DC Generators
- 1. Electric Generators • In the first practical electric generators, called dynamos, the AC was converted into DC with a commutator, a set of rotating switch contacts on the armature shaft. • The initial electromagnetic generator (Faraday disk) was invented by British scientist namely Michael Faraday in the year 1831. • The first type was a generator of direct current (DC) electricity. • The second type was a generator of alternating current (AC) electricity.
- 2. • A DC generator is an electrical device used for generating electrical energy. • The main function of this device is to change mechanical energy into electrical energy. • There are several types of mechanical energy sources available such as hand cranks, internal combustion engines, water turbines, gas and steam turbines.
- 3. CONSTRUCTION OF A DC MACHINE PRACTICAL DC MACHINE SECTIONAL VIEW OF DC MACHINE
- 4. YOKE -Acts as outer frame of the machine - Mechanical support -Low reluctance path for magnetic flux produced by field winding -High Permeability -For Small machines yoke is made of Cast iron -For Large Machines yoke is made of Cast Steel (Rolled steel) Large DC machine Small DC machine
- 5. POLE CORES AND POLE SHOES Pole core (Pole body): Carry the field Act as electromagnet Fitted to yoke through bolts or welding Pole shoe: Reduce the reluctance of magnetic path Pole shoes serve two purposes (i) they support field coils and (ii) spread out the flux in air gap uniformly.
- 6. POLE COILS •Also known as field coils/ magnetizing coils •made of copper •Provide excitation •Field coils are wound and placed on each pole and are connected in series •They are wound in such a way that, when energized, they form alternate North and South poles.
- 7. ARMATURE CORE • Armature core is the rotor of the machine. • It is cylindrical in shape with slots to carry armature winding. • The armature is built up of thin laminated circular steel disks for reducing eddy current losses. • It may be provided with air ducts for the axial air flow for cooling purposes. • Armature is keyed to the shaft.
- 8. ARMATURE WINDING •Winding is made of Copper (or) Aluminum •Conductors are insulated and placed in the slots of the armature core. • The armature winding is the heart of the DC Machine. •Armature winding is a place where conversion of power takes place.
- 9. ARMATURE WINDING • On the basis of connections the armature windings are classified into two types: – Lap Winding – Wave Winding.
- 10. LAP WINDING and WAVE WINDING • The windings may be a lap winding or wave winding. • The difference between the two consists in the arrangement of the end of the connections at the front of the armature. LAP WINDING WAVE WINDING
- 12. LAP WINDING • In the lap winding the finish of each coil is connected to the start of the next coil so that winding or commutator pitch is unity. • The lap winding may be progressive (it progresses in the direction in which the coils are wound) or retrogressive (it progresses in the opposite direction). • Equalizing connections are necessary. Advantages: • This winding is necessarily required for large current application because it has more parallel paths. • It is suitable for low voltage and high current generators.
- 13. WAVE WINDING • In the wave winding the finish of each coil is connected to the start of another coil well away from the first coil. • Equalizing connections are not necessary. Advantages: • For a given number of poles and armature conductors, wave winding gives more EMF than the lap winding. • Suitable for small generators(500-600V). • Wave winding is used for high voltage, low current machines.
- 14. COMMUTATOR •Physical connection to the armature winding is made through a commutator-brush arrangement. •The function of a commutator: in dc generator -to collect the current generated in armature conductors. in dc motor -helps in providing current to the armature conductors. •A commutator consists of a set of copper segments which are insulated from each other. •The number of segments is equal to the number of armature coils. •Each segment is connected to an armature coil and the commutator is keyed to the shaft.
- 15. BEARINGS AND BRUSHES Brushes •Made from carbon or graphite. •They rest on commutator segments and slide on the segments when the commutator rotates keeping the physical contact to collect or supply the current. Shaft and bearings •Made of mild steel with a maximum breaking strength. •Used to transfer mechanical power from or to the machine. •The rotating parts like armature core, commutator, cooling fans, etc. are keyed to the shaft.
- 16. Rotor of a dc machine https://www.youtube.com/watch?v=d_LOXUEFA-o DC Machine Construction
- 17. DC GENERATOR
- 18. *All the generators works on a principle of dynamically induced e.m.f. Faraday's laws of electromagnetic induction – the relationship between electric circuit and magnetic field. – basic working principle of the most of the electric motor, generators, transformers, inductors etc. Faraday's first law: Whenever a conductor is placed in a varying magnetic field an EMF gets induced across the conductor and if the conductor is a closed circuit then induced current flows through it. Working Principle of DC Generator
- 19. Faraday's second law of electromagnetic induction states that, the magnitude of induced emf is equal to the rate of change of flux linkages with the coil. The flux linkages is the product of number of turns and the flux associated with the coil. Phenomenon of Mutual Induction Alternating current flowing in a coil produces alternating magnetic field around it. When two or more coils are magnetically linked to each other, then an alternating current flowing through one coil causes an induced emf across the other linked coils. This phenomenon is called as mutual induction.
- 20. Flemings Right Hand rule As per Faraday's law of electromagnetic induction, whenever a conductor moves inside a magnetic field, there will be an induced current in it. If this conductor gets forcefully moved inside the magnetic field, there will be a relation between the direction of applied force, magnetic field and the current. This relation among these three directions is determined by Fleming's Right Hand Rule. This rule states "Hold out the right hand with the first finger, second finger and thumb at right angle to each other. If forefinger represents the direction of the line of force, the thumb points in the direction of motion or applied force, then second finger points in the direction of the induced current.
- 22. Working principle of DC Generator Let us consider a single turn coil ABCD rotated on a shaft in a clockwise direction. The coil is placed in a uniform magnetic field, between a north pole(N) and south pole(S). The blue color lines in the below figure represent the magnetic flux lines. The coil sides AB and CD are connected to the slip rings a and b respectively. From the slip rings, the current is taken out and given to the load through brushes 1 and 2. When coil sides AB and CD are moving parallel to the magnetic field, the coil sides do not cut the flux. Instead, they move parallel to the flux. Hence, no emf is induced in the coil.
- 23. • At this angle, the coil sides AB and CD are perpendicular to the magnetic field. Now, the flux linked with the coil is minimum but the rate of change of flux is maximum. Hence, maximum emf is induced in the coil at position 2. • As the coil rotates further from θ = 90 to θ = 180, the rate of change of flux linkage reduces steadily, til position 4 is reached. Here, again the coil sides become parallel to the magnetic flux lines. The rate of change of flux linkage will be minimum. So no emf is induced when it reaches position 4. • Thus during the first half rotation of the coil from θ = 0 to θ = 180, no emf is induced at position 0, then increases and becomes maximum at position 2, then decreases and no emf is induced at position 4.
- 24. • In the next half rotation of the coil, that is, from θ = 180 to θ = 360, the variation in the emf induced is similar to that of the first half rotation. No emf is induced at position 4, then increases and becomes maximum at position 6, then decreases, and no emf is induced at position 8. • But the direction of current induced gets reversed in the second half rotation. The path of current flow is from DCFGBA. It is just the reverse of the current flow during the first half rotation of the coil. • If this rotation of the coil is continued, the change in emf is repeated and becomes alternately positive and negative. Such an emf is called as alternating emf. It has both positive and negative half-cycles and is called a bidirectional output.
- 27. Working of DC Generator with split rings or commutator • Split rings are made up of conducting cylinder which is cut into two segments insulated from each other by a thin layer of mica or some insulating materials. The split rings are also called as a commutator.
- 28. • During the first half rotation of the coil, the current flows from ABFGCD. The brush 1 will get in contact with the commutator segment E and brush 2 will be in contact with the segment H. So the current flows from F to G in the load. • During the second half rotation of the coil, the current gets reversed and the path of flow is DCFGBA. The brush 1 will get in contact with the commutator segment H and brush 2 will be in contact with the segment E. So the current flows from F to G in the load. • For each half rotation of the coil, the commutator segments change their position. The current through the load flows in the same direction, from F to G. Thus the current thus produced is unidirectional but not continuous like a pure DC current.
- 29. EMF EQUATION OF A GENERATOR Let = flux/pole in Weber Z =Total number of armature conductors Z=No. of slot × No. of conductors/slot P= No. of generator poles A =No. of parallel paths in armature N= Armature rotation in revolutions per minute (r. p. m) E= e.m.f induced in any parallel path in armature Eg= e.m.f generated in any one of the parallel paths Average e.m.f generated/conductor = d volt dt Now, flux cut/conductor in one revolution d = P wb
- 30. EMF EQUATION OF A GENERATOR Time for one revolution , dt= 60 /N sec According to Faraday’s Law of electro magnetic induction E.M.F generated/conductor = d= PN volts dt 60 No. of conductors (in series) in one parallel path= Z / A E.M.F generated/path= PN × Z Volts 60 A Generated E.M.F, Eg= Z N × P Volts 60 A For i) Wave winding A = 2 ii) Lap winding A = P
- 31. Problems 1.4 pole wave wound armature has 720 conductor and is related 1000 rev/min. If the useful flux is 20 mWb, calculated the generated voltage. Generated E.M.F, Eg= Z N × P Volts 60 A Eg=480 V
- 32. Problems • An 8 pole lap connected has armature 4 slot with 12 conductor an generate of voltage 500 V determine the speed at which running if the flux per pole is 50 mWb. Generated E.M.F, Eg= Z N × P Volts 60 A • N = 1250 r.p.m
- 33. EXCITATION: • The application of energy to something. • DC generators are classified into two main categories based on the method of excitation. (i) Separately excited, (ii) Self-excited. Types of Generators 1. Separately excited: In this type, field coils are energized from an independent external DC source. METHODS OF EXCITATION
- 34. TYPES OF GENERATOR 2. Self excited: (Voltage building process) • In this type, field coils are energized from the current produced by the generator itself. •Initial emf generation is due to residual magnetism in field poles. •The generated emf causes a part of current to flow in the field coils, thus strengthening the field flux and thereby increasing emf generation. •Self excited dc generators can further be divided into three types - (a) Series wound - field winding in series with armature winding (b) Shunt wound - field winding in parallel with armature winding (c) Compound wound - combination of series and shunt winding
- 35. TYPES OF DC GENERATORS
- 36. Voltage drop in the armature = Ia × Ra (R/sub>a is the armature resistance) Let, Ia = IL = I (say) Then, voltage across the load, V = IRa Power generated, Pg = Eg×I E=V t+ I a Ra + V brush Power delivered to the external load, PL = V×I. SEPERATELY EXCITED DC GENERATOR
- 37. Ia = Isc = IL E=V t+ I a Ra + I a Rse + V brush Power generated, Pg = EgI Power delivered to the load, PL = VIL Shunt field current, Ish = V/Rsh E=V t+ I a Ra + V brush Power generated, Pg= EgIa Power delivered to the load, PL = VIL SELF EXCITED DC GENERATOR- SHUNT GENERATOR SELF EXCITED DC GENERATOR- SERIES GENERATOR
- 38. Series field current, Ise = IL Shunt field current, Ish = (V+Ise Rse)/Rsh Armature current, Ia = Ish + IL E=V t+ I a Ra + I L R se + V brush Power generated, Pg = Eg×Ia Power delivered to the load, PL=V×IL Shunt field current, Ish=V/Rsh Armature current, Ia= Ise= IL+Ish Series field current, Ise= IL+Ish E=V t+ I a Ra + I a Rse + V brush Power generated, Pg= EgIa Power delivered to the load, PL=V×IL LONG SHUNT DC COMPOUND GENERATOR SHORT SHUNT DC COMPOUND GENERATOR
- 39. •In a compound wound generator, the shunt field is stronger than the series field. •When the series field assists the shunt field, generator is said to be commutatively compound wound. •On the other hand if series field opposes the shunt field, the generator is said to be differentially compound wound. CUMULATIVE COMPOUND MOTOR DIFFERENTIAL COMPOUND MOTOR
- 40. CHARACTERISTICS OF DC GENERATOR • No load saturation or Open Circuit Characteristic (O.C.C.) (E0/If) or Magnetization characteristics • Internal or Total Characteristic (E/Ia) • External Characteristic (V/IL)
- 41. Separately excited DC Generator Open circuit characteristics
- 42. Separately excited DC Generator 1. Open circuit characteristic It is also known as magnetic characteristic or no-load saturation characteristic. This characteristic shows the relation between generated emf at no load (E0) and the field current (If) at a given fixed speed. Same for both the DC generators.
- 43. Separately excited DC Generator 2. Internal Characteristic • This characteristic shows the relation between the on-load generated emf (Eg) and the armature current (Ia). • The on-load generated emf Eg is always less than E0 due to the armature reaction. • Therefore, internal characteristic curve lies below the O.C.C. 3. External Characteristic • An external characteristic curve shows the relation between terminal voltage (V) and the load current (IL).
- 44. EXTERNAL CHARACTERISTICS SEPERATELY EXCITED DC GENERATOR
- 45. Self excited DC Generator 1. Open circuit characteristic It is also known as magnetic characteristic or no-load saturation characteristic. This characteristic shows the relation between generated emf at no load (E0) and the field current (If) at a given fixed speed. Same for both the DC generators.
- 46. Self excited DC Generator 2. Internal Characteristic • This characteristic shows the relation between the on-load generated emf (Eg) and the armature current (Ia). • The on-load generated emf Eg is always less than E0 due to the armature reaction. • Therefore, internal characteristic curve lies below the O.C.C. 3. External Characteristic An external characteristic curve shows the relation between terminal voltage (V) and the load current (IL).
- 47. INTERNAL and EXTERNAL CHARACTERISTICS SHUNT DC GENERATOR
- 48. INTERNAL and EXTERNAL CHARACTERISTICS SERIES DC GENERATOR
- 49. INTERNAL and EXTERNAL CHARACTERISTICS COMPOUND DC GENERTOR
- 50. Voltage Build up in DC Shunt Generator
- 51. What is Critical Field Resistance of DC Shunt Generator ? • It is defined as the amount of field resistance required to generate emf in the armature winding. (or) • The critical field resistance is defined as the field resistance of the generator which holds the rated voltage of it. (or) • It is the value of field resistance beyond which the generator fails to build up the voltage if there is a further increase in the field resistance.
- 52. Significance of Critical Field Resistance The total field circuit resistance must be equal to or less than the critical field resistance value.
- 53. What is Critical Speed of DC Shunt Generator ? • Critical speed is defined as the speed at which the given shunt field resistance is equal to the critical resistance. It is the speed at which the shunt generator just fails to build up its voltage without any external resistance in the field circuit. It is denoted by Nc.
- 54. Significance of Critical Speed
- 55. Problem 1 A shunt generator delivers 450 A at 230 V and the resistance of the shunt field and armature are 50 Ω and 0.03 Ω respectively. Calculate the generated e.m.f?
- 56. Problem 1 A shunt generator delivers 450 A at 230 V and the resistance of the shunt field and armature are 50 Ω and 0.03 Ω respectively. Calculate the generated e.m.f?
- 57. Problem 2 • A long shunt compound generator delivers a load current of50 A at 500 V, and the resistances of armature, series and shunt fields are 0.05 ohm, 0.03 ohm, and 250 ohms respectively. Calculate the generated emf, and armature current. Allow 1.0 V per brush for contact drop.
- 58. • A long shunt compound generator delivers a load current of 50 A at 500 V, and the resistances of armature, series and shunt fields are 0.05 ohm, 0.03 ohm, and 250 ohms respectively. Calculate the generated emf, and armature current. Allow 1.0 V per brush for contact drop.
- 59. Problem 3 • The armature of a 4–pole, lap-wound shunt generator has 120slots with 4 conductors per slot. The flux per pole is 0.05 Wb. The armature resistance is 0.05 Ω and shunt field resistance is 50 Ω. Find the speed of machine, when supplying 450 A at a terminal voltage of 250 V.
- 60. • The armature of a 4–pole, lap-wound shunt generator has 120slots with 4 conductors per slot. The flux per pole is 0.05 Wb. The armature resistance is 0.05 Ω and shunt field resistance is 50 Ω. Find the speed of machine, when supplying 450 A at a terminal voltage of 250 V.
- 62. APPLICATIONS OF DC GENERATORS SEPARATELY EXCITED DC GENERATORS • Used in laboratories for testing as they have a wide range of voltage output. • Used as a supply source of DC motors. SHUNT WOUND GENERATORS • Used for lighting purposes. • Used to charge the battery. • Providing excitation to the alternators. SERIES WOUND GENERATORS • Used in DC locomotives for regenerative braking for providing field excitation current. • Used as a booster in distribution networks. COMPOUND WOUND GENERATOR • Over compounded cumulative generators are used in lighting and heavy power supply. • Flat compounded generators are used in offices, hotels, homes, schools, etc. • Differentially compounded generators are mainly used for arc welding purpose.