D.C GENERATORS Principle of operation of D.C machines, types of D.C Generators, e.m.f equation of D.C Generator, O.C.C of a D.C Shunt Generator, Load characteristics of D.C.Generators GENERATOR PRINCIPLE: An electric generator is a machine that converts mechanical energy into electrical energy. An electric generator is based on the principal that whenever flux is cut by a conductor, an e.m.f. is induced which will cause a current to flow if the conductor circuit is closed. The direction of induced e.m.f. (and hence current) is given by FLEMING S RIGHT HAND RULE. Mechanical Input Generator Electrical Output The essential components of a generator are: a) A magnetic field b) Conductor or a group of conductors c) Motion of conductor with respect to magnetic field. SIMPLE LOOP GENERATOR: Consider a single turn loop ABCD rotating clockwise in a uniform magnetic field with a constant speed as shown in Fig. 1.1.As the loop rotates, the flux linking the coil sides AB and CD changes continuously. Hence the e.m.f. induced in these coil sides also changes but the e.m.f. induced in one coil side adds to that induced in the other. (It is because the coil sides always remain under the influence of opposite poles i.e., if one coil side is under the influence of the N-pole, then the other coil side will be under the influence of S-pole and vice-versa.) i) When the loop is in position no. 1 [see Fig. 1.1], the generated e.m.f. is zero because the coil sides (AB and CD) are cutting no flux but are moving parallel to it. ii) When the loop is in position no. 2, the coil sides are moving at an angle to the flux and, therefore, a low e.m.f. is generated as indicated by point 2 in Fig. 1.2. iii) When the loop is in position no. 3, the coil sides (AB and CD) are at right angle to the flux and are, therefore, cutting the flux at a maximum rate. Hence at this instant, the generated e.m.f. is maximum as indicated by the point 3 in Fig. 1.2. iv) At position 4, the generated e.m.f. is less because the coil sides are cutting the flux at an angle. v) At position 5, no magnetic lines are cut and hence induced e.m.f. is zero as indicated by point 5 in Fig. 1.2. G.Purushotham, Asst. Professor, Dept. of EEE @SVCE,TPT Page 1
vi) At position 6, the coil sides move under a pole of opposite polarity and hence the direction of generated e.m.f. is reversed. The maximum e.m.f. in this direction (i.e., reverse direction, see fig. 1.2) will be the loop is at position 7 and zero when at position 1. This cycle repeats with each revolution with each revolution of the coil. vii) Note that e.m.f. generated in the loop is alternating one. It is because any coil side, say AB has e.m.f. in one direction when under the influence of N-pole and in the other direction when under the influence of S-pole. If a load is connected across the ends of the loop, then alternating current will flow through the load. The alternating voltage generated in the loop can be converted into direct voltage by a device called Commutator. We then have the D.C. Generator. In fact, a Commutator is a mechanical rectifier. Action of Commutator: If, somehow, connection of the coil side to the external load is reversed at the same instant the current in the coil side reverses, the current through the load will be direct current. This is what a Commutator does. Fig. 1.3 shows a Commutator having two segments C 1 and C 2. It consists of a cylindrical metal ring cut into two halves or segments C 1 and C 2 respectively separated by a thin sheet of mica. The ends of coil sides AB and CD are connected to the segments C 1 and C 2 respectively as shown in Fig. 1.4. Two stationary carbon brushes rest on the Commutator and leads currents to the external load. With this arrangement, the Commutator at all times connects the coil side under S-pole to the positive brush and that under N-pole to the negative brush. G.Purushotham, Asst. Professor, Dept. of EEE @SVCE,TPT Page 2
i) In Fig. 1.4, the coil sides AB and CD are under N-pole and S-pole respectively. Note that segment C 1 connects the coil side AB to point P of the load resistance R and the segment C 2 connects the coil side CD to point Q of the load. Also note the direction of current through load. It is from Q to P. ii) After half a revolution of the loop (i.e., 180 o rotation), the coil side AB is under S-pole and the coil side CD under N-pole as shown in Fig. 1.5. The currents in the coil sides now flow in the reverse direction but the segments C 1 and C 2 have also moved through 180 o i.e., segment C 1 is now in contact with positive brush and segment C 2 in contact with negative brush. Note that Commutator has reversed the coil connections to the load i.e., coil side AB is now connected to point Q of the load and coil side CD to the point P of the load. Also note the direction of current through the load. It is again from Q to P. (The situation is that currents in the coil sides are reversed and at the same time the connections of the coil sides to the external load are reversed. This means that current will flow in the same direction through the load.) Thus the alternating voltage generated in the loop will appear as direct voltage across the brushes. The reader may note that e.m.f. generated in the armature winding of a D.C. generator is alternating one. It is by the use of Commutator that we convert the generated alternating e.m.f. into direct voltage. The purpose of brushes is simply to lead current from the rotating loop or winding to the external stationary load. The variation of voltage across the brushes with the angular displacement of the loop will be as shown in Fig. 1.6. This is not a steady direct voltage but has a pulsating character. It is because the voltage appearing across the brushes varies from zero to maximum value and back to zero twice for each revolution of the loop. A pulsating direct voltage such as is produced by a single loop is not suitable for many commercial uses. What we require is steady state voltage. This can be achieved by using a large number of coils connected in series. The resulting arrangement is known as armature winding. G.Purushotham, Asst. Professor, Dept. of EEE @SVCE,TPT Page 3
CONSTRUCTION OF D.C. GENERATOR: The D.C. generators and D.C. motors have the same general construction. Any D.C. generator can be run as a D.C. motor and viceversa. All D.C. machines have five principal components viz., i) Field system ii) Armature core iii) Armature winding iv) Commutator v) Brushes i. Field system: The function of the field system is to produce uniform magnetic field within which the armature rotates. It consists of a number of salient poles (of course, even number) bolted to the inside of circular frame (generally called yoke). The yoke is usually made of solid cast steel whereas the pole pieces are composed of stacked laminations. Field coils are connected in such a way that adjacent poles have opposite polarity. ii. Armature Core: The armature core is keyed to the machine shaft and rotates between the field poles. It consists of slotted soft-iron laminations (about 0.4 to 0.6 mm thick) that are stacked to form a cylindrical core as shown in Fig. The purpose of laminating the core is to reduce the eddy current losses. iii. Armature Winding: The slots of the armature core hold insulated conductors that are connected in a suitable manner. This is known as armature winding This is the winding in which working e.m.f. is induced. The armature winding of a D.C. machine is a closed-circuit winding; the conductors being connected in a symmetrical manner forming a closed loop or series of closed loops. iv. Commutator: A Commutator is a mechanical rectifier which converts the alternating voltage generated in the armature in the armature winding into direct voltage across the brushes. Depending upon the manner in which the armature conductors are connected to the Commutator segments, there are two types of armature winding in a D.C. machine viz, a) Lap winding b)wave winding G.Purushotham, Asst. Professor, Dept. of EEE @SVCE,TPT Page 4
v. Brushes: The purpose of brushes is to ensure electrical connections between the rotating Commutator and stationary external load circuit. The brushes are made of carbon and rest on the Commutator. E.m.f. Equation of a D.C. Generator: We shall now derive an expression for the e.m.f. generated in a D.C. generator. Let, φ = flux/pole in Wb Z = total number of armature conductors P = number of poles A = number of parallel paths = 2.. For wave winding = P.. For lap winding N = speed of armature in r.p.m. E g = e.m.f. of the generator = e.m.f./parallel path Flux cut by one conductor in one revolution of the armature, dφ = Pφ (webers) As the conductor cuts the flux of all the P poles. Time taken to complete one revolution, dt = 60/N (seconds) E.m.f. generated/conductor, φ = φ = φ ( ) E.m.f. of generator, E g = φ E g = φ = e.m.f. per parallel path = (e.m.f./conductor) No. of conductors/parallel path Where A = 2.. For wave winding volts = P.. For lap winding TYPES OF D.C. GENERATORS: The magnetic field in a D.C. generator is normally produced by electromagnets rather than permanent magnets. Generators are generally classified according to their methods of field excitation. On this basis, d.c. generators are divided in to the following two classes : (i) Separately excited generator (ii) Self excited generator The behavior of a d.c. generator on load depends upon the method of field excitation adopted. G.Purushotham, Asst. Professor, Dept. of EEE @SVCE,TPT Page 5
Separately Excited Generator: A D.C. generator whose field magnet winding is supplied from an independent external D.C. source (e.g., a battery etc.) is called a separately excited generator. The voltage output depends upon the speed of rotation of armature and the field current (E g = P φ Z N/ 60 A). The greater the speed and field current greater is the generated e.m.f. It may be noted that separately excited D.C. generators are rarely used in practice. The D.C. generators are normally of self excited type. Figure shows the connections of a separately excited generator. Armature current, Ia = I L Terminal voltage, V = Eg- I a R a Electric power developed = E g Ia 2 Power delivered to load = E g I a -I a R a = I a (Eg I a R a ) = V I a Self Excited : A d.c generator whose field magnet winding is supplied current from the output of the generator itself is called a self excited generator. 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. Fig. shows the connections of series generator G.Purushotham, Asst. Professor, Dept. of EEE @SVCE,TPT Page 6
Armature current, IA = I se = I L = I (say) Terminal voltage, V = E g I (R a +R se ) Power developed in armature = Eg I a Power delivered to load = E g I a I 2 a R a (R a +R se ) = I a [E g I a ( R a + R se ) ] = V I a (or) V I L (iii) Shunt Generator : In a shunt generator, the field winding is connected in parallel with the armature winding so that terminal voltage of the generator is applied across it. Figure shows the connections of a shunt wound generator. Shunt field current, I sh = V/R a Armature current, IA = I L + I sh Terminal voltage, V = E g I a R a Power developed in armature = E g I a Power delivered to load = V I L (iv) Compound Generator: In a compound - wound generator there are two sets of field winding on each pole one is in series and the other in parallel with the armature. A compound generator may be: (a) Short Shunt (b) Long Shunt G.Purushotham, Asst. Professor, Dept. of EEE @SVCE,TPT Page 7
Short shunt: this generator in which only shunt field winding is in parallel with the armature winding. Series field current, I se = I L Shunt field current, Ish = (V + I se R se )/ R sh Terminal voltage, V = E g I a R a I se R se Power developed in the armature= E g I a Power delivered to load = V I L Long Shunt: this generator in which shunt field winding is in parallel with both series field and armature winding. Series field current, I se = I a = I L + I sh Shunt field current, I sh = V/R sh Terminal voltage, V = E g I a (R a +R se ) Power delivered in the armature = E g I a Power delivered to load = V I L CHARACTERISTICS OF A D.C GENERATOR: Following are the three most important characteristics of curves of a DC generator: 1. No-load saturation characteristic (E o /I f ): It is also known as magnetic or open-circuit characteristic (O.C.C). It shows the relation between the no-load generated e.m.f in armature E o, and the field or exciting current (I f ) at a given fixed speed. It is just the magnetization curve for the material of the electromagnets. Its shape is practically the same for all generators whether separately-excited or self-excited. 2. Internal Characteristic (E/Ia): It gives the relation between the e.m.f, E actually induced in the armature and the armature current (Ia). This is of interest mainly to the designer. 3. External Characteristic (V L /I L ): It is also referred to as performance (c/s) or sometime voltage-regulation curve. It gives relation between the terminal voltage (V L ) and the load current (I L ). This curve lies below the internal (c/s) between it takes into account the voltage drop over the armature circuit resistance. The values of (V L ) are obtain by subtracting (Ia Ra) from corresponding values of (E). This is of great importance in judging the suitability of a generator for a particular purpose. G.Purushotham, Asst. Professor, Dept. of EEE @SVCE,TPT Page 8
Separately- Excited Generator: Open-circuit characteristics (E o /I f ): The arrangement for obtaining the necessary data to plot this curve is shown in fig. The exciting or field current (I f ) is obtained from an external independent D.C source. It can be varied (I f ) from zero upwards by a rheostat and its value read by an ammeter (A) connected in the field circuit as shown. Now, the voltage equation of a D.C generator is: E g = φ Hence, if the speed is constant, the above relation becomes: E = K.Φ It is obvious that when (I f ) is increased from its initial small value, the flux (Φ) and hence generated e.m.f, Eo increase directly as current while the poles are unsaturated. This is represented by straight portion (o d) in fig. But as the flux density increases, the poles become saturated, so a greater increase in (I f ) is required to produce a given increase in voltage than on the lower part of curve. That is why the upper portion (d b) of curve (o d b) bends over as shown. volts Self- Excited Generator: Open-circuit characteristic or magnetization curve for self-excited generator: The (O.C.C) or no-load saturated curves for self-excited generators whether shunt or seriesconnected, 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 as shown in fig. The field or exciting current (I f ) is varied rheostatically and its value read on the ammeter (A). The machine is driven at constant speed by the prime mover and the generator e.m.f on No-load is measured by voltmeter connected across the armature. (I f ) is increased by suitable steps (starting from zero) and the corresponding Values of (E o ) are measured. On plotting the relation between (I f ) and (Eo) a curve of the form shown in fig. is obtained. Due to residual magnetism in the poles, some e.m.f is generated even when (I f =0). G.Purushotham, Asst. Professor, Dept. of EEE @SVCE,TPT Page 9
Hence, the curve starts a little way up. The straight curvature at the lower end is due to magnetic inertia. It is seen that the first part of the curve is practically straight. Now, connect the field windings back to the armature and run the machine as shunt generator. A shunt generator will excite only if the poles have some residual magnetism and the resistance of the shunt circuit is less than some critical value, the actual value depending upon the machine and upon the speed at which the armature is driven. Suppose curve in fig. to represent the open-circuit characteristic of a shunt generator. With increasing excitation. Then, for a shunt current (If) OA, the e.m.f. is AB and. Corresponding resistance of shunt circuit The resistance line OB represents smaller resistance to which the machine will build up and represent the maximum voltage AB. If field resistance is increased, and then slope of the resistance line increase, and hence the maximum voltage to which the generator will build up at a given speed, decreases. If field resistance increased so much that the resistance line does not cut the O.C.C at all (like OE) then obviously the machine will fail to excite, there will be no build up" of the voltage. The value of the resistance represented by the tangent to the curve is known as critical resistance R C for a given speed. G.Purushotham, Asst. Professor, Dept. of EEE @SVCE,TPT Page 10
Load Characteristics of DC Shunt Generator: Internal characteristics: Consider the dc shunt generator shown in fig. the internal characteristics is E vs I L Intially the induced e.m.f is not dependent on the load current I L or armature current I a. but as load current increases, the armature current increases to supply load demand. As I a increases armature, armature flux increases. The effect of armature flux which is produced by the armature current on the main field flux is called armature reaction. Due to armature reaction main flux gets disturbed. Hence lesser flux gets linked with the armature. This reduces the induced e.m.f. Internal Characteristics external characteristics for dc shunt generator we know tha E = V+I a R a neglecting other drops as load current I L increases, I a increases. Hence drop I a R a increases. but the value of armature resistance is very small,the drop in terminal voltage as I L changes from no load to full load is very small.this is shown in below fig. hence dc shunt generator is called as constant voltage generator. External Characteristics G.Purushotham, Asst. Professor, Dept. of EEE @SVCE,TPT Page 11
Load Characteristics of DC Series Generator: Internal characteristics: In case of series generator Ia= =Ise=I L As load current increases, Ise increases. The flux ɸ is directly proportional to Ise. So flux also increases. The induced emf E is proportional to flux. So induced emf also increases. Thus internal characteristics of E is increasing nature. As Ia increases, armature reaction increases but the effect of armature reaction is negligible compared to increase in E. But in case of high load current saturation occurs and flux remains constant.in such case,due to armature reaction E starts decreasing as shown in fig by dotted line. External characteristics: As I L = Ia increases,thus the drop Iaa (Ra+Rse) increases. thus the external characteristics is also of rising nature as E increases but it will be below internal characteristics due to drop Ia (Ra+Rse) as shown in above fig. Note: in self excited seriess generator, open circuit characteristics cannot be obtained. In open circuit, Ia= I L = 0 hence induced e.m.f is zero. Thus open circuit characteristics is possible only by separately excited the field winding. Load Characteristics of DC Compound Generator: The characteristics depends on whether generator is cumulatively compound or differentially compound generator In cumulatively compound ɸ = ɸ sh + ɸ se. as load current increases,ia increases hence Ise also increases producing more flux. Thus individual emf increases and total terminal voltage also increases. But as Ia increases, the various voltage drops and armature reaction drop also increases.hence there is drop in the terminal voltage. G.Purushotham, Asst. Professor, Dept. of EEE @SVCE,TPT Page 12
If drop in voltage due to increasing I L is more dominating than increase in V due to increase in flux then the generator is called under compounded generator and its characteristics is dropping in nature. As shown in fig. If drop in V due to armature reaction and other drops is much less than increase in V due to increase in flux then the generator is called over compounded and its characteristics is rising in nature. If the effects of the two are such that on full load current V is same as no load induced e.m.f. i.e. these effects are neytralising each other on full load then generator is called flat compounded or level compounded. In differentially compound the net flux is difference between the two fluxes.(i.e ɸ = ɸ sh - ɸ se) as I L increases, ɸ sh is almost constant but ɸ se increases rapidly. Hence total flux is reduces. hence the induced emf and terminal voltage also decreases drastically. There is a drop due to armature resistance, series field resistance, armature reaction due to which terminal voltage drops. Thus we get the characteristics of differentially compound generator. ******** All The Best ******** G.Purushotham, Asst. Professor, Dept. of EEE @SVCE,TPT Page 13