pdf book: Problems and Solutions on Solid State Physics, Relativity and Miscellaneous Topics by LIM CLICK HERE TO DOWNLOAD pdf book. (For B.E./ caite.info & other Engg. Examinations). V.K. MEHTA. ROHIT MEHT. ROHIT MEHTA. Scilab Textbook Companion for Principles of Power Systems by. Chapter (1) D.C. Generators Introduction Although a far greater percentage of the electrical machines in service are a.c. machines, the d.c. machines are of.
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Such a fine speed control is generally not possible with a. Thus the ends of any coil are brought out to adjacent commutator segments and the result of this method of connection is that all the coils of the armature. The greater the speed and field current, greater is the generated e. The result is that the operation of other appliances connected to the line may be impaired and in particular cases, they may refuse to work. The conductors connected to these segments lie between the poles in position of zero magnetic flux which is termed as magnetic neutral axis M. The criterion for good commutation is that it should be sparkless. Therefore, when a conductor moves from the influence of N-pole to that of S-pole, the direction of current in the conductor must be reversed.
In order to have sparkless commutation, the brushes on the commutator should be placed at points known as neutral point where no voltage exists between adjacent segments. The conductors connected to these segments lie between the poles in position of zero magnetic flux which is termed as magnetic neutral axis M.
Equation of a D. Generator We shall now derive an expression for the e. Generators The magnetic field in a d. Generators are generally classified according to their methods of field excitation. On this basis, d. Generators A d. The greater the speed and field current, greater is the generated e.
It may be noted that separately excited d. The d. There are three types of self-excited generators depending upon the manner in which the field winding is connected to the armature, namely; i Series generator; ii Shunt generator; iii Compound generator i Series generator In a series wound generator, the field winding is connected in series with armature winding so that whole armature current flows through the field winding as well as the load.
Since the field winding carries the whole of load current, it has a few turns of thick wire having low resistance. Series generators are rarely used except for special purposes e. The shunt field winding has many turns of fine wire having high resistance. Therefore, only a part of armature current flows through shunt field winding and the rest flows through the load. A compound wound generator may be: Obviously, its value will depend upon the amount of current flowing and the value of contact resistance.
This drop is generally small. Machine The losses in a d. All these losses appear as heat and thus raise the temperature of the machine. They also lower the efficiency of the machine. Copper losses These losses occur due to currents in the various windings of the machine. There is also brush contact loss due to brush contact resistance i. This loss is generally included in armature copper loss. Iron or Core losses These losses occur in the armature of a d.
They are of two types viz. Consider a small piece ab of the armature. When the piece ab is under N-pole, the magnetic lines pass from a to b. Half a revolution later, the same piece of iron is under S-pole and magnetic lines pass from b to a so that magnetism in the iron is reversed. In order to reverse continuously the molecular magnets in the armature core, some amount of power has to be spent which is called hysteresis loss. It is given by Steinmetz formula. These voltages produce circulating currents in the armature core as shown in Fig.
These are called eddy currents and power loss due to their flow is called eddy current loss. The eddy current loss appears as heat which raises the temperature of the machine and lowers its efficiency. If a continuous solid iron core is used, the resistance to eddy current path will be small due to large cross-sectional area of the core. Consequently, the magnitude of eddy current and hence eddy current loss will be large. The magnitude of eddy current can be reduced by making core resistance as high as practical.
The laminations are insulated from each other with a coating of varnish. The insulating coating has a high resistance, so very little current flows from one lamination to the other. Also, because each lamination is very thin, the resistance to current flowing through the width of a lamination is also quite large.
Thus laminating a core increases the core resistance which decreases the eddy current and hence the eddy current loss. For this reason, lamination thickness should be kept as small as possible.
Mechanical losses These losses are due to friction and windage. These losses depend upon the speed of the machine. But for a given speed, they are practically constant.
Iron losses and mechanical losses together are called stray losses. The constant losses in a d. The variable losses in a d. Field Cu loss is constant for shunt and compound generators. Consider a shunt generator delivering a load current IL at a terminal voltage V. Fig 1. Although the armature winding is not provided for the purpose of producing a magnetic field, nevertheless the current in the armature winding will also produce magnetic flux called armature flux.
The armature flux distorts and weakens the main flux posing problems for the proper operation of the d. The action of armature flux on the main flux is called armature reaction. In the previous chapter Sec 1. This phenomenon is termed as commutation. The criterion for good commutation is that it should be sparkless. In order to have sparkless commutation, the brushes should lie along magnetic neutral axis.
In this chapter, we shall discuss the various aspects of armature reaction and commutation in a d. However, current flowing through armature conductors also creates a magnetic flux called armature flux that distorts and weakens the flux coming from the poles.
This distortion and field weakening takes place in both generators and motors. The action of armature flux on the main flux is known as armature reaction. The phenomenon of armature reaction in a d. Only one pole is shown for clarity. Referring to Fig 2. This unequal field distribution produces the following two effects: Consequently, the increase in flux at pole tip B is less than the decrease in flux under pole tip A.
As we shall see, the weakening of flux due to armature reaction depends upon the position of brushes. Clearly, it is the axis of symmetry between two adjacent poles. Clearly, no e. With no current in the armature conductors, the M. In order to achieve sparkless commutation, the brushes must lie along M.
However, when current flows in armature conductors, the combined action of main flux and armature flux shifts the M. In case of a generator, the M. In order to achieve sparkless commutation, the brushes have to be moved along the new M. Under such a condition, the armature reaction produces the following two effects: It demagnetizes or weakens the main flux. It cross-magnetizes or distorts the main flux. Let us discuss these effects of armature reaction by considering a 2-pole generator though the following remarks also hold good for a multipolar generator.
The flux across the air gap is uniform. Note that OFm is perpendicular to G. The armature conductors to the left of G. The direction of magnetic lines of force can be found by cork screw rule. It is clear that armature flux is directed downward parallel to the brush axis. The resultant m. Since M. Note that M. Due to brush shift, the m. It is because some of the conductors which were earlier under N-pole now come under S-pole and vice-versa. The result is that armature m.
Now FA can be resolved into rectangular components Fc and Fd. OFm due to main poles. It has a demagnetizing effect on the flux due to main poles. For this reason, it is called the demagnetizing or weakening component of armature reaction. It distorts the main field. For this reason, it is called the cross- magnetizing or distorting component of armature reaction.
It may be noted that with the increase of armature current, both demagnetizing and distorting effects will increase. Conclusions i With brushes located along G. There is only distorting or cross- magnetizing effect of armature reaction. Their relative magnitudes depend on the amount of shift. This shift is directly proportional to the armature current. On the other hand, the distorting component of armature reaction distorts the main flux.
However, when the brushes are shifted from the G. We shall identify the armature conductors that produce demagnetizing effect and those that produce cross-magnetizing effect. These are called demagnetizing armature conductors and constitute the demagnetizing ampere-turns of armature reaction Remember two conductors constitute a turn. These conductors produce the cross-magnetizing or distorting effect i. Therefore, they are called cross-magnetizing conductors and constitute the cross-magnetizing ampere-turns of armature reaction.
This is achieved by adding extra ampere-turns to the main field winding. When a conductor passes a pair of poles, one cycle of voltage is generated. We say one cycle contains electrical degrees. In order to neutralize the cross- magnetizing effect of armature reaction, a compensating winding is used. A compensating winding is an auxiliary winding embedded in slots in the pole faces as shown in Fig.
It is connected in series with armature in a Fig. If Z is the total number of armature conductors and P is the number of poles, then, Z No.
There are two parallel paths between the brushes. Note that the currents in the coils connected to a brush are either all towards the brush positive brush or all directed away from the brush negative brush. Therefore, current in a coil will reverse as the coil passes a brush. When commutation takes place, the coil undergoing commutation is short- circuited by the brush. The brief period during which the coil remains short- circuited is known as commutation period Tc.
If the current reversal is completed by the end of commutation period, it is called ideal commutation. If the current reversal is not completed by that time, then sparking occurs between the brush and the commutator which results in progressive damage to both. Ideal commutation Let us discuss the phenomenon of ideal commutation i.
For this purpose, we consider the coil A. The brush width is equal to the width of one commutator segment and one mica insulation. Suppose the total armature current is 40 A.
Since there are two parallel paths, each coil carries a current of 20 A. The commutator segment 1 conducts a current of 40 A to the brush; 20 A from coil A and 20 A from the adjacent coil as shown. The coil A has yet to undergo commutation. There are now two parallel paths into the brush as long as the short-circuit of coil A exists. For this condition, the resistance of the path through segment 2 is three times the resistance of the path through segment 1 Q contact resistance varies inversely as the area of contact of brush with the segment.
The brush again conducts a current of 40 A; 30 A through segment 1 and 10 A through segment 2. Note that current in coil A the coil undergoing commutation is reduced from 20 A to 10 A.
The brush again conducts 40 A; 20 A through segment 1 and 20 A through segment 2 Q now the resistances of the two parallel paths are equal. Note that now. The brush conducts a current of 40 A; 30 A through segment 2 and 10 A through segment 1. Note that current in coil A is 10 A but in the reverse direction to that before the start of commutation.
The reader may see the action of the commutator in reversing the current in a coil as the coil passes the brush axis. Note that now current in coil A is 20 A but in the reverse direction. Thus the coil A has undergone commutation. Each coil undergoes commutation in this way as it passes the brush axis. Note that during commutation, the coil under consideration remains short- circuited by the brush. The horizontal line AB represents a constant current of 20 A upto the beginning of commutation.
From the finish of commutation, it is represented by another horizontal line CD on the opposite side of the zero line and Fig. The way in which current changes from B to C depends upon the conditions under which the coil undergoes commutation. If the current changes at a uniform rate i. Under such conditions, no sparking will take place between the brush and the commutator.
Practical difficulties The ideal commutation i. This is mainly due to the fact that the armature coils have appreciable inductance. When the current in the coil undergoing commutation changes, self-induced e. This is generally called reactance voltage. This reactance voltage opposes the change of current in the coil undergoing commutation.
The straight line RC represents the ideal commutation whereas the curve BE represents the change in current when self-inductance of the coil is taken into account. This results in sparking just as when any other current- carrying circuit is broken. The sparking results in overheating of commutator- brush contact and causing damage to both. At the end of commutation or short-circuit period, the current in coil A is reversed to a value of 12 A instead of 20 A due to inductance of the coil.
When the brush breaks contact with segment 1, the remaining 8 A current jumps from segment 1 to the brush through air causing sparking between segment 1 and the brush.
Therefore, the time of short circuit or commutation period Tc is equal to the time required by the commutator to move a distance equal to the circumferential thickness of the brush minus the thickness of one insultating strip of mica. The following are the two principal methods of improving commutation: Therefore, there are two parallel paths for the current as long as the short circuit exists.
If the contact resistance between the brush and the commutator is made large, then current would divide in the inverse ratio of contact resistances as for any two resistances in parallel. This is the key point in improving commutation. This is achieved by using carbon brushes instead of Cu brushes which have high contact resistance. This method of improving commutation is called resistance commutation. The coil A is yet to undergo commutation. As the armature rotates, the brush short- circuits the coil A and there are two parallel paths for the current into the brush.
The equivalent electric circuit is shown in Fig. The values of current in the parallel paths of the equivalent circuit are determined by the respective resistances of the paths.
For the condition shown in Fig. Therefore, the current distribution in the paths will be as shown. Note that current in coil A is reduced from 20 A to 10 A due to division of current in he inverse ratio of contact resistances. If the Cu brush is used which has low contact resistance , R1 R2 and the current in coil A would not have reduced to 10 A.
As the carbon brush passes over the commutator, the contact area with segment 2 increases and that with segment 1 decreases i. Therefore, more and more current passes to the brush through segment 2. This is illustrated in Figs. It may be noted that the main cause of sparking during commutation is the production of reactance voltage and carbon brushes cannot prevent it.
Nevertheless, the carbon brushes do help in improving commutation. The other minor advantages of carbon brushes are: Commutation In this method, an arrangement is made to neutralize the reactance voltage by producing a reversing voltage in the coil undergoing commutation. The reversing voltage acts in opposition to the reactance voltage and neutralizes it to some extent. If the reversing voltage is equal to the reactance voltage, the effect of the latter is completely wiped out and we get sparkless commutation.
The reversing voltage may be produced in the following two ways: Since the short-circuited coil is now in the reversing field, the reversing voltage produced cancels the reactance voltage. This method suffers from the following drawbacks: Therefore, the brush shift will depend on the magnitude of armature current which keeps on changing.
This necessitates frequent shifting of brushes. This increases the demagnetizing effect of armature reaction and further weakens the main field. This method is discussed in Sec. These are small poles fixed to the yoke and spaced mid-way between the main poles See Fig. They are wound with comparatively few turns and connected in series with the armature so that they carry armature current. Their polarity is the same as the next main pole ahead in the direction of rotation for a generator See Fig.
Connections for a d. The interpoles perform the following two functions: This leads to sparkless commutation. The e. Since the interpoles carry the armature current and the reactance voltage is also proportional to armature current, the neutralization of reactance voltage is automatic.
It is because the two m. Both these windings are connected in series with the armature and so they carry the armature current. However, the functions they perform must be understood clearly. The main function of commutating winding is to produce reversing or commutating e. The compensating winding neutralizes the cross-magnetizing effect of armature reaction under the pole faces. Because of wear in the bearings, and for other reasons, the air gaps in a generator become unequal and, therefore, the flux in some poles becomes greater than in others.
This causes the voltages of the different paths to be unequal. With unequal voltages in these parallel paths, circulating current will flow even if no current is supplied to an external load. If these currents are large, some of the brushes will be required to carry a greater current at full load than they were designed to carry and this will cause sparking.
To relieve the brushes of these circulating currents, points on the armature that are at the same potential are connected together by means of copper bars called equalizer rings. This is achieved by connecting to the same equalizer ring the coils that occupy the same positions relative to the poles See Fig. Thus referring to Fig. Therefore, the two coils are connected to the same equalizer ring. The equalizers provide a low resistance path for the circulating current.
As a result, the circulating current due to the slight differences in the voltages of the various parallel paths passes through the equalizer rings instead of passing through the brushes. This reduces sparking. For best results, each coil should be connected to an equalizer ring but this is seldom done.
Satisfactory results are obtained by connecting about every third coil to an equalizer ring. In order to distribute the connections to the equalizer rings equally, the number of coils per pole must be divisible by the connection pitch. Equalizer rings are not used in wave winding because there is no imbalance in the voltages of the two parallel paths.
This is due to the fact that conductors in each of the two paths pass under all N and S poles successively unlike a lap winding where all conductors in any parallel path lie under one pair of poles. Therefore, even if there are inequalities in pole flux, they will affect each path equally.
Chapter 3 D. Generator Characteristics Introduction The speed of a d. For general-purpose operation, the prime mover is equipped with a speed governor so that the speed of the generator is practically constant.
Under such condition, the generator performance deals primarily with the relation between excitation, terminal voltage and load. These relations can be best exhibited graphically by means of curves known as generator characteristics. These characteristics show at a glance the behaviour of the generator under different load conditions.
Generator Characteristics The following are the three most important characteristics of a d. Open Circuit Characteristic O. This curve shows the relation between the generated e. It is also known as magnetic characteristic or no-load saturation curve.
Its shape is practically the same for all generators whether separately or self-excited. The data for O.
E is less than E0 due to the demagnetizing effect of armature reaction. Therefore, this curve will lie below the open circuit characteristic O. The internal characteristic is of interest chiefly to the designer. It cannot be obtained directly by experiment.
It is because a voltmeter cannot read the e. The internal characteristic can be obtained from external characteristic if winding resistances are known because armature reaction effect is included in both characteristics. The terminal voltage V will be less than E due to voltage drop in the armature circuit. Therefore, this curve will lie below the internal characteristic. This characteristic is very important in determining the suitability of a generator for a given purpose.
It can be obtained by making simultaneous measurements of terminal voltage and load current with voltmeter and ammeter of a loaded generator. Generator The O. The field winding of the d. The generator is run at fixed speed i. The field current If is increased from zero in steps and the corresponding values of generated e. E0 read off on a voltmeter connected across the armature terminals. On plotting the relation between E0 and If, we get the open circuit characteristic as shown in Fig.
This is due to the residual magnetism in the field poles. It is because in this range, reluctance of iron is negligible as compared with that of air gap. The air gap reluctance is constant and hence linear relationship.
Consequently, the curve deviates from linear relationship. The reader may note that the O. Generator The obvious disadvantage of a separately excited d. But since the output voltage may be controlled more easily and over a wide range from zero to a maximum , this type of excitation finds many applications. The O. Note that if the value of constant speed is increased, the steepness of the curve also increases.
When the field current is zero, the residual magnetism in the poles will give rise to the small initial e. In order to determine the external characteristic, the circuit set up is as shown in Fig. As the load current increases, the terminal voltage falls due to two reasons: Due to these reasons, the external characteristic is a drooping curve [curve 3 in Fig.
Note that in the absence of armature reaction and armature drop, the generated e. The internal characteristic can be determined from external characteristic by adding ILRa drop to the external characteristic. It is because armature reaction drop is included in the external characteristic. If the generator is run at a constant speed, some e. This small e. This process continues and the generator builds up the normal generated voltage following the O.
The field resistance Rf can be represented by a straight line passing through the origin as shown in Fig. The two curves can be shown on the same diagram as they have the same ordinate [See Fig.
An amount AB of the c. Since this surplus voltage is available, it is possible for the field current to increase above the value OA. However, at point D, the available voltage is OM and is all absorbed by i Rf drop. Consequently, the field current cannot increase further and the generator build up stops. Thus in Fig. Hence the generator will build up a voltage OM. The residual voltage will cause a current to flow through the whole series circuit when the circuit is closed. There will then be voltage build up to an equilibrium point exactly analogous to the build up of a shunt generator.
The voltage build up graph will be similar to that of shunt generator except that now load current instead of field current for shunt generator will be taken along x-axis. When the series field is connected in reverse so that its field flux opposes the shunt field flux, the generator is then differential compound.
The easiest way to build up voltage in a compound generator is to start under no load conditions. At no load, only the shunt field is effective. When no-load voltage build up is achieved, the generator is loaded. If under load, the voltage rises, the series field connection is cumulative. If the voltage drops significantly, the connection is differential compound.
As the field circuit resistance is increased, the slope of resistance line also increases. When the field Fig. If the field circuit resistance is increased beyond this point say line OD , the generator will fail to excite. The field circuit resistance represented by line OC tangent to O. It may be defined as under: The maximum field circuit resistance for a given speed with which the shunt generator would just excite is known as its critical field resistance.
It should be noted that shunt generator will build up voltage only if field circuit resistance is less than critical field resistance. Here R1, R2 etc. If the total circuit resistance is R1, then series generator will build up a voltage OL. The Fig. If the total resistance of the circuit is more than RC say line OD , the generator will fail to build up voltage.
Note that Fig. However, R1, R2 etc. However, in Fig. Since there is only one current that which flows through the whole machine , the load current is the same as the exciting current. Curve 1 shows the open circuit characteristic O. It can be obtained experimentally by disconnecting the field winding from the machine and exciting it from a separate d. It gives the relation between the generated e. Due to armature reaction, the flux in the machine will be less than the flux at no load.
Hence, e. E generated under load conditions will be less than the e. E0 generated under no load conditions. It gives the relation between terminal voltage and load current IL:.
The internal and external characteristics of a d. Suppose we are given the internal characteristic of the generator. Let the line OC represent the resistance of the whole machine i. If the load current is OB, drop in the machine is AB i. Then point b will lie on the external characteristic of the generator.
Following similar procedure, other points of external characteristic can be located. It is easy to see that we can also plot internal characteristic from the external characteristic. The armature current Ia splits up into two parts; a small fraction Ish flowing through shunt field winding while the major part IL goes to the external load. The line OA represents the shunt field circuit resistance.
When the generator is run at normal speed, it will build up a voltage OM. E generated on load is less than the e. It gives the relation between terminal voltage V and load current IL. It may be seen from the external characteristic that change in terminal voltage from no-load to full load is small.
The terminal voltage can always be maintained constant by adjusting the field rheostat R automatically 3. However, there is a limit to the increase in load current with the decrease of load resistance. Any decrease of load resistance beyond this point, instead of increasing the current, ultimately results in Fig.
Consequently, the external characteristic turns back dotted curve as shown in Fig. The tangent OA to the curve represents the minimum external resistance required to excite the shunt generator on load and is called critical external resistance. If the resistance of the external circuit is less than the critical external resistance represented by tangent OA in Fig.
There are two critical resistances for a shunt generator viz. For the shunt generator to build up voltage, the former should not be exceeded and the latter must not be gone below. If we are given O. Fig 3. Here we are given O. It is desired to find the O. Therefore, the new value of e. E2 for the same If but at N2 i Fig. Similarly, other points can be located taking different values of If.
The locus of these points will be the O. Clearly, it is the speed for which the given shunt field resistance represents the critical resistance. In other words, the speed of the generator should be higher than the critical speed. The shunt winding can be connected either across the armature only short-shunt connection S or across armature plus series field long-shunt connection G.
The compound generator can be cumulatively compounded or differentially compounded generator. The latter is rarely used in practice.
Therefore, we shall discuss the characteristics of cumulatively- compounded generator. It may be noted that external characteristics of long and short shunt compound generators are almost identical. External characteristic Fig. The series excitation aids the shunt excitation.
In such a case, as the load current increases, the series field m. The increase in generated voltage is greater than the IaRa drop so that instead of decreasing, the terminal voltage increases as shown by curve A in Fig.
The series winding of such a machine has lesser number of turns than the one in over-compounded machine and, therefore, does not increase the flux as much for a given load current. Consequently, the full-load voltage is nearly equal to the no-load voltage as indicated by curve B in Fig 3. Such a machine is called under-compounded generator.
Generators In a d. This is due to the following reasons: However, if power is supplied from a number of small units operating in parallel, then in case of failure of one unit, the continuity of supply can be maintained by other healthy units. Electric power costs less per kWh when the generator producing it is efficiently loaded. Therefore, when load demand on power plant decreases, one or more generators can be shut down and the remaining units can be efficiently loaded.
Therefore, if generators are operated in parallel, the routine or emergency operations can be performed by isolating the affected generator while load is being supplied by other units. This leads to both safety and economy. When added capacity is required, the new unit can be simply paralleled with the old units. In that case a number of smaller units can be operated in parallel to meet the load requirement. Generally a single large unit is more expensive.
The positive terminals of the generators are. When the load on the power plant increases beyond the capacity of this generator, the second shunt generator 2 is connected in parallel wish the first to meet the increased load demand.
The procedure for paralleling generator 2 with generator 1 is as under: Now switch S4 in the field circuit of the generator 2 is closed. This is indicated by voltmeter V2. The main switch S3, is closed, thus putting generator 2 in parallel with generator 1. Note that generator 2 is not supplying any load because its generated e. By increasing the field current and hence induced e. E , the generator 2 can be made to supply proper amount of load.
Thus if generator 1 is to be shut down, the whole load can be shifted onto generator 2 provided it has the capacity to supply that load. In that case, reduce the current supplied by generator 1 to zero This will be indicated by ammeter A1 open C.
The load may be shifted from one generator to another merely by adjusting the field excitation. Let us discuss the load sharing of two generators which have unequal no-load voltages. These values may be changed by field rheostats.
The common terminal voltage or bus-bars voltage will depend upon i the e. It is generally desired to keep the bus- bars voltage constant. This can be achieved by adjusting the field excitations of the generators operating in parallel. This is achieved by connecting two negative brushes together as shown in Fig.
The conductor used to connect these brushes is generally called equalizer bar. Suppose that an attempt is made to operate the two generators in Fig. If, for any reason, the current supplied by generator 1 increases slightly, the current in its series field will increase and raise the generated voltage. This will cause generator 1 to take more load. Since this effect is cumulative, the generator 1 will take the entire load and drive generator 2 as a motor.
Under such conditions, the current in the two machines will be in the direction shown in Fig.
After machine 2 changes from a generator to a motor, the current in the shunt field will remain in the same direction, but the current in the armature and series field will reverse.
Thus the magnetizing action, of the series field opposes that of the shunt field. As the current taken by the machine 2 increases, the demagnetizing action of series field becomes greater and the resultant field becomes weaker. The resultant field will finally become zero and at that time machine 2 will short- circuit machine 1, opening the breaker of either or both machines. To consider this, suppose that current delivered by generator 1 increases [See Fig.
The increased current will not only pass through the series field of generator 1 but also through the equalizer bar and series field of generator 2. Therefore, the voltage of both the machines increases and the generator 2 will take a part of the load. Chapter 4 D. Motors Introduction D. Therefore, it is not surprising to note that for industrial drives, d.
Like d. The use of a particular motor depends upon the mechanical load it has to drive. Motor Principle A machine that converts d. Its operation is based on the principle that when a current carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force. The same d. Motor Consider a part of a multipolar d.
When the terminals of the motor are connected to an external source of d. All conductors under N-pole carry currents in one direction while all the conductors under S-pole carry currents in the opposite direction.
Suppose the conductors under N-pole carry currents into the plane of the paper and those under S-pole carry currents out of the plane of the paper as shown in Fig. All these forces add together to produce a driving torque which sets the armature rotating. Consequently, the direction of force on the conductor remains the same. When the armature of a d. The back e. Consider a shunt wound motor shown in Fig. When d. Therefore, driving torque acts on the armature which begins to rotate.
As the armature rotates, back e. Eb is induced which opposes the applied voltage V. The applied voltage V has to Fig. The electric work done in overcoming and causing the current to flow against Eb is converted into mechanical energy developed in the armature. It follows, therefore, that energy conversion in a d. If the speed of the motor is high, then back e. The presence of back e.
Therefore, the armature current Ia is small and the back e. Therefore, the speed at which the armature conductors move through the field is reduced and hence the back e. Eb falls. The decreased back e. Thus, the driving torque increases as the motor slows down.
The motor will stop slowing down when the armature current is just sufficient to produce the increased torque required by the load. As the armature speed increases, the back e. Eb also increases and causes the armature current Ia to decrease.
The motor will stop accelerating when the armature current is just sufficient to produce the reduced torque required by the load. It follows, therefore, that back e. Motor Let in a d. Eb acts in opposition to the Fig. Limitations In practice, we never aim at achieving maximum power due to the following reasons: Motors Like generators, there are three types of d. The current through the shunt field winding is not the same as the armature current.
Shunt field windings are designed to produce the necessary m. Therefore, shunt field current is relatively small compared with the armature current. Therefore, series field winding carries the armature current. Since the current passing through a series field winding is the same as the armature current, series field windings must be designed with much fewer turns than shunt field windings for the same m.
Therefore, a series field winding has a relatively small number of turns of thick wire and, therefore, will possess a low resistance. There are two types of compound motor connections like generators. When the shunt field winding is directly connected across the armature terminals [See Fig. Therefore, shunt field in compound machines is the basic dominant factor in the production of the magnetic field in the machine.
Motor Torque is the turning moment of a force about an axis and is measured by the product of force F and radius r at right angle to which the force acts i. Therefore, each conductor exerts a torque, tending to rotate the armature. The sum of the torques due to all armature conductors is known as gross or armature torque Ta. Let in a d. It is represented by Tsh. The total or gross torque Ta developed in the armature of a motor is not available Fig. Therefore, shaft torque Tsh is somewhat less than the armature torque Ta.
If the speed of the motor is N r. The horse power developed by the shaft torque is known as brake horsepower B. If the motor is running at N r.
Motor For any motor, the torque and speed are very important factors. When the torque increases, the speed of a motor increases and vice-versa. We have seen that for a d. This is not possible because the increase in motor speed must be the result of increased torque. Indeed, it is so in this case. When the flux decreases slightly, the armature current increases to a large value. As a result, in spite of the weakened field, the torque is momentarily increased to a high value and will exceed considerably the value corresponding to the load.
The surplus torque available causes the motor to accelerate and back e. Steady conditions of speed will ultimately be achieved when back e. Illustration Let us illustrate the above point with a numerical example. Suppose a V shunt motor is running at r. The armature resistance is 0. This will result in the production of high value of torque.
However, soon the steady conditions will prevail. This will depend on the system inertia; the more rapidly the motor can alter the speed, the sooner the e. Motors As in a d. This is expected because when current flows through the armature conductors of a d. For a motor with the same polarity and direction of rotation as is for generator, the direction of armature reaction field is reversed. Eg whereas in a motor, the armature current flows against the induced e.
Therefore, it should be expected that for the same direction of rotation and field polarity, the armature flux of the motor will be in the opposite direction to that of the generator.
Hence instead of the main flux being distorted in the direction of rotation as in a generator, it is distorted opposite to the direction of rotation. We can conclude that: Armature reaction in a d. However, in case of a d. With no commutating poles used, the brushes are given a forward lead in a d. Since commutating poles windings carry the armature current, then, when a machine changes from generator to motor with consequent reversal of current , the polarities of commutating poles must be of opposite sign.
Therefore, in a d. This is the opposite of the corresponding relation in a d. Motors Since the armature of a motor is the same as that of a generator, the current from the supply line must divide and pass through the paths of the armature windings.
In order to produce unidirectional force or torque on the armature conductors of a motor, the conductors under any pole must carry the current in the same direction at all times. In this case, the current flows away from the observer in the conductors under the N-pole and towards the observer in the conductors under the S-pole. Therefore, when a conductor moves from the influence of N-pole to that of S-pole, the direction of current in the conductor must be reversed. This is termed as commutation.
The function of the commutator and the brush gear in a d. For good commutation, the following points may be noted: For a d. When the operation of a d. Since commutating poles winding carries armature current, the polarity of commutating pole reverses automatically to the correct polarity. These are [See Fig. The following points may be noted: Since d. Motor Like a d. Motor Characteristics There are three principal types of d. Both shunt and series types have only one field winding wound on the core of each pole of the motor.
The compound type has two separate field windings wound on the core of each pole. The performance of a d. It is also known as electrical characteristic of the motor.
It is very important characteristic as it is often the deciding factor in the selection of the motor for a particular application. It is also known as mechanical characteristic.
The field current Ish is constant since the field winding is directly connected to the supply voltage V which is assumed to be constant. Hence, the flux in a shunt motor is approximately constant. We know that in a d. The questions provided in this book have been selected from various potent resources to provide the students with an idea of how the questions are set and what type of questions to expect on the final day. Indian educational consultant and authors of competitive examinations, V.
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