Electrical World

The characteristics depends on whether generator is cumulatively compound or differentially compound generator. In cumulatively compound, Φ 2r = Φ 2r + Φ 2r . As load current increases,  I a increases hence  I se  also increases producing more flux. But as  I a increases, the various voltage drops and armature reaction drop also increases. Hence there is drop in the terminal voltage.        If drop in V t due to increasing  I L is more dominating than increase in V t due to increase in flux then generator is called under compounded and its characteristics is dropping in nature, as shown in the Fig. 1. Fig. 1 Characteristics of compound generator        If drop in V t due to armature reaction and other drops is much less than increase in V t due to increase in flux then generator is called over compound and its characteristics is rising in nature, as shown in the Fig. 1. If the effects of the two are such that on full load current V t is same as no load induced e.m.

Consider a series generator shown in the Fig. 1        In case of series generator,         I a = I se =  I L        As load current increases,  I se increases. The flux Φ is directly proportional to. So flux also increases. The induced e.m.f. E is proportional to  I se flux hence induced e.m.f. also increases. Thus the characteristics of E against i.e. internal characteristics is of increasing nature. As increases I a , armature reaction increases but its effect is negligible compared to increase in E. But for high load current, saturation occurs and flux remains constant. In such case, due to the armature reaction E starts decreasing as shown by dotted line in the Fig. 2. Fig. 1 Characteristics of d.c. series motor        Now as I L = I a  increases, thus the drop I a  (R a  +R se ) increases.       V t = E - I a  (R a  +R se )       Thus the external characteristics is also of rising nature as E increases but it will be below internal characteristic

Consider the d.c. shunt generator shown in the Fig. 1. The internal characteristics is E V s I L while the external characteristics is V t  against I L . Fig. 1 Internal characteristics        Let us see the nature of these two characteristics.        Ideally 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 I a increases to supply load demand. As I a increases, armature flux increases. Note : The effect of flux produced by armature on the main flux produced by the field winding is called an armature reaction.        Due to the armature reaction, main flux pattern gets distorted. Hence lesser flux gets linked with the armature conductors. This reduces the induced e.m.f. Note : Thus the armature conductors. This reduces the induced e.m.f.        This is shown in the Fig. 2. Fig. 2  Internal characteristics 1.2 External Characteristics         For d.c. shunt generator we know that, E = V t + I a

The characteristics is separately excited d.c. generator are divided into two types, 1) Magnetization   and         2) Load characteristics. 1.1 Magnetization or Open Circuit Characteristics         The arrangement to obtain this characteristics is shown in the Fig. 1. Fig. 1  Obtaining O.C.C. of separately excited generator        The rheostat as a potential driver is used to control the field current and the flux. It is varied from zero and is measured on ammeter connected.        E o  = (ΦPNZ) / (60A)        As I f is varied, then Φ change and hence induced e.m.f. E o  also varies. It is measured on voltmeter connected across armature. No Load is connected to machine, hence characteristics are also called no load characteristics which is graph of E o  against field current I f as shown in the Fig. 2. As I f increases, flux Φ increases and E o increases. After point A, saturation occurs when Φ becomes constant and hence E o  saturates. Fig. 2  Open circuit characteristics

The d.c. generators have following characteristics in general, 1) Magnetization characteristics 2) Load characteristics 1.1 Magnetization Characteristics        This characteristics is the graph of generated no load voltage E against the field current I f , when speed of, generator is maintained constant. As it is plotted without load with open output terminals it is also called No load characteristics or Open circuit characteristics.        E o V s I f  is magnetization characteristics        Where  E o = No load induced e.m.f.        But for generator,        E = (ΦPNZ) / (60A) . . .     E α   Φ                 with (PNZ) / (60A)  constant . . .     E  α   I f                    as   Φ  α   I f        Thus induced e.m.f. increases directly as I f  increases. But after certain I f  core gets saturated and flux also remain constant though I f  increases. Hence after saturation, voltage also remains constatnt. Note : Thus characteristics is linear till saturation and after tha

In this type, the part of the field winding is connected in parallel with armature and part in series with the armature. Both series and shunt field windings are mounted on the same poles. Depending upon the connection of shunt and series field winding, compound generator is further classified as : i) Long shunt compound generator, ii) Short shunt compound generator. 1.1 Long Shunt Compound Generator        In this type, shunt field winding is connected across the series combination of armature and series field winding as shown in the Fig. 1. Fig. 1 Long shunt compound generator        Voltage and current relations are as follows.        From the Fig. 1.        I a = I se         and  I a = I sh +  I L        Voltage across shunt field winding is V t .        I sh = V t /R sh       where R sh = Resistance of shunt field winding        And voltage equation is,        E = V t + I a R a + I a R se + V brush        Where R se = Resistance of series field winding 1.2 Short Shunt Co

   When the field winding is connected in series with the armature winding while supplying the load then the generator is called series generator. It is shown in the Fig. 1.        Field winding, in this case is denoted as S 1 and S 2 . The resistance of series field winding is very small and hence naturally it has less number of turns of thick cross-section wire as shown in the Fig. 1. Fig. 1 Series generators       Let R se be the resistance of the series field winding. 1.1 Voltage and current Relations        As all armature, field and load are in series they carry the same current. . . .         I a = I se =  I L Where   I se = Current through series field winding.        Now in addition to drop I a R a , induced e.m.f. has to supply voltage drop across series field winding too. This is I se R se i.e. I a R se as I a = I se . So voltage equations can be written as,         E = V t + I a R a + I a R se + V brush . . .      E = V t + I a (R a + R se ) + Vbrush        where 

When the field winding is connected in parallel with the armature and the combination across the load then the generator is called shunt generator.        The field winding has large number of turns of thin wire so it has high resistance. Let R sh be the resistance of the field winding. Fig. 1 Shunt generator 1.1 Voltage and Current Relations        From the Fig. 1, we can write           I a = I L + I sh        Now voltage across load is V t which is same across field winding as both are in parallel with each other. . . .          I sh = V t /R sh        While induced e.m.f. E, still requires to supply voltage drop I a R a and brush contact drop. . . .          E = V t + I a R a + V brush Where    E = (ΦPNZ) / (60A)        In practical, brush contact drop can be neglected.

The magnetic field required for the operation of a d.c. generator is produced by an electromagnet. This electromagnet carries a field winding which produces required magnetic flux when current is passed through it. Note : The field winding is also called exciting winding and current carried by the field winding is called an exciting current.        Thus supplying current to field winding is called excitation and the way of supplying the exciting current is called method of excitation.        There are two methods of excitation used for d.c. generators, 1. Separate excitation. 2. Self excitation.        Depending on the method of excitation used, the d.c. generators are classified as, 1. Separately excited generators  2. Self excited generator        In separately excited generator, a separate external d.c. supply is used to provide exciting current through the field winding.        The d.c. generator produces d.c. voltage. If this generated voltage itself is used to excite the

The armature is denoted by a circle with two brushes. Mechanically it is connected to another device called prime mover. The two ends of armature are denoted as A 1 - A 2 . The field winding is shown near armature and the two ends are denoted as . The field winding is shown near armature and the two ends are denoted as F 1 - F 2 . The representation of field very little bit, depending on the type of generator.        The symbolic representation is shown in the Fig. 1. Many times an arrow (↑) is indicated near armature. This arrow denotes the direction of current which induced e.m.f. will set up, when connected to an external load. Fig. 1 Symbolic representation of d.c. generator Note : Every practical generator needs a prime mover to rotate its armature. Hence to avoid complexity of the diagram, prime mover need not to be included in the symbolic representation of generator.

When the field winding is supplied from the armature of the generator itself then it is said to be self excited generator. Now without generated e.m.f., field can not be excited in such generator and without excitation there can not be generated e.m.f. So one may obviously wonder, how this type of generator works. The answer to this is residual magnetism possessed by the field poles, under normal condition.        Practically through the generator is not working, without any current through field winding, the field poles possess some magnetic flux. This is called residual flux and the property is called residual magnetism. Thus when the generator is started, due to such residual flux, it develops a small e.m.f. which now drives a small current through the field winding. This tends to increase the flux produced. This in turn increases the induced e.m.f. This further increases the field current and the flux. The process is cumulative and continues till the generator develop

When the field winding is supplied from external, separate d.c. supply i.e. excitation of field winding is separate then the generator is called separately excited generator. Schematic representation of this type is shown in the Fig.1. Fig. 1  Separately excited generator        The field winding of this type of generator has large number of turns of thin wire. So length of such winding is more with less cross-sectional area. So resistance of this field winding is high in order to limit the field current. 1.1 Voltage and Current Relations        The field winding is excited separately, so the field current depends on supply voltage and resistance of the field winding.        For armature side, we can see that it is supplying a load, demanding a load current of I L at a voltage of V t which is called terminal voltage.        Now   I a = I L        The internally induced e.m.f. E is supplying the voltage of the load hence terminal voltage V t is a part of E. But E is not equal

The magnetic field required for the operation of a d.c. generator is produced by an electromagnet. This electromagnet carries a field winding which produces required magnetic flux when current is passed through it. Note : The field winding is also called exciting winding and current carried by the field winding is called an exciting current.        Thus supplying current to the field winding is called excitation and the way of supplying the exciting current is called method of excitation.        There are two methods of excitation used for d.c. generators, 1. Separate excitation                   2. Self excitation        Depending on the method of excitation used, the d.c. generators are classified as, 1. Separately excited generators  2. Self excited generators        In separately excited generators, a separate external d.c. supply is used to provide exciting current through the field winding.        The d.c. generators produces d.c. voltage. If this generated voltage itself

There are two practical ways by which commutation may be improved. These methods are, 1. Resistance commutation and 2. E.M.F. commutation. 1.1 Resistance Commutation        In this method of improving commutation, the low resistance copper brushes are replaced by high resistance carbon brushes.        From the Fig. 1 it can be seen that the current I from coil C when passing through commutator segment 'b' has two parallel paths. One is straight from 'b' to brush while the other is through short circuited coil B to segment 'a' and then to the brush. By using low resistance copper brush the current will not prefer second path as it will prefer first low resistance path. Fig. 1 Resistance Commutation        When carbon brushes having comparatively high resistance are used then current I through coil C will select the second path as resistance of first path will be increasing due to decrease in contact area of 'b' with brush and resistance r 2  of

The e.m.f. induced in each coil of armature is alternating in nature. If load is connected, the current flowing will also be alternating. But the flow of current in a d.c. generator must be undirectional. This can be achieved by the use of commutator. When the armature conductors are under the influence of one pole they carry current in one direction whereas the current is reversed when the conductors are under the influence of other pole. This reversal of current takes place along the magnetic neutral axis. Fig. 1 Note : The reversal of current is likely to take place in short interval when a coil is short circuited by a brush so that transfer of current from one direction to other is carried out without any sparking. This process is called commutation.        Thus a process by which current in the short circuited coil is reversed while it crosses the MNA is called commutation. The time during which the coil remains short circuited is known as commutation period. This pe

It is seen that, the e.m.f. induced in the conductors is always sinusoidal and commutator converts this sinusoidal e.m.f. to unidirectional e.m.f. Let us see, how it happens.        For simplicity of understanding the commutator action, consider commutator in its simplest form. Commutator is divided into number of copper segments insulated from each other. In its simplest form, it is assumed to be divided into two segments, each is nothing but the half of the entire commutator drum, separated by insulating material. So in its simplest form it is a  ring with two halves separated by insulation as shown in the Fig. 1. Fig. 1 Split ring        Such a ring is called split ring. The brushes P and Q are stationary and pressed on the surface of split ring. Split ring is mounted on the shaft and rotates as armature rotates.        Consider a single turn generator with conductors (1) and (2). These armature conductors are connected to the two segments of split ring. The external re

  In order to reduce the effect of armature reaction following methods are used. 1) The armature reaction causes the distortion in main field flux. This can be reduced if the reluctance of the path of the cross-magnetizing field is increased. The armature teeth and air gap at pole tips offer reluctance to armature flux. Thus by increasing length of air gap, the armature reaction effect is reduced. 2) If reluctance at pole tips is increased it will reduce distorting effect of armature reaction. By using special construction in which leading and trialing pole tip portions of laminations are alternately omitted. 3) The effect of armature reaction can be neutralized by use of compensating winding. It is always placed in series with armature winding. The armature ampere conductors under pole shoe must be equal to compensating winding ampere conductors which will compensate armature m.m.f. perfectly. 4) The armature reaction causes shifting the magnetic neutral axis. Therefore th