ADVANTAGES OF ROTATING FIELD ALTERNATOR

Most alternators have the rotating field and the stationary armature.The rotating-field type alternator has several advantages over the rotating-armature type alternator.

(1)A stationary armature is more easily insulated for the high voltage for which the alternator is designed.This generated voltage may be as high as 33 kV.

(2)The armature windings can be braced better mechanically against high electromagnetic forces due to large short-circuit currents when the armature windings are in the stator.

(3)The armature windings, being stationary,are not subjected to vibration and centrifugal forces.

(4)The output current can be taken directly from fixed terminals on the stationary armature without using slip rings,brushes,etc.

(5)The rotating field is supplied with direct current.Usually the field voltage is between 100 to 500 volts.Only two slip rings are required to provide direct current for the rotating field,while at least three slip rings would be required for a rotating armature.The insulation of the relatively low voltage slip rings from the shaft can be provided easily.

(6)The bulk and weight of the armature winding are substantially greater than the windings of the field poles.The size of the machine is,therefore,reduced.

(7)Rotating field is comparatively light and can be constructed for high speed rotation.The armatures of large alternators are forced cooled with circulating gas or liquids.

(8)The stationary  armature may be cooled more easily because the armature can be made large to provide a number of cooling ducts. 

Synchronous Generators(Alternators)

INTRODUCTION

An alternating voltage is generated in a single conductor or armature coil rotating in a uniform magnetic field with stationary field poles.An alternating voltage will also be generated in stationary armature conductors when the field poles.state past the conductors.Thus,we see that as long as there is a relative motion between the armature conductors and the field flux there will be a voltage generated in the armature conductors.In both the cases the wave shape of the voltage is a sine curve.

In d.c. generators,the field poles are stationary and the armature conductors rotate.The voltage generated in the armature conductors is of alternating nature.This generated alternating voltage is converted to a direct voltage at the brushes with the help of the commutator.

A.c. generators  are usually called alternators.They are also called synchronous generators.Rotating machines that rotate at a speed fixed by the supply frequency and the number of poles are called synchronous machines.

A Synchronous generator is a machine for converting mechanical power from a prime mover to ac electric power at a specific voltage and frequency.A synchronous motor is a synchronous machine that converts electric power to mechanical power.Synchronous generators are usually of  3-phase type because of the several advantage of 3-phase generation,transmission and distribution.Large synchronous generators are used to generated bulk power at thermal,hydro and nuclear power stations.Synchronous generators with power ratings of several hundred MVA are very commonly used in generating stations.The biggest size used in India has a rating of 500 MVA used in super-power thermal power stations.Synchronous generators are the primary sources of worlds electric power systems today.For bulk power generation, stators windings of synchronous generators are desidgned for voltages ranging from 6.6 kV to 33 kV.

THERMOCOUPLES

Thermocouples are perhaps the most simple and most widely used devices for measurement of temperature.It essentially consists of two dissimilar metal wires A and B,insulated from each other but welded or brazed together at their ends forming two junctions,as shown in fig a.
when one end of each wire is connected to a measuring instrument,as shown in fig b,it becomes an accurate and sensitive temperature measurement device.
The operation of a thermocouple is based on see back effect,i.e.,if two wire of different metals are joined together ateach end and form a complete  electric circuit then current flows in the circuit when the two junctions are kept at different temperatures.This current is caused y an emf,called the thermo electric emf, set up in the circuit and is a function of temperature difference of the two junctions(the hot junction,whose temperature is to be measured and the cold or reference junction which is maintained at a constant temperature).The thermoelectric emf set up so is the same for any particular temperature,and is not afffected by the size of the conductors,the area in contact,or the method of joining them.

RESISTIVE TRANSDUCERS

In such a transducer,resistance between the output terminals of a transducer gets varied according to the measurand. Resistive transducers are preferred over other transducers because dc and ac both are suitable for resistance measurement.

Resistance of any metal conductor is given by the equation 

R=ρ l/a

Where ρ  is the resistivity of the material of conductor in Ω-m,l is the length of conductor in meters,and a is the cross-sectional area of conductor in m^2.

Physical phenomenon i.e., input signal to the transducer causes variation in resistance by changing any one of the quantities,ρ ,l,and a,.For measurement of displacement length of conductor is varied in potentiometer thereby resulting in change in resistance.For measurement of force and pressure resistance of a conductor or semiconductor is varied in strain gauges.Variation in temperature causes change in the resistivity of the conductor material and so change in resistance  takes place which is noted for measurement of temperature.with some devices resistance varies with the change in light intensity because of photoconductive effect,while with others resistance varies on exposure to magnetic field due to magnetoresistive effect.Thus resistive transducer have a wide field of application.

Though every wire is associated with self inductance and capacitance across its ends but these are important only at low and high values of R respectively while the resistance of most of the resistive transducer lies in the mid-range 100 Ω-100k Ω.

SELECTION OF TRANSDUCERS

The points to be considered in determining a transducer suitability for a specified measurement are as follows:

1.Range. The range of the transducer should be large enough to encompass all the expected magnitudes of the measurand.

2.Sensitivity.The transducer should give a sufficient output signal per unit of measured input in order to yield meaningful data.

3.Electrical output Characteristics.The electrical characteristics-the output impedance output signal should be compatible with the recording device and the rest of the measuring system equipment.

4.Physical Environment. The transducer selected should be able to withstand the environmental condition to which it is likely to be subjected while carrying out measurements and tests.such parameters are temperature,acceleration,shock and vibration,moisture,and corrosive chemicals that might damage some transducers but not others.

5.Errors.The errors inherent in the operation of the transducer itself,or those errors caused by environmental conditions of the measurement,should be small enough or controlled enough that they allow meaningful data to be taken. 

Active and Passive Transducers

The transducers may be classified in various ways such as on the basis of electrical principle involved,methods of application,methods of energy conversion used,nature of output signal etc.

Active and Passive Transducer:

Transducers,on the basis methods of energy conversion used.may be classified into active and passive transducer.

self generating type transducers i.e., the transducers,which develop their output in the form of electrical voltage or current without any auxiliary source,are called the active transducers.Such transducers draw energy from the system under and,therefore,use of amplifier becomes essential. Tachogenerators used for measurement of angular velocity,thermocouples used for measurement of temperature,piezoelectric crystal used for measurement of force fall in this category.

Transducers,in which electrical parameters i.e.,resistance,inductance or capacitance changes with the change in input signal,are called the passive transducers.These transducer require external power source for energy conversion.In such transducer electrical parameters i.e., resistance,inductance or capacitance causes a change in voltage,current or frequency of the external power source.These transducers may draw some energy from the system under measurement. Resistive,inductive and capacitive transducer fall in this category.

ACCELERATION

Acceleration of an object in motion is defined as the ratio of change in velocity and the corresponding time taken by the object i.e.

acceleration=(change in velocity)/time taken

Acceleration is a vector quantity.It is positive if the velocity is decreasing.The negative acceleration is also called retardation or deceleration.

The unit of acceleration is m s⁻²   

^{(a)Uniform acceleration.An object is said to be moving with a uniform acceleration if its velocity}

 ^{changes by equal amounts in equal intervals of time.}

^{(b)Variable acceleration.An object is said to be moving with a variable acceleration if its velocity} 

^{changes by unequal amounts in equal intervals of time.}

^{(c)Average acceleration.When an object is moving with a variable acceleration,then the average} 

^{acceleration of the object for the given motion is defined as the ratio of the total change in}

 ^{velocity of the object during motion to the total time taken i.e.,
   →}

   ^{aav =(total change in velocity)/(total time taken)}

         
<sup>The average acceleration can be positive or negative depending upon the sign of slope of 
velocity of the object in the given interval of time is zero.

(d) Instantaneous acceleration.When an object is moving with variable acceleration,then the 
object  possesses different accelerations at different instants.The acceleration of the object at a 
given instant of time or at a given points of motion,is called its instantaneous acceleration.</sup>

ACCELERATED MOTION

In non-uniform motion of an object,the velocity of the object is different  at difference instants i.e.,the velocity of the object keeps on changing with time.Such a motion is said to be an accelerated motion.For example,a vehicle while going from one position to another on a road,moves with different velocities at different instants,hence it is the illustration of accelerated motion.

In the accelerated motion,if the change in velocity of an object in each unit of time is constant,the object is said to be moving with constant,the object is said to be moving with constant accelerated and such a motion is called uniformly accelerated motion.On the other hand,if the change in velocity in each unit of time is not constant,the object is said to be moving with variable acceleration and such a motion is called non-uniformly accelerated motion.   

NON-UNIFORM MOTION

An object is said to be in non-uniform motion if it undergoes equal displacements in unequal intervals of time, howsoever small these intervals may be.

Clearly,in non-uniform motion,the velocity of the object is different at different instants.

For example,when we run or travel in a  vehicle from one position to another,we start from the condition of rest.We gain speed for some time and then may travel with constant velocity for sometime and finally slow down and come to a stop.In such a case,the velocity of object is different instants and hence this motion is non-uniform motion.

In non-uniform motion,the velocity of object in different forms is explained below.

(i) Variable velocity: An object is said to be moving with a variable velocity,if either its speed or its direction of motion or both change with time.

(ii)Average velocity: For a given motions of an object,the average velocity is that uniform velocity with which the object will cover the same displacement in a given interval of time as it does with its actual variable velocity during the giving interval of time.

The average velocity of an object can be positive or negative depending on the sign of displacement.It is zero if the displacement is zero.

(iii)Instantaneous velocity:The average velocity of the object in motion tells us how fast an object has been moving over a given interval of time.But it does not tell,how fast the given object is moving at different instants of time during motion.For this,we define instantaneous velocity.

CURRENT TRANSFORMER (C.T.)

A Current transformer operates on the same principle as a power transformer.The cross-sectional area of the core of a C.T. is made large so that the magnetic flux density is low.The core is in the form of a toroid and is made of high-permeability steel.This reduces magnetising current and thus the radio of primary and secondary currents is constant under all working conditions.
The primary of a C.T. is generally a single-turn winding B(called a bar primary)and carries full-load current.The secondary winding S has a large number of turns.Current transformer ratios are generally specified in terms of full-load primary and secondary currents.Usually,the secondary winding are designed for rated values of 5 A,although 1A and 2A ranges are also used.For example,1000/5 A current transformer may be used with 5 A ammeter to measure currents upto 1000 A.fig.shows the connections of a current transformer.

There is one important difference between a current transformer and a power transformer and a power transformer.The power transformer is supplied at constant voltage,and the primary current automatically adjusts itself so that the primary ampere-turns balance the secondary ampere-turns.In the current transformer,the primary current is determined by the external circuit and,therefore,the variation of the load on the secondary side does not affect the primary current.The secondary current adjusts itself to a value so that the primary ampere-turns balance the secondary ampere turns.   

ELECTRODYNAMIC WATTMETER

The electrodynamic wattmeter consists of two coils CC placed parallel to one another to produced a uniform magnetic field.They are connected in series with the load,and are called the current coils.The two fixed coils can be connected in series or parallel to give two current ratings,The current coils carry the full-load current  or a fraction of the full-load current.Thus,the current in the current coils is proportional to the load current.The moving coil VC,in series with a high non-inductive resistance Rs,is connected across the supply Thus,the current flowing in the moving coil is proportional to,and practically in phase with,the supply voltage.The moving coil is also called the voltage coil. Current coils have few turns of thick wire,while voltage coil has large number of turns of thin wire.The moving coil,VC is mounted centrally such that it does not project outside the fixed coils at its maximum deflected position.It is carried on a pivoted spindle and carries the pointer which moves over a calibrated scale.

Two springs S1,S2 are used for providing the corntrolling torque torque and for leading current into and out of the moving coil.Damping is done by air controlling friction.An aluminium vane moving V in a closed sector-shaped chamber B is generally used for this purpose.The control springs and the air damber are shown in fig.
In practice,there are two methods in which a wattmeter,may be connected in a circuit.In the voltage coil VC is connected on the supply side.The current coil CC carries the load current and the voltage across the voltage coil VC is higher than the load voltage because of small voltage drop across the current coil.

TRANSFORMER CONSTRUCTIONS(CORE-TYPE & SHELL-TYPE)

There are two basic types of transformer constructions,the core type and the shell type.


CORE-TYPE CONSTRUCTION 

In the core-type transformer,the magnetic circuit consists of two vertical legs or limbs with two horizontal sections,called yokes.To keep the leakage flux to a maximum,half of each winding is placed on each leg of the core as shown in fig.The low-voltage winding is placed next to the core and the high-voltage winding is placed around the low-voltage winding to reduce the insulating material required.Thus,the two winding are arranged as concentric coils.Such a winding is, therefore,called concentric winding or cylindrical winding.

SHELL-TYPE TRANSFORMER

In the shell-type transformer fig.,both primary and secondary windings are wound on the central limb,and the two outer limbs complete the low-reluctance flux paths.Each winding is subdivided into sections.Low-voltage(lv) and high-voltage(hv) subsections are alternately put in the form of a sandwich.such a winding is,therefore,called sandwich or discwinding.
The core-type transformer is easier to dismantle for repair.The shell-type transformer gives better support against electromagnetic forces between the current-carrying conductors.These forces are of considerable magnitude under short-circuit conditions.Shell-type transformers provide a shorter magnetic path,and hence magnetizing current is lesser than that in the core-type transformer.The natural cooling is poor in a shall-type transformer due to the embedding of the coils.

READ ONLY MEMORIES(ROM'S) Application

The rise in READ ONLY MEMORY applications has been exponential in nature.Presently there is hardly a new digital system that doesn't boast of a specification such as "microprogram controlled","new ROM or PLA has made this product reliable and economical," "microprocessor controlled",and so forth.The ROM,though simple in concept,had to wait until the semiconductor industry could produce these devices economically;since then they hence become one of the most widely used components ever produced.

A ROM is a "digital dictionary," a memory device whose output code is dependent on an input code.The ROM is sometimes referred to as READ ONLY - STORAGE,fixed memory,permanent storage,or dead memory,but basically it is an array-type multiple-output combinational  circuit having basically the model shown in fig:

ROM's are characterized or specified by their "size" This size or bit number is determine by the following expression:

ROM size= 2^n X m

where n=number of inputs

               m=number of outputs 
For example, a ROM with ten inputs and eight outputs would be specified as an
8k ROM
1024 X 8 ROM

Detection of circularly polarised light

To confirm whether light emerging out of the quarter-wave plate is circularly whether light emerging out of the quarter-wave plate is circularly polarised,it is first passed through another quarter-wave plate with optic axis in any position.The light on 

viscosity

Q.Define clearly coefficient of viscosity and find its dimensions. 
Viscosity.When the motion of a liquid over a horizontal solid surface is slow and steady,its layer in contact with the solid surface is stationary .In other words,its velocity along the surface is zero.The velocity of any other layer is proportional to its distance from the stationary layer and is maximum for the top-most layer.
if we consider any particular layer of the liquid,we find that the layer immediately above.it is moving faster than the layer immediately below it.
Hence the upper layer tends to increase the velocity of the lower layer whereas the lower layer tends to decrease the velocity of the upper layer.The two layers together tend to destroy their relative motion as if there is some backward dragging force acting tangentially on the layers.Consequently,if a relative velocity is to be maintained between the two layers of a liquid,an external force is required to overcome this backward drag.
This property by virtue of which a liquid opposes relative motion between its different layers is called viscosity.
Coefficient of viscosity.Let a layer RS of a liquid move with velocity v relative to a parallel layer PQ which is at a distance r from it.Let the force on an area A required to produce this motion be F.This force must act on RS in the direction of motion.An equal force will,therefore ,act on it in the opposite direction due to viscosity.

This backward dragging force F depends upon the following factors: