applied voltage, up until the knee point. The knee point is defined as the point at which an increase of applied voltage of 10% results in an increase in magnetizing current of 50%. From the knee point upwards, the magnetizing current increases abruptly even with small increments in voltage across the secondary terminals. The knee-point voltage is less applicable for metering current transformers as their accuracy is generally much tighter but constrained within a very small bandwidth of the current transformer rating, typically 1.2 to 1.5 times rated current. However, the concept of knee point voltage is very pertinent to protection current. 3.2.5 Rating factor Rating factor is a factor by which the nominal full load current of a CT can be multiplied to determine its absolute maximum measurable primary current. Conversely, the minimum primary current a CT can accurately measure is light load. Most CTs have rating factors for 35 degrees Celsius and 55 degrees Celsius.
  19. 19. 19 It is important to be mindful of ambient temperatures and resultant rating factors when CTs are installed inside pad-mounted transformers or poorly ventilated mechanical rooms. Recently, manufacturers have been moving towards. 3.3 POTENTIAL TRANSFORMER Voltage transformers or potential transformers are another type of instrument transformers used for metering and protection in high-voltage circuits. They are designed to present negligible load to the supply being measured and to have a precise voltage ratio to accurately step down high voltages so that metering and protective relay equipment can be operated at a lower potential. Typically the secondary of a voltage transformer is rated for 69 V or 120 V at rated primary voltage, to match the input ratings of protective relays. The transformer winding high-voltage connection points are typically labeled as H1, H2 and X1, X2 and sometimes an X3 tap may be present. Sometimes a second isolated winding (Y1, Y2, Y3) may also be available on the same voltage transformer. The high side may be connected phase to ground or phase to phase. The low side is usually phase to ground. The terminal identifications are often referred to as polarity. This applies to current transformers as well. At any instant terminals with the same suffix numerals have the same polarity and phase. Correct identification of terminals and wiring is essential for proper operation of metering and protective relays. Some meters operate directly on the secondary service voltages at or below 600 V. VTs are typically used for higher voltages, or where isolation is desired between the meter and the measured circuit.
  20. 20. 20 3.4 RECTIFIER Converts the ac transformer voltage to dc, reduces the ac ripple component of the dc and determines the output hold-up time when the input is interrupted. 3.4.1 Diodes The diode is a two-terminal device whose function is to pass current in one direction but not in the other. A conventional diode is formed from the junction of p-type and n- type silicon. The ideal device has a “brick-wall” V-I characteristic the practical silicon diode has an exponential characteristic which approximates to the brick wall, if viewed. 3.4.2Diode Bridge Figure 3.5 bridge rectifier A diode bridge is an arrangement of four diodes in a bridge circuit configuration that provides the same polarity of output for either polarity of input. When used in its most common application, for conversion of an alternating current input into direct current a DC output, it is known as a bridge rectifier. A bridge rectifier provides full-wave rectification from a two wire AC input, resulting in lower cost and weight as compared to a rectifier with a 3-wire input from a transformer with a center tapped secondary winding. In the diagrams below, when the input connected to the left corner of the diamond is positive, and the input connected to the right corner
  21. 21. 21 is negative, current flows from the upper supply terminal to the right along the red positive path to the output, and returns to the lower supply terminal via the blue negative path. Figure 3.6 Positive half conduction When the input connected to the left corner is negative, and the input connected to the right corner is positive, current flows from the lower supply terminal to the right along the red positive path to the output, and returns to the upper supply terminal via the blue negative path. Figure 3.7 Negative half conduction In each case, the upper right output remains positive and lower right output negative. Since this is true whether the input is AC or DC, this circuit not only produces a DC output from an AC input, it can also provide what is sometimes called "reverse polarity protection". That is, it permits normal functioning of DC-powered equipment when batteries have been installed backwards, or when the leads wires from a DC power source have been
  22. 22. 22 reversed, and protects the equipment from potential damage caused by reverse polarity. Figure 3.8 Bridge rectifier with load The function of this capacitor, known as a reservoir capacitor is to lessen the variation in the rectified AC output voltage waveform from the bridge. There is still some variation, known as "ripple". One explanation of 'smoothing' is that the capacitor provides a low impedance path to the AC component of the output, reducing the AC voltage across, and AC current through, the resistive load. In less technical terms, any drop in the output voltage and current of the bridge tends to be canceled by loss of charge in the capacitor. This charge flows out as additional current through the load. Thus the change of load current and voltage is reduced relative to what would occur without the capacitor. Increases of voltage correspondingly store excess charge in the capacitor, thus moderating the change in output voltage / current. The simplified circuit shown has a well-deserved reputation for being dangerous, because, in some applications, the capacitor can retain a lethal charge after the AC power source is removed. If supplying a dangerous voltage, a practical circuit should include a reliable way to discharge the capacitor safely. If the normal load cannot be guaranteed to perform this function, perhaps because it can be disconnected, the circuit should include a bleeder resistor connected as close as practical across the
  23. 23. 23 capacitor. This resistor should consume a current large enough to discharge the capacitor in a reasonable time, but small enough to minimize unnecessary power waste. The capacitor and the load resistance have a typical time constant τ = RC Where C and R are the capacitance and load resistance respectively. As long as the load resistor is large enough so that this time constant is much longer than the time of one ripple cycle, the above configuration will produce a smoothed DC voltage across the load. When the capacitor is connected directly to the bridge, as shown, current flows in only a small portion of each cycle, which may be undesirable. The transformer and bridge diodes must be sized to withstand the current surge that occurs when the power is turned on at the peak of the AC voltage and the capacitor is fully discharged. Sometimes a small series resistor is included before the capacitor to limit this current, though in most applications the power supply transformer's resistance is already sufficient. Adding a resistor, or better yet, an inductor, between the bridge and capacitor can ensure that current is drawn over a large portion of each cycle and a large current surge does not occur. In older times, this crude power supply was often followed by passive filters to reduce the ripple further. When an inductor is used this way it is often called a choke. The choke tends to keep the current more constant. Although the inductor gives the best performance, usually the resistor is chosen for cost reasons. The idealized waveforms shown above are seen for both voltage and current when the load on the bridge is resistive. When the load includes a smoothing capacitor, both the voltage and the current waveforms will be
  24. 24. 24 greatly changed. While the voltage is smoothed, as described above, current will flow through the bridge only during the time when the input voltage is greater than the capacitor voltage. For example, if the load draws an average current of n Amps, and the diodes conduct for 10% of the time, the average diode current during conduction must be 10n Amps. This non- sinusoidal current leads to harmonic distortion and a poor power factor in the AC supply. Some early console radios created the speaker's constant field with the current from the high voltage power supply, which was then routed to the consuming circuits to create the speaker's constant magnetic field. The speaker field coil thus performed 2 jobs in one: it acted as a choke, filtering the power supply, and it produced the magnetic field to operate the speaker. 3.4.3 Poly Phase Bridge The diode bridge can be generalized to rectify poly phase AC inputs. For example, for a three phase AC input, a half-wave rectifier consists of three diodes, but a full wave bridge rectifier consists of six diodes. Figure 3.9 Six diodes bridge rectifier
  25. 25. 25 RECTIFIED OUTPUT WAVEFORM Figure 3.10 Three phase AC input waveform (top), half wave rectified waveform (center), and wave of rectified waveform (bottom)
  26. 26. 26 3.5 DC-TO-DC CONVERTER Figure 3.11 Regulator IC DC to DC converters are important in portable electronic devices such as cellular phones and laptop computers, which are supplied with power from batteries primarily. Such electronic devices often contain several sub-circuits, each with its own voltage level requirement different from that supplied by the battery or an external supply. Additionally, the battery voltage declines as its stored power is drained. Switched DC to DC converters offer a method to increase voltage from a partially lowered battery voltage thereby saving space instead of using multiple batteries to accomplish the same thing. Most DC to DC converters also regulate the output. Some exceptions include high-efficiency LED power sources, which are a kind of DC to DC converter that regulates the current through the LEDs, and simple charge pumps which double or triple the input voltage. 3.5.1 CONVERSION METHODS 3.5.1 Linear method Linear regulators can only output at lower voltages from the input. They are very inefficient when the voltage drop is large and the current is high as they dissipate heat equal to the product of the output current and the
  27. 27. 27 voltage drop; consequently they are not normally used for large-drop high- current applications. The inefficiency wastes power and requires higher-rated, and consequently more expensive and larger, components. The heat dissipated by high-power supplies is a problem in itself as it must be removed from the circuitry to prevent unacceptable temperature rises. Linear regulators are inexpensive, reliable if good heat sinking is used and much simpler than switching regulators. As part of a power supply they may require a transformer, which is larger for a given power level than that required by a switch-mode power supply. Linear regulators can provide a very low-noise output voltage, and are very suitable for powering noise- sensitive low-power analog and radio frequency circuits. A popular design approach is to use a Low Drop-out Regulator that provides a local "point of load" DC supply to a low power circuit. 3.5.2 Switched-mode conversion Electronic switch-mode DC to DC converters convert one DC voltage level to another, by storing the input energy temporarily and then releasing that energy to the output at a different voltage. The storage may be in either magnetic field storage components. This conversion method is more power efficient than linear voltage regulation. This efficiency is beneficial to increasing the running time of battery operated devices, which incur more switching losses and require a more complicated drive circuit. Another important innovation in DC-DC converters is the use of synchronous rectification replacing the flywheel diode with a power FET with low "On" resistance, thereby reducing switching losses.
  28. 28. 28 Most DC to DC converters are designed to move power in only one direction, from the input to the output. However, all switching regulator topologies can be made bi-directional by replacing all diodes with independently controlled active rectification. A bi-directional converter can move power in either direction, which is useful in applications requiring regenerative braking. Drawbacks of switching converters include complexity, electronic noise and to some extent cost, although this has come down with advances in chip design, DC to DC converters are now available as integrated circuits needing minimal additional components. DC to DC converters are also available as a complete hybrid circuit component, ready for use within an electronic assembly. 3.5.3 Magnetic In these DC to DC converters, energy is periodically stored into and released from a magnetic field in an inductor or a transformer, typically in the range from 300 kHz to 10 MHz By adjusting the duty cycle of the charging voltage, the amount of power transferred can be controlled. Usually, this is applied to control the output voltage, though it could be applied to control the input current, the output current, or maintain a constant power. Transformer- based converters may provide isolation between the input and the output. In general, the term "DC to DC converter" refers to one of these switching converters. These circuits are the heart of a switched-mode power supply. Many topologies exist. This table shows the most common.  Hard switched - transistors switch quickly while exposed to both full voltage and full current
  29. 29. 29  Resonant - an LC circuit shapes the voltage across the transistor and current through it so that the transistor switches when either the voltage or the current is zero Magnetic DC to DC converters may be operated in two modes, according to the current in its main magnetic component  Continuous - the current fluctuates but never goes down to zero  Discontinuous - the current fluctuates during the cycle, going down to zero at or before the end of each cycle The Half bridge and Fly back topologies are similar in that energy stored in the magnetic core needs to be dissipated so that the core does not saturate. Power transmission in a fly back circuit is limited by the amount of energy that can be stored in the core, while forward circuits are usually limited by the I/V characteristics of the switches. MOSFET switches can tolerate simultaneous full current and voltage; bipolar switches generally can't so require the use of a snubber. 3.5.4 Capacitive Switched capacitor converters rely on alternately connecting capacitors to the input and output in differing topologies. For example, a switched- capacitor reducing converter might charge two capacitors in series and then discharge them in parallel. This would produce an output voltage of half the input voltage, but at twice the current. Because they operate on discrete quantities of charge, these are also sometimes referred to as charge pump converters. They are typically used in applications requiring relatively small amounts of current, as at higher current loads the increased efficiency and smaller size of switch-mode converters makes them a better choice.
  30. 30. 30 3.5.5 Electrochemical A further means of DC to DC conversion in the kilowatt to many Megawatts range is presented by using redox flow batteries such as the vanadium redox battery, although this technique has not been applied commercially to date. 3.5.6 Terminology: Step-down–A converter where output voltage is lower than the input voltage. Like a Buck converter. Step-up–A converter that outputs a voltage higher than the input voltage. Like a Boost converter. Continuous Current Mode–Current and thus the magnetic field in the inductive energy storage never reach zero. Discontinuous Current Mode–Current and thus the magnetic field in the inductive energy storage may reach or cross zero. Noise - Since all properly designed DC to DC converters are completely inaudible, "noise" in discussing them always refers to unwanted electrical and electromagnetic signal noise. Output noise - The output of a DC to DC converter is designed to have a flat, constant output voltage. Unfortunately, all real DC to DC converters produce an output that constantly varies up and down from the nominal designed output voltage. This varying voltage on the output is the output noise. All DC to DC converters, including linear regulators, have some thermal output noise. Switching converters have, in addition, switching noise at the switching frequency and its harmonics. Some sensitive radio frequency and analog circuits require a power supply with so little noise that it can only be provided by a linear regulator.
  31. 31. 31 Input noise - If the converter loads the input with sharp load edges. Electrical noise can be emitted from the supplying power lines as RF noise. This should be prevented with proper filtering in the input stage of the converter. RF noise - Switching converters inherently emit radio waves at the switching frequency and its harmonics. Switching converters that produce triangular switching current, such as the Split-Pi or Ćuk converter in continuous current mode, produce less harmonic noise than other switching converters. Linear converters produce practically no RF noise. Too much RF noise causes electromagnetic interference. 3.6 INVERTOR An invertor is an electrical device that converts direct current to alternating current. The converted AC can be at any required voltage and frequency with the use of appropriate transformer, switching, and control circuit. Solid-state inverters have no moving parts and are used in a wide range of applications, from small switching power supplies in computers, to large electric utility high-voltage direct current applications that transport bulk power. Inverters are commonly used to supply AC power from DC sources such as solar panels or batteries. 3.6.1 Types 1. Modified sine wave The output of a modified sine wave inverter is similar to a square wave output except that the output goes to zero volts for a time before switching positive or negative. It is simple and low cost and is compatible with most electronic devices, except for sensitive or specialized equipment, for example certain laser printers, fluorescent lighting, and audio equipment.
  32. 32. 32 2. Pure sine wave A pure sine wave inverter produces a nearly perfect sine wave output that is essentially the same as utility-supplied grid power. Thus it is compatible with all AC electronic devices. This is the type used in grid-tie inverters. Its design is more complex, and costs more per unit power. The electrical inverter is a high-power electronic oscillator. It is so named because early mechanical AC to DC converters was made to work in reverse, and thus was "inverted", to convert DC to AC. 3. Uninterruptible power supplies An uninterruptible power supply (UPS) uses batteries and an inverter to supply AC power when main power is not available. 3.6.2 Applications  Induction heating  Inverters convert low frequency main AC power to higher frequency for use in induction heating. To do this, AC power is first rectified to provide DC power. The inverter then changes the DC power to high frequency AC power.  HVDC power transmission  With HVDC power transmission, AC power is rectified and high  Voltage DC power is transmitted to another location. At the receiving location, an inverter in an inverter plant converts the power back to AC.  Variable-frequency drives  A variable-frequency drive controls the operating speed of an AC motor by controlling the frequency and voltage of the power supplied to the motor. An inverter provides the controlled power.
  33. 33. 33 3.7 PULSE WIDTH MODULATION Pulse-width modulation or pulse duration modulation is a commonly used technique for controlling power to inertial electrical devices, made practical by modern electronic power switches. The switch is ON compared to the OFF periods, the higher the power supplied to the load is the average value of voltage and current fed to the load is controlled by turning the switch between supply and load ON and OFF at a fast pace. The term duty cycle describes the proportion of 'on' time to the regular interval or 'period' of time; a low duty cycle corresponds to low power, because the power is off for most of the time. Duty cycle is expressed in percent, 100% being fully on. The main advantage of PWM is that power loss in the switching devices is very low. When a switch is off there is practically no current, and when it is on, there is almost no voltage drop across the switch. Power loss, being the product of voltage and current, is thus in both cases close to zero. PWM also works well with digital controls, which, because of their on/off nature, can easily set the needed duty cycle. 3.7.1 Working Principle Pulse width modulation uses a rectangular pulse we whose pulse width is modulated resulting in the variation of the average value of the waveform. If we consider a pulse waveform f(t) with a low value ymin, a high value ymax and a duty cycle D, the average value of the waveform is given by:
  34. 34. 34 Figure 3.12 Rectangular square pulses 3.7.2 Generate PWM Pulse Figure 3.13 PWM pulse signal The signal here the red sine wave is compared with a saw tooth waveform blue. When the latter is less than the former, the PWM signal magenta is in
  35. 35. 35 high state 1. Otherwise it is in the low state 0.The simplest way to generate a PWM signal is the introspective method, which requires only a saw tooth or a triangle waveform easily generated using a simple oscillator and a comparator. When the value of the reference signal the red sine wave in is more than the modulation waveform blue, the PWM signal magenta is in the high state, otherwise it is in the low state. 3.7.3 Delta Modulation In the use of delta modulation for PWM control, the output signal is integrated, and the result is compared with limits, which correspond to a reference signal offset by a constant. Every time the integral of the output signal reaches one of the limits, the PWM signal changes state. Figure 3.14 Delta modulation waveform The Principle of the delta PWM output signal is compared with the limits. These limits correspond to the reference signal offset by a given value. Every time the output signal reaches one of the limits, the PWM signal changes state.
  36. 36. 36 3.7.4 Direct Torque Control Direct torque control is a method used to control AC motors. It is closely related with the delta modulation. Motor torque and magnetic flux are estimated and these are controlled to stay within their hysteresis bands by turning on new combination of the device's semiconductor switches each time either of the signal tries to deviate out of the band. 3.7.5 Types of Pulse-Width Modulation 1. The pulse center may be fixed in the center of the time window and both edges of the pulse moved to compress or expand the width. 2. The lead edge can be held at the lead edge of the window and the tail edge modulated. 3. The tail edge can be fixed and the lead edge modulated. Figure 3.15 Pulse-Width Modulations
  37. 37. 37 3.8 ATMEGA 8 MICROCONTROLLER 3.8.1 Features High-performance, Low-power AVR 8-bit Microcontroller -Advanced RISC Architecture –130 Powerful Instructions – Most Single-clock Cycle Execution – 32 x 8 General Purpose Working Registers – Fully Static Operation – Up to 16 MIPS Throughput at 16 MHz – On-chip 2-cycle Multiplier High Endurance Non-volatile Memory segments – 8K Bytes of In-System Self-programmable Flash program memory – 512 Bytes EEPROM – 1K Byte Internal SRAM – Write/Erase Cycles: 10,000 Flash/100,000 EEPROM – Data retention: 20 years at 85°C/100 years at 25°C – Optional Boot Code Section with Independent Lock Bits In-System Programming by On-chip Boot Program – Programming Lock for Software Security 3.8.2 Peripheral Features – Two 8-bit Timer/Counters with Separate Presale, one Compare Mode – One 16-bit Timer/Counter – Real Time Counter with Separate Oscillator – Three PWM Channels – 8-channel ADC in TQFP and QFN/MLF package Eight Channels 10-bit Accuracy – 6-channel ADC in PDIP package Six Channels 10-bit Accuracy
  38. 38. 38 – Byte-oriented Two-wire Serial Interface – Programmable Serial USART – Master/Slave SPI Serial Interface – Programmable Watchdog Timer with Separate On-chip Oscillator – On-chip Analog Comparator 3.8.3 Special Microcontroller Features – Power-on Reset and Programmable Brown-out Detection – Internal Calibrated RC Oscillator – External and Internal Interrupt Sources – Five Sleep Modes: Idle, ADC Noise Reduction, Power-save, Power- down, an Standby I/O and Packages – 23 Programmable I/O Lines Operating Voltages – 2.7 - 5.5V (ATmega8L) – 4.5 - 5.5V (ATmega8) Speed Grades – 0 - 8 MHz (ATmega8L) – 0 - 16 MHz (ATmega8) Power Consumption at 4 MHz, 3V, 25°C – Active: 3.6 mA – Idle Mode: 1.0 mA – Power-down Mode: 0.5 3.8.4 Overview The ATmega8 is a low-power CMOS 8-bit microcontroller based on the AVR RISC architecture. By executing powerful instructions in a single clock cycle, the ATmega8 achieves throughputs approaching 1 MIPS per MHz, allowing the system designed to optimize power consumption versus processing speed.
  39. 39. 39 The resulting architecture is more code efficient while achieving throughputs up to ten times faster than conventional CISC microcontrollers. The device is manufactured using Atmel’s high density non-volatile memory technology. The Flash Program memory can be reprogrammed In-System through an SPI serial interface, by a conventional non-volatile memory programmer, or by an On-chip boot program running on the AVR core. The boot program can use any interface to download the application program in the Application Flash memory. Software in the Boot Flash Section will continue to run while the Application Flash Section is updated, providing true Read While Write operation. 3.8.5 Pin Configurations Figure 3.16 Pin details
  40. 40. 40 3.8.6 Pin Descriptions VCC: 5v dc supply voltage. GND: Ground. Port B: is an 8-bit bi-directional I/O port with internal pull-up XTAL1/XTAL2/TOSC1/ Port B output buffers have symmetrical drive Oscillator amplifier. If the Internal Calibrated RC Oscillator is used as chip clock input for the Asynchronous Timer/Counter2 if the AS2 bit in The various special features of Port B are elaborated in “Al 58 and “System Clock and Clock Options” Port C (PC5-PC0): Port C is an7-bit bi-directional I/O port with internal pull- up Port C output buffers have symmetrical drive character is capability. As inputs, Port C pins that are externally pulled resistors are activated. The Port C pins are tri-stated when even if the clock is not running. PC6/RESET If the RSTDISBL Port If the RSTDISBL Fuse is un programmed, generate a Reset. The various Special features of Port C are elaborated on Port D . Port D (PD7-PD0): is an 8-bit bidirectional I/O port with internal pull-up Port D output buffers has symmetrical drive characters capability. As inputs, Port 3.8.7 General Purpose The Register File is optimized for the AVR Enhanced RISC instruction set. In order to achieve Register File the required performance and flexibility, the following input/output schemes are supported by the file. Register File
  41. 41. 41 One 8-bit output operand and one 8-bit result input. Two 8-bit output operands and one 8-bit result input. Two 8-bit output operands and one 16-bit result input. One 16-bit output operand and one 16-bit result input. 3.9 ANALOG-TO-DIGITAL CONVERTER An analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement. The reverse operation is performed by a converter. Typically, an ADC is an electronic device that converts an input analog voltage or current to a digital number proportional to the magnitude of the voltage or current. However, some non-electronic or only partially electronic devices, such as rotary encoders, can also be considered ADCs. 3.9.1 8-level ADC The resolution of the converter indicates the number of discrete values it can produce over the range of analog values. The values are usually stored electronically in binary form, so the resolution is usually expressed in bits. In consequence, the number of discrete values available, or "levels", is a power of two. For example, an ADC with a resolution of 8 bits can encode an analog input to one in 256 different levels, since 28 = 256. The values can represent the ranges from 0 to 255.
  42. 42. 42 Resolution can also be defined electrically, and expressed in volts. The minimum change in voltage required to guarantee a change in the output code level is called the least significant bit voltage. The resolution Q of the ADC is equal to the LSB voltage. The voltage resolution of an ADC is equal to its overall voltage measurement range divided by the number of discrete voltage intervals: Where, N is the number of voltage intervals and EFSR is the full scale voltage range. EFSR is given by Where VRefHi and VRefLow are the upper and lower extremes, respectively, of the voltages that can be coded. Normally, the number of voltage intervals is given by Where, M is the ADC's resolution in bits. 3.9.2 Response type Most ADCs are linear types. The term linear implies that the range of input values has a linear relationship with the output value. Some early converters had a logarithmic response to directly implement A-law or µ-law coding. These encodings are now achieved by using a higher-resolution linear ADC and mapping its output to the 8-bit coded values.
  43. 43. 43 1. Accuracy An ADC has several sources of errors. Quantization error and non- linearity are intrinsic to any analog-to-digital conversion. There is also a so- called aperture error which is due to a clock jitter and is revealed when digitizing a time-variant signal. These errors are measured in a unit called the least significant bit. In the above example of an eight-bit ADC, an error of one LSB is 1/256 of the full signal range, or about 0.4%. 2. Quantization error Quantization error is the difference between the original signal and the digitized signal. Hence, the magnitude of the quantization error at the sampling instant is between zero and half of one LSB. Quantization error is due to the finite resolution of the digital representation of the signal, and is an unavoidable imperfection in all types of ADCs. 3. Non-linearity All ADCs suffer from non-linearity errors caused by their physical imperfections, causing their output to deviate from a linear function of their input. These errors can sometimes be mitigated by calibration, or prevented by testing. Important parameters for linearity are integral non-linearity and differential non-linearity. These nonlinearities reduce the dynamic range of the signals that can be digitized by the ADC, also reducing the effective resolution of the ADC. 4. Sampling rate The analog signal is continuous in time and it is necessary to convert this to a flow of digital values. It is therefore required to define the rate at
  44. 44. 44 which new digital values are sampled from the analog signal. The rate of new values is called the sampling rate or sampling frequency of the converter. A continuously varying band limited signal can be sampled and then the original signal can be exactly reproduced from the discrete-time values by an interpolation formula. The accuracy is limited by quantization error. However, this faithful reproduction is only possible if the sampling rate is higher than twice the highest frequency of the signal. This is essentially what is embodied in the Shannon-Nyquist sampling theorem. 5. Aliasing All ADCs work by sampling their input at discrete intervals of time. Their output is therefore an incomplete picture of the behavior of the input. There is no way of knowing, by looking at the output, what the input was doing between one sampling instant and the next. If the input is known to be changing slowly compared to the sampling rate, then it can be assumed that the value of the signal between two sample instants was somewhere between the two sampled values. If, however, the input signal is changing rapidly compared to the sample rate, then this assumption is not valid. If the digital values produced by the ADC are, at some later stage in the system, converted back to analog values by a digital to analog converter or DAC, it is desirable that the output of the DAC be a faithful representation of the original signal. If the input signal is changing much faster than the sample rate, then this will not be the case, and spurious signals called aliases will be produced at the output of the DAC. The frequency of the aliased signal is the difference between the signal frequency and the sampling rate. For example, a 2 kHz sine wave being sampled at 1.5 kHz would be reconstructed as a 500 Hz sine wave. This problem is called aliasing.
  45. 45. 45 6. Dither In A-to-D converters, performance can usually be improved using dither. This is a very small amount of random noise, which is added to the input before conversion. Its effect is to cause the state of the LSB to randomly oscillate between 0 and 1 in the presence of very low levels of input, rather than sticking at a fixed value. Rather than the signal simply getting cut off altogether at this low level, it extends the effective range of signals that the A- to-D converter can convert at the expense of a slight increase in noise effectively the quantization error is diffused across a series of noise values which is far less objectionable than a hard cutoff. The result is an accurate representation of the signal over time. A suitable filter at the output of the system can thus recover this small signal variation. 7. Oversampling Usually, signals are sampled at the minimum rate required, for economy, with the result that the quantization noise introduced is white noise spread over the whole pass band of the converter. If a signal is sampled at a rate much higher than the Nyquist frequency and then digitally filtered to limit it to the signal bandwidth and effective resolution larger than that provided by the converter alone to improvement in SNR is 3 dB per octave of oversampling which is not sufficient for many applications. Therefore, oversampling is usually coupled with noise shaping. With noise shaping, the improvement is 6L+3 dB per octave where L is the order of loop filter used for noise shaping.
  46. 46. 46 3.9.3 ADC types These are the most common ways of implementing an electronic ADC. A direct-conversion ADC or flash ADC has a bank of comparators sampling the input signal in parallel, each firing for their decoded voltage range. The comparator bank feeds a logic circuit that generates a code for each voltage range. Direct conversion is very fast, capable of gigahertz sampling rates, but usually has only 8 bits of resolution or fewer, since the number of comparators needed, 2N - 1, doubles with each additional bit, requiring a large, expensive circuit. ADCs of this type have a large die size, a high input capacitance, high power dissipation, and are prone to produce glitches at the output. Scaling to newer submicron meters technologies does not help as the device mismatch is the dominant design limitation. They are often used for video, wideband communications or other fast signals in optical storage. 1. Successive-Approximation DC Uses a comparator to reject ranges of voltages, eventually settling on a final voltage range. Successive approximation works by constantly comparing the input voltage to the output of an internal digital to analog converter until the best approximation is achieved. At each step in this process, a binary value of the approximation is stored in a successive approximation register. The SAR uses a reference voltage for comparisons. For example if the input voltage is 60 V and the reference voltage is 100 V, in the 1st clock cycle, 60 V is compared to 50 V, and the voltage from the comparator is positive. At this point the first binary digit is set to a '1'. In the 2nd clock cycle the input voltage is compared to 75 V because 60 V is less than 75 V, the comparator output is now negative. The second binary digit is therefore set to a '0'. In the 3rd clock cycle, the input voltage is compared with 62.5 V. The output of the
  47. 47. 47 comparator is negative or '0' so the third binary digit is set to a 0. The fourth clock cycle similarly results in the fourth digit being a '1'. The result of this would be in the binary form 1001. This is also called bit-weighting conversion, and is similar to a binary search. The analogue value is rounded to the nearest binary value below, meaning this converter type is mid-rise. Because the approximations are successive, the conversion takes one clock- cycle for each bit of resolution desired. The clock frequency must be equal to the sampling frequency multiplied by the number of bits of resolution desired. 2. Ramp Type ADC Produces a saw-tooth signal that ramps up or down then quickly returns to zero. When the ramp starts, a timer starts counting. When the ramp voltage matches the input, a comparator fires, and the timer's value is recorded. Timed ramp converters require the least number of transistors. The ramp time is sensitive to temperature because the circuit generating the ramp is often just some simple oscillator. There are two solutions: use a clocked counter driving a DAC and then use the comparator to preserve the counter's value, or calibrate the timed ramp. A special advantage of the ramp-compare system is that comparing a second signal just requires another comparator, and another register to store the voltage value. 4. Integrating ADC Applies the unknown input voltage to the input of an integrator and allows the voltage to ramp for a fixed time period. Then a known reference voltage of opposite polarity is applied to the integrator and is allowed to ramp until the integrator output returns to zero. The input voltage is computed as a function of the reference voltage, the constant run-up time period, and the measured run-down time period.
  48. 48. 48 3.9.4 Pipeline ADC Uses two or more steps of sub ranging. First, a coarse conversion is done Analog to digital converter. This difference is then converted finer, and the results are combined in a last step. This can be considered a refinement of the successive-approximation ADC wherein the feedback reference signal consists of the interim conversion of a whole range of bits rather than just the next-most-significant bit. 3.10 LIQUID CRYSTAL DISPLAYS Figure 3.17 LCD Display 3.10.1 General Craft data BC1602A An LCD is a small low cost display. It is easy to interface with a micro- controller Because of an embedded controller. This controller is standard across many displays (HD 44780) which means many Micro-controllers have libraries that make displaying 3.10.2 Testing Testing your LCD with an Adriano is really simple. Wire up your display using the schematic or breadboard layout sheet.
  49. 49. 49 CHAPTER 4 IMPLEMENTION OF PROPOSED PROJECT 4.1 Schematic Circuit Diagram Figure 4.1 Circuit diagram
  50. 50. 50 4.1.1 Power Supply A transformer is a static device that transfers electrical energy from one circuit to another through inductively coupled conductors the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF) or "voltage" in the secondary winding. This effect is called mutual induction. If a load is connected to the secondary, an electric current will flow in the secondary winding and electrical energy will be transferred from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding (Vs) is in proportion to the primary voltage (Vp), and is given by the ratio of the number of turns in the secondary (Ns) to the number of turns in the primary (Np) as follows: Primary coil changes the magnetic flux that is developed. The changing magnetic flux induces a voltage in the secondary coil by appropriate selection of the ratio of turns, a transformer thus allows an alternating current (AC) voltage to be "stepped up" by making Ns greater than Np, or "stepped down" by making Ns less than Np. In the vast majority of transformers, the windings are coils wound around a ferromagnetic core, air-core transformers being a notable exception. Transformers range in size from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge units weighing hundreds of tons used to interconnect portions of power grids. All operate
  51. 51. 51 with the same basic principles, although the range of designs is wide. While new technologies have eliminated the need for transformers in some electronic circuits, transformers are still found in nearly all electronic devices designed for household voltage. Transformers are essential for high-voltage electric power transmission, which makes long-distance transmission economically practical. The transformer is based on two principles: first, that an electric current can produce a magnetic field, second that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil. Figure 4.2 step down transformer coil An ideal transformer is shown in the adjacent figure. Current passing through the primary coil creates a magnetic field. The primary and secondary coils are wrapped around a core of very high magnetic permeability, such as iron, so that most of the magnetic flux passes through both the primary and secondary coils.
  52. 52. 52 4.1.2 Diodes The diode is a two-terminal device whose function is to pass current in one direction but not in the other. A conventional diode is formed from the junction of p-type and n- type silicon. The ideal device has a “brick-wall” V-I characteristic: the practical silicon diode has an exponential characteristic which approximates to the brick wall, if viewed Forward bias: The first thing to notice is that the forward voltage V is not constant, nor is it zero. It has two determinants, forward current I and temperature T. They are related by the I = I [exp (V · q/kT) - 1] Reverse bias: So far we have only considered the forward characteristic that is for positive applied voltage. An ideal diode would block all current flow in the reverse direction. There are two main reverse characteristics, reverse leakage current I and reverse breakdown voltage VBR. The diode equation holds good in the reverse direction until VBR is approached; in the low-voltage region IR is almost equal to IS. Breakdown VBR is that voltage at which the reverse-biased junction can no longer withstand the applied electric field. At this point, avalanche breakdown occurs and a current limited mainly by the external source impedance will flow. If the device maximum power dissipation is exceeded the junction will be destroyed. Diodes operated conventionally, as opposed to Zener diodes to which we will return shortly, are always run at reverse voltages lower than VBR.
  53. 53. 53 Positive Voltage Regulator: A monolithic regulator IC includes the voltage reference on-chip, along with other circuitry and the series pass element. This means that the reference is subject to a thermal shift when the power dissipation of the series pass element changes. This gives rise to a separate longer term component of regulation, called thermal regulation, defined as the change in output voltage caused by a change in dissipated power for a specified time. Provided the chip has been well-designed, thermal regulation is not a significant factor for most purposes, but it is rarely specified in data sheets and for some precision applications may render monolithic regulators unsuitable. 4.1.3 Rectifier In the diagrams below, when the input connected to the left corner of the diamond is positive, and the input connected to the right corner is negative, current flows from the upper supply terminal to the right along the red positive path to the output, and returns to the lower supply terminal via the blue negative path. When the input connected to the left corner is negative, and the input connected to the right corner is positive, current flows from the lower supply terminal to the right along the red positive path to the output, and returns to the upper supply terminal via the blue negative path. In each case, the upper right output remains positive and lower right output negative. Since this is true whether the input is AC or DC, this circuit not only produces a DC output from an AC input, it can also provide what is sometimes called "reverse polarity protection". That is, it permits normal functioning of DC-powered equipment when batteries have been installed backwards.
  54. 54. 54 4.1.4 Boost circuit Electronic switch-mode DC to DC converters convert one DC voltage level to another, by storing the input energy temporarily and then releasing that energy to the output at a different voltage. The storage may be in either magnetic field storage components. This conversion method is more power efficient than linear voltage regulation. This efficiency is beneficial to increasing the running time of battery operated devices. 4.1.5 ADC An analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement. The reverse operation is performed by a digital-to- analog converter. Typically, an ADC is an electronic device that converts an input analog voltage or current to a digital number proportional to the magnitude of the voltage or current. However, some non-electronic or only partially electronic devices, such as rotary encoders, can also be considered ADCs. The digital output may use different coding schemes. Typically the digital output will be a two's complement binary number that is proportional to the input, but there are other possibilities. An encoder, for example, might output a Gray code. 4.1.6 Pulse width modulation Pulse width modulation uses a rectangular pulse we whose pulse width is modulated resulting in the variation of the average value of the waveform. If we consider a pulse waveform f(t) with a low value ymin, a high value ymax and a duty cycle D
  55. 55. 55 Figure 4.3 PWM signal 4.1.7 Three Phase Inverter The BLDC motor control consists of generating DC currents in the motor phases. This control is subdivided into two independent operations: stator and rotor flux synchronization and control of the current value. Both operations are realized through the three phase inverter depicted in the following scheme. Figure 4.4 Three Phase Inverter
  56. 56. 56 The flux synchronization is derived from the position information coming from sensors, or from sensor less techniques. From the position, the controller determines the appropriate pair of transistors which must be driven. The regulation of the current to a fixed 60 degrees reference can be realized in either of the two different modes: 4.1.8 BLDC Motor Brushless D.C. motors operate in conjunction with an electronic controller and a rotor position feedback sensor. Based upon the actual rotor position, the controller sequentially energizes or switches “on” the stator’s phase windings so that torque is continuously generated as the permanent magnet (PM) rotor rotates. This switching action is called electronic commutation. To sense the rotor’s angular position, position sensors are used. When the PM rotor passes one set of phase windings, a signal from the position sensor is sent to the controller which then sends a signal to switch “on” the next set of phase windings so that the magnetic fields of the rotor and the stator’s phase windings remain synchronized. The torque/speed characteristic of the motor is determined by the magnitude of the signal and the switching rate of the controller. For example, in a 2-phase motor, when the phase 1 winding is energized, the PM rotor will rotate to align itself with magnetic field produced by the phase 1 winding. When the phase 1 winding is turned “off”, the phase 2 winding is turned “on” and the rotor will continue to rotate to align itself with the magnetic field of the phase 2 winding. This “on” and “off” switching of the phase windings will maintain torque of the PM rotor.
  57. 57. 57 CHAPTER 5 SIMULATION IMPLEMENTATION 5.1 SIMULATION CIRCUIT DIAGRAM Figure 5.1 simulation circuit diagram
  58. 58. 58 5.2 SIMULATION OUTPUT SPEED WAVEFORM Figure 5.2 speed waveform 5.3 TORQUE WAVEFORM Figure 5.3 torque waveform
  59. 59. 59 5.4 PULSE WITH MODULATION SIMULATION OUTPUT WAVEFORM Figure 5.4 PWM waveform
  60. 60. 60 5.5 POWER FACTOR SIMULATION OUTPUT Figure 5.5 power factor waveform 5.6 STATOR CURRENT SHAPING SIMULATION Figure 5.6 Stator current waveform
  61. 61. 61 CHAPTER 6 CONCLUSION Power Factor correction of a 3-phase Four Switch Inverter fed BLDC Motor with PI control is analyzed. Design of Boost Converter, 3- phase Four Switch Inverter, programing processor, LCD display, hall sensor feedback control, and PMBLDC Motor etc. are done. Hardware for the topology with machine design is implemented. Power Factor Correction using Boost Converter reduces the line harmonics and hence, the line current becomes sinusoidal in shape and will be in same phase with the voltage. The efficiency of the utility system will be high so that the total working of the system get better motor control. 3-Phase 4-Switch technology has been used number of switches, switching losses; cost, complexity etc. can be reduced. THD is reduced to 3% in simulation and achieve the same in hardware. The cost off the drive is reduced from Rs- 2800 to less than Rs- 1000.
  62. 62. 62 REFERENCES 1. Atef, S.O. and Al-Mashakbeh, 2009. “Proportional Integral and derivative control of brushless DC motor”.Eur. J. Sci. Res., 35(2): 198-203c. 2. A. Monti, D. Patterson, E. Santi, F. Ponci, J. Bastos, D. Franzoni, W. Jiang, D. Kovuri, S. Krooswyk, S. Lentijo, D. Li, T. Lovett, B. Lu, L. Lu, J. Mcclanahan, K. Patel, A. Smith, K. Veturi, X. Wu, "An innovative low-cost drive for permanent magnet motors: a Future Energy Challenge experience," Proc. IEEE Applied Power Electronics Conference (APEC'04), pp. 1137-1142, 2004 3. Krishnan, R., 2003. “A Text Book on Electric Motor Drives, Modelling, Analysis and Control”. Prentice Hall of India Pvt Ltd., New Delhi. 4. Byoung-Kuk, L., K. Tae-Hyung and J. Mehrdad Ehsani, 2001. “On the feasibility of four-switch three phase BLDC motor drives for low cost commercial applications topology and control”. IEEE Tran. Power Electron. APEC, 18(1): 428-433. 5. R. W. Erickson, D. Maksimovic, “Fundamentals of Power Electronics,” Kluwer Academic Publishers, Second Edition, 2001 6. F. Blaabjerg, D. O. Neacsu and J. K. Pedersen, “Adaptive SVM to Compensate Dc-Link Voltage Ripple for Four-Switch, Three-Phase Voltage Source Inverter”, IEEE Trans. on Power Electronics, Vol. 14, No. 4, July 1999, pp.743-751.
  63. 63. 63 7. G. Spiazzi, “Analysis of Buck converters used as power factor preregulators,” Power Electronics Specialists Conference, vol.1, pp. 564 – 570, 1997 8. K.M. Smedley, S. Cuk, “One-cycle control of switching converters,” IEEE Transactions on Power Electronics, Volume 10, Issue 6, Nov. 1995 pp. 625 – 633 9. W. Tang, F.C. Lee, R.B. Ridley, I. Cohen, “Charge control: modeling, analysis, and design,” IEEE Transactions on Power Electronics, Volume 8, Issue 4, Oct. 1993 pp. 396 – 403 10 . J. Skinner, T.A. Lipo, “Input current shaping in brushless DC motor drives utilizing inverter current control,” Fifth International Conference on Electrical Machines and Drives, 1991, 11-13 Sep 1991 pp. 121 – 125

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