In Fig.3 is shown the resultant mmf OF (The new position of M.N.A.) which is found by vectorially combining OFm and OFA. And the new position of

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1 Armature Reaction: The effect of magnetic field set up by armature current on the distribution of flux under main poles of a generator. The armature magnetic field has two effects: (i) It demagnetises or weakens the main flux and (ii) It cross-magnetises or distorts it. Fig 1 shows the flux distribution of a bipolar generator when there is no current in the armature conductors. The brushes are touching the armature conductors directly, although in practice, they touch commutator segments, it is seen that: (a) the flux is distributed symmetrically with respect to the polar axis, which is the line joining the centres of NS poles. (b) The magnetic neutral axis (M.N.A.) coincides with the geometrical neutral axis (G.N.A.). Magnetic neutral axis may be defined as the axis along which no emf is produced in the armature conductors because they move parallel to the lines of flux. Or M.N.A. is the axis which is perpendicular to the flux passing through the armature. Fig (1) Brushes are always placed along M.N.A. Hence, M.N.A. is also called axis of commutation because reversal of current in armature conductors takes place across this axis. Vector OFm which represents, both magnitude and direction, the mmf of producing the main flux. Fig 2 shows the field (or flux) set up by the armature conductors alone when carrying current, the field coils being unexcited. The current direction is downwards in conductors under N-pole and upwards in those under S-pole. 26

2 Fig (2) The armature mmf (depending on the strength of the armature current) is shown separately both in magnitude and direction by the vector OFA. Under actual load conditions, the two mmf exist simultaneously in the generator as shown in Fig. 3. It is seen that the flux through the armature is no longer uniform and symmetrical about the pole axis, rather it has been distorted. The flux is seen to be crowded at the trailing pole tips but weakened or thinned out at the leading pole tips (the pole tip which is first met during rotation by armature conductors is known as the leading pole tip and the other as trailing pole tip). The strengthening and weakening of flux is separately shown for a four-pole machine in Fig. 4. Fig (3) Fig (4) 27

3 In Fig.3 is shown the resultant mmf OF (The new position of M.N.A.) which is found by vectorially combining OFm and OFA. And the new position of M.N.A which is always perpendicular to the resultant mmf vector OF, is also shown in the figure. With the shift of M.N.A., say through an angle θ brushes are also shifted so as to lie along the new position of M.N.A. Due to this brush shift, the armature conductors and hence armature current is redistributed. All conductors to the left of new position of M.N.A. but between the two brushes, carry current downwards and those to the right carry current upwards. The armature mmf is found to lie in the direction of the new position of M.N.A. (or brush axis). The armature mmf is now represented by the vector OFA. OFA can now be resolved into two rectangular components, OFd parallel to polar axis and OFC perpendicular to this axis. We find that: (i) Component OFC is at right angles to the vector OFm representing the main mmf It produces distortion in the main field and is hence called the cross-magnetising or distorting component of the armature reaction. (ii) The component OFd is in direct opposition of OFm which represents the main mmf It exerts a demagnetising influence on the main pole flux. Hence, it is called the demagnetising or weakening component of the armature reaction. It should be noted that both distorting and demagnetising effects will increase with increase in the armature current. 28

4 Demagnetising and Cross-magnetising Conductors: All conductors lying within angles AOC = BOD = 2 at the top and bottom of the armature, are carrying current in such a direction as to send the flux through the armature from right to left. It is these conductors which act in direct opposition to the main field and are hence called the demagnetising armature conductors. Now consider the remaining armature conductors lying between angles AOD and COB. These conductors carry current in such a direction as to produce a flux at right angles to the main flux. This results in distortion of the main field. Hence, these conductors are known as cross-magnetising conductors and constitute distorting ampere-conductors. Since armature demagnetising ampere-turns are neutralized by adding extra ampereturns to the main field winding, it is essential to calculate their number. But before proceeding further, it should be remembered that the number of turns is equal to half the number of conductors because two conductors-constitute one turn. Let Z = total number of armature conductors I = current in each armature conductor = Ia/2... for simplex wave winding = Ia/P... for simplex lap winding θm = forward lead in mechanical or geometrical or angular degrees. Total number of armature conductors in angles AOC and BOD is conductors constitute one turn, 4 m 360 Z. As two 29

5 2 Total number of turns in these angles = m Z 360 Demagnetising amp turns per pair of pole = 2 m 360 ZI AT d per pole = m ZI 360 The conductors lying between angles AOD and BOC constitute what are known as distorting or cross-magnetising conductors. Their number is found as under: Total armature-conductors/pole both cross and demagnetising = Z / P Corss-magnetising conductors/pole = Z P Z 2 m 1 2 m Z 360 p 360 Cross-magnetising amp-conductors/pole = 1 2 m ZI p 360 Corss-magnetising amp-turns/pole = 1 m ZI 2 p m ATc/pole = ZI 2 p 360 For neutralizing the demagnetising effect of armature-reaction, an extra number of turns may be put on each pole. AT No. of extra turns/pole = I f d I f field current If the leakage coefficient is given, then multiply each of the above expressions by it. If lead angle is given in electrical degrees, it should be converted into mechanical degrees by the following relation: m e 2 e p 2 p Compensating Windings: These are used for large direct current machines which are subjected to large fluctuations in load i.e. rolling mill motors and turbo-generators etc. Their function is to neutralize the cross magnetizing effect of armature reaction. In the absence of compensating windings, the flux will be suddenly shifting backward and forward with every change in load. This shifting of flux will induce statically induced emf in the armature coils. The magnitude of this emf will depend upon the rapidity of changes in load and the amount of change. It may be so high as to strike an arc between 30

6 the consecutive commutator segments across the top of the mica sheets separating them. This may further develop into a flashover around the whole commutator thereby shortcircuiting the whole armature. These windings are embedded in slots in the pole shoes and are connected in series with 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. Compensating winding must provide sufficient m.m.f so as to counterbalance the armature mmf Let Zc = No. of compensating conductos/pole face Za = No. of active armature conductors/pole, Ia = Total armature current Ia/A = current of armature conductor ZcIa = Za (Ia/A) or Zc = Za/A No. of armature conductors/pole = Z/P No. of armature turns/pole = Z/2P No. of armature-turns immediately under one pole (apprixamatelly) Z pole arc Z =0.7 2 P pole pitch 2 p 31

7 Z No. of armature amp-turns/pole for compensating winding=0.7 2 p amp-turns/pole =0.7armature Commutation: the currents induced in armature conductors of a d.c. generator are alternating. These currents flow in one direction when armature conductors are under N-pole and in the opposite direction when they are under S-pole. As conductors pass out of the influence of a N-pole and enter that of S-pole, the current in them is reversed. This reversal of current takes place along magnetic neutral axis or brush axis i.e. when the brush spans and hence shortcircuits that particular coil undergoing reversal of current through it. This process by which current in the short-circuited coil is reversed while it crosses the M.N.A. is called commutation. The brief period during which coil remains short-circuited is known as commutation period Tc. If the current reversal i.e. the change from + I to zero and then to I is completed by the end of short circuit or commutation period, then the commutation is ideal. If current reversal is not complete by that time, then sparking is produced between the brush and the commutator which results in progressive damage to both. The brush width is equal to the width of one commutator segment and one mica insulation. In Fig (a) coil B is about to be short circuited because brush is about to come in touch with commutator segment a. It is assumed that each coil carries 20 A, so that brush current is 40 A. Prior to the beginning of short circuit, coil B belongs to the group of coils lying to the left of the brush and carries 20 A from left to right. In Fig (b) the current through coil B has reduced down from 20 A to 10 A. As area of contact of the brush is more with segment b than with segment a, it receives 30 A from the former, the total again being 40 A. Fig (c) shows the coil B in the middle of its short-circuit period, the brush contact areas with the two segments b and a are equal. The current through it has decreased to zero. The two currents of value 20 A each, pass to the brush directly from coil A and C. 32

8 In Fig (d), coil B has become part of the group of coils lying to the right of the brush. Coil B now carries 10 A in the reverse direction which combines with 20 A supplied by coil A to make up 30 A that passes from segment a to the brush. The other 10 A is supplied by coil C and passes from segment b to the brush, again giving a total of 40 A at the brush. Fig (e) depicts the moment when coil B is almost at the end of commutation or short circuit period. For ideal commutation, current through it should have reversed but it is carrying 15 A only instead of 20 A. If the current varies at a uniform rate i.e. if BC is a straight line, then it is referred to as linear commutation. However, due to the production of self-induced emf in the coil the variations follow the dotted curve. It is seen that, in that case, current in coil B has reached only a value of KF = 15 A in the reversed direction, hence the difference of 5 A (20-15 A) passes as a spark. So, we conclude that sparking at the brushes, which results in poor commutation is due to the inability of the current in the short-circuited coil to reverse completely by the end of short-circuit period (which is usually of the order of 1/500 second). The main cause which retards or delays this quick reversal is the production of self-induced emf in the coil undergoing commutation. It may be pointed out that the coil possesses appreciable amount of self inductance because it lies embedded in the armature which is 33

9 built up of a material of high magnetic permeability. This self-induced emf is known as reactance voltage. Methods of Improving Commutation: There are two practical ways of improving commutation i.e. of making current reversal in the short-circuited coil as sparkless as possible. These methods are known as (i) resistance commutation and (ii) emf. commutation (which is done with the help of either brush lead or interpoles, usually the later). Resistance Commutation: This method of improving commutation consists of replacing low-resistance Cu brushes by comparatively high-resistance carbon brushes. When current I from coil C reaches the commutator segment b, it has two parallel paths open to it. The first part is straight from bar b to the brush and the other parallel path is via the short-circuited coil B to bar a and then to the brush. If the Cu brushes are used, then there is no inducement for the current to follow the second longer path, it would preferably follow the first path. But when carbon brushes having high resistance are used, then current I coming from C will prefer to pass through the second path. The additional advantages of carbon brushes are that (i) they are to some degree selflubricating and polish the commutator and (ii) should sparking occur, they would damage the commutator less than when Cu brushes are used. But some of their minor disadvantages are: (i) Due to their high contact resistance (which is beneficial to sparkless commutation) a loss of approximately 2 volt is caused. Hence, they are not much suitable for small machines where this voltage forms an appreciable percentage loss. (ii) Owing to this large loss, the commutator has to be made some what larger than with Cu brushes in order to dissipate heat efficiently without greater rise of temperature. (iii) because of their lower current density (about 7-8 A/cm2 as compared to A/cm2 for Cu brushes) they need larger brush holders. 34

10 EMF Commutation: In this method, arrangement is made to neutralize the reactance voltage by producing a reversing emf in the short-circuited coil under commutation. This reversing emf, as the name shows, is an emf in opposition to the reactance voltage and if its value is made equal to the latter, it will completely wipe it off, thereby producing quick reversal of current in the short-circuited coil which will result in sparkless commutation. The reversing emf may be produced in two ways: (i) either by giving the brushes a forward lead sufficient enough to bring the short-circuited coil under the influence of next pole of opposite polarity or (ii) by using interpoles. The first method was used in the early machines but has now been abandoned due to many other difficulties it brings along with. Interpoles of Compoles: These are small poles fixed to the yoke and spaced in between the main poles. They are wound with comparatively few heavy gauge Cu wire turns and are connected in series with the armature so that they carry full armature current. Their polarity, in the case of a generator, is the same as that of the main pole ahead in the direction of rotation. The function of interpoles is two-fold: (i) As their polarity is the same as that of the main pole ahead, they induce an emf in the coil (under commutation) which helps the reversal of current. The emf induced by the compoles is known as commutating or reversing emf. The commutating emf neutralizes the reactance emf thereby making commutation sparkless. With interpoles, sparkless commutation can be obtained up to 20 to 30% overload with fixed brush position. In fact, interpoles raise sparking limit of a machine to almost the same value as heating 35

11 limit. Hence, for a given output, an interpole machine can be made smaller and, therefore, cheaper than a non-interpolar machine. As interpoles carry armature current, their commutating emf is proportional to the armature current. This ensures automatic neutralization of reactance voltage which is also due to armature current. (ii) Another function of the interpoles is to neutralize the cross-magnetising effect of armature reaction. Hence, brushes are not to be shifted from the original position. OF as before, represents the mmf due to main poles. OA represents the crossmagnetising mmf due to armature. BC which represents mmf due to interpoles, is obviously in opposition to OA, hence they cancel each other out. This cancellation of crossmagnetisation is automatic and for all loads because both are produced by the same armature current. The distinction between the interpoles and compensating windings should be clearly understood. Both are connected in series and thier m.m.fs. are such as to neutralize armature reaction. But compoles additionally supply mmf for counteracting the reactance voltage induced in the coil undergoing commutation. Moreover, the action of the compoles is localized; they have negligible effect on the armature reaction occurring on the remainder of the armature periphery. 36

12 Characteristics of D.C. Generators: Following are the three most important characteristics or curves of a d.c. generator: 1. No-load saturation Characteristic (E 0 /If): It is also known as Magnetic Characteristic or Open-circuit Characteristic (O.C.C.). It shows the relation between the no-load generated mmf in armature, E 0 and the field or exciting current I f at a given fixed speed. It is just the magnetisation curve for the material of the electromagnets. Its shape is practically the same for all generators whether separately-excited or self-excited. 2. Internal or Total Characteristic (E/I a ): It gives the relation between the mmf E actually induces in the armature (after allowing for the demagnetising effect of armature reaction) and the armature current I a. This characteristic is of interest mainly to the designer. 3. External Characteristic (V/I): It is also referred to as performance characteristic or sometimes voltage-regulating curve. It gives relation between that terminal voltage V and the load current I. This curve lies below the internal characteristic because it takes into account the voltage drop over the armature circuit resistance. The values of V are obtained by subtracting I a R a from corresponding values of E. This characteristic is of great importance 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. 37

13 provided the armature and field resistances are known and also if the demagnetising effect (under rated load conditions) or the armature reaction (from the short-circuit test) is known. Separately-excited Generator: (a) No-load Saturation Characteristic (E 0 /If): the voltage equation of a d.c. generator is, ZN p E g 60 A volt Hence, if speed is constant, the above relation becomes E = k It is obvious that when If is increased from its initial small value, the flux and hence generated mmf Eg increase directly as current so long as the poles are unsaturated. This is represented by the straight portion Od. But as the flux density increases, the poles become saturated, so a greater increase in If is required to produce a given increase in voltage than on the lower part of the curve. That is why the upper portion db of the curve Odb bends over as shown. (b) Internal and External Characteristics: Let us consider a separately-excited generator giving its rated no-load voltage of E 0 for a certain constant field current (line I). But when the generator is loaded, the voltage falls due to armature reaction and armature voltage drop, thereby giving 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 emf actually induced in the armature under load conditions. Curve II is plotted in this way and is known as the internal characteristic. The straight line Oa 38

14 represents the I a R a drops corresponding to different armature currents. If we subtract from E the armature drop I a R a, we get terminal voltage V. Curve III represents the external characteristic and is obtained by subtracting ordinates the line Oa from those of curve II. No-load Curve for Self-excited Generator: The O.C.C. or no-load saturated curves for self-excited generators whether shunt or series connected, are obtained in a similar way. The field winding of the generator (whether shunt or series wound) is disconnected from the machine and connected to an external source of direct current. The field or exciting current I f is increased by suitable steps (starting from zero) and the corresponding values of E 0 are measured. On plotting the relation between I f and E 0, a curve of this form is obtained. Due to residual magnetism in the poles, some emf (= OA) is generated even when I f = 0. Hence, the curve starts a little way up. It is seen that the first part of the curve is practically straight. This is due to the fact that the flux and consequently, the generated emf is directly proportional to the exciting current. However, at high flux densities saturation of poles starts, and straight relation between E and If no longer holds good. It should be noted that O.C.C. for a higher speed would lie above this curve and for a lower speed, would lie below it. 39

15 Voltage Built up and Critical Resistance for Shunt Generator: For shunt generator, due to residual magnetism in the poles, some emf and hence current, would be generated. This current while passing through the field coils will strengthen the magnetism of the poles (provided field coils are properly connected as regards polarity). This will increase the pole flux which will further increase the generated mmf Increased mmf means more current which further increases the flux and so on. This mutual reinforcement of mmf and flux proceeds on till equilibrium is reached at some point like P. The point lies on the resistance line OA of the field winding. Let R be the resistance of the field winding. Line OA is drawn such that its slope equals the field winding resistance i.e. every point on this curve is such that volt/ampere = R. The voltage OL corresponding to point P represents the maximum voltage to which the machine will build up with R as field resistance. OB represents smaller resistance and the corresponding voltage OM is slightly greater than OL. If field resistance is increased, then slope of the resistance line increased, and hence the maximum voltage to which the generator will build up at a given speed, decreases. If R is increased so much that the resistance line does not cut the O.C.C. at all (like OI), then obviously the machine will fail to excite i.e. there will be no build up of the voltage. If the resistance line just lies along the slope, then with that value of field resistance, the machine will just excite. The value of the resistance represented by the tangent to the curve, is known as critical resistance R c for a given speed. 40

16 How to Find Critical Resistance Rc : First, O.C.C. is plotted from the given data. Then, tangent is drawn to its initial portion. The slope of this curve gives the critical resistance for the speed at which the data was obtained. How to Draw O.C.C. at Different Speeds : Suppose we are given the data for O.C.C. of a generator run at a fixed speed, say, N 1. It will be given that O.C.C. at any other constant speed N 2 can be deduced from the O.C.C. for N 1. If the O.C.C. for speed N 1, is given, since E N for any fixed excitation, then E2 N2 N or 2 E2 E1 E1 N1 N1. As seen, for I f N2 =OH, E 1 =HC. The value of new voltage for the same I f but at N 2 : E2 HC HD. In N this way, point D is located. In a similar way, other such points can be found and the new O.C.C. at N 2 drawn. 1 41

17 Critical Speed Nc: Critical speed of a shunt generator is that speed for which the given shunt field resistance represents critical resistance. Curve 2 corresponds to critical speed because R sh line is tangential to it. Obviously BC AC Nc Nc AC N c Full speed(n) Full Speed N BC Conditions for Build-up of a Shunt Generator: We may summarize the conditions necessary for the build-up of a (self-excited) short generator as follows: 1. There must be some residual magnetism in the generator poles. 42

18 2. For the given direction of rotation, the shunt field coils should be correctly connected to the armature i.e. they should be so connected that the induced current reinforces the mmf produced initially due to residual magnetism. 3. If excited on open circuit, its shunt field resistance should be less than the critical resistance (which can be found from its O.C.C.) 4. If excited on load, then its load resistance should be more than a certain minimum value of resistance which is given by internal characteristic External Characteristic: It is found that if after building up, a shunt generator is loaded, then its terminal voltage V drops with increase in load current. Such a drop in voltage is undesirable especially when the generator is supplying current for light and power for which purpose it is desirable that V should remain practically constant and independent of the load. This condition of constant voltage is almost impossible to be fulfilled with a shunt generator unless the field current is being automatically adjusted by an automatic regulator. Without such regulation terminal voltage drops considerably as the load on the generator is increased. These are three main reasons for the drop in terminal voltage of a shunt generator when under load. (i) Armature resistance drop: As the load current increases, more and more voltage is consumed in the ohmic resistance of the armature circuit. Hence, the terminal voltage V=E I a R a is decreased where E is the induced mmf in the armature under load condition. (ii) Armature reaction drop: Due to the demagnetising effect of armature reaction, pole flux is weakened and so the induced mmf in the armature is decreased. (iii) The drop in terminal voltage V due to (i) and (ii) results in a decreased field current I f which further reduces the induced mmf For obtaining the relation between the terminal voltage and load current, the generator is connected as shown below. The shunt generator is first excited on no-load so that it gives its full open circuit voltage = Oa. Then, the load is gradually applied and, at suitable intervals, the terminal voltage V and the load current I are noted. The field current as recorded by ammeter A1 is kept constant by a rheostat (because during the test, due to heating, shunt field resistance is 43

19 increased). By plotting these readings, the external characteristic is obtained. The portion ab is the working part of this curve. Over this part, if the load resistance is decreased, load current is increased as usual, although this results in a comparatively small additional drop in voltage. These conditions hold good till point b is reached. This point is known as breakdown point. It is found that beyond this point (where load is maximum = OB) any effort to increase load current by further decreasing load resistance results in decreased load current (like OA) due to a very rapid decrease in terminal voltage. Voltage Regulation: By voltage regulation of a generator is meant the change in its terminal voltage with the change in load current when it is running at a constant speed. If the change in voltage between no-load and full load is small, then the generator is said to have good regulation but if the change in voltage is large, then it has poor regulation. The voltage regulation of a d.c. generator is the change in voltage when the load is reduced from rated value to zero, expressed as percentage of the rated load voltage. If no-load voltage of a certain generator is 240 V and rated-load voltage is 220 V, then, regulation = ( )/220 = or 9.1 % Internal or Total Characteristic: Internal characteristic gives the relation between E and I a. Hence, E/I a curve can be obtained from V/I curve. In this figure, ab represents the external characteristic as discussed above. The field resistance line OB is drawn as usual. The horizontal distances from OY line to the line OB give the values of field currents for different terminal voltages. If we add these distances horizontally to the external characteristic ab, then we get the curve for the total armature current i.e. dotted 44

20 curve ac. For example, point d on ac is obtained by making gd = ef. The armature resistance drop line Or is then plotted as usual. If brush contact resistance is assumed constant, then armature voltage drop is proportional to the armature current. For any armature current = OK, armature voltage drop I a R a = mk. If we add these drops to the ordinates of curve ac, we get the internal characteristic. For example, St = mk. The point t lies on the internal characteristic. Other points like t can be found similarly at different armature currents as the total characteristic can be drawn. It may be noted here, in passing, that product EI a gives the total power developed within the armature. Some of this power goes to meet I 2 R losses in armature and shunt field windings and the rest appears as output. If load resistance is decreased, the armature current increases up to a certain load current value. After that, any decrease in load resistance is not accompanied by increase in load current. Rather, it is decreased and the curve turns back. If the load resistance is too small, then the generator is short-circuited and there is no generated mmf due to heavy demagnetisation of main poles. Line OP is tangential to the internal characteristic MB and its slope gives the value of the minimum resistance with which the generator will excite if excited on load. Series Generator: In this generator, because field windings are in series with the armature [Fig.(a)], they carry full armature current I a. As I a is increased, flux and hence 45

21 generated mmf 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. If the ordinate bc = gh = armature voltage drop, then point c lies on the external characteristic [Fig (b)]. It will be noticed that a series generator has rising voltage characteristic i.e. with increase in load; its voltage is also increased. But it is seen that at high loads, the voltage starts decreasing due to excessive demagnetising effects of armature reaction. In fact, terminal voltage starts decreasing as load current is increased as shown by the dotted curve. For a load current OC, the terminal voltage is reduced to zero as shown. Compound-wound Generator: A shunt generator is unsuitable where constancy of terminal voltage is essential, because its terminal voltage decreases as the load on it increases. This decrease in V is particularly objectionable for lighting circuit where even slight change in the voltage makes an appreciable change in the candle power of the incandescent lamps. A shunt generator may be made to supply substantially constant voltage (or even a rise in voltage as the load increases) by adding to it a few turns joined in series with either the armature or the load. These turns are so connected as to aid to shunt turns when the generator supplies load. As the load current increases, the current through the series windings also increase thereby increasing the flux. Due to the 46

22 increase in flux, induced mmf is also increased. By adjusting the number of series turns (or series amp-turns), this increase in mmf can be made to balance the combined voltage drop in the generator due to armature reaction and the armature drop. Hence, V remains practically constant which means that field current is also almost unchanged. We have already discussed the three causes which decrease the terminal voltage of a shunt generator. Out of these three, the first two are neutralized by the series field ampturns and the third one, therefore, does not occur. If the series field amp-turns are such as to produce the same voltage at rated load as at no-load, then the generator is flatcompounded. It should be noted, however, that even in the case of a flat-compounded generator, the voltage is not constant from no-load to rated- load. At half the load, the voltage is actually greater than the rated voltage. If the series field amp-turns are such that the rated-load voltage is greater than the no-load voltage, then generator is overcompounded. If rated-load voltage is less than the no-load voltage, then the generator is under-compounded but such generators are seldom used. For short distances such as in hotels and office buildings, flat-compound generators are used because the loss of voltage over small lengths of the feeder is negligible. But when it is necessary to maintain a constant voltage then an over compounded generator, which combines the functions of a generator and a booster, is invariably used. 47

23 How to Calculate Required Series Turns: Consider a 110-V, 250-ampere generator. Suppose it gives its rated no-load voltage with a field current of 5.8 A. If, now, the series windings are disconnected and the shunt field rheostat is left unchanged then the machine will act as shunt generator, hence its voltage will fall with increase in load current. Further, supply that the field current has to be increased to 6.3 A in order to maintain the rated terminal voltage at full load. If the number of turns of the shunt field winding is 2000, then 2000( )=1000 amp-turns represent the additional excitation that has to be supplied by the series windings. As series turns will be carrying a full load current of 250 A, hence number of series turns = 1000/250 = 4. In general, let I sh = increase in shunt field current required to keep voltage constant from no-load to full load. N sh = No. of shunt field turns per pole (or the total number of turns) N se = No. of series turns per pole (or the total number of turns) I se = current through series winding = armature current Ia for long-shunt = load current I for short-shunt It is seen that while running as a simple generator, the increase in shunt field ampereturns necessary for keeping its voltage constant from no-load to full-load is N sh I sh. This increase in field excitation can be alternatively achieved by adding a few series turns to the shunt generator [Fig. (a)] thereby converting it into a compound generator. N sh I sh = N se I se If other things are known, N se may be found from the above equation. In practice, a few extra series amp-turns are taken in order to allow for the drop in armature. Any surplus amp-turns can be changed with the help of a divertor across the series winding as shown in Fig. (b). As said above, the degree of compounding can be adjusted with the help of a variable-resistance, divertor as shown in Fig. (b). If Id is the current through the divertor of resistance Rd, then remembering that series windings and divertor are in parallel, I se R se =I d R d or R d =I se R se /I d 48

24 Uses of D.C. Generators: 1. Shunt generators with field regulators are used for ordinary lighting and power supply purposes. They are also used for charging batteries because their terminal voltages are almost constant or can be kept constant. 2. Series generators are not used for power supply because of their rising characteristics. However, their rising characteristic makes them suitable for being used as boosters in certain types of distribution systems particularly in railway service. 3. Compound generators: The cumulatively-compound generator is the most widely used d.c. generator because its external characteristic can be adjusted for compensating the voltage drop in the line resistance. Hence, such generators are used for motor driving which require d.c. supply at constant voltage, for lamp loads and for heavy power service such as electric railways. The differential-compound generator has an external characteristic similar to that of a shunt generator but with large demagnetization armature reaction. Hence, it is widely used in arc welding where larger voltage drop is desirable with increase in current. End of Part (2) 49

25 Parallel Operation of DC Generators: In a dc power plant, power is usually supplied from several generators of small ratings connected in parallel instead of from one large generator. This is due to the following reasons: (i) Continuity of service: If a single large generator is used in the power plant, then in case of its breakdown, the whole plant will be shut down. 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. (ii) Efficiency: Generators run most efficiently when loaded to their rated capacity. Therefore, when load demand on power plant decreases, one or more generators can be shut down and the remaining units can be efficiently loaded. (iii) Maintenance and repair: Generators generally require routinemaintenance and repair. 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. (iv) Increasing plant capacity: In the modern world of increasing population, the use of electricity is continuously increasing. When added capacity is required, the new unit can be simply paralleled with the old units. (v) Non-availability of single large unit: In many situations, a single unit of desired large capacity may not be available. 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. Connecting Shunt Generators in Parallel: The generators in a power plant are connected in parallel through bus-bars. The bus-bars are heavy thick copper bars and they act as +ve and -ve terminals. The positive terminals of the generators are.connected to the +ve side of bus-bars and negative terminals to the negative side of bus-bars. Fig. (1) shows shunt generator 1 connected to the bus-bars and supplying load. When the load on the power plant increases beyond the capacity of this generator, the second shunt generator 2 is connected in parallel with the first to meet the increased load demand. The procedure for paralleling generator 2 with generator 1 is as under: (i) The prime mover of generator 2 is brought up to the rated speed. Now switch S4 in the field circuit of the generator 2 is closed. (ii) Next circuit breaker CB-2 is closed and the excitation of generator 2 is adjusted till it generates voltage equal to the bus-bars voltage. This is indicated by voltmeter V2. (iii) Now the generator 2 is ready to be paralleled with generator 1. The main switch S3 is closed, thus putting generator 2 in parallel with 50

26 generator 1. Note that generator 2 is not supplying any load because its generated emf is equal to bus-bars voltage. The generator is said to be floating (i.e. not supplying any load) on the bus-bars. Figure(1) (iv) If generator 2 is to deliver any current, then its generated voltage E should be greater than the bus-bars voltage V. In that case, current supplied by it is I = (E - V)/Ra where Ra is the resistance of the armature circuit. By increasing the field current (and hence induced emf E), the generator 2 can be made to supply proper amount of load. (v) The load may be shifted from one shunt generator to another merely by adjusting the field excitation. 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.B.-1 and then open the main switch S1. Load Sharing: The load sharing between shunt generators in parallel can be easily regulated because of their drooping characteristics. 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. Let E1, E2 = no-load voltages of the two generators R1, R2 = their armature resistances V = common terminal voltage (Bus-bars voltage). Then I1 E1 V and R1 I2 E2 V R2 Thus current output of the generators depends upon the values of E1 and E2. These values may be changed by field rheostats. The common terminal voltage (or bus-bars voltage) will depend upon (i) the emfs of individual generators and (ii) the total load current supplied. It is generally desired to keep the busbars voltage constant. This can be 51

27 achieved by adjusting the field excitations of the generators operating in parallel. Compound Generators in Parallel: Under-compounded generators also operate satisfactorily in parallel but over compounded generators will not operate satisfactorily unless their series fields are paralleled. This is achieved by connecting two negative brushes together as shown in Fig. (2) (i). The conductor used to connect these brushes is generally called equalizer bar. Suppose that an attempt is made to operate the two generators in parallel without an equalizer bar. 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 total load supplied to the system is constant, the current in generator 2 must decrease and as a result its series field is weakened. Since this effect is cumulative, the generator 1 will take the entire load and drive generator 2 as a motor. 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 be short circuited machine 1, opening the breaker of either or both machines. Figure (2) When the equalizer bar is used, a stabilizing action exists and neither machine tends to take all the load. To consider this, suppose that current delivered by generator 1 increases. 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. 52

28 DC MOTORS DC motors are seldom used in ordinary applications because all electric supply companies furnish alternating current However, for special applications such as in steel mills, mines and electric trains, it is advantageous to convert alternating current into direct current in order to use dc motors. The reason is that speed/torque characteristics of dc motors are much more superior to that of ac motors. Therefore, it is not surprising to note that for industrial drives, dc motors are as popular as 3phase induction motors. DC Motor Principle: A machine that converts dc power into mechanical power is known as a dc motor. 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 direction of this force is given by Fleming s left hand rule and magnitude is given by; F BI newtons Basically, there is no constructional difference between a dc motor and a dc generator. The same dc machine can be run as a generator or motor. Working of DC Motor: Consider a part of a multipolar dc motor as shown in Fig. (1). When the terminals of the motor are connected to an external source of dc supply: (i) The field magnets are excited developing alternate N and S poles; (ii) The armature conductors carry ^currents. All conductors under Npole 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.(1). Since each armature conductor is carrying current and is placed in the magnetic field, mechanical force acts on it. Referring to Fig.(1) and applying Fleming s left hand rule, it is clear that force on each conductor is tending to rotate the armature in anticlockwise direction. All these forces add together to produce a driving torque which sets the armature rotating. When the conductor moves from one side of a brush to the other, the current in that conductor is reversed and at the same time it comes under the influence of next pole which is of opposite polarity. Consequently, the direction of force on the conductor remains the same. Fig. (1) 53

29 Back or Counter EMF: When the armature of a dc motor rotates under the influence of the driving torque, the armature conductors move through the magnetic field and hence emf is induced in them as in a generator The induced emf acts in opposite direction to the applied voltage V(Lenz s law) and in known as back or counter emf E. The back emf E(= P ZN/60 A) is always less than the applied voltage V, although this difference is small when the motor is running under normal conditions. If Ra is the armature circuit resistance, then, I a V E Ra Since V and Ra are usually fixed, the value of E will determine the current drawn by the motor. If the speed of the motor is high, then back emf (E=P ZN/60A) is large and hence the motor will draw less armature current and vice-versa. Figure (2) Significance of Back EMF: The presence of back emf makes the dc motor a self-regulating machine i.e., it makes the motor to draw as much armature current as is just sufficient to develop the torque required by the load. When the motor is running on no load, small torque is required to overcome the friction and windage losses. Therefore, the armature current Ia is small and the back emf is nearly equal to the applied voltage. If the motor is loaded, the first effect is to cause the armature to slow down and hence the back emf E falls. The decreased back emf allows a larger current to flow through the armature and larger current means increased driving torque. 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. 54

30 If the load on the motor is decreased, the driving torque is momentarily in excess of the requirement so that armature is accelerated. As the armature speed increases, the back emf E 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 emf in a dc motor regulates the flow of armature current i.e., it automatically changes the armature current to meet the load requirement. Voltage and power Equations of DC Motor: Let in a dc motor, Figure (3) V = applied voltage E = back e.m.f. Ra = armature resistance Ia = armature current V E IaRa. By multiplying this equation by la, we get, VIa EIa I2aRa This is known as power equation of the dc motor. VIa = electric power supplied to armature (armature input) EIa = power developed by armature (armature output) Ia2 Ra = electric power wasted in armature (armature Cu loss) Thus out of the armature input, a small portion (about 5%) is wasted as a Ia2Ra and the remaining portion EIa is converted into mechanical power within the armature. Types of DC Motors: Like generators, there are three types of dc motors characterized by the connections of field winding in relation to the armature: (i) Shunt-wound motor (ii) Series-wound motor (iii) Compound-wound motor Armature Torque of DC Motor: Consider a pulley of radius r meter acted upon by a circumferential force of F Newton which causes it to rotate at N r.p.m. 55

31 Then torque T = F r Newton-meter (N.m) Work done by this force in one revolution =Force distance=f 2 r Joule Power developed = F 2 r N Joule/second or Watt (N in r.p.s unit) = (F r) 2 N Watt 2 N = Angular velocity ω in radian/second Power developed = T ω watt or P = T ω Watt Moreover, if N is in r.p.m., then ω= 2 N/60 rad/s p 2 N T 60 Armature Torque of a Motor: Let Ta be the torque developed by the armature of a motor running at N r.p.m. If Ta is in N.m, then Power developed = Ta 2 N/60 watt...(i), Electrical power converted into mechanical power in the armature=eia watt...(ii) Equating (i) and (ii), we get Ta 2 N/60 = EIa...(iii) Since E = ZNP/60A volt, we have Ta Ta Ia. 1 P ZI a N.m 2 A In the case of a series motor, is directly proportional to Ia (before saturation) because field windings carry full armature current. 2 Ta I a For shunt motors, is practically constant. Ta Ia Shaft Torque (Tsh): The whole of the armature torque, as calculated above, is not available for doing useful work, because a certain percentage of it is required for supplying iron and friction losses in the motor. The torque which is available for doing useful work is known as shaft torque Tsh. It is so called because it is available at the shaft: Tsh Pout N.m 2 N / 60 The difference (Ta Tsh) is known as lost torque and is due to iron and friction losses of the motor. 56

32 Speed of a DC Motor: P ZN 60 A (V I a Ra ) 60 A (V I a Ra ) Then N where K 60A/PZ K PZ E E or N N K E V IaRa and E Therefore, in a dc motor, speed is directly proportional to back emf E and inversely proportional to flux per pole. If a dc motor has initial values of speed, flux per pole and back emf as and E1 respectively and the corresponding final values are N2, 2 and E2, then, N1 E1 1 and N 2 E2 2 then N 2 E2 1 N1 E1 2 (i) For a shunt motor, flux practically remains constant so that 1 = 2. N 2 E2 N1 E1 (ii) For a series motor, Ia prior to saturation. N 2 E2 I a1 where N1 E1 I a 2 Ia1 = initial armature current Ia2 = final armature current Speed Regulation: The speed regulation of a motor is the change in speed from full-load to no-loud and is expressed as a percentage of the speed at full-load i.e. Speed regulation No N 100 % where N N0 = No - load.speed N = Full - load speed Armature Reaction in DC Motors: As in a dc generator, armature reaction also occurs in a dc motor. This is expected because when current flows through the armature conductors of a dc motor, it produces flux (armature flux) which lets on the flux produced by the main poles. For a motor with the same polarity and direction of rotation as is for generator, the direction of armature reaction field is reversed. (i) In a generator, the armature current flows in the direction of the induced emf (i.e. generated emf E) whereas in a motor, the armature current flows against the induced emf (i.e. back emf 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 dc 57

33 generator weakens the flux at leading pole tips and strengthens the flux at trailing pole tips while the armature reaction in a dc motor produces the opposite effect. (ii) In case of a dc generator, with brushes along G.N.A. and no commutating poles used, the brushes must be shifted in the direction of rotation (forward lead) for satisfactory commutation. However, in case of a dc motor, the brushes are given a negative lead i.e., they are shifted against the direction of rotation. (iii) By using commutating poles (compoles), a dc machine can be operated with fixed brush positions for all conditions of load. 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 dc motor, the commutating poles must have the same polarity as the main poles directly back of them. This is the opposite of the corresponding relation in a dc generator. Commutation in DC 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 Spole. 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 dc motor is to cause the reversal of current in a conductor as it moves from one side of a brush to the other. For good commutation, the following points may be noted: 58

34 (i) If a motor does not have commutating poles (compoles), the brushes must be given a negative lead i.e., they must be shifted from G.N.A. against the direction of rotation of, the motor. (ii) By using interpoles, a dc motor can be operated with fixed brush positions for all conditions of load. For a dc motor, the commutating poles must have the same polarity as the main poles directly back of them. This is the opposite of the corresponding relation in a dc generator. A dc machine may be used as a motor or a generator without changing the commutating poles connections. When the operation of a dc machine changes from generator to motor, the direction of the armature current reverses. Since commutating poles winding carries armature current, the polarity of commutating pole reverses automatically to the correct polarity. Losses in a DC Motor: The losses occurring in a dc motor are the same as in a dc generator. These are: (i) copper losses and Iron losses or magnetic losses (ii) Mechanical losses As in a generator, these losses cause (a) an increase of machine temperature and (b) Reduction in the efficiency of the dc motor. The following points may be noted: (i) Apart from armature Cu loss, field Cu loss and brush contact loss, Cu losses also occur in interpoles (commutating poles) and compensating windings. Since these windings carry armature current (Ia), Loss in interpole winding = I2a x Resistance of interpole winding Loss in compensating winding =I2a x Resistance of compensating winding (ii) Since dc machines (generators or motors) are generally operated at constant flux density and constant speed, the iron losses are nearly constant. (iii) The mechanical losses (i.e. friction and windage) vary as the cube of the speed of rotation of the dc machine (generator or motor). Since dc machines are generally operated at constant speed, mechanical losses are considered to be constant. Efficiency of a DC Motor: Like a dc generator, the efficiency of a dc motor is the ratio of output power to the input power i.e. As for a generator, the efficiency of a dc motor will be maximum when: Variable losses = Constant losses Therefore, the efficiency curve of a dc motor is similar in shape to that of a dc generator. A - B = Copper losses B - C = Iron and friction losses 59

35 Overall efficiency, ηc = C/A Electrical efficiency, ηe = B/A Mechanical efficiency, ηm = C/B DC Motor Characteristics: There are three principal types of dc motors: shunt motors, series motors and compound motors. The performance of a dc motor can be judged from its characteristic curves known as motor characteristics; following are the three important characteristics of a dc motor: (i) Torque and Armature current characteristic (Ta/Ia): It is known as electrical characteristic of the motor. (ii) Speed and armature current characteristic (N/Ia): It is very important characteristic as it is often the deciding factor in the selection of the motor for a particular application. (iii) Speed and torque characteristic (N/Ta): It is also known as mechanical characteristic. Characteristics of Series Motors 1. Ta/Ia Characteristic. We have seen that Ta ΦIa. In this case, as field windings also carry the armature current, Φ Ia up to the point of magnetic saturation. Hence, before saturation, Ta ΦIa and Ta I2a At light loads, Ia and hence Φ is small. But as Ia increases, Ta increases as the square of the current. Hence, Ta/Ia curve is a parabola. After saturation, Φ is almost independent of Ia hence Ta Ia only. So the characteristic becomes a straight line. The shaft torque Tsh is less than armature torque due to stray losses. It is shown dotted in the figure. So we conclude that (prior to magnetic saturation) on heavy loads, a series motor exerts a torque proportional to the square of armature current. Hence, in cases where huge starting torque is required for accelerating heavy masses quickly as in hoists and electric trains etc., series motors are used. 60

36 2. N/Ia Characteristics. Variations of speed can be deduced from the formula: N E Change in E, for various load currents is small and hence may be neglected for the time being. With increased Ia, Φ also increases. Hence, speed varies inversely as armature current. When load is heavy, Ia is large. Hence, speed is low (this decreases E and allows more armature current to flow). But when load current and hence Ia falls to a small value, speed becomes dangerously high. Hence, a series motor should never be started without some mechanical (not belt-driven) load on it otherwise it may develop excessive speed and get damaged due to heavy centrifugal forces so produced. It should be noted that series motor is a variable speed motor. 3. N/Ta or mechanical characteristic. It is found from above that when speed is high, torque is low and vice-versa. Characteristics of Shunt Motors 1. Ta/Ia Characteristic: Assuming Φ to be practically constant (though at heavy loads, φ decreases somewhat due to increased armature reaction) we find that Ta Ia. Hence, the electrical characteristic, is practically a straight line through the origin. Shaft torque is shown dotted. Since a heavy starting load will need a heavy starting current, shunt motor should never be started on (heavy) load. 2. N/Ia Characteristic: If Φ is assumed constant, then N E. As E is also practically constant, speed is, for most purposes, constant. 61

37 But strictly speaking, both E and Φ decrease with increasing load. However, E decreases slightly more than φ so that on the whole, there is some decrease in speed. The drop varies from 5 to 15% of full-load speed, being dependent on saturation, armature reaction and brush position. Hence, the actual speed curve is slightly drooping as shown by the dotted line in the figure. But, for all practical purposes, shunt motor is taken as a constant-speed motor. Because there is no appreciable change in the speed of a shunt motor from no-load to full load, it may be connected to loads which are totally and suddenly thrown off without any fear of excessive speed resulting. Due to the constancy of their speed, shunt motors are suitable for driving shafting, machine tools, lathes, wood-working machines and for all other purposes where an approximately constant speed is required. 3. N/Ta Characteristic can be deduced from (1) and (2) above. Compound Motors: These motors have both series and shunt windings. If series excitation helps the shunt excitation i.e. series flux is in the same direction; then the motor is said to be cummulatively compounded. If on the other hand, series field opposes the shunt field, then the motor is said to be differentially compounded. The characteristics of such motors lie in between those of shunt and series motors. (a) Cumulative-compound Motors: Such machines are used where series characteristics are required and where, in addition, the load is likely to be removed totally such as in some types of coal cutting machines or for driving heavy machine tools which have to take sudden cuts quite often. Due to shunt windings, speed will not become excessively high but due to series windings, it will be able to take heavy loads. In conjunction with fly-wheel (functioning as load equalizer), it is employed where there are sudden temporary loads as in rolling mills. The fly-wheel supplies its stored kinetic energy when motor slows down due to sudden heavy load. And when due to the removal of load motor speeds up, it gathers up its kinetic energy. Compound-wound motors have greatest application with loads that require high starting torques or pulsating loads (because such motors smooth out the energy demand required of a pulsating load). They are used to drive electric shovels, metal-stamping machines, reciprocating pumps, hoists and compressors etc. (b) Differential-compound Motors: Since series field opposes the shunt field, the flux is decreased as load is applied to the motor. This results in the motor speed remaining almost constant or even increasing with increase in load (because, N E/(Φ). Due to this reason, there is a decrease in the rate at which the motor torque increases with load. Such motors are not in common use. But because they can be designed to give an accurately constant speed under all conditions, they find limited 62

38 application for experimental and research work. One of the biggest drawback of such a motor is that due to weakening of flux with increases in load, there is a tendency towards speed instability and motor running away unless designed properly. 63

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