Pwm Based Induction Motor Speed Control

A MAJOR PROJECT REPORT

ON

PWM BASED INDUCTION MOTOR SPEED CONTROL

Submitted in partial fulfillment of the

Award of Degree

BACHELOR OF TECHNOLOGY

IN

ELECTRONICS & COMMUNICATION ENGINEERING

Submitted by

Md. Wasim

Rajkumar

Tarun Dhiman

Guided by

Mr. Vipul Sharma

FACULTY OF ENGINEERING AND TECHNOLOGYGURUKULA KANGRI VISHWAVIDYALAYA

HARIDWAR-249404

Contents

Certificate

Acknowledgement

1. Introduction of induction motor

2. Apparatus Used

2.1 Capacitor

2.2 Diode

2.3 Resistance

2.4 Solder

3. NE556 TIMER IC

4. Making of Printing Circuit Board

5. To Construct a Lamp Dimmer Using Traic

6. Power Supply

7.Pulse Width Modulation

8. PWM Speed Control

CERTIFICATE

We, hereby declare that the work which is being presented in the major project entitled “PWM BASED INDUCTION MOTOR SPEED CONTROL” in the partial

fulfillment of the requirement for the award of degree of Bachelor of Technology in Electronics & Communication Engineering, is an authentic record of our own work, carried out during the period from February 2011 to March 2011,under the guidance of Mr. Vipul Sharma, Incharge, Department of Electronics & Communication Engineering, Faculty of Engineering, Gurukula Kangri Vishwavidyalaya, Haridwar.The matter embodied in this record has not been submitted by us for the award of any other degree or diploma.

Date: MD. WASIM

RAJKUMAR

TARUN DHIMAN

This is to certify that the above statement made by the candidates is correct to the best of my knowledge.

Mr. Vipul Sharma Incharge,

Department of Electronics and Communication Engineering,Faculty of Engineering and Technology,

Gurukula Kangri Vishwavidyalaya,Haridwar-249404

U.K. (India)

Acknowledgement

We are highly indebted to Mr.VIPUL SHARMA and obliged for giving us

the autonomy of functioning and experimenting with ideas. We should like to take

this opportunity to express our profound gratitude to him not only for his academic

guidance but also for his personal interest in our project and constant support

coupled with confidence boosting and motivating sessions which proved very

fruitful and were instrumental in infusing self-assurance and trust within us. The

nurturing and blossoming of the present work was mainly due to his valuable

guidance, suggestions, astute judgment, constructive criticism and an eye for

perfection.

Finally, we are grateful to our parents , friends and colleagues whose constant

encouragement served to renew our spirit refocus our attention and energy and

helped us in carrying out this work.

INTRODUCTION OF INDUCTION MOTOR

SYNCHRONOUS MOTORS

The construction of the synchronous motors is essentially the same as the construction of the salient- pole alternator. In fact, such an alternator may be run as an ac motor. It is similar to the drawing in figure 4-6. Synchronous motors have the characteristic of constant speed between no load and full load. They are capable of correcting the low power factor of an inductive load when they are operated under certain conditions. They are often used to drive dc generators. Synchronous motors are designed in sizes up to thousands of horsepower. They may be designed as either single-phase or multiphase machines. The discussion that follows is based on a three-phase design. Figure 4-6.—Revolving-field synchronous motor. To understand how the synchronous motor works, assume that the application of three-phase ac power to the stator causes a rotating magnetic field to be set up around the rotor. The rotor is energized with dc (it acts like a bar magnet). The strong rotating magnetic field attracts the strong rotor field activated by the dc. This results in a strong turning force on the rotor shaft. The rotor is therefore able to turn a load as it rotates in step with the rotating magnetic field. It works this way once it’s started. However, one of the disadvantages of a synchronous motor is that it cannot be started from a standstill by applying three-phase ac power to the stator. When ac is applied to the stator, a high-speed rotating magnetic field appears immediately. This rotating field rushes past the rotor poles so quickly that the rotor does not have a chance to get started. In effect, the rotor is repelled first in one direction and then the other. A synchronous motor in its purest form has no starting torque. It has torque only when it is running at synchronous speed. A squirrel-cage type of winding is added to the rotor of a synchronous motor to cause it to start. The squirrel cage is shown as the outer part of the rotor in figure 4-7. It is so named because it is shaped and looks something like a turnable squirrel cage. Simply, the windings are heavy copper bars shorted

Figure 4-6.—Revolving-field synchronous motor.

APPARATUS USED

CAPACITORSIt is an electronic component whose function is to accumulate charges and then release it.

To understand the concept of capacitance, consider a pair of metal plates which all are placed near to each other without touching. If a battery is connected to these plates the positive pole to one and the negative pole to the other, electrons from the battery will be attracted from the plate connected to the positive terminal of the battery. If the battery is then disconnected, one plate will be left with an excess of electrons, the other with a shortage, and a potential or voltage difference will exists between them. These plates will be acting as capacitors. Capacitors are of two types: - (1) fixed type like ceramic, polyester, electrolytic capacitors-these names refer to the material they are made of aluminium foil. (2) Variable type like gang condenser in radio or trimmer. In fixed type capacitors, it has two leads and its value is written over its body and variable

type has three leads. Unit of measurement of a capacitor is farad denoted by the symbol F. It is a very big unit of capacitance. Small unit capacitor are pico-farad denoted by pf (Ipf=1/1000,000,000,000 f) Above all, in case of electrolytic capacitors, it's two terminal are marked as (-) and (+) so check it while using capacitors in the

circuit in right direction. Mistake can destroy the capacitor or entire circuit in operational.

DIODE

The simplest semiconductor device is made up of a sandwich of P-type semiconducting material, with contacts provided to connect the p-and n-type layers to an external circuit. This is a junction Diode. If the positive terminal of the battery is connected to the p-type material (cathode) and the negative terminal to the N-type material (Anode), a large current will flow. This is called forward current or forward biased.If the connections are reversed, a very little current will flow. This is because under this condition, the p-type material will accept the electrons from the negative terminal of the battery and the N-type material will give up its free electrons to the battery, resulting in the state of electrical equilibrium since the N-type material has no more electrons. Thus there will be a small current to flow and the diode is called Reverse biased.

Thus the Diode allows direct current to pass only in one direction while blocking it in the other direction. Power diodes are used in concerting AC into DC. In this, current will flow freely during the first half cycle (forward biased) and practically not at all during the other half cycle (reverse biased). This makes the diode an effective rectifier, which convert ac into pulsating dc. Signal diodes are used in radio circuits for detection. Zener diodes are used in the circuit to control the voltage.

Some common diodes are:-

1. Zener diode.

2. Photo diode.

3. Light Emitting diode.

1. ZENER DIODE:-

A zener diode is specially designed junction diode, which can operate continuously without being damaged in the region of reverse break down voltage. One of the most important applications of zener diode is the design of constant

voltage power supply. The zener diode is joined in reverse bias to d.c. through a resistance R of suitable value.

2. PHOTO DIODE:-

A photo diode is a junction diode made from photo- sensitive semiconductor or material. In such a diode, there is a provision to allow the light of suitable frequency to fall on the p-n junction. It is reverse biased, but the voltage applied is less than the break down voltage. As the intensity of incident light is increased, current goes on increasing till it becomes maximum. The maximum current is called saturation current.

3. LIGHT EMITTING DIODE (LED):-

When a junction diode is forward biased, energy is released at the junction diode is forward biased, energy is released at the junction due to recombination of electrons and holes. In case of silicon and germanium diodes, the energy released is in infrared region. In the junction diode made of gallium arsenate or indium phosphide, the energy is released in visible region. Such a junction diode is called a light emitting diode or LED.

RESISTANCE

Resistance is the opposition of a material to the current. It is measured in Ohms (). All conductors represent a certain amount of resistance, since no conductor is

100% efficient. To control the electron flow (current) in a predictable manner, we use resistors. Electronic circuits use calibrated lumped resistance to control the flow of current. Broadly speaking, resistor can be divided into two groups viz. fixed & adjustable (variable) resistors. In fixed resistors, the value is fixed & cannot be varied. In variable resistors, the resistance value can be varied by an adjuster knob. It can be divided into (a) Carbon composition (b) Wire wound (c) Special type. The most common type of resistors used in our projects is carbon type. The resistance value is normally indicated by colour bands. Each resistance has four colours, one of the band on either side will be gold or silver, this is called fourth band and indicates the tolerance, others three band will give the value of resistance (see table). For example if a resistor has the following marking on it say red, violet, gold. Comparing these coloured rings with the colour code, its value is 27000 ohms or 27 kilo ohms and its tolerance is ±5%. Resistor comes in various sizes (Power rating). The bigger, the size, the more power rating of 1/4 watts. The four colour rings on its body tells us the value of resistor value as given below.

COLOURS CODE

Black-------------------------------------------------0

Brown-----------------------------------------------1

Red---------------------------------------------------2

Orange----------------------------------------------3

Yellow-----------------------------------------------4

Green------------------------------------------------5

Blue--------------------------------------------------6

Violet------------------------------------------------7

Grey--------------------------------------------------8

White------------------------------------------------9

The first rings give the first digit. The second ring gives the second digit. The third ring indicates the number of zeroes to be placed after the digits. The fourth ring gives tolerance (gold ±5%, silver ± 10%, No colour ± 20%).

In variable resistors, we have the dial type of resistance boxes. There is a knob with a metal pointer. This presses over brass pieces placed along a circle with some space b/w each of them.

Resistance coils of different values are connected b/w the gaps. When the knob is rotated, the pointer also moves over the brass pieces. If a gap is skipped over, its resistance is included in the circuit. If two gaps are skipped over, the resistances of both together are included in the circuit and so on.

A dial type of resistance box contains many dials depending upon the range, which it has to cover. If a resistance box has to read upto 10,000, it will have three dials each having ten gaps i.e. ten resistance coils each of resistance 10. The third dial will have ten resistances each of 100.

The dial type of resistance boxes is better because the contact resistance in this case is small & constant.

PROCEDURE FOR MAKING PROJECTBuilding project in the proper manner is really an art, something which must be

prectised and learned through trial and error, it is not all that difficult. The main thing is to remember to take each step slowly and carefully according to the instructions giving making since that everything at it should be before proceeding further.

TOOLS: The electronics workbench is an actual place of work with comfortably & conveniently & should be supplied with compliment of those tools must often use in project building. Probably the most important device is a soldering tool. Other tool which should be at the electronic work bench includes a pair of needle nose pliers, diagonal wire cutter, a small knife, an assortment of screw driver, nut driver, few nuts & bolts, electrical tape, plucker etc. Diagonal wire cutter will be used to cut away any excess lead length from copper side of P.C.B. 7 to cut section of the board after the circuit is complete. The needle nose pliers are most often using to bend wire leads & wrap them in order to form a strong mechanical connection.

MOUNTING & SOLDERING: Soldering is process of joining together two metallic parts. It is actually a process of function in which an alloy, the solder, with a comparatively low melting point penetrates the surface of the metal being joined & makes a firm joint between them on cooling & solidifying.

I. THE SOLDERING KIT

1. SOLDERING IRON:

As soldering is a process of joining together two metallic parts, the instrument, which is used, for doing this job is known as soldering Iron. Thus it is meant for melting the solder and to setup the metal parts being joined. Soldering Iron is rated according to their wattage, which varies from 10- 200 watts.

2. SOLDER:

The raw material used for soldering is solder. It is composition of lead & tin. The good quality solder (a type of flexible naked wire) is 60% Tin +40% Lead which will melt between 180 degree to 200 degree C temperature.

3. FLUXES OR SOLDERING PASTE:

When the points to solder are heated, an oxide film forms. This must be removed at once so that solder may get to the surface of the metal parts. This is done by applying chemical substance called Flux, which boils under the heat of the iron remove the oxide formation and enable the metal to receive the solder.4. BLADES OR KNIFE:

To clean the surface & leads of components to be soldered is done by this common instrument.

5. SAND PAPER:

The oxide formation may attack at the tip of your soldering iron & create the problem. To prevent this, clean the tip with the help of sand paper time to time or you may use blade for doing this job. Apart from all these tools, the working bench for soldering also includes desoldering pump, wink wire (used for desoldering purpose), file etc.

II. HOW TO SOLDER?Mount components at their appropriate place; bend the leads slightly outwards to prevent them from falling out when the board is turned over for soldering. No cut the leads so that you may solder them easily. Apply a small amount of flux at these components leads with the help of a screwdriver. Now fix the bit or iron with a small amount of solder and flow freely at the point and the P.C.B copper track at the same time. A good solder joint will appear smooth & shiny. If all appear well, you may continue to the next solder connections.

TIPS FOR GOOD SOLDERING

1. Use right type of soldering iron. A small efficient soldering iron (about 10-25 watts with 1/8 or 1/4 inch tip) is ideal for this work.

2. Keep the hot tip of the soldering iron on a piece of metal so that excess heat is dissipated.

3. Make sure that connection to the soldered is clean. Wax frayed insulation and other substances cause poor soldering connection. Clean the leads, wires, tags etc. before soldering.

4. Use just enough solder to cover the lead to be soldered. Excess solder can cause a short circuit.

5. Use sufficient heat. This is the essence of good soldering. Apply enough heat to the component lead. You are not using enough heat, if the solder barely melts and forms a round ball of rough flaky solder. A good solder joint will look smooth, shining and spread type. The difference between good & bad soldering is just a few seconds extra with a hot iron applied firmly.

PRECAUTIONS1. Mount the components at the appropriate places before soldering. Follow the circuit description and components details, leads identification etc. Do not start soldering before making it confirm that all the components are mounted at the right place.

2. Do not use a spread solder on the board, it may cause short circuit.

3. Do not sit under the fan while soldering.

4. Position the board so that gravity tends to keep the solder where you want it.

5. Do not over heat the components at the board. Excess heat may damage the components or board.

6. The board should not vibrate while soldering otherwise you have a dry or a cold joint.

7. Do not put the kit under or over voltage source. Be sure about the voltage either dc or ac while operating the gadget.

8. Do spare the bare ends of the components leads otherwise it may short circuit with the other components. To prevent this use sleeves at the component leads or use sleeved wire for connections.

9. Do not use old dark colour solder. It may give dry joint. Be sure that all the joints are clean and well shiny.

10. Do make loose wire connections especially with cell holder, speaker, probes etc. Put knots while connections to the circuit board, otherwise it may get loose.

GENERAL PURPOSE DUAL BIPOLAR TIMERS

NE556

SA556 - SE556

LOW TURN OFF TIME

MAXIMUM OPERATING FREQUENCY

GREATER THAN 500kHz TIMING FROM MICROSECONDS TO HOURS OPERATES IN BOTH ASTABLE ANDMONOSTABLE MODES

HIGH OUTPUT CURRENT CAN SOURCE OR SINK 200mA ADJUSTABLE DUTY CYCLE

TTL COMPATIBLE

TEMPERATURE STABILITY OF 0.005% PER oC

DESCRIPTION

The NE556 dual monolithic timing circuit is a highly stable controller capable of producing accurate time delays or oscillation. In the time delay mode of

operation, the time is precisely controlled by one external resistor and capacitor. For a stable operation as an oscillator, the free running frequency and the duty cycle are both accurately controlled with two external resistors and one capacitor. The circuit may be triggered and reset on falling waveforms, and the output structure can source or sink up to 200mA.

a) TONE BURST GENERATOR

For a tone burst generator the first timer is used as a monostable and determines the tone duration when triggered by a positive pulse at pin 6. The second timer is enabled by the high output os the monostable. It is connected as an astable and determines the frequency of the tone.

MAKING PRINTED CIRCUIT BOARD (P.C.B.)

INTRODUCTION--

Making a Printed Circuit Board is the first step towards building electronic equipment by any electronic industry. A number of methods are available for making P.C.B., the simplest method is of drawing pattern on a copper clad board with acid resistant (etchants) ink or paint or simple nail polish on a copper clad board and do the etching process for dissolving the rest of copper pattern in acid liquid.

MATERIAL REQUIRED

The apparatus needs for making a P.C.B. is :-

* Copper Clad Sheet

* Nail Polish or Paint

* Ferric Chloride Powder. (Fecl)

* Plastic Tray

* Tap Water etc.

PROCEDURE

The first and foremost in the process is to clean all dirt from copper sheet with

say spirit or trichloro ethylene to remove traces grease or oil etc. and then wash

the board under running tap water. Dry the surface with forced warm air or just

leave the board to dry naturally for some time.

Making of the P.C.B. drawing involves some preliminary consideration such as

thickness of lines/ holes according to the components. Now draw the sketch of

P.C.B. design (tracks, rows, square) as per circuit diagram with the help of nail

polish or enamel paint or any other acid resistant liquid. Dry the point surface in

open air, when it is completely dried, the marked holes in P.C.B. may be drilled

using 1Mm drill bits. In case there is any shorting of lines due to spilling of paint,

these may be removed by scraping with a blade or a knife, after the paint has

dried.

After drying, 22-30 grams of ferric chloride in 75 ml of water may be heated to

about 60 degree and poured over the P.C.B. , placed with its copper side upwards

in a plastic tray of about 15*20 cm. Stirring the solution helps speedy etching. The

dissolution of unwanted copper would take about 45 minutes. If etching takes

longer, the solution may be heated again and the process repeated. The paint on

the pattern can be removed P.C.B. may then be washed and dried. Put a coat of

varnish to retain the shine. Your P.C.B. is ready.

REACTION

Fecl3 + Cu ----- CuCl3 + Fe

Fecl3 + 3H2O --------- Fe (OH)3 + 3HCL

PRECAUTION

1. Add Ferric Chloride (Fecl3) carefully, without any splashing. Fecl3 is

irritating to the skin and will stain the clothes.

2. Place the board in solution with copper side up.

3. Try not to breathe the vapours. Stir the solution by giving see-saw motion

to the dish and solution in it.

4. Occasionally warm if the solution over a heater-not to boiling. After some

time the unshaded parts change their colour continue to etch. Gradually the base

material will become visible. Etch for two minutes more to get a neat pattern.

5. Don't throw away the remaining Fecl3 solution. It can be used again for

next Printed Circuit Board P.C.B.

USES

Printed Circuit Board are used for housing components to make a circuit for

compactness, simplicity of servicing and case of interconnection. Thus we can

define the P.C.B. as : Prinked Circuit Boards is actually a sheet of bakelite (an

insulating material) on the one side of which copper patterns are made with holes

and from another side, leads of electronic components are inserted in the proper

holes and soldered to the copper points on the back. Thus leads of electronic

components terminals are joined to make electronic circuit.

In the boards copper cladding is done by pasting thin copper foil on the boards

during curing. The copper on the board is about 2 mm thick and weights an ounce

per square foot.

The process of making a Printed Circuit for any application has the following steps

(opted professionally):

* Preparing the layout of the track.

* Transferring this layout photographically M the copper.

* Removing the copper in places which are not needed, by the process of

etching (chemical process)

* Drilling holes for components mounting.

PRINTED CIRCUIT BOARD

Printed circuit boards are used for housing components to make a circuit, for

comactness, simplicity of servicing and ease of interconnection. Single sided,

double sided and double sided with plated-through-hold (PYH) types of p.c boards

are common today.

Boards are of two types of material (1) phenolic paper based material (2) Glass

epoxy material. Both materials are available as laminate sheets with copper

cladding.

Printed circuit boards have a copper cladding on one or both sides. In both

boards, pasting thin copper foil on the board during curing does this. Boards are

prepared in sizes of 1 to 5 metre wide and upto 2 metres long. The thickness of

the boards is 1.42 to 1.8mm. The copper on the boards is about 0.2 thick and

weighs and ounce per square foot.

TO CONSTRUCT A LAMP DIMMER USING TRIAC

INTRODUCTION

In the conventional resistor type (rheostats) and inductor type (variac) regulators, the voltage regulation is achieved by taking power from different tappings. With these methods of voltage regulation, the total power consumption remains the same irrespective of the power drawn by the load. This results in a significant wastage of power.

To overcome this disadvantage, electronic regulations are used. Triacs and SCRs are commonly used in electronic regulators. Silicon controlled rectifier. (SCR) are used for DC regulation of a half wave supply. Triac regulation is most commonly used and the regulation is achieved by varying the voltage at the gate of triac. The triacs can be used both for positive and negative regulation.

The lamp dimmer describes here uses a triac and controls the light intensity from almost "zero to full brilliancy" without expensive and bulky rheostats and without wastage of power.

THEORY

POWER CONTROL USING TRIAC

The triac is semiconductor devices that can once triggered, conduct current in the forward as well as in the reverse direction. This makes them useful for AC or mains power control. The main reason of popularity of a triac is its capability to conduct and control current in both direction, like SCRm the triac also has three electrodes but they are called as main terminal No.1, main symbol and the voltage current characteristics of a triac are shown in Fig. 3(a), (b) and (c), respectively.

As is evident from Fig. 3 (C), the triac also exhibits the same forward blocking and forward conducting characteristics of an SCR; but for either polarity of the voltage applied to the main terminal. The triac can thus be called a bi-directional thyristor. Like the SCR, the break over voltage of the triac can be controlled or varied by application of a positive or negative current pulse or to the gate electrode. As the amplitude of the current pulse is increased, the break over point of the triac is decreased. The triac can be thus considered as two SCRs connected back to back in parallel as shown in Fig.4.

DIACS

A diac is a two electrode three device which behaves like a low voltage gas discharge tube. It can be switched from the OFF state to ON state for either polarity of the applied voltage. The junction diagrams, the schematic symbol at the voltage current characteristics are shown in Fig. 2.

Diacs are primarily used as triggering devices in triac phase control circuits used for light dimming, universal motor-speed control, heat control and similar applications.

THE CIRCUIT

In many situations it is desirable to control continuously the amount of power dissipated in a load. Triac can be used to control both negative and the positive hay cycles of the ac power line.

Output power and thus light intensity are varied by controlling the phase of conduction of the triac. The RC time constant provides a phase shifted ac signal to the diac.

In this project we are discussing the triac regulator as shown in Fig. 1. The triac used in this circuit is a 400V, 4A device. To control the gate voltage of atrica, a D32SS type of diac (D1) is used. This diac conducts at 32votls+/_ 5% ac or dc. The diac voltage is controlled by R1 and VR1. When about 32V is available at the junction of VR1 and C1, the diac D1 fires and this ultimately forces the triac to conduct. As the value of VR1. When about 32V is available at the junction of VR1 and C1, the diac D1 fires and this ultimately forces the triac to conduct. As the value of VR1 is changed so that the voltage applied at the gate of triac increases, the current flow also increases. Because of the increased voltage drop, the desired regulation is achieved.

PRECAUTIONS

* This circuit can be used with loads upto 400 watts without any heat sink for the triac. For power outputs between 400W to about 1KW the triac tag should be screwed to a heat sink of about 16 Sq inches.

* To protect triacs and diacs from damages, surge protection is necessary. This is achieved by avoiding switching ON the regulator at full voltage. The regulator should be switched ON at minimum voltage i.e. VR1 should be set for maximum value initially. After switchin ON at this position the required voltage should be set.

* All phase control circuits give rise to radio frequency interference (RFI) and thus suitable suppression is required. The described arrangement removes the need for a separate radio frequency interference (RFI) filter.

TRIACS DATA

ABBREVIATION

VDRM : Repetitive Peak Off-State Voltage, Open Gate

VRRM : Repetitive Peak Reverse Voltage, Open Gate

IT(RMS) : RMS On-State Current

VGT : Average Trigger Voltage DC Value

IGT : Average Trigger Current DC Value

ITSM : Surge Non-Repetitive On-State Current

COMPONENTS USED

SEMICONDUCTOR :-

1. TRIAC...............................ST044A (400V)

2. DIAC.................................5532D

RESISTORS :-

1. R1.....................................100

2. R2.....................................3.9 K

3. R3.....................................470 K

4. VR1..................................600 K (on/off-linear potmeter)

CAPACITOR :-

1. C1 0.1 or 0.2F (Ceramic / Polyester)

Fig.2: (a) Junction Diagram (b) Schematic Symbol

Fig.3: (a) Junction Diagram (b) Schematic symbol

(c) and Voltage current characteristics for a Triac.

Fig.4 : Equivalent Circuit for a Triac

POWER SUPPLY

In alternating current the electron flow is alternate, i.e. the electron flow increases to maximum in one direction, decreases back to zero. It then increases in the other direction and then decreases to zero again. Direct current flows in one direction only. Rectifier converts alternating current to flow in one direction only. When the anode of the diode is positive with respect to its cathode, it is forward biased, allowing current to flow. But when its anode is negative with respect to the cathode, it is reverse biased and does not allow current to flow. This unidirectional property of the diode is useful for rectification. A single diode arranged back-to-back might allow the electrons to flow during positive half cycles only and suppress the negative half cycles. Double diodes arranged back-to-back might act as full wave rectifiers as they may allow the electron flow during both positive and negative half cycles. Four diodes can be arranged to make a full wave bridge rectifier. Different types of filter circuits are used to smooth out the pulsations in amplitude of the output voltage from a rectifier. The property of capacitor to oppose any change in the voltage applied across them by storing energy in the electric field of the capacitor and of inductors to oppose any change in the current flowing through them by storing energy in the magnetic field of coil may be utilized. To remove pulsation of the direct current obtained from the rectifier, different types of combination of capacitor, inductors and resistors may be also be used to increase to action of filtering.

NEED OF POWER SUPPLYPerhaps all of you are aware that a ‘power supply’ is a primary requirement for the ‘Test Bench’ of a home experimenter’s mini lab. A battery eliminator can eliminate or replace the batteries of solid-state electronic equipment and the equipment thus can be operated by 230v A.C. mains instead of the batteries or dry cells. Nowadays, the use of commercial battery eliminator or power supply unit has become increasingly popular as power source for household appliances like transreceivers, record player, cassette players, digital clock etc.

THEORY

U SE OF DIODES IN RECTIFIERS:

Electric energy is available in homes and industries in India, in the form of alternating voltage. The supply has a voltage of 220V (rms) at a frequency of 50 Hz. In the USA, it is 110V at 60 Hz. For the operation of most of the devices in electronic equipment, a dc voltage is needed. For instance, a transistor radio requires a dc supply for its operation. Usually, this supply is provided by dry cells. But sometime we use a battery eliminator in place of dry cells. The battery eliminator converts the ac voltage into dc voltage and thus eliminates the need for dry cells. Nowadays, almost all-electronic equipment includes a circuit that converts ac voltage of mains supply into dc voltage. This part of the equipment is called Power Supply. In general, at the input of the power supply, there is a power transformer. It is followed by a diode circuit called Rectifier. The output of the rectifier goes to a smoothing filter, and then to a voltage regulator circuit. The rectifier circuit is the heart of a power supply.

RECTIFICATIONRectification is a process of rendering an alternating current or voltage into a unidirectional one. The component used for rectification is called ‘Rectifier’. A rectifier permits current to flow only during the positive half cycles of the applied AC voltage by eliminating the negative half cycles or alternations of the applied AC voltage. Thus pulsating DC is obtained. To obtain smooth DC power, additional filter circuits are required.

A diode can be used as rectifier. There are various types of diodes. But, semiconductor diodes are very popularly used as rectifiers. A semiconductor diode is a solid-state device consisting of two elements is being an electron emitter or cathode, the other an electron collector or anode. Since electrons in a semiconductor diode can flow in one direction only-from emitter to collector- the diode provides the unilateral conduction necessary for rectification. Out of the semiconductor diodes, copper oxide and selenium rectifier are also commonly used.

FULL WAVE RECTIFIER

It is possible to rectify both alternations of the input voltage by using two diodes in the circuit arrangement. Assume 6.3 V rms (18 V p-p) is applied to the circuit. Assume further that two equal-valued series-connected resistors R are placed in parallel with the ac source. The 18 V p-p appears across the two resistors connected between points AC and CB, and point C is the electrical midpoint between A and B. Hence 9 V p-p appears across each resistor. At any moment during a cycle of vin, if point A is positive relative to C, point B is negative relative

to C. When A is negative to C, point B is positive relative to C. The effective voltage in proper time phase which each diode "sees" is in Fig. The voltage applied to the anode of each diode is equal but opposite in polarity at any given instant.

When A is positive relative to C, the anode of D1 is positive with respect to its

cathode. Hence D1 will conduct but D2 will not. During the second alternation, B

is positive relative to C. The anode of D2 is therefore positive with respect to its

cathode, and D2 conducts while D1 is cut off.

There is conduction then by either D1 or D2 during the entire input-voltage cycle.

Since the two diodes have a common-cathode load resistor RL, the output voltage

across RL will result from the alternate conduction of D1 and D2. The output

waveform vout across RL, therefore has no gaps as in the case of the half-wave

rectifier.

The output of a full-wave rectifier is also pulsating direct current. In the diagram, the two equal resistors R across the input voltage are necessary to provide a voltage midpoint C for circuit connection and zero reference. Note that the load resistor RL is connected from the cathodes to this center reference point C.

An interesting fact about the output waveform vout is that its peak amplitude is

not 9 V as in the case of the half-wave rectifier using the same power source, but is less than 4½ V. The reason, of course, is that the peak positive voltage of A relative to C is 4½ V, not 9 V, and part of the 4½ V is lost across R.

Though the full wave rectifier fills in the conduction gaps, it delivers less than half the peak output voltage that results from half-wave rectification.

BRIDGE RECTIFIERA more widely used full-wave rectifier circuit is the bridge rectifier. It requires four diodes instead of two, but avoids the need for a centre-tapped transformer. During the positive half-cycle of the secondary voltage, diodes D2 and D4 are conducting and diodes D1 and D3 are non-conducting. Therefore, current flows through the secondary winding, diode D2, load resistor RL and diode D4. During negative half-cycles of the secondary voltage, diodes D1 and D3 conduct, and the diodes D2 and D4 do not conduct. The current therefore flows through the secondary winding, diode D1, load resistor RL and diode D3. In both cases, the current passes through the load resistor in the same direction. Therefore, a fluctuating, unidirectional voltage is developed across the load.

Filtration

The rectifier circuits we have discussed above deliver an output voltage that always has the same polarity: but however, this output is not suitable as DC power supply for solid-state circuits. This is due to the pulsation or ripples of the output voltage. This should be removed out before the output voltage can be supplied to any circuit. This smoothing is done by incorporating filter networks. The filter network consists of inductors and capacitors. The inductors or choke coils are generally connected in series with the rectifier output and the load. The inductors oppose any change in the magnitude of a current flowing through them by storing up energy in a magnetic field. An inductor offers very low resistance for DC whereas; it offers very high resistance to AC. Thus, a series connected choke coil in a rectifier circuit helps to reduce the pulsations or ripples to a great extent in the output voltage. The fitter capacitors are usually connected in parallel with

the rectifier output and the load. As, AC can pass through a capacitor but DC cannot, the ripples are thus limited and the output becomes smoothed. When the voltage across its plates tends to rise, it stores up energy back into voltage and current. Thus, the fluctuations in the output voltage are reduced considerable. Filter network circuits may be of two types in general:

CHOKE INPUT FILTERIf a choke coil or an inductor is used as the ‘first- components’ in the filter network, the filter is called ‘choke input filter’. The D.C. along with AC pulsation from the rectifier circuit at first passes through the choke (L). It opposes the AC pulsations but allows the DC to pass through it freely. Thus AC pulsations are largely reduced. The further ripples are by passed through the parallel capacitor C. But, however, a little nipple remains unaffected, which are considered negligible. This little ripple may be reduced by incorporating a series a choke input filters.

CAPACITOR INPUT FILTERIf a capacitor is placed before the inductors of a choke-input filter network, the filter is called capacitor input filter. The D.C. along with AC ripples from the rectifier circuit starts charging the capacitor C. to about peak value. The AC ripples are then diminished slightly. Now the capacitor C, discharges through the inductor or choke coil, which opposes the AC ripples, except the DC. The second capacitor C by passes the further AC ripples. A small ripple is still present in the output of DC, which may be reduced by adding additional filter network in series.

Pulse Width Modulation (PWM) Basics There are many forms of modulation used for communicating information. When a high frequency signal has amplitude varied in response to a lower frequency signal we have AM (amplitude modulation). When the signal frequency is varied in response to the modulating signal we have FM (frequency modulation. These signals are used for radio modulation because the high frequency carrier signal is needs for efficient radiation of the signal. When communication by pulses was introduced, the amplitude, frequency and pulse width become possible modulation options. In many power electronic converters where the output voltage can be one of two values the only option is modulation of average conduction time.

Fig. 1: Unmodulated, sine modulated pulses

B. 1. Linear ModulationThe simplest modulation to interpret is where the average ON time of the pulses varies proportionally with the modulating signal. The advantage of linear processing for this application lies in the ease of de-modulation. The modulating signal can be recovered from the PWM by low pass filtering. For a single low frequency sine wave as modulating signal modulating the width of a fixed frequency (fs) pulse train the spectra is as shown in Fig 2. Clearly a low pass filter can extract the modulating component fm.

Fig. 2: Spectra of PWM

C. 2. Sawtooth PWMThe simplest analog form of generating fixed frequency PWM is by comparison with a linear slope waveform such as a sawtooth. As seen in Fig 2 the output signal goes high when the sine wave is higher than the sawtooth. This is implemented using a comparitor whose outputvoltage goes to logic HIGH when ne input is greater than the other.

Fig. 3: Sine Sawtooth PWMOther signals with straight edges can be used for modulation a rising ramp carrier will generate PWM with Trailing Edge Modulation.

Fig. 4: Trailing Edge Modulation

It is easier to have an integrator with a reset to generate the ramp in Fig 4 but the modulation is inferior to double edge modulation.

D. 3. Regular Sampled PWMThe scheme illustrated above generates a switching edge at the instant of crossing of the sine wave and the triangle. This is an easy scheme to implement using analog electronics but suffers the imprecision and drift of all analog computation as well as having difficulties of generating multiple edges when the signal has even a small added noise. Many modulators are now implemented digitally but there is difficulty is computing the precise intercept of the modulating wave and the carrier. Regular sampled PWM makes the width of the pulse proportional to the value of the modulating signal at the beginning of the carrier period. In Fig 5 the intercept of the sample values with the traingle determine the edges of the Pulses. For a sawtooth wave of frequency fs the samples are at 2fs.

Fig. 5: Regular Sampled PWM There are many ways to generate a Pulse Width Modulated signal other than fixed frquency sine sawtooth. For three phase systems the modulation of a Voltage Source Inverter can generate a PWM signal for each phase leg by comparison of the desired output voltage waveform for each phase with the same sawtooth. One alternative which is easier to implement in a computer and gives a larger MODULATION DEPTH is using SPACE VECTOR MODULATION.

1. 4. Modulation DepthFor a single phase inverter modulated by a sine-sawtooth comparison, if we compare a sine wave of magnitude from -2 to +2 with a traingle from -1 to +1 the linear relation between the input signal and the average output signal will be lost.

Once the sine wave reaches the peak of the transgle the pulses will be of maximum width and the modulation will then saturate. The Modulation depth is the ratio of the current signal to the case when saturation is just starting. Thus sine wave of peak 1.2 compared with a triangle with peak 2.0 will have a modulation depth of m=0.6.

Fig. 6: Saturated Pulse Width Modulation

III. PULSE-WIDTH MODULATION

Pulse-width modulation of a signal or power source involves the modulation of its duty cycle, to either convey information over a communications channel or control the amount of power sent to a load.

A. Principle

Fig. 1: a square wave, showing the definitions of ymin, ymax and D.

Pulse-width modulation uses a square wave whose duty cycle is modulated resulting in the variation of the average value of the waveform. If we consider a

square waveform f(t) with a low value ymin, a high value ymax and a duty cycle D (see figure 1), the average value of the waveform is given by:

As f(t) is a square wave, its value is ymax for and ymin for . The above expression then becomes:

This latter expression can be fairly simplified in many cases where ymin = 0 as . From this, it is obvious that the average value of the signal ( ) is

directly dependent on the duty cycle D.

B. Technique 1. Generation

a) Intersective

Fig. 2: A simple method to generate the PWM pulse train corresponding to a given signal is the intersective PWM: the signal (here the green sinewave) is compared with a sawtooth waveform (blue). When the latter is less than the former, the PWM signal (magenta) is in high state (1). Otherwise it is in the low state (0).

The simplest way to generate a PWM signal is the intersective method, which requires only a sawtooth or a triangle waveform (easily generated using a simple

oscillator) and a comparator. When the value of the reference signal (the green sine wave in figure 2) is more than the modulation waveform (blue), the PWM signal (magenta) is in the high state, otherwise it is in the low state.

b) DeltaMain article: Delta modulation

The output signal is compared with limits, which correspond to a reference signal offset by a constant. Every time the output signal reaches one of the limits, the PWM signal changes state.

Fig. 3 : Principle of the delta PWM. The output signal (blue) is compared with the limits (gn). These limits correspond to the reference signal (red), offset by a given value. Every time the output signal reaches one of the limits, the PWM signal changes state.

c) Sigma-DeltaMain article: Delta-sigma modulation

The output signal is subtracted from a reference signal to form an error signal. This error is integrated, and when the integral of the error exceeds the limits, the output changes state.

Fig. 4 : Principle of the sigma-delta PWM. The top green waveform is the reference signal, on which the output signal (PWM, in the middle plot) is subtracted to form the error signal (blue, in top plot). This error is integrated (bottom plot), and when the integral of the error exceeds the limits (red lines), the output changes state.

d) DigitalMany digital circuits can generate PWM signals (e.g many microcontrollers have PWM outputs to control an electrical motor). They usually use a counter that increments periodically (it is connected directly or indirectly to the clock of the circuit) and is reset at the end of every period of the PWM. When the counter value is more than the reference value, the PWM output changes state from high to low.

2. Types

Fig. 5 : Three types of PWM signals (blue): leading edge modulation (top), trailing edge modulation (middle) and centered pulses (both edges are modulated, bottom). The green lines are the sawtooth signals used to generate the PWM waveforms using the intersective method.

Three types of pulse-width modulation (PWM) are possible.

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.

3.4.5. Spectrum

The resulting spectra (of the three cases) are similar, and each contains a dc component, a base sideband containing the modulating signal and phase modulated carriers at each harmonic of the frequency of the pulse. The amplitudes of the harmonic groups are restricted by a sinx / x envelope (sinc function) and extend to infinity.

C. Applications1. Telecommunications

In telecommunications, the widths of the pulses correspond to specific data values encoded at one end and decoded at the other.Pulses of various lengths (the information itself) will be sent at regular intervals (the carrier frequency of the modulation). _ _ _ _ _ _ _ _ | | | | | | | | | | | | | | | | Clock | | | | | | | | | | | | | | | | __| |____| |____| |____| |____| |____| |____| |____| |____

_ __ ____ ____ _Data | | | | | | | | | | | | | | | | | | | | _________| |____| |___| |________| |_| |___________

Data 0 1 2 4 0 4 1 0The inclusion of a clock signal is not necessary, as the leading edge of the data signal can be used as the clock if a small offset is added to the data value in order to avoid a data value with a zero length pulse.

2. Power deliveryPWM can be used to reduce the total amount of power delivered to a load without losses normally incurred when a power source is limited by resistive means. This is because the average power delivered is proportional to the modulation duty cycle. With a sufficiently high modulation rate, passive electronic filters can be used to smooth the pulse train and recover an average analog waveform.High frequency PWM power control systems are easily realisable with semiconductor switches. The discrete on/off states of the modulation are used to control the state of the switch(es) which correspondingly control the voltage across or current through the load. The major advantage of this system is the switches are either off and not conducting any current, or on and have (ideally) no voltage drop across them. The product of the current and the voltage at any given time defines the power dissipated by the switch, thus (ideally) no power is dissipated by the switch. Reallistically, semiconductor switches such as MOSFETs or BJTs are non-ideal switches, but high efficiency controllers can still be built.PWM is also often used to control the supply of electrical power to another device such as in speed control of electric motors, volume control of Class D audio

amplifiers or brightness control of light sources and many other power electronics applications. For example, light dimmers for home use employ a specific type of PWM control. Home use light dimmers typically include electronic circuitry which suppresses current flow during defined portions of each cycle of the AC line voltage. Adjusting the brightness of light emitted by a light source is then merely a matter of setting at what voltage (or phase) in the AC cycle the dimmer begins to provide electrical current to the light source (e.g. by using an electronic switch such as a triac). In this case the PWM duty cycle is defined by the frequency of the AC line voltage (50 Hz or 60 Hz depending on the country). These rather simple types of dimmers can be effectively used with inert (or relatively slow reacting) light sources such as incandescent lamps, for example, for which the additional modulation in supplied electrical energy which is caused by the dimmer causes only negligible additional fluctuations in the emitted light. Some other types of light sources such as light-emitting diodes (LEDs), however, turn on and off extremely rapidly and would perceivably flicker if supplied with low frequency drive voltages. Perceivable flicker effects from such rapid response light sources can be reduced by increasing the PWM frequency. If the light fluctuations are sufficiently rapid, the human visual system can no longer resolve them and the eye perceives the time average intensity without flicker (see flicker fusion threshold).

3. Voltage regulationMain article: Switched-mode power supply

PWM is also used in efficient voltage regulators. By switching voltage to the load with the appropriate duty cycle, the output will approximate a voltage at the desired level. The switching noise is usually filtered with an inductor and a capacitor.One method measures the output voltage. When it is lower than the desired voltage, it turns on the switch. When the output voltage is above the desired voltage, it turns off the switch.

4. Audio effects and amplificationPWM is sometimes used in sound synthesis, in particular subtractive synthesis, as it gives a nice effect similar to chorus or slightly detuned oscillators played together. (In fact, PWM is equivalent to the difference of two sawtooth waves. [1]) The ratio between the high and low level is typically modulated with a low frequency oscillator, or LFO.A new class of audio amplifiers based on the PWM principle is becoming popular. Called "Class-D amplifiers", these amplifiers produce a PWM equivalent of the

analogue input signal which is fed to the loudspeaker via a suitable filter network to block the carrier and recover the original audio. These amplifiers are characterized by very good efficiency figures (≥ 90%) and compact size/light weight for large power outputs.Historically, a crude form of PWM has been used to play back PCM digital sound on the PC speaker, which is only capable of outputting two sound levels. By carefully timing the duration of the pulses, and by relying on the speaker's physical filtering properties (limited frequency response, self-inductance, etc.) it was possible to obtain an approximate playback of mono PCM samples, although at a very low quality, and with wildly varying results between implementations.

PWM-BASED SPEED CONTROL

CIRCUIT DESCRIPTION

There are several methods for controlling the speed of DC motors. One simple method is to add series resistance using a rheostat. As considerable power is consumed in the rheostat, this method is not economical. Another method is to use a series switch that can be closed/ opened rapidly. This type of control is termed as chopper control. Here a PWM based chopper circuit that smoothly controls the speed of general-purpose dc motors.

Fig. 1 shows the block diagram of a basic PWM-based chopper. The circuit shown in fig. 2 is designed as per this diagram. A dual timer IC (NE556) is used to configure both the astable as well as the monostable multivibrator. Timing components for the astable are chosen to provide a frequency of 546 Hz, while the monostable components are selected to obtain a maximum pulsewidth of 2.42 ms. Diode D1 improves duty factor of the astable oscillator output, whereas D2 acts as a free wheeling diode. Transistor SL100 drives the motor, while the 22-ohm, 2W resistor (R4) serves as a current limiter, avoiding overheating of the transistor. The DPDT switch enables direction reversal of the motor, as desired.

Adjusting VR1, which changes the threshold value to which capacitor C1 in the monostable circuit is charged, can vary the speed. This, in turn, determines its output pulsewidth and hence the average voltage applied to the motor. Waveforms shown in Fig. 3 depict the average voltage for controlling various speeds.

For effective speed control, ‘on’ period (TON) of the astable should be equal to the maximum pulsewidth (TON) of the monostable.

For higher voltage and power requirements, SL100 can be replaced by an appropriate MOSFET or IGBT with relevant changes in the drive circuitry.

Fig.1 : Block Diagram of PWM-based speed controller

Fig. 2 : The circuit of PWM-Based Speed Controller

Fig. 3 : waveforms at different conditions of VR1