Synchronous induction motor

Introduction :

In the applications where high starting torque and constant speed are desired then synchronous induction motors can be used. It has the advantages of  both synchronous and induction motors. The synchronous motor gives constant speed whereas induction motors can be started against full load torque.

connections and working principles :

Consider a normal slip ring induction motor having three phase winding on the rotor as shown in the Fig.

The motor is connected to the exciter which gives d.c. supply to the motor through slip rings. One phase carries full d.c. current while the other two carries half of the full d.c. current as they are in

parallel. Due to this d.c. excitation, permanent poles (N and S) are formed on the rotor.

Initially it is run as an slip ring induction motor with the help of starting resistances. When the resistance is cut out the motor runs with a slip. Now the connections are changed and the exciter is

connected in series with the rotor winding's which will remain in the circuit permanently.

 As the motor is running as induction motor initially high starting

torque (upto twice full load value) can be developed. When d.c.

excitation is provided it is pulled into synchronism and starts running at constant speed. The synchronous induction motor provides constant speed, large starting torque, low starting current and power factor correction.

Semiconductors and its types

A Semiconductor is a substance which has resistivity in between conductor and insulator .Copper is a good conductor which is resistivity is 1.7*10^-8.Germanium is a semiconductor which is reistivity is 0.6 ohm meter.A Glass is a good insulator which is resistivity is 9*10^ ohm meter.A Nichrome is a resistance material which is resistivity is 10^-4 ohm meter.

Comparing the resistivity of above materials it is apparent that resistivity of germanium is quite high as compared to copper but is quite low when compared with glass.This is shows that resistivity of a semiconductor lies in between conductors and insulators.

Types of semiconductors :

The semiconductors are classifieds as followed

(i) intrinsic semiconductors

(ii)Extrinsic semiconductors

In every elements,the atoms are tied together by the bonding action of valence electrons.Si and Ge atoms contains only 4 valence electrons.These electrons have tendency to fill the outer most orbit.In this way the electrons placed to in the last orbit of an atom share the electrons with their neighbor atoms. Similarly all the electrons are tied together with their neighboring atoms.

(i)Intrinsic Semiconductors :

A pure semiconductor is called intrinsic semiconductor.The silicon and Ge atoms contain only four electrons in the outermost orbit.So they are called tetravalent bonds.In an intrinsic semiconductors even at room temperature hole pair are created.

When electric field is applied across an intrinsic semiconductor ,the current conduction takes same place by two process which is free electrons and holes.Now no electrons get away from the co-valent bonds.So current flows is zero.

Metal oxide Semiconductor FET

Introduction :

A field effect transistor that can be operated in the enhancement -mode is called a MOSFET. A MOSFET is a important semiconductor device and can be used in any of the circuits covered for JFET.

The main draw back of a JFET is that gate must be reverse biased for proper operation of the device which means that negative gate operation for n-channel and positive gate operation for p-channel.This means that we can only decrease width of the channel.This type of operation is called depletion mode operation.There fore JFET can only operate in the depletion mode.

Symbols for MOSFET :

Types of MOSFET :

there are two basic types available in MOSFET 

1.D-MOSFET or Depletion type MOSFET .This type of MOSFET can be operated in both depletion mode and enhancement mode.

2.E-MOSFET or Enhancement type MOSFET.This type of MOSFET can be operated only in enhancement -mode.

1.D-MOSFET : 

The above figure shows that n-channel MOSFET is a piece of n-type material with p-type region on the right and insulated gate on the left side then free electrons are moving from source to drain must passes through the narrow channel between the gate and the n-type region.

It is usual practice to connect the substrate to the source internally so that a MOSFET has three terminals which are source,drain,gate.

gate insulated from the channel we can apply either positive or negative voltage to the gate.therefore D-MOSFET can be operated in both modes then JFET can operated only one mode which means that depletion mode.

The figure given in below the n-channel D-MOSFET 

2.E-MOSFET :

The E-MOSFET has no channel between source and drain unlike the D-MOSFET. The E-MOSFET requires a proper gate voltage to form a channel .It is reminded that E-MOSFET can operate only enhancement mode only.The constructional details of a n-channel E-MOSFET figure given in below.

Lead acid battery

Battery :

Battery is an electrochemical  device,which delivers electric energy by chemical reaction.An emf is produced by chemical means,whenever two dissimilar solid conductors known as electrodes are placed in conducting liquid known as electrolyte current will flow through the external circuit connected across the two electrodes.

classification of cells :
The cells are classified into two types 
1.primary cell
2.secondary cell
1.primary cell :

A cell which cannot be recharged is called Primary cell.The Primary cells convert chemical energy into electrical energy.

Example : Dry cell,Volatile cell,Daniel cell.

In a primary cell ,the chemical action is irreversible.The electrodes and electrolytes undergo changes during discharge which cannot be reversed.

2.Secondary cell :

The cell which can be recharged and brought back to the original state is called a Secondary cell.

Example : Lead acid cell,Alkaline cells.

The secondary cell converts the electrical energy into chemical energy and it is stored into the cells during charges.When the cells discharges the stored chemical energy is converted in to electrical energy.These are known as storage cells.

What is Battery ?
When number of cells are grouped together it is called Battery.
Types of Battery :
1.primary cell :
a) Dry cell b) Volatile cell c) Daniel cell d) Weston cell
2.secondary cell 
a) Lead acid battery 
b) Nickel-cadmium battery 
c) Nickel-iron battery
d) nickel-zinc battery

Care and Maintenance of Lead Acid Battery :

• Install the Battery firmly.
• Tighten all the connection periodically.
• Keep the battery dry and clean to avoid discharging through moisture.
• Keep the battery surface free of any electrolyte.
• When cleaning or washing high voltage batteries,connection may be opened at several points to avoid shock.
•  Apply petroleum jelly to terminals to prevent corrosion.
• The level of electrolyte must be kept above the top of the plates.
• The level of the electrolyte should be checked at regular intervals.
• The cell  should be recharged as soon as possible after discharge.
• Do not leave the battery in a discharged condition for a long period.
• Do not charge the battery at high rate.
• If battery being used as a stand by source to give emergence lighting etc.It should be given trickle charge.
• The temperature of the cell should not exceed above 40 degree.
• The battery terminals should not be short circuited.
• Carefully follow the instructions of the manufactures.
• During charging the positive terminals of the battery is to be connected to the positive terminal of the supply and negative to the negative terminals of the supply. 

TYPES OF D.C GENERATOR

The power station of modern design generate practically only three phase alternating current.A large part of this power is used in the form of alternating current in industry,for lighting and domestic needs.When industrial needs make it necessary too or when it is of greater advantage to use direct current .It is generally obtained by converting A.C to D.C with help of converters of ionic or machines type.

As primary sources of power,D.C generators are mainly used in self contained plants such as automobiles and airplanes,for electric arc welding,train car lighting,in submarines etc,.

Types of D.C generator :

sending current through the field winding for production of magnetic flux is called Excitation.Depending on the method of excitation D.C generator are classified as

1.separately excited D.C generators

2.self excited D.C generators.

1.Separately excited D.C generators

These generators whose field magnets are energised from independent external source of D.C current.

 In separately excited D.C generators,the exciting field current is supplied by a separate source as shown in above.

2.Self excited D.C generators :

These generators whose field magnets are energised by the current produced by the generators themselves.Due to residual magnetism there is always present some flux in the poles .When the armature is rotated,some emf and hence some induced current produced which is partly or fully passed through the field coils there by strengthening the pole flux.

self excited generators can be divided in to accordance with how field winding is connected into generators as follows,

(i) D.C shunt generators

(ii) D.C series generators

(iii)D.C compound generators

(i)D.C shunt generators:

In shunt generators the field winding is connected in parallel with the armature winding terminals and have the full voltage of the generator across them.

some important relations are

The shunt generator field winding has many turns of fine wire having high resistance.In DC shunt generator the armature current (Ia) will be the sum of load current (I) and the field current(Ish).

Electric shock and safety measures

INTRODUCTION ELECTRIC SHOCK:

Our human body is a conductor.When the live wire is touches the body,current completes its path through body and earth.Hence the muscular function body are paralysed due to the currents action on the nervous system and causing breathing to stop,severe burns and resulting death.

This is the symbol of electric shock is

Prevention and precaution against electric shock:

To prevent electric shock and danger the following precaution should be observed before using the electric equipment's.

1.Always be careful.

2.All electrical equipment's are connected with good quality wires.

3.Before replacing  a blown out fuse always put the main switch off.

4.Always keep the earth connection in good condition.

5.Before using portable electrical things like table fan,iron box heaters etc,see that these are well earthed.

6.Line wires should always be connected through the switch.

7.Replace immediately broken or damaged switches and plug etc.

8.A plug point should never disconnected by pulling the flexible cable.

9.Plug point connection should be made only by plug tops not by bare wires.

10.Check proper working devices.

11.Keep proper condition of electrical hand tools.

The purpose of earthing the electrical equipment's is to bring it to zero potential and thereby to avoid the shock to the operator. 

Due to failure of insulation of a live conductor it may be in contact with the frame of the machine.If the frame is not earthed when somebody is touches the frames of the equipment's they will get shock.

But if the frame of the equipment is earthed the voltage of parted with respect to the earth doesn't rise to dangerously higher value.Thus severe shock is prevented. 

Corona effect in Electrical energy

INTRODUCTION :
This type of effect can be noticed that near the overhead lines there exists a hissing noise and sometimes a faint violet glow.
CORONA EFFECT:

When normal alternating voltages is applied across two conductors which conductors are enough spacing in between,then there is no change in the atmospheric conditions surrounding conductors.but if the applied voltage exceeds a particular limits value then the air surrounding the conductors gets ionized due to which hissing noise or faint violet glow appears.

The phenomenon of violet glow, hissing noise and production of ozone gas in an overhead transmission line is known as corona.

If the conductors are polished and smooth, the corona glow will be uniform throughout the length of the conductors, otherwise the rough points will appear brighter. With d.c. voltage, there is difference in the appearance of the two wires. The positive wire has uniform glow about it, while the negative conductor has spotty glow.

Theory of corona formation in transmission lines :

The corona effect takes place when applied voltage exceeds the particular value which is called critical disruptive voltage.

TRANSFORMER-BASICs

BASIC DEFINITIONS :

Transformer is a static device which is used for transferring the electric power from one circuit to another circuit without changes in the frequency and it accomplishes this by electromagnetic induction or mutual inductance.

• The transformer has no rotating part so it is called static device.
• Transformer is a electromagnetic device designed for the transform of alternating current in to another one without changes in the frequency.
• Transformer with two winding's is called double-winding transformer and three or more winding's is termed is called multi-winding one.
• A tapped transformer is one whose winding's fitted with special taps for changing voltage.

step-up transformer:
A transformer raises the voltage level from the input voltage is called step-up transformer.
step-down transformer:
A transformer reduces the voltage level from the input voltage is step-down transformer.

WHAT IS ELECTRIC HEATING

INTRODUCTION:

Electric heating is extensively used for both domestic and industrial applications.Domestic application are room heaters,hot plates for cooking,electric kettles,electric ovens for bakeries,electric toasters etc...

Then industrial applications of electric heating melting of metals,heat treatment metals like soldering,moulding of glass,baking of insulators,enameling copper wires.

BATTERY OPERATED HEATERS:

This is the simple and basic circuit diagram for battery operated

transistor biasing circuit explanation

 Transistor biasing

what is transistor?

     A transistor consists of two pn junction formed by sandwiching either p-type or n-type semiconductor pair of opposite types.

     There are two types of transistors names 

1. n-p-n transistor
2. p-n-p transistor

transistor biasing:

    The basic function of transistor is to do amplification.so first we give a weak signal to the base of transistor and amplified output is obtained in the collector circuit.

    The proper flow of zero signal collector current and the maintenance of proper collector-emitter voltage during the passage of signal is known as Transistor biasing.

faithful amplification :

    The process of raising the strength of a weak signal without any changes in its general shape .This process is known as faithful amplification.

    This process is revealed that it will function properly if its input circuit base emitter junction is forward biased and output circuit of collector base junction remains reversed biased.

    The following basic condition must be satisfied

1. proper zero signal collector current.
2. Minimum proper base-emitter voltage (Vbe)at any instant.
3. Minimum proper collector-emitter voltage (Vce) at any instant.   

     The basic purpose of transistor biasing is keep the base-emitter junction properly forward biased and collector base junction is properly reverse biased during the application of signal.this is can be achieved with a bias battery or associationg a circuit with a transistor.