Generator Chracteristics

The three most important characteristics or curves of a d.c generator are
1. No-load saturation characteristic (E0/If)
It is also know as Magnetic characteristic or Open circuit Characteristic (O.C.C). It shows the reation between the no-load generated e.m.f in armature, E0 and the field or exciting current If at a given fixed speed. It is just te magnetisation curve for the material of the electromagnets.Its shape is practically the same for all generators whether separately-excited or self-excited.

A typical no load saturation curve is shown in Figure.It has generator output voltage plotted against field current.The lower straight line portion of the curve represents the air gap because the magnetic parts are not saturated. When the magnetic parts start to saturate, the curve bends over until complete saturation is reached. Then the curve becomes a straight line again.

Separately-excited Generator
The No-load saturation curve of a separately excited generator will be as shown in the above figure.It is obvous that when If is increased from its initial small value, the flux and hence generated e.m.f Eg increase irectly as curent so long as the poles are unsaturated.This is represented by straight portion in figure.But as the flux denity increases,the poles become saturated, so a greater increase If is required to produce a given increase in voltage than on the lower part of the curve.That is why the upper portion of the curve bends.

Self-excited Generator (Series or Shunt )

The O.C.C curve for self-excited generators whether shunt or series wound is shown in above figure.Due to the residal magnetism in the poles, some e.m.f (=OA) is gnerated even when If =0.Hence, the curve starts a little way up.The slight curvature at the lower end is due to magnetic inertia.It is seen that the first part of the curve is practically straight.This is due to fact that at low flux densities reluctance of iron path being negligible,total reluctance is given by the air gap reluctance which is constant.Hence,the flux and consequently,the generated e.m.f is directly proportional to the exciting current.However, at high flux densities, where μ is small,iron path reluctance becomes appreciable and straight relation between E and If no longer holds good.In other words,after point B, saturation of pole starts.However, the initial slope of the curve is determined by air-gap width.O.C.C for higher speed would lie above this curve and for lower speed,would lie below it.
Compound-wound Generator

If the series field amp-turns are such as to produce the same voltage at rate load as at no load.then th generator is flat-compounded. It should be noted, however, that even in the case of a flat-cmpounded generator, the voltage is not constant form no load to rated load. At half load, the voltage is actually greater than the rated voltage as seen from figure.

If the series field amp-turns are such that the rated voltage is greater than the no-load voltage, then generator is over-compounded.If rated voltage is less than the no -load voltage, then the generator is under-compound.

2. Internal or Total characteristic (E/Ia)
It gives the relation between the e.m.f E atually induces in the armature (after allowing for the demagnetising effect of armature reaction) and the armature current Ia.

Separately-excited Generator
Let us consider a separately-excited generator giving its rated no-load voltage of E0 for a certain constant field current.If there were no armature reaction and armature voltage drop,then this voltage would have remained constant as shown in figure by the horizontal line 1. But when the generator is loaded, the voltage falls due to these two causes, thereby gving slightly dropping characteristics.If we subtract from E0 the values of voltage drops due to armature reaction for different loads, then we get the value of E-the e.m.f actually induced in the armature under load conditions.Curve 2 is plotted in this way and is known as the internal characteristic. Series Generator
In this genarator, because field windings are in series with the armature, they carry full armature current Ia. As Ia is increased, flux and hence generated e.m.f. is also increased as shown by the curve. Curve Oa is the O.C.C. The extra exciting current necessary to neutralize the weakening effect of armature reaction at full load is given by the horizontal distance ab. Hence, point b is on the internal characteristic.
3. External characteristic (V/I)It is also referred to as performance characteristic or sometimes voltage-regulating curve.
It gives relation between the terminal voltage V and the load current I.This curve lies below the internal characteristic because it takes in to account the voltage drop over the armature circuit resistance.The values of V are obtained by subtracting IaRa from corresponding values of E.This characteristic is of great importnce in judging the suitability of a generator for a particular purpose.It may be obtained in two ways (i) by making simultaneous measurements with a suitable voltmeter and an ammeter on a loaded generator or (ii) graphically from the O.C.C provided the armature and field resistances are known and also if the demagnetising effect or the armature reaction is known.
Figure above shows the external characteristic curves for generators with various types of excitation. If a generator, which is separately excited, is driven at constant speed and has a fixed field current, the output voltage will decrease with increased load current as shown. This decrease is due to the armature resistance and armature reaction effects. If the field flux remained constant, the generated voltage would tend to remain constant and the output voltage would be equal to the generated voltage minus the IR drop of the armature circuit. However, the demagnetizing component of armature reactions tends to decrease the flux, thus adding an additional factor, which decreases the output voltage.

In a shunt excited generator, it can be seen that the output voltage decreases faster than with separate excitation. This is due to the fact that, since the output voltage is reduced because of the armature reaction effect and armature IR drop, the field voltage is also reduced which further reduces the flux. It can also be seen that beyond a certain critical value, the shunt generator shows a reversal in trend of current values with decreasing voltages. This point of maximum current output is known as the breakdown point. At the short circuit condition, the only flux available to produce current is the residual magnetism of the armature.
To build up the voltage on a series generator, the external circuit must be connected and its resistance reduced to a comparatively low value. Since the armature is in series with the field, load current must be flowing to obtain flux in the field. As the voltage and current rise the load resistance may be increased to its normal value. As the external characteristic curve shows, the voltage output starts at zero, reaches a peak, and then falls back to zero.
The combination of a shunt field and a series field gives the best external characteristic as illustrated in Figure. The voltage drop, which occurs in the shunt machine, is compensated for by the voltage rise, which occurs in the series machine. The addition of a sufficient number of series turns offsets the armature IR drop and armature reaction effect, resulting in a flat-compound generator, which has a nearly constant voltage. If more series turns are added, the voltage may rise with load and the machine is known as an over-compound generator.

D.C Generator E.M.F Equation,Terminal Voltage and Ratings

E.M.F Equation
Let
Φ = flux/pole in weber
Z = total number of armture conductors
= No.of slots x No.of conductors/slot
P = No.of generator poles
A = No.of parallel paths in armature
N = armature rotation in revolutions per minute (r.p.m)
E = e.m.f induced in any parallel path in armature
Generated e.m.f Eg = e.m.f generated in any one of the parallel paths i.e E.
Average e.m.f geneated /conductor = dΦ/dt volt (n=1)
Now, flux cut/conductor in one revolution dΦ = ΦP Wb
No.of revolutions/second = N/60
Time for one revolution, dt = 60/N second
Hence, according to Faraday's Laws of Electroagnetic Induction,
E.M.F generated/conductor is

For a simplex wave-wound generator
No.of parallel paths = 2
No.of conductors (in series) in one path = Z/2
E.M.F. generated/path is

For a simplex lap-wound generator
No.of parallel paths = P
No.of conductors (in series) in one path = Z/P
E.M.F.generated/path

In general generated e.m.f


where A = 2 - for simplex wave-winding
= P - for simplex lap-winding

Terminal VoltageDC generator output voltage is dependent on three factors : (1) the number of conductor loops in series in the armature, (2) armature speed, and (3) magnetic field strength. In order to change the generator output, one of these three factors must be varied. The number of conductors in the armature can not be changed in a normally operating generator, and it is usually impractical to change the speed at which the armature rotates. The strength of the magnetic field, however, can be changed quite easily by varying the current through the field winding. This is the most widely used method for regulating the output voltage of a DC generator.

DC Generator Ratings
A DC generator contains four ratings.
Voltage: Voltage rating of a machine is based on the insulation type and design of
the machine.
Current: The current rating is based on the size of the conductor and the amount of
heat that can be dissipated in the generator.
Power: The power rating is based on the mechanical limitations of the device that is used to turn the generator and on the thermal limits of conductors,bearings, and other components of the generator.
Speed: Speed rating, at the upper limit, is determined by the speed at which mechanical damage is done to the machine. The lower speed rating is based on the limit for field current (as speed increases, a higher field current is necessary to produce the same voltage).

Generator Efficiency

Various power stages in the case of a d.c generator are shown below



Following are the three gnerator efficiencies

1. Mechanical Efficiency


2. Electrical Efficiency

3.Overall or Commercial Efficiency

It is obvious that overall efficiency is the product of mechanical and electrical efficiencies. For good generators,its value may be as high as 95%.

Condition for Maximum Efficiency
In general generator efficiency = Output / (Output + losses)
The condtion for maximum efficiency of generator is given by

i.e Variable loss = Constant loss.

Generator Losses - Copper,Hysteresis, Eddy Current and Mechanical losses

In dc generators, as in most electrical devices, certain forces act to decrease the efficiency. These forces, as they affect the armature, are considered as losses and may be defined as follows:
1. Copper loss in the winding 2. Magnetic Losses 3. Mechanical Losses

Copper lossThe power lost in the form of heat in the armature winding of a generator is known as Copper loss. Heat is generated any time current flows in a conductor.
loss is the Copper loss, which increases as current increases. The amount of heat generated is also proportional to the resistance of the conductor. The resistance of the conductor varies directly with its length and inversely with its cross- sectional area. Copper loss is minimized in armature windings by using large diameter wire.Copper loss is again divided as

(i) Armature copper loss
= Armature copper loss. Where Ra =resistance of armature and interpoles and series field winding etc. This loss is about 30 to 40% of full -load losses.

(ii) Field copper loss : It is the loss in series or shunt field of generator.
is the field copper loss in case of series generators, where Rse is the resistance of the series field widing.

is the field copper loss in case of shunt generators.


This loss is about 20 to 30% of F.L losses.
(iii) The loss due to brsh contact resistance.It is usually inluded in the armture copper loss.

Magnetic Losses (also known as iron or core losses)(i) Hysteresis loss (Wh)Hysteresis loss is a heat loss caused by the magnetic properties of the armature. When an armature core is in a magnetic field, the magnetic particles of the core tend to line up with the magnetic field. When the armature core is rotating, its magnetic field keeps changing direction. The continuous movement of the magnetic particles, as they try to align themselves with the magnetic field, produces molecular friction. This, in turn, produces heat. This heat is transmitted to the armature windings. The heat causes armature resistances to increase. To compensate for hysteresis losses, heat-treated silicon steel laminations are used in most dc generator armatures. After the steel has been formed to the proper shape, the laminations are heated and allowed to cool. This annealing process reduces the hysteresis loss to a low value.


(ii) Eddy Current Loss (We)The core of a generator armature is made from soft iron, which is a conducting material with desirable magnetic characteristics. Any conductor will have currents induced in it when it is rotated in a magnetic field. These currents that are induced in the generator armature core are called EDDY CURRENTS. The power dissipated in the form of heat, as a result of the eddy currents, is considered a loss.

Eddy currents, just like any other electrical currents, are affected by the resistance of the material in which the currents flow. The resistance of any material is inversely proportional to its cross-sectional area. Figure, view A, shows the eddy currents induced in an armature core that is a solid piece of soft iron. Figure, view B, shows a soft iron core of the same size, but made up of several small pieces insulated from each other. This process is called lamination. The currents in each piece of the laminated core are considerably less than in the solid core because the resistance of the pieces is much higher. (Resistance is inversely proportional to cross-sectional area.) The currents in the individual pieces of the laminated core are so small that the sum of the individual currents is much less than the total of eddy currents in the solid iron core.

As you can see, eddy current losses are kept low when the core material is made up of many thin sheets of metal. Laminations in a small generator armature may be as thin as 1/64 inch. The laminations are insulated from each other by a thin coat of lacquer or, in some instances, simply by the oxidation of the surfaces. Oxidation is caused by contact with the air while the laminations are being annealed. The insulation value need not be high because the voltages induced are very small.

Most generators use armatures with laminated cores to reduce eddy current losses.

These magnetic losses are practically constant for shunt and compound-wound generators, because in their case, field current is constant.

Mechanical or Rotational Losses
These consist of
(i) friction loss at bearings and comutator.
(ii) air-friction or windage loss of rotating armature
These are about 10 to 20% of F.L losses.

Careful maintenance can be instrumental in keeping bearing friction to a minimum. Clean bearings and proper lubrication are essential to the reduction of bearing friction.Brush friction is reduced by assuring proper brush seating, using proper brushes, and maintaining proper brush tension. A smooth and clean commutator also aids in the reduction of brush friction.
Usually, magnetic and mechanical losses are collectively known as Stray Losses. These are also known as rotational losses for obvious reasons.

As said above, field Cu loss is constant for shunt and compound generators.Hence, stray losses and shunt Cu loss are constant in their case.These losses are together known as standing or constant losses Wc.

Hence, for shunt and compound generators,
Total loss = armature copper loss + Wc
Armature Cu loss is known as variable loss because it varies with the load current.
Total loss = Variable loss + constant losses Wc

Armature Reaction in Generator, Compensating Windings and InterPoles

All current-carrying conductors produce magnetic fields. The magnetic field produced by current in the armature of a dc generator affects the flux pattern and distorts the main field. This distortion causes a shift in the neutral plane, which affects commutation. This change in the neutral plane and the reaction of the magnetic field is called armature reaction.

You know that for proper commutation, the coil short-circuited by the brushes must be in the neutral plane. Consider the operation of a simple two-pole dc generator, shown in figure. View A of the figure shows the field poles and the main magnetic field. In this the flux is distributed symmetrically with respect to polar axis,which is the line joining the centres of NS poles.The Magnetic neutral axis or plane (M.N.A)coincides with the geometrical neutral axis or plane (G.N.A).M.N.A may be defined as the axis along which no e.m.f is produced in the armature conductors or the axis which is perpendicular to the flux passing through armature.

The armature is shown in a simplified view in views B and C with the cross section of its coil represented as little circles. The symbols within the circles represent arrows. The dot (0)represents the point of the arrow coming toward you, and the cross (+) represents the tail, or feathered end, going away from you. When the armature rotates clockwise, the sides of the coil to the left will have current flowing toward you, as indicated by the dot. The side of the coil to the right will have current flowing away from you, as indicated by the cross. The field generated around each side of the coil is shown in view B of figure. This field increases in strength for each wire in the armature coil, and sets up a magnetic field almost perpendicular to the main field.

Now you have two fields — the main field, view A, and the field around the armature coil, view B. View C of figure shows how the armature field distorts the main field and how the neutral plane is shifted in the direction of rotation. If the brushes remain in the old neutral plane, they will be short- circuiting coils that have voltage induced in them. Consequently, there will be arcing between the brushes and commutator. To prevent arcing, the brushes must be shifted to the new neutral plane.Due to this brush shift,the armature conductors and hence armature current is redistribued. Some armature conductors which were earlier under the influence of N-pole come under the influence of S-pole and vice-versa.

Compensating Windings and InterPolesShifting the brushes to the advanced position (the new neutral plane) does not completely solve the problems of armature reaction. The effect of armature reaction varies with the load current. Therefore, each time the load current varies, the neutral plane shifts. This means the brush position must be changed each time the load current varies.
In small generators, the effects of armature reaction are reduced by actually mechanically shifting the position of the brushes. The practice of shifting the brush position for each current variation is not practiced except in small generators. In larger generators, other means are taken to eliminate armature reaction. COMPENSATING WINDINGS or INTERPOLES are used for this purpose.Their function is to neutralize the cross magnetizing effect of armature reaction. The compensating windings consist of a series of coils embedded in slots in the pole faces. These coils are connected in series with the armature in such a way that the current in them flows in opposite direction to that flowing in armature conductors directly below the pole shoes.
The series-connected compensating windings produce a magnetic field, which varies directly with armature current. Because the compensating windings are wound to produce a field that opposes the magnetic field of the armature, they tend to cancel the effects of the armature magnetic field. The neutral plane will remain stationary and in its original position for all values of armature current. Because of this, once the brushes have been set correctly, they do not have to be moved again.
It should be carefully noted that compensating winding must provide sufficient m.m.f so as to counter balance the armature m.m.f. Let
Zc = No.of compensating conuctors/pole face
Za = No.of active armature conductors/pole
Ia = Total armature current
Ia/A = Current/armature conductor
ZcIa = Za (Ia/A) or Zc = Za/A

Another way to reduce the effects of armature reaction is to place small auxiliary poles called "interpoles" between the main field poles. The interpoles have a few turns of large wire and are connected in series with the armature. Interpoles are wound and placed so that each interpole has the same magnetic polarity as the main pole ahead of it, in the direction of rotation. The field generated by the interpoles produces the same effect as the compensating winding. This field, in effect, cancels the armature reaction for all values of load current by causing a shift in the neutral plane opposite to the shift caused by armature reaction. The amount of shift caused by the interpoles will equal the shift caused by armature reaction since both shifts are a result of armature current.

Ferro Resonance - Introduction,Classification and Characterstics

Introduction
The term "Ferro-resonance ", which appeared in the literature for the first time in 1920, refers to all oscillating phenomena occurring in an electric circuit which must contain at least:

• a non-linear inductance (ferromagnetic and
  saturable),
• a capacitor,
• a voltage source (generally sinusoidal),
• low losses.

Power networks are made up of a large number of saturable inductances (power transformers, voltage measurement inductive transformers (VT), shunt reactors), as well as capacitors cables, long lines, capacitor voltage transformers, series or shunt capacitor banks,voltage grading capacitors in circuit-breakers,metalclad substations). They thus present scenarios under which ferroresonance can occur.
The main feature of this phenomenon is that more than one stable steady state response is possible for the same set of the network parameters. Transients, lightning overvoltages,energizing or deenergizing transformers or loads, occurrence or removal of faults, live works, etc...may initiate ferroresonance. The response can suddenly jump from one normal steady state response (sinusoidal at the same frequency as the source) to an another ferroresonant steady state response characterised by high overvoltages and harmonic levels which can lead to serious damage to the equipment.
A practical example of such behaviour (surprising for the uninitiated) is the deenergization of a voltage transformer by the opening of a circuit-breaker. As the transformer is still fed through grading capacitors accross the circuit-breaker, this may lead either to zero voltage at the transformer terminals or to permanent highly distorted voltage of an amplitude well over normal voltage.
To prevent the consequences of ferroresonance (untimely tripping of protection devices,destruction of equipment such as power transformers or voltage transformers, production losses,...), it is necessary to:

• understand the phenomenon,
• predict it,
• identify it and
• avoid or eliminate it.

Little is known about this complex phenomenon as it is rare and cannot be analysed or predicted by the computation methods (based on linear approximation) normally used by electrical
engineers. This lack of knowledge means that it is readily considered responsible for a number of unexplained destructions or malfunctionings of equipment.
A distinction drawn between resonance and ferroresonance will highlight the specific and some times disconcerting characteristics of ferroresonance.
Practical examples of electrical power system configurations at risk from ferroresonance are used to identify and emphasise the variety of potentially dangerous configurations.Well-informed system designers avoid putting themselves in such risky situations.
Ferro Resonance
The main differences between a ferroresonant circuit and a linear resonant circuit are for a given ω :

• its resonance possibility in a wide range of
  values of C,
• the frequency of the voltage and current waves
  which may be different from that of the sinusoidal
  voltage source,
• the existence of several stable steady state
  responses for a given configuration and values

Classification of ferroresonant modes
Experience of waveforms appearing on power systems, experiments conducted on reduced system models, together with numerical simulations, enable classification of ferroresonance states into four different types.
This classification corresponds to the steady state condition, i.e. once the transient state is over, as it is difficult for a ferroresonant circuit to distinguish the normal transient state from ferroresonant transient states. However, this in no way implies that transient ferroresonance phenomena do not present a risk for electrical equipment. Dangerous transient overvoltages can occur several system periods after an event (for example following energizing of an unloaded transformer) and persist for several power system cycles.
The four different ferroresonance types are:

• fundamental mode,
• subharmonic mode,
• quasi-periodic mode,
• chaotic mode.

The type of ferroresonance can be identified:

• either by the spectrum of the current and voltage signals,
• or by a stroboscopic image obtained by measuring current i and voltage v at a given point of the system and by plotting in plane v, i the instantaneous values at instants separated
  by a system period.

The characteristics of each type of ferroresonance are defined below.
Fundamental modeVoltages and currents are periodic with a period T equal to the system period, and can contain a varying rate of harmonics. The signal spectrum is a discontinuous spectrum made up of the fundamental f0 of the power system and of its harmonics (2f0, 3f0 ...). The stroboscopic image is reduced to a point far removed from the point representing the normal state.

Subharmonic mode
The signals are periodic with a period nT which is a multiple of the source period. This state is
known as subharmonic n or harmonic 1/n.Subharmonic ferroresonant states are normally of odd order. The spectrum presents a fundamental equal to f0/n (where f0 is the source frequency and n is an integer) and its harmonics (frequency f0 is thus part of the spectrum).A stroboscopic plotted line reveals n points.

Quasi-periodic mode
This mode (also called pseudo-periodic) is not periodic. The spectrum is a discontinuous spectrum whose frequencies are expressed in the form: nf1+mf2 (where n and m are integers
and f1/f2 an irrational real number). The stroboscopic image shows a closed curve.


Chaotic mode
The corresponding spectrum is continuous, i.e. it is not cancelled for any frequency. The stroboscopic image is made up of completely separate points occupying an area in plane v, i known as the strange attractor.

Since it is not possible to discuss the different case studies here iam giving the links related to different cases of ferroresonance

Ferroresonace - Link1 Link2
Examples of ferroresonance in a high voltage power system - click here
Modeling Ferroresonance Phenomena in an Underground Distribution System - click here
Examples of Ferroresonance in Distribution sysems - click here
Summary of a recent ferroresonance study - click here

Classification of Generators - Series,Shunt,Compound

Generators are usually classified according to the way in which their fields are excited.The field windings provide the excitation necessary to set up the magnetic fields in the machine. There are various types of field windings that can be used in the generator or motor circuit. In addition to the following field winding types, permanent magnet fields are used on some smaller DC products.Generators may be divided in to (a) Separately-excited generators and (b) Self-excited generators.
(a) Separately-excited generators are those whoe field magnets are energised from an independent external source of DC current.
(b) Self-excited generators are those whose field magnets are energused by the current produced by the generators themselves.Due to residual magnetism, there is always present someflux in the poles.When the armature is rotated, some e.m.f and hence some induced current is produced which is partly or fully passed through the field coils thereby strengthening the residual pole flux.

Self-excited generators are classed according to the type of field connection they use. There are three general types of field connections — SERIES-WOUND, SHUNT-WOUND (parallel), and COMPOUND-WOUND. Compound-wound generators are further classified as cumulative-compound and differential-compound.

Series-wound generator
In the series-wound generator, shown in figure, the field windings are connected in series with the armature. Current that flows in the armature flows through the external circuit and through the field windings. The external circuit connected to the generator is called the load circuit
A series-wound generator uses very low resistance field coils, which consist of a few turns of large diameter wire.

The voltage output increases as the load circuit starts drawing more current. Under low-load current conditions, the current that flows in the load and through the generator is small. Since small current means that a small magnetic field is set up by the field poles, only a small voltage is induced in the armature. If the resistance of the load decreases, the load current increases. Under this condition, more current flows through the field. This increases the magnetic field and increases the output voltage. A series-wound dc generator has the characteristic that the output voltage varies with load current. This is undesirable in most applications. For this reason, this type of generator is rarely used in everyday practice.

Shunt wound
In this field winding is connected in parallel with the armature conductors and have the full voltage of the generator applied across them.The field coils consist of many turns of small wire. They are connected in parallel with the load. In other words, they are connected across the output voltage of the armature.

Current in the field windings of a shunt-wound generator is independent of the load current (currents in parallel branches are independent of each other). Since field current, and therefore field strength, is not affected by load current, the output voltage remains more nearly constant than does the output voltage of the series-wound generator.

In actual use, the output voltage in a dc shunt-wound generator varies inversely as load current varies. The output voltage decreases as load current increases because the voltage drop across the armature resistance increases (E = IR).

In a series-wound generator, output voltage varies directly with load current. In the shunt-wound generator, output voltage varies inversely with load current. A combination of the two types can overcome the disadvantages of both. This combination of windings is called the compound-wound dc generator.

Compound-wound generator :
Compound-wound generators have a series-field winding in addition to a shunt-field winding, as shown in figure. The shunt and series windings are wound on the same pole pieces. They can be either short-shunt or long-shunt as shown in figures. In a comound generator, the shunt field is stronger than the series field.When series field aids the shunt field, generator is said to be commutatively-compounded.On the other hand if series field opposes the shunt field,the generator is said to be differentially compounded.

In the compound-wound generator when load current increases, the armature voltage decreases just as in the shunt-wound generator. This causes the voltage applied to the shunt-field winding to decrease, which results in a decrease in the magnetic field. This same increase in load current, since it flows through the series winding, causes an increase in the magnetic field produced by that winding.

By proportioning the two fields so that the decrease in the shunt field is just compensated by the increase in the series field, the output voltage remains constant. This is shown in figure, which shows the voltage characteristics of the series-, shunt-, and compound-wound generators. As you can see, by proportioning the effects of the two fields (series and shunt), a compound-wound generator provides a constant output voltage under varying load conditions. Actual curves are seldom, if ever, as perfect as shown.

Armature and its Windings

Armature :
Gramme -Ring armatureThe old Gramme-Ring armature,now obselete is shown in figure view A. Each coil is connected to two commutator segments as shown. One end of coil 1 goes to segment A, and the other end of coil 1 goes to segment B. One end of coil 2 goes to segment C, and the other end of coil 2 goes to segment B. The rest of the coils are connected in a like manner, in series, around the armature. To complete the series arrangement, coil 8 connects to segment A. Therefore, each coil is in series with every other coil.

View B shows a composite view of a Gramme-ring armature. It illustrates more graphically the physical relationship of the coils and commutator locations.
The windings of a Gramme-ring armature are placed on an iron ring. A disadvantage of this arrangement is that the windings located on the inner side of the iron ring cut few lines of flux. Therefore, they have little, if any, voltage induced in them. For this reason, the Gramme-ring armature is not widely used.
Drum-type armature :
A drum-type armature is shown in figure.The armature windings are placed in slots cut in a drum-shaped iron core. Each winding completely surrounds the core so that the entire length of the conductor cuts the main magnetic field. Therefore, the total voltage induced in the armature is greater than in the Gramme-ring. You can see that the drum-type armature is much more efficient than the Gramme-ring. This accounts for the almost universal use of the drum-type armature in modem dc generators.

Armature Windings :Drum-type armatures are wound with either of two types of windings - the Lap Winding or the Wave Winding. The difference beween the two is merely due to the different arrangement of the end connections at the front or commutator end of armature.Each winding can be arranged progressively or retrogressively and connected in simplex,duplex and triplex.The following rules,however,apply to both types of the windings:

(i)The front and back pitch are each approximately equal to the pole-pitch i.e windings should be full-pitched.This results in increased e.m.f round the coils.For special purposes,fractional-pitched windings are deliberately used.
(ii)Both pitches should be odd, otherwise it would be difficult to place the coils properly on the armature.For example if YB and YF were both even,then all the coil sides and conductors would lie either in the upper half of slots or in the lower half.Hence, it would become impossible for one side of the coil to lie in the upper half of one slot and the other side of the same coil to lie in the lower half of some other slot.
(iii) The number of commutator segments is equa to the number of slots or coils because the front ends of conductors are joined to the segments in pairs.
(iv) The winding must close upon itself i.e if we start from agiven point and move from one coil to another,then all conuctors should be traversed and we should reach the same point again without a break or discontinuty in betwen.

Lap Winding :
View A This type of winding is used in dc generators designed for high-current applications. The windings are connected to provide several parallel paths for current in the armature. For this reason, lap-wound armatures used in dc generators require several pairs of poles and brushes.

In lap winding, the finishing end of one coil is connected to a commutator segment and to the starting end of the adjacent coil situated under the same pole an so on,till all the coils have been connected.This type of winding derives its name from the fact it doubles or laps back with its succeding coils.Following points regarding simplex lap winding should be noted:

1. The back and front pitches are odd and of opposite sign.But they can't be equal. They differ by 2 or some multiple thereof.
2. Both YB and YF shpuld be nearly equal to a pole pitch.
3. The average pitch YA = (YB + YF)/2.It equals pole pitch = Z/P.
4. Commutator pitch YC = ±1.
5. Resultant pitch YR is even, being the arithmetical difference of two odd numbers i.e YR = YB - YF.
6. The number of slots for a 2-layer winding is equal to the number of coils.The number of commutator segments is also the same.
7. The number of parallel paths in the armature = mP where 'm' is the multiplicity of the winding and 'P' the number of poles.Taking the first condition, we have YB = YF ± 2m where m=1 fo simplex lap and m =2 for duplex winding etc.

• If YB > YF i.e YB = YF + 2, then we get a progressive or right-handed winding i.e a winding which progresses in the clockwise direction as seen from the comutator end.In this case YC = +1.
• If YB < size="1">F i.e YB = YF - 2,then we get a retrogressive or left-handed winding i.e one which advances in the anti-clockwise direction when seen from the commutator side.In this case YC = -1.
• Hence, it is obvious that for
  

The figures below shows the simplex lap winding in circular form and in development form.


































Wave WindingView B, shows a wave winding on a drum-type armature. This type of winding is used in dc generators employed in high-voltage applications. Notice that the two ends of each coil are connected to commutator segments separated by the distance between poles. This configuration allows the series addition of the voltages in all the windings between brushes. This type of winding only requires one pair of brushes. In practice, a practical generator may have several pairs to improve commutation.
When the end connections of the coils are spread apart as shown in Figure a wave or series winding is formed. In a wave winding there are only two paths regardless of the number of poles. Therefore, this type winding requires only two brushes but can use as many brushes as poles. Because the winding progresses in one direction round the armature in a series of 'waves' it is know as wave winding.If, after passing once round the armature,the winding falls in a slot to the left of its starting point then winding is said to be retrogressive.If, however, it falls one slot to the right, then it is progressive.
The figures below shows the simplex wave winding in circular form and in development form.





















Points to note in case of Wave winding :

1. Both pitches YB and YF are odd and of the same sign.
2. Back and front pitches are nearly equal to the pole pitch and may be equal or differ by 2, in which case, they are respectively one more or one less than the average pitch.
3. Resultant pitch YR = YF + YB.
4. Commutator pitch, YC = YA (in lap winding YC = ±1 ). Also YC = (No.of commutator bars ± 1 ) / No.of pair of poles.
5. The average pitch which must be an integer is given by YA = (Z ± 2)/P = (No.of commutator bars ± 1)/No.of pair of poles.
6. The number of coils i.e NC can be found from the relation NC = (PYA ± 2)/2.
7. It is obvious from 5 that for a wave winding, the number of armature conductors with 2 either added or subtracted must be a multiple of the number of poles of the generator.This restriction eliminates many even numbers which are unsuitable for this winding.
8. The number of armature parallel paths = 2m where 'm' is the multiplicity of the winding.

Pole pitch,Coil pitch,Back pitch,Front pitch,Resultant pitch,Commutator pitch,Conductor and Coil

Pole-pitch :

It may be variously defined as

(i) The periphery of the amature divided by the number of poles of the generator i.e the distance between two adjacent poles .

(ii)It is equal to the nmber of armature conductors per pole

Conductor

The length of a wire lying in the magnetic field and in which an e.m.f is induced,is called a conductor as,for example length AB or CD in fig.

Coil and Winding Element

With reference to right side fig, the two conductors AB and CD along wth their end connections constitute one coil of the armature winding.The coil may be single turn or multi-turn coil.A single turn coil wil have two conductors.But a multi-turn coil may have many conductors per coil side.

Coil-span or Coil-pitch (Ys)

It is the distance, measured in tems o armature slots between two sides of a coil.It is,in fact, the periphery of the armature spanned by the two sides of the coil.

If the coil-pitch is equal to the pole-pitch,then winding is called full-pitched.It means that coil span is 180 electrical degrees.In this case,the coil sides lie under opposite poles,hence the induced e.m.fs in them are additive.Therefore,maximum e.m.f is induced in he coil as a whole.it being the sum of the e.m.fs induced in the two coil sides.

If the coil-pitch is less than the pole-pitch,then the winding is fractional-pitched.In tis case,there is a phase difference between the e.m.fs in the two sides of the coil.

Pitch of a Winding (Y)

In general, it may be defined as the distance round the armature between two successive conductors which are directly connected together.Or it is the istance between the beginings of two consecutive turns.

Y = YB - YF -------- for lap winding

Y = YB + YF -------- for wave winding

Back Pitch (YB)

The distance, measured in terms of the armature conductors,whic a coil advances on the bak of the armature is called back pitch.

Front Pitch (YF)

The number of armature conductors or elements spanned by a coil on the front is called the front pitch.

Alternatively, the front pitch ma be defined as the distance between the second conductor of one coil and the first conductor of the next coil which are connected together at the front i.e commutator end of the armature. Both front and back pitches for lap winding are shown in fig.

Resultant Pitch (YR)

It is the distance between the begining of one coil and the begining of the next coilto which it is connected.It is also shown in fig.

Commutator Pitch (YC)

It is the distance between the segments to which the two ends of a coil are connected.From fig it is clear that for lap winding , Yc is the difference of YB and YF where as for wave winding it is the sum of the two.

Rankine Cycle,Processes and its Variations

The Rankine cycle is a thermodynamic cycle. Like other thermodynamic cycles, the maximum efficiency of the Rankine cycle is given by calculating the maximum efficiency of the Carnot cycle. It is named after William John Macquorn Rankine, a Scottish polymath.

Processes of the Rankine cycle


There are four processes in the Rankine cycle, each changing the state of the working fluid. These states are identified by number in the diagram above.

• Process 4-1: First, the working fluid is pumped (ideally isentropically) from low to high pressure by a pump. Pumping requires a power input (for example mechanical or electrical).
• Process 1-2: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a saturated vapor. Common heat sources for power plant systems are coal (or other chemical energy), natural gas, or nuclear power.
• Process 2-3: The superheated vapor expands through a turbine to generate power output. Ideally, this expansion is isentropic. This decreases the temperature and pressure of the vapor.
• Process 3-4: The vapor then enters a condenser where it is cooled to become a saturated liquid. This liquid then re-enters the pump and the cycle repeats.
  The exposed Rankine cycle can also present vapor overheating , which reduces the amount of liquid condensed after the expansion in the turbine.

Rankine cycles describe the operation of steam heat engines commonly found in power generation plants. In such vapour power plants, power is generated by alternately vaporizing and condensing a working fluid (in many cases water, although refrigerants such as ammonia may also be used).
The working fluid in a Rankine cycle follows a closed loop and is re-used constantly. Water vapour seen billowing from power plants is evaporating cooling water, not working fluid. (NB: steam is invisible until it comes in contact with cool, saturated air, at which point it condenses and forms the white billowy clouds seen leaving cooling towers).
Variables and Equations
heat input rate (energy per unit time)

mass flow rate (mass per unit time)

mechanical power used by or provided to the system (energy per unit time)

η thermodynamic efficiency of the process (power used for turbine per heat input, unitless)
h1,h2,h3,h4 these are the "specific enthalpies" at indicated points on the T-S diagram
Each of the first four equations are easily derived from the energy and mass balance for a control volume. The fifth equation defines the thermodynamic efficiency of the cycle as the ratio of net power output to heat input.





Real Rankine cycle (non-ideal)
In a real Rankine cycle, the compression by the pump and the expansion in the turbine are not isentropic. In other words, these processes are non-reversible and entropy is increased during the two processes (indicated in the figure as ΔS). This somewhat increases the power required by the pump and decreases the power generated by the turbine. It also makes calculations more involved and difficult.

Variations of the basic Rankine cycle
Two main variations of the basic Rankine cycle are used in modern practice.
Rankine cycle with reheat
In this variation, two turbines work in series. The first accepts vapor from the boiler at high pressure. After the vapor has passed through the first turbine, it re-enters the boiler and is reheated before passing through a second, lower pressure turbine. Among other advantages, this prevents the vapor from condensing during its expansion which can seriously damage the turbine blades.
Regenerative Rankine cycle
The regenerative Rankine cycle is so named because after emerging from the condenser (possibly as a subcooled liquid) the working fluid is heated by steam tapped from the hot portion of the cycle. This increases the average temperature of heat addition which in turn increases the thermodynamic efficiency of the cycle.
Organic Rankine cycle
The Organic Rankine Cycle (ORC) uses organic fluids (such as toluene) in place of water and steam. For example, this allows use of lower temperature heat sources such as solar ponds, which typically operated at around 70-90 °C. The efficiency of the cycle is much lower as a result of the lower temperature range, but this can be worthwhile, because of the lower cost involved in gathering heat at this lower temperature.
Reverse Rankine cycle
A Rankine cycle that is driven in reverse, via net work input, is the vapor-compression refrigeration cycle. Its purpose is to move heat, rather than produce work.

For more details click on the below links
ttp://en.wikipedia.org/wiki/Rankine_cycle
http://www.taftan.com/thermodynamics/RANKINE.HTM