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UNIT - I D.C. GENERATORS

UNIT - I D.C. GENERATORS. D.C. GENERATORS-CONSTRUCTION & OPERATION DC Generators Principle of operation Action of Commutator Constructional details of DC Machine Types of DC generators EMF Equation. DC Generator. DC motor. D.C. GENERATORS PRINCIPLE OF OPERATION

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UNIT - I D.C. GENERATORS

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  1. UNIT - I D.C. GENERATORS

  2. D.C. GENERATORS-CONSTRUCTION & OPERATION • DC Generators • Principle of operation • Action of Commutator • Constructional details of DC Machine • Types of DC generators • EMF Equation

  3. DC Generator

  4. DC motor

  5. D.C. GENERATORS PRINCIPLE OF OPERATION • DC generator converts mechanical energy into electrical energy. when a conductor move in a magnetic field in such a way conductors cuts across a magnetic flux of lines and e.m.f. produces in a generator and it is defined by faradays law of electromagnetic induction e.m.f. causes current to flow if the conductor circuit is closed.

  6. First Law :   Whenever the magnetic flux linked with a circuit changes, an e.m.f. is always induced in it.                                          or   Whenever a conductor cuts magnetic flux, an e.m.f. is induced in that conductor.  Second Law :   The magnitude of the induced e.m.f. is equal to the rate of change of flux linkages.

  7. Faradays Law of Electromagnetic Induction A changing magnetic flux through a  loop or loops of wire induces an electromotive force (voltage) in each loop.

  8. Lenz’s Law “The induced currents in a conductor are in such a direction as to oppose the change in magnetic field that produces them..” OR “The direction of induced E.M.F in a coil (conductor) is such that it opposes the cause of producing it..”

  9. Fleming's Right Hand Rule E.M.F • The Thumb represents the direction of Motion of the conductor. • The First finger (four finger) represents Field. • The Second finger (Middle finger) represents Current

  10. Fleming's Right Hand Rule

  11. The following are the basic requirements to be satisfied for generation of E.M.F • Magnetic field :- Permanent Magnet (or) Electro Magnet (practical) • Conductor :- Copper (or) Aluminum bars placed in slots cut around the periphery of cylindrical rotor • Relative motion:- By Prime Mover Turbine I.C Engine (Internal combustion) 1.A uniform Magnetic field 2.A System of conductors 3.Relative motion between the magnetic field and conductors

  12. Simple loop generator

  13. Basic Generator

  14. Generators

  15. Simple loop generator with slip ring

  16. Generators • Basic operation of the generator • As the loop rotates, the magnetic flux through it changes with time • This induces an e.m.f and a current in the external circuit • The ends of the loop are connected to slip rings that rotate with the loop • Connections to the external circuit are made by stationary brushes in contact with the slip rings

  17. Simple loop generator with split ring

  18. Simple loop generator with split ring

  19. Working Principle of D.C Generator Schematic diagram of a simple DC Generator 1st half cycle(00 to 1800 ) Path of current ABR1B1MLR2B2CD 2st half cycle(1800 to 3600) Path of current DCR2B1MLB2R1BA

  20. The output voltage always has the same polarity The current is a pulsating current To produce a steady current, many loops and commutators around the axis of rotation are used The multiple outputs are superimposed and the output is almost free of fluctuations DC Generators, cont

  21. Unidirectional current wave shape Resultant current wave shape when number of conductors used result current wave shape

  22. Constructional Details Of DC Machine • Yoke: • Rotor: • Stator:  • Field electromagnets:  • Pole core and pole shoe: • Brushes: • Shaft: • Armature: • Coil: • Commutator: • Bearings:

  23. Construction details of DC generator Cross section view of dc machine N shaft S

  24. Main parts of a 4-pole d. c machine

  25. Practical Dc Machine

  26. 1)Yoke 1)Yoke:- - Acts as frame of the machine - Mechanical support - low reluctance for magnetic flux - High Permeability -- For Small machines -- Cast iron—low cost -- For Large Machines -- Cast Steel (Rolled steel) Small DC machine Large DC machine

  27. 2)pole cores and pole shoes 2)Field Magnets:- a) Pole core (Pole body) :- --Carry the field coils --Rectangle Cross sections -- Laminated to reduce heat losses --Fitted to yoke through bolts b) Pole shoe:- Acts as support to field poles and spreads out flux Pole core & Pole shoe are laminated of annealed steel (Of thickness of 1mm to 0.25 mm)

  28. 2)pole cores and pole shoes 2)Field Magnets:- c) Field coils (Magnetizing coils):- -- Provide excitation (exciting coils) I . e field flux --Number of poles depends speed of armature on and the output for which the machine designed --Frame to used for design for exciting coils Different types of fields i) Separately Exciting ii) Self Exciting

  29. 3)Armature core

  30. 3)Conductor system:- a) Armature core (Armature):- -- To support armature windings --To rotate conductors in a magnetic field -- it is cylindrical or drum shaped is built --Made of high permeability silicon steel stampings (of 0.5 mm thick) -- Each stamping is separated from its neighboring one by thin varnish as insulation --Laminated to reduce eddy current losses -- A small air gap between pole pieces and armature so that no rubbing between them -- High grade silicon steel used to reduce i) Hysteresis loss ii) Eddy current loss -- Ventilating ducts are provided to dissipate heat to dissipate heat generated by above losses b) Armature Winding:- Main flux cuts armature and hence E.M.F is induced --winding made of Copper (or) Aluminum --windings are insulated each other

  31. 4)commutator 4) Commutator:--Hard drawn copper bars segments insulated from each other by mica segments (insulation) -- Between armature & External circuit -- Split-Rings (acts like Rectifier AC to DC )

  32. 5&6 Bearings and Brushes 5)Brushes and brush gear:- Carbon, Carbon graphite, copper used to Collects current from commutation (in case of Generator) 6)Shaft and bearings:- Shaft-- Mechanical link between prime over and armature Bearings– For free rotation

  33. DC Machine Construction

  34. DC Machine Construction Rotor of a dc machine

  35. DC Machine Construction Cutaway view of a dc machine

  36. Armature Winding Armature Winding is classified into two types: • Lap winding • Wave windings

  37. Armature windings

  38. Lap Winding: • are used in machines designed for low voltage and high current • armatures are constructed with large wire because of high current • Eg: - are used is in the starter motor of almost all automobiles • The windings of a lap wound armature are connected in parallel. This permits the current capacity of each winding to be added and provides a higher operating current. • No of parallel path, A=P ; P = no. of poles

  39. Wave winding: • are used in machines designed for high voltage and low current • their windings connected in series • When the windings are connected in series, the voltage of each winding adds, but the current capacity remains the same • are used is in the small generator. • No of parallel path, A=2,

  40. Commutation process in D.C Generator • Commutation is the positioning of the DC generator brushes so that the commutator segments change brushes at the same time the armature current changes direction.

  41. Generated EMF or EMF Equation of a generator Let  = flux/pole in Weber Z =Total number of armature conductors =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 Generated e.m.f Eg= e.m.f generated in any one of the parallel paths i.e E Average e.m.f generated/conductor = d  volt dt Now, flux cut/conductor in one revolution d  = P wb

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