1. DC GENERATOR

2.  Constructional features –  Principle of operation of DC generator –  Armature winding - types –  E.m.f. equation –  Armature reaction – effects of armature reaction - demagnetizing and cross magnetizing ampere- turns - compensating winding -  Commutation – methods to improve commutation – e.m.f. in coil undergoing commutation –  Reactance e.m.f.-  Effect of brush shift-  Inter poles.

3.  Types of excitation – separately excited- self excited shunt, series and compound machines –  The magnetization curve –  Condition for self excitation-  Field critical resistance- critical speed-  Load characteristics of generators-load critical resistance –  Voltage regulation –  Parallel operation of shunt, series and compound generators –  Power flow diagram-  Losses and efficiency-  Condition for maximum efficiency-  Applications.

4.  Electrical machines/converters  Electromechanical energy conversion

5.  An electric generator is a machine that converts mechanical energy into electrical energy.  An electric generator is based on the principle 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.  Therefore, the essential components of a generator are: (a) A magnetic field (b) Conductor or a group of conductors (c)Relative motion of conductor w.r.t. magnetic field. Generator Principle

6. Flemings Right-hand rule (often called the geneRator rule): First finger-Field thuMb – Motion sEcond finger – E.m.f

9.  It is possible to convert the alternating current that is induced into the armature into a form of direct current. This conversion of AC into DC is accomplished through the use of a commutator. (split rings)  The conductors of the armature of a DC generator are connected to commutator segments. Action Of Commutator

14. A D.C. machine consists mainly of two part The stationary part called stator and The rotating part called rotor. The stator consists of main poles used to produce magnetic flux,commutating poles or interpoles in between the main poles to avoid sparking at the commutator and finally the frame or yoke which forms the supporting structure of the machine. The rotor consist of an armature a cylindrical metallic body or core with slots in it to place armature windings or bars, a commutator and brush gears. The major parts can be identified as, 1. Yoke 2. Poles 3.Field winding 4. Armature 5. Commutator and 6.Brushes & Brush gear

15. 1.Yoke i. Functions: The ring shaped body portion of the frame is called Yoke(the magnetic path for the magnetic fluxes from the main poles and interpoles) The main poles and commutator poles are bolted. It forms the supporting structure to the poles. Why we use cast steel instead of cast iron for the construction of Yoke? Cast iron flux density is 0.8 Wb/sq.m where as saturation with cast iron steel is about 1.5 Wb/sq.m. Cross section area needed for cast steel is less than cast iron The weight of the machine too. If we use cast iron there may be chances of blow holes in it while casting. Steels are developed and these have consistent magnetic and mechanical properties.

16. 3.Main Poles Each pole is divided into two parts I. Pole core II.Pole shoe Functions Of Pole Core & Pole Shoe Pole core basically carries a field winding which is necessary to produce the flux It directs the flux produced through air gap to armature core, to the next pole. Solid poles of fabricated steel with separate/integral pole shoes are fastened to the frame by means of bolts. Pole shoes are generally laminated(Sometimes pole body and pole shoe are formed from the same laminations). The pole shoes are shaped so as to have a slightly increased air gap at the tips.

17. Inter-poles are small additional poles located in between the main poles. These can be solid, or laminated just as the main poles. These are also fastened to the yoke by bolts. Sometimes the yoke may be slotted to receive these poles. These are also called as commutating poles or com poles.

18. interpoles Main poles Pole shoe

19.  The field winding is wound on the pole core with a definite direction.  Function: To carry current due to which pole core, on which the field winding is placed behaves as an electromagnet, producing necessary FLUX. Helps in producing the magnetic field i.e. exciting the pole as an electromagnet it is called Field Winding or Exciting winding.  Choice of Material: It has to carry current hence it should be made up of conducting material.(aluminum or copper) Field coils are required to take any type of shape and bend about the pole core(copper has good pliability, so copper is the best choice)

20. 4.Armature  The armature is where the moving conductors are located.  The armature is constructed by stacking laminated sheets of silicon steel.  Thickness of these lamination is kept low to reduce eddy current losses. As the laminations carry alternating flux the choice of suitable material, insulation coating on the laminations, stacking it etc are to be done more carefully.  The winding cannot be placed on the surface of the rotor due to the mechanical forces coming on the same. Open parallel sided equally spaced slots are normally punched in the rotor laminations. These slots house the armature winding.

22. 5.Commutator:  Function:  To facilitate the collection of current from the armature conductors.  Convert internally developed AC to DC e.m.f.  It consists of copper segments tightly fastened together with mica/minacity insulating separators on an insulated base.  The whole commutator forms a rigid and solid assembly of insulated copper strips and can rotate at high speeds.

23. 6.Brush and Brush Gear: Brushes rest on the surface of the commutator. Normally electro-graphite is used as brush material. The brush holders provide slots for the brushes to be placed. The brushes are kept pressed on the commutator with the help of springs. This is to ensure proper contact between the brushes and the commutator even under high speeds of operation. A flexible copper conductor is used called as PIG-TAIL which connect the brush to the external circuit(conveys current from brushes to the holder). They support the bearings which are on the shaft.

24. Pig Tail

27.  The resistance offered by the armature circuit is known as armature resistance (Ra) and includes: (i) resistance of armature winding (ii) resistance of brushes  The armature resistance depends upon the construction of machine. Except for small machines, its value is generally less than 1 Ω.

28.  D.C Generators are classified according to the way in which a magnetic field is developed(excitation of the field windings) in the stator of the machine.  Thus, there are three basic classification DC generators Permanent-magnet field Separately-excited field Self-excited field.

30.  Permanent-magnet DC machines are widely found in a wide variety of low-power applications.  The field winding is replaced by a permanent magnet, resulting in simpler construction.  They do not require external excitation.

31.  Separately-excited generators are those whose field magnets are energized from an independent external source of DC current.

32.  Separately-excited generators are generally more expensive than self-excited DC generators because of their requirement of separate excitation source. Because of that their applications are restricted.self-excited DC generators Because of their ability of giving wide range of voltage output, they are generally used for testing purpose in the laboratories.voltage Separately excited generators operate in a stable condition with any variation in field excitation. Because of this property they are used as supply source of DC motors, whose speeds are to be controlled for various applications. Example- Ward Leonard Systems of speed control.

33.  Self-excited generators are those whose field magnets are energized by the current produced by the generators themselves.  Due to residual magnetism, there is always present some flux in poles.  When the armature is rotated, some e.m.f and hence some induced current is produced which is partly or fully passed through the field coils thereby strengthening the residual pole flux.  There are three types of self-excited generators named according to the manner in which their field coils ( or windings) are connected to armature.

35.  The field windings are connected across or in parallel with the armature conductors.  The shunt field winding has many turns of fine wire having high resistance. Therefore, only a part of armature current flows through shunt field winding and the rest flows through the load.

36.  The application of shunt generators are very much restricted for its dropping voltage characteristic.voltage  These type of DC generators generally give constant terminal voltage for small distance operation with the help of field regulators from no load to full load.DC generatorsvoltage They are used for general lighting. They are used to charge battery because they can be made to give constant output voltage.battery They are used for giving the excitation to the alternators. They are also used for small power supply.

37.  The field winding is connected in series with armature winding so that whole armature current flows through the field winding as well as the load.  Since the field winding carries the whole of load current, it has a few turns of thick wire having low resistance.  Series generators are rarely used except for special purposes.

38.  They give constant current.current They are used for supplying field excitation current in DC locomotives for regenerative breaking.current This types of generators are used as boosters to compensate the voltage drop in the feeder in various types of distribution systems such as railway service.voltage In series arc lightening this type of generators are mainly used.

39.  In a compound-wound generator, there are two sets of field windings on each pole—one is in series and the other in parallel with the armature.  A compound wound generator may be: (a) Short Shunt in which only shunt field winding is in parallel with the armature winding. (b) Long Shunt in which shunt field winding is in parallel with both series field and armature winding.

41.  Among various types of DC generators, the compound wound DC generators are most widely used because of its compensating property.types of DC generatorscompound wound DC generators  We can get desired terminal voltage by compensating the drop due to armature reaction and ohmic drop in the in the linevoltage Cumulative compound wound generators are generally used lighting, power supply purpose and for heavy power services because of their constant voltage property.voltage Cumulative compound wound generators are also used for driving a motor. For small distance operation, such as power supply for hotels, offices, homes and lodges, the flat compounded generators are generally used. The differential compound wound generators, because of their large demagnetization armature reaction, are used for arc welding where huge voltage drop and constant current is required.voltagecurrent

42.  It is the voltage drop over the brush contact resistance when current flows.  Obviously, its value will depend upon the amount of current flowing and the value of contact resistance.  This drop is generally small.

43.  Following are the three most important characteristics of curves of a DC generator: I. No-load saturation characteristic ( Eo / I f )  It is also known as magnetic (c/s) or open-circuit characteristic (O.C.C).  It shows the relation between the no-load generated e.m.f in armature Eo, and the field or exciting current ( If ) at a given fixed speed.  Its shape is practically the same for all generators whether separately- excited or self-excited. II. 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 (c/s) is of interest mainly to the designer.

44.  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 (c/s) is of great importance in judging the suitability of a generator for a particular purpose.

46.  When the field is zero, there is some generated e.m.f OA,this is due to Residual magnetism.  It can be varied (If) 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:  Hence, if the speed is constant, the above relation becomes: E = K. φ  It is obvious that when (If) 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(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 curve. That is why the upper portion (d b) of curve (o d b) bends over as shown.

48. I)Let us consider a separately-excited generator giving its rated no- load voltage of (Eo) for a certain constant field current. a)If there were no armature reaction and armature voltage drop, then this voltage would have remained constant as shown by the dotted horizontal line (I). II) When the generator is loaded, the voltage falls due to these two causes, thereby giving slightly drooping (c/s). a) If we subtract from (Eo) the values of voltage drops due to armature reaction for different loads. Then we get the value of (E) the e.m.f actually induced in the armature under load conditions. b) Curve (II) is plotted in this way and is know as the internal characteristic.

49. III)The straight line (o a) represents the (IaRa) drops corresponding to different armature currents. a)If we subtract from (E) the armature drop (IaRa) we get terminal voltage (VL). b)Curve (III) represents the external (c/s) and is obtained by subtracting ordinates of line (o a) from those of curve (II).

50. The (O.C.C) or no-load saturated curves for self-excited generators whether shunt or series-connected, are obtained in a similar way.

51.  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.  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, then slope of the resistance line increase, and hence the maximum voltage to which the generator will build up at a given speed, decreases.

52.  If field resistance increased so much that the resistance line dose 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 Rc for a given speed.  How to draw O.C.C at Different Speed  Suppose we are given the data for (O.C.C) of a generator run at a fixed speed, say, N1. It will be shown that (O.C.C) at any other constant speed N2 can be deduced from the (O.C.C) for N1.

54.  It is found that if after building up, a shunt generator is loaded, then its terminal voltage (VL) drops with increase in load current. Such a drop in voltage is undesirable especially when the generator is supplying current for load and power for which purposes it is desirable that (VL) should remain practically constant and independent of the load.  There are three main reasons for the drop in terminal voltage of a shunt generator when under load. I) Armature resistance drop. II) Armature reaction. III) The drop in terminal voltage due to armature resistance and armature reaction results in a decreased field current (If) which further reduces the induced (e.m.f).

55.  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(O A) due to a very rapid decrease in terminal voltage.  The shunt generator is first excited on no-load so that it gives its full open circuit voltage (o a).  The load is gradually applied and, at suitable intervals, the terminal voltage(VL) (as read by the voltmeter) and the load current (IL) ( as read by the ammeter A2) are noted.

56.  The field current as recorded by ammeter (A1), is kept constant by a rheostat. By plotting these reading. The external (c/s)is obtained.  The portion (a b) is 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 condition hold good till point (b) is reached. This point is known as break-down point.  It is found that beyond this point(where load is max=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.

58.  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 (Ia.Ra= MK).  If we add these drops to the ordinates of curve(ac),we get the internal characteristic.

59.  The magnetization curve of a series DC generator looks very much like the magnetization curve of any other generator.  At no load, however, there is no field current, so (VL) is reduced to a small level given by the residual flux in the machine.  As the load increases, the field current rises, so (E) rises rapidly.  The Ia.(Ra+Rf) drop goes up too, but at first the increase in (E) goes up more rapidly than Ia.(Ra+Rf) drop rises, so (VL) increase.

60.  After a while, the machine approaches saturation, and (E) becomes almost constant. At that point, the resistive drop is predominant effect, and VL starts to fall.  It is obvious that this machine would make a bad constant- voltage source. In fact, its voltage regulation is a large negative number.

61.  If the full-load voltage is thereby made the same as the no-load voltage, this is known as a level-compound characteristic.  The curve is not actually flat because armature reaction demagnetizing effects are not exactly linear with current.  If the series field amp-turns are such that the rated-load voltage is greater than the no-load voltage, then generator is over- compounded.  If rated-load voltage is less than the no-load voltage, then the generator is under-compounded but such generators are seldom used.

63. Condition for Build up of a self-excited  The conditions necessary for the build-up of a (self- excited) generator as follows: There must be some residual magnetism in the generator poles. For the given direction of rotation, the (shunt or series) field coils should be correctly connected to the armature i.e. they should be so connected that the induced current reinforces the e.m.f produced initially due to residual magnetism.

68. ARMATURE REACTION

69. Armature Reaction in a d.c. machine is basically the effect of armature produced flux on the main flux or field flux.

70. The armature reaction produces the following two undesirable effects: 1. It demagnetizes or weakens the main flux. 2. It cross-magnetizes or distorts the main flux.

71. Reduction in main flux per voltage reduces the generated voltage and torque whereas distortion of the main- field flux influences the limits of successful commutation in d.c. machines.

72.  To understand this process let us first assume a 2-pole d.c. machine at no load.  At that instant there is no armature current.  So the flux due to mmf produced by field current in the machine at north pole of the magnet will flow towards the south pole of the magnet.

74. The net / resultant flux of the system can be taken as a straight horizontal line OA and can be shown in phasor as :- Also at that instant the Magnetic Neutral Axis (M.N.A) of the machine will coincide with the Geometrical Neutral Axis (G.N.A) of the machine as the M.N.A is always perpendicular to the net flux.

75.  Now when the dc machine is loaded, current flows in armature windings. This armature current set up armature flux. With field windings unexcited, the flux can be shown as vertical lines across armature conductors.  The conductors on the left side of the M.N.A will have current flowing in inside direction whereas on right side of MNA, the current will flow in outside direction. The direction of the flux thus produced can be determined by using Maxwell‘s Right hand Screw rule.

77. The resultant flux of the system is a straight vertical line OB and can be shown in phasor as :- Note that the magnitude of OB will always be less than OA since the cause of armature flux is field flux and it is known to us that effect is always less than cause. Here armature flux is the effect and field flux is its cause.

78. An examination to the above two phasor reveals that the path of armature flux is perpendicular to the main field flux. In other words, the path of the armature flux crosses the path of the main field flux. This can be shown in phasor as :- Thus the effect of armature flux on the main field flux is entirely ‘cross-magnetizing’ and it is for this reason that the flux produced by armature mmf is also called as cross-flux.

79.  When the current flows in both the armature and field windings, the resultant flux distribution is obtained by superimposing theses two fluxes. i.e.

80.  It is observed that the armature flux aids the main field flux at the lower end of the N-pole and at the upper end of the S-pole, therefore at these two poles, the armature flux strengthens the main field flux.  Likewise, the armature flux weakens the main field flux at Upper end of the N-pole and at lower end of the S-pole.

82. Fig 1 Fig 2

83. 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 demagnetizing 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.

84.  Now, if there is no magnetic saturation, then the amount of strengthening and weakening of the main field flux are equal and the resultant flux per pole remains unaltered from its no load value.  Actually, the magnetic saturation does occurs and as a consequence, the strengthening effect is less as compared to the weakening effect and the resultant flux is decreased from its no-load value. This is called ‘Demagnetizing effect of armature reaction’

85.  So when the machine is run loaded, M.N.A will shift from G.N.A of the machine.  The resultant shift is completely dependent on the magnitude of armature current.  Thus, greater the value of armature current, greater is the shift of MNA from GNA.

86. It may therefore be stated from the above that net effect of armature flux on the main field flux is:- 1. To distort the main field flux thereby causing non- uniform distribution of flux under the main poles. 2. To shift the MNA in the direction of the rotation for a generator and against the direction of rotation for a motor. 3. To reduce the main field flux from its no-load value due to magnetic saturation.

87. De-magnetising and Cross-magnetising Conductors:

90. By high Reluctance at POLE TIPS: At the time of construction we use chamfered poles. These poles have larger air gap on the tips and smaller air gap at the centre. These poles provide non-uniform air gap. The effect of armature reaction is more near to edge of poles and negligible near the centre of pole. If air gap is kept non uniform i.e., larger air gap at the edges(Pole Tip) and smaller near the centre of the pole and then armature flux near the pole tip decreases and armature reaction decreases.

92. By Laminated Pole Shoe: We insert Laminated objects in the pole. By having Laminated pole shoe the reluctance in the armature flux path increases. Hence the armature flux gap gets reduced.

95. The compensating windings consist of a series of coils embedded in slots in the pole faces. These coils are connected in series with the armature in such a way that the current in them flows in opposite direction to that flowing in armature conductors directly below the pole shoes.  The series-connected compensating windings produce a magnetic field, which varies directly with armature current.  As the compensating windings are wound to produce a field that opposes the magnetic field of the armature, they tend to cancel the effects of the armature magnetic field.

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

99.  Methods to improve commutation  e.m.f. in coil undergoing commutation  Reactance e.m.f

101.  A d.c. machine (generator or motor) generally employs windings distributed in slots over the circumference of the armature core.  Each conductor lies at right angles to the magnetic flux and to the direction of its movement Therefore, the induced e.m.f. in the conductor is given by: e = Bl v volts  The armature conductors are connected to form coils.. It has two conductors or coil sides connected at the back of the armature.

102. Commutator Pitch (Y C) The commutator pitch is the number of commutator segments spanned by each coil of the winding. It is denoted by YC. In Fig. (1), one side of the coil is connected to commutator segment 1 and the other side connected to commutator segment 2. Therefore, the number of commutator segments spanned by the coil is 1 i.e., YC = 1. In Fig. (2), one side of the coil is connected to commutator segment 1 and the other side to commutator segment 8. Therefore, the number of commutator segments spanned by the coil = 8 - 1 = 7 segments i.e., YC = 7. Since each coil has two ends and as two coil connections are joined at each commutator segment, Number of coils = Number of commutator segments For example, if an armature has 30 conductors, the number of coils will be 30/2 = 15. Therefore, number of commutator segments is also 15. Note : commutator pitch is the most important factor in determining the type of d.c. armature winding.

104. Note: The distance between two adjacent poles

106.  Coil span is defined as peripheral distance between two sides of a coil, measured in terms of number of armature slots between them.  That means, after placing one side of the coil in a particular slot, after how many conjugative slots, the other side of the same coil is placed on the armature. This number is known as coil span.  a. Full-Pitched Coil: If the coil-span or coil pitch is equal to pole pitch, it is called full-pitched coil.  b.Fractional pitched coil. If the coil span or coil pitch is less than the pole pitch, then it is called fractional pitched coil.

110.  The different armature coils in a d.c. armature Winding must be connected in series with each other by means of end connections (back connection and front connection) in a manner so that the generated voltages of the respective coils will aid each other in the production of the terminal e.m.f. of the winding.  Two basic methods of making these end connections are: Simplex lap winding Simplex wave winding