## speed control of a single phase induction motor

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“SPEED CONTROL OF A SINGLE PHASE INDUCTION MOTOR”

Introduction:

An induction motor or a synchronous motor is a type of alternating current motor where power is supplied to the rotor by means of electromagnetic induction. An electric motor turns because of magnetic force exerted between a stationary electromagnet called the stator and a rotating electromagnet called the rotor. In an induction motor, by contrast, the current is induced in the rotor without contacts by the magnetic field of the stator, through electromagnetic induction.

The speed of an induction motor is dependent upon its terminal voltage and operating frequency. The operating frequency of an induction motor is varied by using PWM. In this project the output frequency is varied by firing thyristor. If the firing sequence of thyristor is controlled, then we can get various frequencies.

Our project work (device) is a direct-frequency changer that converts AC power at one frequency to AC power at another frequency by AC-AC conversion without an intermediate conversion link.

We know that the speed of an induction motor is dependent upon the voltage and frequency. If voltage and frequency is changed then the speed of induction motor is changed.

In the project work the voltage and frequency is changed and controllable then the speed of an induction motor is controlled.

Here in chapter one discuss about the theoretical background and circuit components.

Here in chapter two, discuss about some component which is needed to form this circuit.

In chapter three, discuss about power supply unit. This is needed for many IC’s. Here a 12 volts power supply is used.

In chapter four, discuss about the IC’s used in the circuit.

In chapter five discuss about circuit diagram, circuit operation and design fabrications.

Objectives of the present project work:

1) To introduce some commonly used electronic components.

2) Generation of firing pulse.

3) Speed control of an induction motor by using Voltage and Frequency.

4) To improve the performance.

5) To lower the costs of drive.

Some view of project is given below:

Fig: 1.1 some view of project.

Now it is our first target to control the firing angle so that we can control the load voltage. For firing angle –? input and required load voltage waveform are shown in figure 1.2

Applications:

A motor controller is a device or group of devices that serves to govern in some predetermined manner the performance of an electric motor. In the recent years, many mills and factory are using this device, some are given below:

1. In a sewage lift station sewage usually flows through sewer pipes under the force of gravity to a wet well location.

2. Airflow can be regulated by using a damper to restrict the flow, but it is more efficient to regulate the airflow by regulating the speed of the motor.

3. This Device is used to cut off central air conditioning (heating or cooling) to an unused room, or to regulate it for room-by-room temperature and climate control.

4. On a wood burning stove or similar device, it is usually a handle on the vent duct as in an air conditioning system.

5. Ship propulsion drives.

6. Cement mill drives.

7. Rolling mill drives.

8. Paper machines.

9. Conveyer belt.

10. Water plant.

THEORITICAL BACKGROUND

An induction motor or a synchronous motor is a type of alternating current motor where power is supplied to the rotor by means of electromagnetic induction. An electric motor turns because of magnetic force exerted between a stationary electromagnet called the stator and a rotating electromagnet called the rotor. Different types of electric motors are distinguished by how electric current is supplied to the moving rotor. In a DC motor and a slip-ring AC motor, current is provided to the rotor directly through sliding electrical contacts called commentators and slip rings. In an induction motor, by contrast, the current is induced in the rotor without contacts by the magnetic field of the stator, through electromagnetic induction.

An induction motor is sometimes called a rotating transformer because the stator (stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is the secondary side. Unlike the normal transformer which changes the current by using time varying flux, induction motors use rotating magnetic fields to transform the voltage. The current in the primary side creates an electromagnetic field which interacts with the electromagnetic field of the secondary side to produce a resultant torque, thereby transforming the electrical energy into mechanical energy. Induction motors are widely used, especially polyphase induction motors, which are frequently used in industrial drives.

Fig: 2.2 Torque-speed curve of single phase induction motor.

Induction motors are now the preferred choice for industrial motors due to their rugged construction, absence of brushes and—thanks to modern power electronics—the ability to control the speed of the motor.

Principle of operation and comparison to synchronous motors:

The basic difference between an induction motor and a synchronous AC motor with a permanent magnet rotor is that in the latter the rotating magnetic field of the stator will impose an electromagnetic torque on the magnetic field of the rotor causing it to move (about a shaft) and a steady rotation of the rotor is produced. It is called synchronous because at steady state the speed of the rotor is the same as the speed of the rotating magnetic field in the stator. By contrast, the induction motor does not have any permanent magnets on the rotor; instead, a current is induced in the rotor. To achieve this, stator windings are arranged around the rotor so that when energized with a polyphase supply they create a rotating magnetic field pattern which sweeps past the rotor. This changing magnetic field pattern induces current in the rotor conductors. This current interacts with the rotating magnetic field created by the stator and in effect causes a rotational motion on the rotor.

However, for these currents to be induced the speed of the physical rotor must be less than the speed of the rotating magnetic field in the stator (the synchronous frequency ns) or else the magnetic field will not be moving relative to the rotor conductors and no currents will be induced. If by some chance this happens, the rotor typically slows slightly until a current is re-induced and then the rotor continues as before. This difference between the speed of the rotor and speed of the rotating magnetic field in the stator is called slip. It is unit less and is the ratio between the relative speeds of the magnetic field as seen by the rotor (the slip speed) to the speed of the rotating stator field. Due to this, an induction motor is sometimes referred to as an asynchronous machine.

### Single phase:

In a single phase induction motor, it is necessary to provide a starting circuit to start rotation of the rotor. If this is not done, rotation may be commenced by manually giving a slight turn to the rotor. The single phase induction motor may rotate in either direction and it is only the starting circuit which determines rotational direction.

For small motors of a few watts, the start rotation is done by means of one or two single turn(s) of heavy copper wire around one corner of the pole. The current induced in the single turn is out of phase with the supply current and so causes an out-of-phase component in the magnetic field, which imparts to the field sufficient rotational character to start the motor. These poles are known as shaded poles. Starting torque is very low and efficiency is also reduced. Such shaded-pole motors are typically used in low-power applications with low or zero starting torque requirements, such as desk fans and record players.

Larger motors are provided with a second stator winding which is fed with an out-of-phase current to create a rotating magnetic field. The out-of-phase current may be derived by feeding the winding through a capacitor or it may derive from the winding having different values of inductance and resistance from the main winding.

In some designs, the second winding is disconnected once the motor is up to speed, usually either by means of a switch operated by centrifugal force acting on weights on the motor shaft or by a positive temperature coefficient thyristors which, after a few seconds of operation, heats up and increases its resistance to a high value thereby reducing the current through the second winding to an insignificant level. Other designs keep the second winding continuously energized when running, which improves torque.

Torque curve 4 different a synchronous electric motors:

A) Single-phase motor.

B) A single multi-phase motors squirrel cage.

C) A single multi-phase motors squirrel cage bar deep.

D) Multi-phase motors with double squirrel cage.

Motor Formula:

Calculating Motor Speed:

A squirrel cage induction motor is a constant speed device. It cannot operate for any length of time at speeds below those shown on the nameplate without danger of burning out.

To calculate the speed of an induction motor, apply this formula:

Spry = 120 x F

P

Spry = synchronous revolutions per minute.

120 = constant

F = supply frequency (in cycles/sec)

P = number of motor winding poles

Example: What is the synchronous of a motor having 4 poles connected to a 60 hz power supply?

Srpm = 120 x F

P

Srpm = 120 x 60

4

Srpm = 7200

4

Srpm = 1800 rpm

Calculating Horse power:

Electrical power is rated in horsepower or watts. A horsepower is a unit of power equal to 746 watts or 33,0000 lb-ft per minute (550 lb-ft per second). A watt is a unit of measure equal to the power produced by a current of 1 amp across the potential difference of 1 volt. It is 1/746 of 1 horsepower. The watt is the base unit of electrical power. Motor power is rated in horsepower and watts. Horsepower is used to measure the energy produced by an electric motor while doing work.

To calculate the horsepower of a motor when current and efficiency, and voltage are known, apply this formula:

HP = V x I x Eff

746

HP = horsepower

V = voltage

I = current (amps)

Eff. = efficiency

Example: What is the horsepower of a 230v motor pulling 4 amps and having 82% efficiency?

HP = V x I x Eff

746

HP = 230 x 4 x .82

746

HP = 754.4

746

HP = 1 Hp

Eff = efficiency / HP = horsepower / V = volts / A = amps / PF = power factor

 Horse power Formulas To Find Use Formula Example Given Find Solution HP HP = I X E X Eff. 746 240V, 20A, 85% Eff. HP HP = 240V x 20A x 85% 746 HP=5.5 I I = HP x 746 E X Eff x PF 10HP, 240V, 90% Eff., 88% PF I I = 10HP x 746 240V x 90% x 88% I = 39 A

To calculate the horse power of a motor when the speed and torque are known, apply this formula:

HP = rpm x T(torque)

5252(constant)

Example: What is the horse power of a 1725 rpm motor with a FLT 3.1 lb-ft?

HP = rpm x T

5252

HP = 1725 x 3.1

5252

HP = 5347.5

5252

HP = 1 hp

2.2 Resistance:

Resistance restricts the flow of electric current, for example a resistor is placed in series with a light-emitting diode (LED) to limit the current passing through the LED.

Fig: 2.3 Symbol of a Resistance

The flow of charge though any material encounters an opposing force similar in many respects to mechanical friction. This opposition, due to the collisions between electrons and other atoms in the material, which converts electrical energy into another form of energy such as heat, is called the resistance of the material. The unit of measurement of resistance is the ohm, for which the symbol is ?, the capital Greek letter omega.

The resistance of any material with a uniform cross-sectional area is determined by the following four factors:

1. Material
2. Length
3. Cross-sectional area
4. Temperature

The chosen material, with its unique molecular structure, will react differentially to pressures to establish current through its core. Conductors that permit a generous flow of charge with little external pressure will have low resistance levels, while insulators will have high resistance characteristics. Resistance is directly proportional to length and inversely proportional to area.

As the temperature of most conductors increases, the increased motion of the particles within the molecular structure makes it increasingly difficult for the “free” carriers to pass though, and the resistance level increases.

At a fixed temperature of 20®C (room temperature), the resistance is related to the other three factors by

R=

Where,

R=Resistance of the conductor

=Conductance

=Length of the conductor

A=Area of the conductor

Resistance values are normally shown in color bands. Each color represents a number as in the table.

Most resistance has 4 bands:

Ø The first band gives the first digit.

Ø The second band gives the second digit.

Ø The third band indicates the number of zeros.

Ø The fourth band is used to shows the tolerance (precision) of the resistance.

Capacitance:

A capacitance is a device that stores energy in the electric field created between a pair of conductors on which electric charges of equal magnitude, but opposite sign, have been placed. A capacitor is occasionally referred to using the older term condenser.

Fig: 2.4 Symbol of a Capacitance

Function: Capacitance store electric charge. They are used with resistors in timing circuits because it takes time for a capacitor to fill with charge. They are used to smooth varying dc supplies by acting as a reservoir of charge. They are also used in filter circuits because capacitors easily pass AC (changing) signals but they block DC (constant) signals.

This is a measure of a capacitances ability to store charge. A large means that more charge can be stored. Capacitance is measured in farads, Symbol F.

Types of Capacitance: Like resistances, all capacitances can be included under either of two general headings: fixed or variable. The curved line represents the plate that is usually connected to the point of lower potential.

Fixed Capacitance: Many types of fixed capacitance are available. Some of the most common are the mica, ceramic, electrolytic, tantalum, and polyester-film capacitors. The typical flat mica capacitor consists basically of mica sheets separated by sheets of metal foil. The plates are connected to two electrodes. The total area is the area of one sheet times the number of dielectric sheets. The entire system is encased in a plastic insulating material for the two central units. The mica capacitor exhibits excellent characteristics under stress of temperature variations and high voltage applications. Its leakage current is also very small. Mica capacitors are typically between a few microfarads and 0.2µF with voltage of 100V or more.

The electrolytic capacitor is used most commonly in situation where capacitances of the order of one to several thousand microfarads are required. They are designed primarily for use in networks where only dc voltage will be applied across the capacitor because they have good insulating characteristics between the plates in one direction but take on the characteristics of a conductor in the other direction. Electrolytic capacitors are available that can be used in ac circuit and in case where the polarity of the dc voltage will reverse across the capacitor for short period of time.

Variable Capacitance: The dielectric for each capacitance is air. The capacitance is changed by turning the shaft at one end to vary the common area of the movable and fixed plates. The greater the common area the larger the capacitance as determined by equation. The capacitance of the trimmer capacitor is changed by turning the screw, which will vary the distance between the plates and thereby the capacitance.

A digital reading capacitance meter appears. Simply place the capacitor between the provided clips with the proper polarity and the meter will display the level of capacitance. The best check of a capacitor is to use a meter designed to perform the necessary tests.

Capacitance in series and parallel:

Capacitances, like resistances, can be placed in series and in parallel. Increasing levels of capacitance can be obtained by placing capacitors in parallel, while decreasing levels can be obtained by placing capacitors in series.

Energy Stored by a Capacitance:

The ideal capacitance does not dissipate any of the energy supplied to it. It stores the energy in the form of an electric field between the conducing surfaces. A plot of the voltage, current, and power to a capacitor during the charging phase. The power curve can be obtained by finding the product of the voltage and current at selected instants of time and connecting the points obtained. The energy stored is represented by the shaded area under the power curve.

Diode:

A diode is a two-terminal electronic component that conducts electric current in only one direction The most common function of a diode is to allow an electric current to pass in one direction (called the diode’s forward direction), while blocking current in the opposite direction (the reverse direction).

Fig: 2.5 Symbol of Diode

However, diodes can have more complicated behavior than this simple on-off action. This is due to their complex non-linear electrical characteristics, which can be tailored by varying the construction of their P-N junction. These are exploited in special purpose diodes that perform many different functions. For example, specialized diodes are used to regulate

Voltage (Zener diodes), to electronically tune radio and TV receivers (varactor diodes), to generate radio frequency oscillations (tunnel diodes), and to produce light (light emitting diodes). Tunnel diodes exhibit negative resistance, which makes them useful in some types of circuits.

A modern semiconductor diode is made of a crystal of semiconductor like silicon that has impurities added to it to create a region on one side that contains negative charge carriers (electrons), called n-type semiconductor, and a region on the other side that contains positive charge carriers (holes), called p-type semiconductor. The diode’s terminals are attached to each of these regions. The boundary within the crystal between these two regions, called a PN junction, is where the action of the diode takes place. The crystal conducts a current of electrons in a direction from the N-type side (called the cathode) to the P-type side (called the anode), but not in the opposite direction; that is, a conventional current flows from anode to cathode (opposite to the electron flow, since electrons have negative charge).

Another type of semiconductor diode, the Schottky diode, is formed from the contact between a metal and a semiconductor rather than by a p-n junction.

Current–voltage characteristic:

If an external voltage is placed across the diode with the same polarity as the built-in potential, the depletion zone continues to act as an insulator, preventing any significant electric current flow (unless electron/hole pairs are actively being created in the junction by, for instance, light. see photodiode). This is the reverse bias phenomenon.

However, if the polarity of the external voltage opposes the built-in potential, recombination can once again proceed, resulting in substantial electric current through the p-n junction (i.e. substantial numbers of electrons and holes recombine at the junction). For silicon diodes, the built-in potential is approximately 0.7 V (0.3 V for Germanium and 0.2 V for Scottky). Thus, if an external current is passed through the diode, about 0.7 V will be developed across the diode such that the P-doped region is positive with respect to the N-doped region and the diode is said to be “turned on” as it has a forward bias.

At very large reverse bias, beyond the peak inverse voltage or PIV, a process called reverse breakdown occurs which causes a large increase in current (i.e. a large number of electrons and holes are created at, and move away from the pn junction) that usually damages the device permanently.

The current-voltage characteristic of a diode is shown below:

Fig: 2.6 Current–voltage characteristic of diode

2.5 Transistor:

A transistor can control its output in proportion to the input signal; that is, it can act as an amplifier. Alternatively, the transistor can be used to turn current on or off in a circuit as an electrically controlled switch, where the amount of current is determined by other circuit elements. The essential usefulness of a transistor comes from its ability to use a small signal applied between one pair of its terminals to control a much larger signal at another pair of terminals. This property is called gain.

Fig: 2.7 Symbol of a transistor

The two types of transistors have slight differences in how they are used in a circuit. A bipolar transistor has terminals labeled base, collector, and emitter. A small current at the base terminal (that is, flowing from the base to the emitter) can control or switch a much larger current between the collector and emitter terminals. For a field-effect transistor, the terminals are labeled gate, source, and drain, and a voltage at the gate can control a current between source and drain.

The image to the right represents a typical bipolar transistor in a circuit. Charge will flow between emitter and collector terminals depending on the current in the base. Since internally the base and emitter connections behave like a semiconductor diode, a voltage drop develops between base and emitter while the base current exists. The amount of this voltage depends on the material the transistor is made from, and is referred to as VBE.

Transistor as a switch

Transistors are commonly used as electronic switches, both for high-power applications such as switched-mode power supplies and for low-power applications such as logic gates.

In a grounded-emitter transistor circuit, such as the light-switch circuit shown, as the base voltage raises the base and collector current rise exponentially, and the collector voltage drops because of the collector load resistor. The relevant equations:

VRC = ICE × RC, the voltage across the load (the lamp with resistance RC)

VRC + VCE = VCC, the supply voltage shown as 6V

If VCE could fall to 0 (perfect closed switch) then Ic could go no higher than VCC / RC, even with higher base voltage and current. The transistor is then said to be saturated. Hence, values of input voltage can be chosen such that the output is either completely off, or completely on. The transistor is acting as a switch, and this type of operation is common in digital circuits where only “on” and “off” values are relevant.

The common-emitter amplifier is designed so that a small change in voltage in (Vin) changes the small current through the base of the transistor and the transistor’s current amplification combined with the properties of the circuit mean that small swings in Vin produce large changes in Vout.

Amplifier circuit, common-emitter configuration.

Operational amplifier:

An operational amplifier (“op-amp”) is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. An op-amp produces an output voltage that is typically hundreds of thousands times larger than the voltage difference between its input terminals.

A Signetics ?a operational amplifier, one of the most successful op-amps.

Operational amplifiers are important building blocks for a wide range of electronic circuits. They had their origins in analog computers where they were used in many linear, non-linear and frequency-dependent circuits. Their popularity in circuit design largely stems from the fact that characteristics of the final elements (such as their gain) are set by external components with little dependence on temperature changes and manufacturing variations in the op-amp itself.

Operation:

The amplifier’s differential inputs consist of an input and an input, and ideally the op-amp amplifies only the difference in voltage between the two, which is called the differential input voltage. The output voltage of the op-amp is given by the equation,

Where the voltage at the non-inverting terminal is, is the voltage at the inverting terminal and AOL is the open-loop gain of the amplifier (the term “open-loop” refers to the absence of a feedback loop from the output to the input).

Typically the op-amp’s very large gain is controlled by negative feedback, which largely determines the magnitude of its output (“closed-loop”) voltage gain in amplifier applications, or the transfer function required (in analog computers). Without negative feedback, and perhaps with positive feedback for regeneration, an op-amp acts as a comparator. High input impedance at the input terminals and low output impedance at the output terminal(s) are important typical characteristics.

With no negative feedback, the op-amp acts as a comparator. The inverting input is held at ground (0 V) by the resistor, so if the Vin applied to the non-inverting input is positive, the output will be maximum positive, and if Vin is negative, the output will be maximum negative. Since there is no feedback from the output to either input, this is an open loop circuit. The circuit’s gain is just the GOL of the op-amp.

Adding negative feedback via the voltage divider Rf,Rg reduces the gain. Equilibrium will be established when Vout is just sufficient to reach around and “pull” the inverting input to the same voltage as Vin. As a simple example, if Vin = 1 V and Rf = Rg, Vout will be 2 V, the amount required to keep V at 1 V. Because of the feedback provided by Rf,Rg this is a closed loop circuit. Its overall gain Vout / Vin is called the closed-loop gain ACL. Because the feedback is negative, in this case ACL is less than the AOL of the op-amp.

If no negative feedback is used, the op-amp functions as a switch or comparator.

Applications:

Use in electronics system design

The use of op-amps as circuit blocks is much easier and clearer than specifying all their individual circuit elements (transistors, resistors, etc.), whether the amplifiers used are integrated or discrete. In the first approximation op-amps can be used as if they were ideal differential gain blocks; at a later stage limits can be placed on the acceptable range of parameters for each op-amp.

Non-inverting amplifier:

An op-amp connected in the non-inverting amplifier configuration

In a non-inverting amplifier, the output voltage changes in the same direction as the input voltage.

The gain equation for the op-amp is:

However, in this circuit V is a function of Vout because of the negative feedback through the R1R2 network. R1 and R2 form a voltage divider, and as V is a high-impedance input, it does not load it appreciably. Consequently:

where

Substituting this into the gain equation, we obtain:

Solving for Vout:

If AOL is very large, this simplifies to

Inverting amplifier:

An op-amp connected in the inverting amplifier configuration

In an inverting amplifier, the output voltage changes in an opposite direction to the input voltage.

As with the non-inverting amplifier, we start with the gain equation of the op-amp:

This time, V is a function of both Vout and Vin due to the voltage divider formed by Rf and Rin. Again, the op-amp input does not apply an appreciable load, so:

Substituting this into the gain equation and solving for Vout:

If AOL is very large, this simplifies to

A resistor is often inserted between the non-inverting input and ground (so both inputs “see” similar resistances), reducing the input offset voltage due to different voltage drops due to bias current, and may reduce distortion in some op-amps.

A DC-blocking capacitor may be inserted in series with the input resistor when a frequency response down to DC is not needed and any DC voltage on the input is unwanted. That is, the capacitive component of the input impedance inserts a DC zero and a low-frequency pole that gives the circuit a band pass or high-pass characteristic.

2.7 Thyristor:

A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bitable switches, conducting when their gate receives a current pulse, and continue to conduct while they are forward biased (that is, while the voltage across the device is not reversed).Some sources define silicon controlled rectifiers and thyristors as synonymous.

Fig: 2.8 symbol of Thyristor

Other sources define thyristors as a larger set of devices with at least four layers of alternating N and P-type material, including:

Function:

The thyristor is a four-layer, three terminal semi conducting device, with each layer consisting of alternately N-type or P-type material, for example P-N-P-N. The main terminals, labeled anode and cathode, are across the full four layers, and the control terminal, called the gate, is attached to p-type material near to the cathode. (A variant called an SCS—Silicon Controlled Switch—brings all four layers out to terminals.) The operation of a thyristor can be understood in terms of a pair of tightly coupled bipolar junction transistors, arranged to cause the self-latching action:

Thyristors have three states:

1. Reverse blocking mode — Voltage is applied in the direction that would be blocked by a diode
2. Forward blocking mode — Voltage is applied in the direction that would cause a diode to conduct, but the thyristor has not yet been triggered into conduction
3. Forward conducting mode — The thyristor has been triggered into conduction and will remain conducting until the forward current drops below a threshold value known as the “holding current”

Function of the gate terminal:

The thyristor has three p-n junctions (serially named J1, J2, J3 from the anode).

Layer diagram of thyristor.

When the anode is at a positive potential VAK with respect to the cathode with no voltage applied at the gate, junctions J1 and J3 are forward biased, while junction J2 is reverse biased. As J2 is reverse biased, no conduction takes place (Off state). Now if VAK is increased beyond the breakdown voltage VBO of the thyristor, avalanche breakdown of J2 takes place and the thyristor starts conducting (On state).

If a positive potential VG is applied at the gate terminal with respect to the cathode, the breakdown of the junction J2 occurs at a lower value of VAK. By selecting an appropriate value of VG, the thyristor can be switched into the on state suddenly.

Once avalanche breakdown has occurred, the thyristor continues to conduct, irrespective of the gate voltage, until: (a) the potential VAK is removed or (b) the current through the device (anode cathode) is less than the holding current specified by the manufacturer. Hence VG can be a voltage pulse, such as the voltage output from a UJT relaxation oscillator.

These gate pulses are characterized in terms of gate trigger voltage (VGT) and gate trigger current (IGT). Gate trigger current varies inversely with gate pulse width in such a way that it is evident that there is a minimum gate charge required to trigger the thyristor.

V – I characteristics: The current-voltage characteristic of a thyristor is shown below:

Fig: 2.9 V – I characteristics of thyristor.

Applications

Thyristors are mainly used where high currents and voltages are involved, and are often used to control alternating currents, where the change of polarity of the current causes the device to switch off automatically; referred to as Zero Cross operation.

Thyristors can be used as the control elements for phase angle triggered controllers, also known as phase fired controllers.

They can also be found in power supplies for digital circuits, where they are used as a sort of “circuit breaker” or “crowbar” to prevent a failure in the power supply from damaging downstream components.

Types of thyristor:

SCR — Silicon Controlled Rectifier

ASCR — Asymmetrical SCR

RCT — Reverse Conducting Thyristor

LASCR — Light Activated SCR, or LTT — Light triggered thyristor

BOD — Break over Diode — A gateless thyristor triggered by avalanche current

• Shockley diode — Unidirectional trigger and switching device
• Dynistor — Unidirectional switching device
• DIAC — Bidirectional trigger device
• SIDAC — Bidirectional switching device
• Trisil, SIDACtor — Bidirectional protection devices

TRIAC — Triode for Alternating Current — a bidirectional switching device containing two thyristor structures with common gate contact

• BCT — Bidirectional Control Thyristor — A bidirectional switching device containing two thyristor structures with separate gate contacts

GTO — Gate Turn-Off thyristor

IGCT — Integrated Gate Commutated Thyristor

• MA-GTO — Modified Anode Gate Turn-Off thyristor
• DB-GTO — Distributed Buffer Gate Turn-Off thyristor

MCT — MOSFET Controlled Thyristor — It contains two additional FET structures for on/off control.

• BRT — Base Resistance Controlled Thyristor

LASS — Light Activated Semi conducting Switch

• AGT — Anode Gate Thyristor — A thyristor with gate on n-type layer near to the anode

• PUT or PUJT — Programmable Injunction Transistor — A thyristor with gate on n-type layer near to the anode used as a functional replacement for injunction transistor

• SCS — Silicon Controlled Switch or Thyristor Tetrad — A thyristor with both cathode and anode gates.

2.8 Mosfet:

The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a transistor used for amplifying or switching electronic signals. In MOSFETs, a voltage on the oxide-insulated gate electrode can induce a conducting channel between the two other contacts called source and drain. The channel can be of n-type or p-type (see article on semiconductor devices), and is accordingly called an nMOSFET or a pMOSFET (also commonly nMOS, pMOS). It is by far the most common transistor in both digital and analog circuits, though the bipolar junction transistor was at one time much more common.

Fig: 2.10 Symbol of Mosfet

Two power MOSFETs in the surface-mount package D2PAK. Operating as switches, each of these components can sustain a blocking voltage of 120 volts in the OFF state, and can conduct a continuous current of 30 amperes in the ON state, dissipating up to about 100 watts and controlling a load of over 2000 watts. A matchstick is pictured for scale.

IGFET is a related term meaning insulated-gate field-effect transistor, and is used almost synonymously with MOSFET, being more accurate since many “MOSFETs” use a gate that is not metal and a gate insulator that is not oxide. Another synonym is MISFET for metal–insulator–semiconductor FET

Circuit symbols:

A variety of symbols are used for the MOSFET. The basic design is generally a line for the channel with the source and drain leaving it at right angles and then bending back at right angles into the same direction as the channel. Sometimes three line segments are used for enhancement mode and a solid line for depletion mode. Another line is drawn parallel to the channel for the gate.

The bulk connection, if shown, is shown connected to the back of the channel with an arrow indicating PMOS or NMOS. Arrows always point from P to N, so an NMOS (N-channel in P-well or P-substrate) has the arrow pointing in (from the bulk to the channel). If the bulk is connected to the source (as is generally the case with discrete devices) it is sometimes angled to meet up with the source leaving the transistor. If the bulk is not shown (as is often the case in IC design as they are generally common bulk) an inversion symbol is sometimes used to indicate PMOS, alternatively an arrow on the source may be used in the same way as for bipolar transistors (out for nMOS, in for pMOS).

Comparison of enhancement-mode and depletion-mode MOSFET symbols, along withJFET symbols (drawn with source and drain ordered such that higher voltages appear higher on the page than lower voltages):

 P-channel N-channel JFET MOSFET enh MOSFET enh (no bulk) MOSFET dep

For the symbols in which the bulk, or body, terminal is shown, it is here shown internally connected to the source. This is a typical configuration, but by no means the only important configuration. In general, the MOSFET is a four-terminal device, and in integrated circuits many of the MOSFETs share a body connection, not necessarily connected to the source terminals of all the transistors.

Mosfet Operation:

Metal–oxide–semiconductor structure

A traditional metal–oxide–semiconductor (MOS) structure is obtained by growing a layer of silicon dioxide (SiO2) on top of a silicon substrate and depositing a layer of metal or polycrystalline silicon (the latter is commonly used). As the silicon dioxide is a dielectric material, its structure is equivalent to a planar capacitor, with one of the electrodes replaced by a semiconductor.

Example application of an N-Channel MOSFET. When the switch is pushed the LED lights up.

Metal–oxide–semiconductor structure on P-type silicon

When a voltage is applied across a MOS structure, it modifies the distribution of charges in the semiconductor. If we consider a P-type semiconductor (with NA the density of acceptors, p the density of holes; p = NA in neutral bulk), a positive voltage, VGB, from gate to body (see figure) creates a depletion layer by forcing the positively charged holes away from the gate-insulator/semiconductor interface, leaving exposed a carrier-free region of immobile, negatively charged acceptor ions (see doping (semiconductor)). If VGB is high enough, a high concentration of negative charge carriers forms in an inversion layer located in a thin layer next to the interface between the semiconductor and the insulator. Unlike the MOSFET, where the inversion layer electrons are supplied rapidly from the source/drain electrodes, in the MOS capacitor they are produced much more slowly by thermal generation through carrier generation and recombination centers in the depletion region. Conventionally, the gate voltage at which the volume density of electrons in the inversion layer is the same as the volume density of holes in the body is called the threshold voltage.

This structure with p-type body is the basis of the N-type MOSFET, which requires the addition of an N-type source and drain regions.

POWER SUPPLY UNIT

Introduction:

Power supply units which can give sinusoidal wave (12sinwt), +12V,-12V understand the basic construction and operation principles, short description of such devices and components are discussed in this chapter.

Transformer:

A transformer is a 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.

Fig: 3.1Transformer.

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:

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.

Basic principles:

The transformer is based on two principles: first, that an electric current can produce a magnetic field (electromagnetism), and, second that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). Changing the current in the primary coil changes the magnetic flux that is developed. The changing magnetic flux induces a voltage in the secondary coil.

### Ideal power equation:

If the secondary coil is attached to a load that allows current to flow, electrical power is transmitted from the primary circuit to the secondary circuit. Ideally, the transformer is perfectly efficient; all the incoming energy is transformed from the primary circuit to the magnetic field and into the secondary circuit. If this condition is met, the incoming electric power must equal the outgoing power:

giving the ideal transformer equation

Transformers normally have high efficiency, so this formula is a reasonable approximation.

Fig: 3.2 Ideal Transformers

If the voltage is increased, then the current is decreased by the same factor. The impedance in one circuit is transformed by the square of the turn’s ratio. For example, if an impedance Zs is attached across the terminals of the secondary coil, it appears to the primary circuit to have an impedance of (Np/Ns)2Zs. This relationship is reciprocal, so that the impedance Zp of the primary circuit appears to the secondary to be (Ns/Np)2Zp.

### Detailed operation:

The simplified description above neglects several practical factors, in particular the primary current required to establish a magnetic field in the core, and the contribution to the field due to current in the secondary circuit.

Models of an ideal transformer typically assume a core of negligible reluctance with two windings of zero resistance. When a voltage is applied to the primary winding, a small current flows, driving flux around the magnetic circuit of the core. The current required to create the flux is termed the magnetizing current; since the ideal core has been assumed to have near-zero reluctance, the magnetizing current is negligible, although still required to create the magnetic field.

The changing magnetic field induces an electromotive force (EMF) across each winding. Since the ideal windings have no impedance, they have no associated voltage drop, and so the voltages VP and VS measured at the terminals of the transformer, are equal to the corresponding EMFs. The primary EMF, acting as it does in opposition to the primary voltage, is sometimes termed the “back EMF”. This is due to Lenz’s law which states that the induction of EMF would always be such that it will oppose development of any such change in magnetic field.

Types:

· Autotransformer

· Polyphase transformers

· Leakage transformers

· Resonant transformers

· Audio transformers

· Instrument transformers

## Classification:

Transformers can be considered a class of electric machine with no moving parts; as such they are described as static electric machines. They can be classified in many different ways; an incomplete list is:

• By power capacity: from a fraction of a volt-ampere (VA) to over a thousand MVA;
• By frequency range: power-, audio-, or radio frequency;
• By voltage class: from a few volts to hundreds of kilovolts;
• By cooling type: air-cooled, oil-filled, fan-cooled, or water-cooled;
• By application: such as power supply, impedance matching, output voltage and current stabilizer, or circuit isolation;
• By purpose: distribution, rectifier, arc furnace, amplifier output, etc.;
• By winding turns ratio: step-up, step-down, isolating with equal or near-equal ratio, variable, and multiple windings.

## Applications:

Transformers are used extensively in electronic products to step down the supply voltage to a level suitable for the low voltage circuits they contain. The transformer also electrically isolates the end user from contact with the supply voltage.

Signal and audio transformers are used to couple stages of amplifiers and to match devices such as microphones and record players to the input of amplifiers. Audio transformers allowed telephone circuits to carry on a two-way conversation over a single pair of wires. A balun transformer converts a signal that is referenced to ground to a signal that has balanced voltages to ground, such as between external cables and internal circuits.

The principle of open-circuit (unloaded) transformer is widely used for characterization of soft magnetic materials, for example in the internationally standardized Epstein frame method.

3.3 Rectifier:

A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which is in only one direction, a process known as rectification. Rectifiers have many uses including as components of power supplies and as detectors of radio signals. Rectifiers may be made of solid state diodes, silicon-controlled rectifiers, vacuum tube diodes, mercury arc valves, and other components.

Half-wave rectification

In half wave rectification, either the positive or negative half of the AC wave is passed, while the other half is blocked. Because only one half of the input waveform reaches the output, it is very inefficient if used for power transfer. Half-wave rectification can be achieved with a single diode in a one-phase supply, or with three diodes in a three-phase supply.

The output DC voltage of a half wave rectifier can be calculated with the following two ideal equations:

Full-wave rectification

A full-wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output. Full-wave rectification converts both polarities of the input waveform to DC (direct current), and is more efficient. However, in a circuit with a non-center tapped transformer, four diodes are required instead of the one needed for half-wave rectification. (See semiconductors, diode). Four diodes arranged this way are called a diode bridge or bridge rectifier.

Fig: 3.3 Grates bridge rectifier: a full-wave rectifier using 4 diodes.

For single-phase AC, if the transformer is center-tapped, then two diodes back-to-back (i.e. anodes-to-anode or cathode-to-cathode) can form a full-wave rectifier. Twice as many windings are required on the transformer secondary to obtain the same output voltage compared to